RELIABILITY EFFICIENCY ASSET MANAGEMENT

PERFORMANCE | RELIABILIT Y | EFFICIENCY | ASSE T MANAGEMENT

&Compressors compressed air systems

2005 - 2006 collection

JANUARY 2005WWW.PLANTSERVICES.COM42

RELIABILITYCompressors

K NOWN AS THE FOURTH UTILITY, compressed air is used in manyfacets of manufacturing. Many plants use it in one capacity or another and,for the most part, these systems provide similar outputs. However, not all com-

pressed air is identical. In several situations, specially engineered air powers a varietyof machinery and tools used across several different industries, from automobile man-ufacturing to beverage processing.

Engineered air is a term that describescompressed air that has been tailoredto meet at least three specific industryneeds — 100% oil-free, particulate-freeand completely reliable. It goes beyondidentifying only a specified pressure.Engineered air provides the right typeof air for the application. Engineered airreliability is a function of system redun-dancy, accessibility and maintenance, andtechnological advances in control andmonitoring.

Consider redundancyWhen an air compressor is vital to plantoperations, a redundant system mini-mizes the possibility of system failurethat can ruin production quotas. Exam-ine your operation to see where air sys-tem redundancies might be useful. Forexample, the air compressor’s oil pump,the heart of the lubrication system,keepsthe machine running smoothly. If thepump fails, the machine comes to a

grinding halt.A redundant system wouldinclude two full-capacity, full-pressurepumps, one a shaft-driven main pumpand the other a motor-driven auxiliary.During regular operations, the shaft-driven main pump operates, while themotor-driven auxiliary is on perpetualstandby for emergency situations.Such a system provides full capacitybackup.Without this redundant system,the entire compressor would need to beshut down in the event of an oil sys-tem malfunction.

Make repairs straightforwardAlthough it requires additional time andattention from plant professionals, com-pressor cleaning and maintenance rep-resent a sound investment.As with otherplant assets, clean,well-maintained com-pressed air systems are less likely to breakdown. Simply put, less downtime allowsfor more production. In addition, con-sistent cleaning and maintenance min-

imize wear and tear, which saves moneyin replacement parts.

Many vendors design compressors withaccessibility and maintenance in mind.The intercooler is a good example. Forinstance,using either U-shaped or straightintercooler tubes is standard industrypractice. However, unlike the U-benddesign, straight tubes are easier to clean.A plant engineer can simply remove thecooling water lines,unbolt the water boxand rod the tubes in place. Roddingisn’t possible with U-bend tubes used insome compressors. In addition, intercool-er tubes with a water-in-tube design areeasier to clean and maintain than thosewith an air-in-tube design that requireswire brush or chemical bath cleaning.Thelonger it takes to clean the intercooler,the longer the engineered airflow isunavailable.

Journal bearings are another goodexample of important compressor partsthat benefit from diligent maintenance

AIR

Maintaining reliability requires attention to three keypoints that minimize downtime By Addison Kelley

ENGINEERED

and cleaning. Properly installed andmaintained, these bearings can last forextended periods of time. Horizontallysplit bearings are easier to maintain,clean and replace because accessing themrequires only removing the top half ofthe gear case. No other disassembly isrequired. Some compressors, on theother hand, have one-piece bearings thatrequire complete compressor disassem-bly for cleaning and maintenance.

Interchangeable parts that save timeand money are another factor thatsimplifies compressor maintenance. Forexample, multistage compressors use abull gear and pinion system to power theimpellers. The quality of the bull gearsdirectly determines whether parts areinterchangeable. The American GearManufacturers Association (AGMA)provides established gear quality ratingsthat range from 3 to 15.

If a compressor gear train featuresAGMA Quality Level 12 (or less) andeither a bull gear or one of the pinionsfail, all three will need to be replaced.Onthe other hand, AGMA Quality Level13 gears, otherwise known as aircraft-quality gearing, are generally regarded ashigh-precision gears. They produce lessnoise and, under normal operating con-ditions, have a longer life. More impor-tantly, though, gears and pinionsmanufactured to this standard are inter-changeable.The plant maintenance tech-nician only needs to swap out thedamaged piece in question, which savesmaintenance time and money.

Monitor and controlPlants that use multiple compressors tofeed a single air system need to coordi-nate, monitor and control compressoroperation. An initial investment in mon-itoring technology can ultimately payfor itself.

A PLC-based automatic sequencercan allow for as many as eight compres-sors to communicate with one anotherand operate as a team as it follows a pro-grammed schedule.The sequencers mon-itor and match compressor supply todemand. For example, they can select

which compressors to use, shutting downthose not necessary to plant operations,even choosing backup units as needed.An automatic sequencer can ensure a sta-ble system pressure, allowing your entireoperation to run as efficiently as possi-ble, saving both time and money.

PLC-based modular control systemscan allow your plant operations engineersto monitor and perform diagnosticchecks on your compressed air systemsremotely, helping to predict and preventsystems malfunctions that could resultin engineered-air downtime.These con-trol systems should be easy to operate,resulting in less training time.

Engineered air is produced withincreased reliability and efficiency witha specific plant application in mind.Rec-ognizing that engineered air is not iden-tical to compressed air is the first stepto minimizing downtime and achieving

better productivity. The systems thatproduce engineered air must, themselves,be engineered to operate as reliably aspossible. Three points can help yourplant engineers ensure the most reliableflow of engineered air possible:• System redundancy, such as a dual-

pump oil system, ensures engineeredair is always available at full capacity.

• Accessibility and maintenance reducesthe wear and tear on key machine parts,keeping the system running smoothly.

• Technological advances in controland monitoring provide the infor-mation needed to keep engineeredairflow stable. p

Addison Kelley is vice president of global

customer support at FS-Elliott. Contact him

at [email protected] and (724) 600-8900.

Figure: FS-Elliott

JANUARY 2005 WWW.PLANTSERVICES.COM 43


Figure 1. This compressor lubrication flow diagram features a redundant oil pump.

Go with the flow

DriverMainoil pump

PAPcompressor

Oilmist

eliminator

Filter Cooler

Oilreservoir

Auxiliaryoil pump

Oiltemperaturetransmitter

Oilpressure

transmitter

Panel

Checkvalve 3

Checkvalve 2

Checkvalve 1

Reliefvalve

Thermalvalve

Bleedline

FEBRUARY 2005 WWW.PLANTSERVICES.COM 57

A common error we see in compressed airsystems, in addition to poor piping practice,is line sizes too small for the desired air flow.This isn’t limited to the interconnecting piping from compres-sor discharge to dryer to header. It also applies to the distrib-ution lines conveying air to production areas and within theequipment found there. Undersized piping restricts the flowand reduces the discharge pressure, thereby robbing the user ofexpensive compressed air power. Small piping exacerbates poorpiping practices by increasing velocity- and turbulence-inducedbackpressure. (See “There’s a Gremlin in your air system — Itsname is turbulence,” Plant Services, July 2002, p. 37).

Pipe size and layout design are the most important variablesin moving air from the compressor to the point of use. Poorsystems not only consume significant energy dollars, but alsodegrade productivity and quality. How does one properlysize compressed air piping for the job at hand? You could ask

the pipefitter, but the answer probably will be,“What we alwaysdo,” and often that’s way off base.

Another approach is matching the discharge connection ofthe upstream piece of equipment (filter, dryer, regulator or com-pressor). Well, a 150-hp, two-stage, reciprocating, double-acting,water-cooled compressor delivers about 750 cfm at 100 psigthrough a 6-in. port. But most 150-hp rotary-screw compres-sors, on the other hand, deliver the same volume and pres-sure through a 2-in. or 3-in. connection. So, which one is right?It’s obvious which is cheaper, but port size isn’t a good guideto pipe size.

Charts and graphsMany people use charts that show the so-called standard pres-sure drop as a function of pipe size and fittings, which sizesthe line for the what is referred to as an acceptable pressuredrop.This practice, too, can be misleading because the charts

THERE’S NO SUCH THING AS TOOLARGE A COMPRESSED AIR LINE

PERFORMANCECompressors

The secretis in thepipe

By Hank van Ormer, Don van Ormer and Scott van Ormer

can’t accommodate velocity- and flow-induced turbulence.Think about it. According to the charts, a short run of small-bore pipe exhibits a low total frictional pressure drop, but thehigh velocity causes an extremely large, turbulence-driven pres-sure drop.Then there’s the question of the meaning of accept-able pressure drop. The answer to this question often isn’tsupported by data, such as the plant’s electric power cost toproduce an additional psig.

We’ve audited many plants during the past 20 yearsand found the unit cost of air for positive-displace-ment compressors runs f rom several hundred dollarsper psig per year to several thousand dollars per psigper year. At current energy costs, you don’t want thepipe to be a source of pressure drop.

Shooting blindNot knowing the energy cost of lost pressure as a function ofline size can lead to a blind decision.Unfortunately, this is whatwe find in most of the air piping systems installed during thepast 30 years. Older systems that were designed with care areoften right on the mark, except if they’ve been modified afterthe original installation.

Some might call pipe sizing a lost art, but we see theissue as a lack of attention to detail, basic piping prin-ciples and guidelines. Read on to learn how to size airpiping using velocity, which, when combined withappropriate piping practice, ensures an efficient com-pressed-air distribution system. As compressed-air sys-tem consultants and troubleshooters, we use theseguidelines to design new piping systems and to analyzeexisting system performance and opportunities forimprovement.

Interconnects and headersThe interconnecting piping is a critical element that must deliv-er air to the distribution headers with little pressure loss, if any.This isn’t only an energy question. It also ensures the capacitycontrols will have sufficient effective storage to allow them toreact to real demand and translate less air usage to a compara-ble reduction in input electrical energy.

The main distr ibution headers not only move airthroughout the plant, they also supply the appropriatelocal storage that ensures the process feeds have ade-quate entry pressure and flow. The main header systemrepresents storage that supports the operating pressureband for capacity control. You want the pressure dropbetween compressor discharge and point of use to besignificantly less than the normal operating control band (10 psig maximum).

The targetsThe objective in sizing interconnecting piping is to transportthe maximum expected volumetric flow from the compressordischarge through the dryers, filters and receivers to the maindistribution header with minimum pressure drop. Contempo-

rary designs that consider the true cost of compressed air tar-get a total pressure drop of less than 3 psi.

Beyond this point, the objective for the main header isto transport the maximum anticipated flow to the pro-duction area and provide an acceptable supply volume fordrops or feeder lines. Again, modern designs consider anacceptable header pressure drop to be 0 psi.

Final l y, for the drops or feeder l ines , the objec-tive is to del iver the maximum anticipated f low tothe work stat ion or process with minimum or nop r e s s u r e l o s s . A g a i n , t h e l i n e s i z e s h o u l d b es ized for near-zero loss . Of course, the controls ,regu la tors , ac tuators and a i r motors a t the workstation or process have requirements for minimumin le t p re s su re to be ab l e to pe r fo rm the i r func-t ions . In many p lant s , the s i z e o f the l ine f eed-ing a work stat ion of ten is selected by people whodon’t know the f low demand and aren’t aware ofhow to s ize piping.

In our opinion, new air-system piping should be sizedaccording to these guidelines. For a system that does-n’t meet the criteria, the cost of modification must beweighed against the energy savings and any improve-ments in productivity and quality.

Obviously, the lower the pressure drop in transporting air,the lower the system’s energy input. Lower header pressurealso reduces unregulated air flow (including leaks) by about1% per psi of pressure reduction.

Eliminate the dropMost charts show frictional pressure drop for a given flowat constant pressure. Wall friction causes most of this loss,which is usually denominated as pressure drop per 100 ft. ofpipe. Similar charts express the estimated pressure loss for fit-tings in terms of additional length of pipe. When added tothe length of straight pipe, the sum is called total equivalentlength.These charts reflect the basic calculations for pressureloss, which include:• Air density at a given pressure and temperature.• Flow rate.• Velocity at pipeline conditions.• The Reynolds number.• Other factors, including a friction factor based on the size

and type of pipe.The calculations and chart data help to identify only the

probable minimum pressure drop. Internal roughness and scal-ing dramatically affect the pipe’s resistance to flow (frictionloss). Resistance increases with time as the inner wall rusts,scales and collects more dirt.This is particularly true of blackiron pipe.

Pressure drop is proportional to the square of the velocity.Any high-volume, intermittent demand produces dramaticpressure drop during peak periods. Ignoring this fact affectsevery process connected to the header. For more detail, see“The compressed air receiver: The endless question,” Plant

FEBRUARY 2005WWW.PLANTSERVICES.COM58


Services, May 1997, p. 49, and Appendix 1, Tables and Out-line from “DOE/CAC Air Master Training Manual.” For agiven size pipe:• At constant pressure, the greater the flow, the greater the

loss per foot of pipe.• At constant flow rate, the lower the inlet pressure, the greater

the loss per foot of pipe.• At any condition, smooth-bore pipe (copper, stainless steel)

exhibits lower friction losses.

Air velocityThe most overlooked idea in piping layout and design is airvelocity. Excessive velocity can be a root cause of backpressure,erratic control signals, turbulence and turbulence-driven pres-sure drop.

The British Compressed Air Society suggests that a veloc-ity of 20 fps or less prevents carrying moisture and debris pastdrain legs and into controls. A velocity greater than 30 fps issufficient to transport any water and debris in the air stream.Thus, the recommended design pipeline velocity for intercon-necting piping and main headers is 20 fps or less, and neverto exceed 30 fps. Field testing reveals that, under these condi-tions, air stream turbulence is generally negligible. Linedrops, feed lines or branch lines less than 50 ft. long workwell at a velocity of 30 fps, but velocity here should notexceed 50 fps.

Crunching numbersFirst, look at the velocity at maximum anticipated flow con-ditions using the following equation:

V = 3.056 * Q/D2 (Eqn 1)Where V = air velocity (ft./sec.)Q = volumetric flow rate (cfm)D = conduit inside diameter (inches)Although this method of determining the minimum pipe

size on the basis of air velocity is easy, you also must considerthat the compressed air volume is expressed in cubic feet per

minute of free air, which is the air volume at ambient atmos-pheric conditions, not the compressed volume.

To adjust the inlet air volumetric flow rate to actual pipelineconditions, you’ll need to divide the volume of free air by thecompression ratio using the following equation:

CR = (P+Pa)/Pa (Eqn 2)Where P = line pressure (psig)Pa = average atmospheric pressure at your elevation (psi)Table 1 shows the compress ion rat io as a func-

t ion of gauge pressure for a locat ion at sea level ,w h e re t h e a t m o s ph e r i c p re s s u re i s 1 4 . 7 p s i . Athigher e levat ions , the average atmospher ic pres-su re d rops and the compre s s ion r a t io r i s e s . Forexample, F lagstaff, Ar iz . , at a 7,000-f t . e levat ion,has an average atmospher ic pressure of about 11psi . At 100 psig, the compression rat io is equal to10 ( i .e . 111/11) .

To determine the pipeline velocity at conditions, merelydivide the velocity given in Equation 1 by the compressionratio given in Equation 2. After selecting the minimumpipe size on the basis of velocity, check any long runs for exces-sive pressure drop using an appropriate drop chart. For exam-ple, a velocity of 25 fps in black iron pipe represents about0.25 psi loss per 100 ft. of run. Although this is a littleabove the recommended minimum of 20 fps and, depend-ing on the layout, would probably be acceptable from a tur-bulence standpoint, a high total frictional loss might dictateusing a larger pipe.

This might seem to be somewhat complicated at first, butit’s the most accurate way to avoid problems in sizing com-pressed air piping. Once you get the hang of it, it’s easy to use.

After carefully selecting a conduit size that eliminates unnec-essary loss, be sure to pay the same attention to downstreamitems such as quick disconnects, regulators, filters, controls,fittings, number of drops from a given header and number ofconnections per header, so as not to offset the gains made withthe pipe. Good piping performance is not an accident — ittakes planning. p

Hank van Ormer, Don van Ormer and Scott van Ormer are owners of

AirPower USA, Pickerington, Ohio. Contact them at

[email protected] and (740) 862-4112.

Figures: AirPower USA

FEBRUARY 2005 WWW.PLANTSERVICES.COM 59


Table 1. Compression ratios at gauge pressures

psig Compression ratio

60 5.05

70 5.76

80 6.44

90 7.12

100 7.8

110 8.48

120 9.16

130 9.84

140 10.52

150 11.20

200 14.5

The calculations and chartdata help to identify onlythe probable minimumpressure drop.

April 2005 www.PLANTSERVICES.com 41

PerformanceCompressors

Air compressors are key components in many manufacturing and process industries. They’re interesting from an engineering standpoint because of the many disci-plines involved in their design and application. Also, they’re interesting historically because they are among the earliest machines, and most people have an intuitive understanding of compressor operation. Anyone who has used a bicycle pump or a bellows has operated an air compressor, and they know that work is required to compress air. Anyone knows that compression heats air.

Big pictureThe major compressor classes are positive-displacement and dynamic. An example of the positive-displacement class is the bicycle pump or fireplace bellows, both of which change the volume of a chamber to compress air. If a piston inside a cylinder forms the chamber, the compressor is known as

a recipro-cating type. These are fur-ther subdivided into single-acting and double-acting. In a single-acting type, only one piston face compresses the air; double-acting types use both faces alter-nately. Reciprocating compressor sizes range from fractional horsepower to more than 600 hp.

Another type is the rotary positive-displacement com-pressor, in either helical screw or sliding vane varieties. The helical screw compresses air between a meshing rotat-ing rotor and screw assembly. Helical screw compressors are available in sizes from about 3 hp to several thousand horsepower.

The sliding-vane compressor uses a set of sliding vanes

pressureApplying the

By Ben J. Sliwinski

These are the elements that affect air-compressor

performance

Photo: Ingersoll-Rand

April 2005www.PLANTSERVICES.com42


placed in slots on a rotor eccentrically mounted in a cylindri-cal casing. As the rotor spins, centrifugal force presses the vanes against the casing wall to compress air between the vanes and the casing.

The second major compressor class is the dynamic type, which compresses by converting air velocity into air pressure using blades mounted on a rotating shaft. In centrifugal com-pressors, air enters near the base of the impeller blades, accel-erates along the blade and exits near the ends of the blades at the circumference of the compressor case. Centrifugal com-pressors range in size from about 100 hp to several thousand horsepower.

In an axial-flow compressor, the air enters and exits along the axis of the shaft, usually after passing through several stages of rotor blades. Each set of rotating blades is separated from the next by nonrotating stator blades. Air compressors in aircraft jet engines are a common example of the axial type. Axial-flow compressors are available in sizes from a few

hundred horsepower to several thousand horsepower.Both positive-displacement and dynamic compressors can

be single- or multistage. Multiple-stage compressors need two or more stages to reach the final output pressure; the output of one stage being the input to the next. Cooling the air between stages improves compressor efficiency.

Each compressor type — reciprocating, screw, rotary vane, centrifugal and axial — has typical operating charac-teristics. There is, however, overlap and, for a given appli-cation, one might have a choice of types. Some important characteristics are flow, pressure, capacity control and lubrication.

The outputThe higher operating speed and continuous flow through dynamic compressors gives them the greatest flow capacity. Axial units provide the greatest flow capacity, but there’s overlap in flow capacity between centrifugal and axial com-pressors. A rough ranking of the flow capacity of the posi-tive-displacement compressors from highest to lowest would be screw, sliding vane and reciprocating, but there’s a great deal of overlap.

Output pressures from positive-displacement units are sim-ilar, with reciprocating units developing the greatest pressure. Within the dynamic compressor family, centrifugal compres-sors provide greater pressure capabilities than the axial type. Figure 1 shows the approximate range of flow and pressure for various compressor types.

Holding backSome methods of capacity control are unique to a compres-sor type, whereas other methods apply to all types. Cylinder unloading is uniquely applied to reciprocating compressors. It controls capacity by delaying the closing of the suction valves so that air drawn into the cylinder can leak back into the suction plenum before compression starts. Keeping the valve open through the entire compression stroke completely unloads the cylinder. More sophisticated systems that allow the valve to close at any time during the compression stroke achieve 100% to 0% capacity variation.

Other control schemes for reciprocating compressors include start-stop, variable-speed and bypass control (in which compressed air is bypassed to the suction). Vibra-tion and bearing lubrication might limit variable-speed control in reciprocating compressors to about 40% of nomi-nal speed.

Slide-valve control is unique to rotary-screw compressors. The slide valve varies compressor displacement by returning air back to the suction. Some slide valve applications also vary the discharge port location, which varies the volume ratio. Lift valve unloaders also allow air to return to the suction. The fixed location of the lift valves results in stepped capacity control as opposed to a slide valve’s stepless control.

Start-stop, suction throttling and variable-speed operation also can control rotary-screw compressor capacity. Several manufacturers offer VFD-drive screw compressor packages.

Centrifugal compressors use inlet-vane control, which pre-rotates the incoming air to alter the compressor’s per-formance curve. Variable-speed control also is effective for centrifugal compressor capacity reduction. Output pres-sure, however, is proportional to the square of the rota-tional speed. Inlet-vane capacity control results in less of

The lay of the land

Figure 1. This chart shows the typical operating regimes for different compressor types.

Reciprocating

Oil-flooded screw

Sliding-vane

Dry rotary-screw

Axial

Centrifugal

1

1

10

100

1,000

10,000

100,000

10 100 1,000 10,000 100,000 1,000,000

acfm

psi

aSome methods of capacity control are unique to a compressor type,

whereas other methods apply to all types.

Figu

re: R

esea

rch

Ass

ocia

tes


a reduction in pressure output than capacity control using variable-speed control.

Below a minimum flow, air bypass might be necessary to avoid surge conditions. Other types of control methods for centrifugal compressors include suction throttling, adjustable diffuser vanes and movable diffuser walls.

Axial compressor capacity can be controlled with vari-able-speed drives or with adjustable stator vanes. Continu-ously variable vanes with automatic control are usually sup-plied on constant-speed applications with frequent changes or fluctuations in operating conditions. With continuously variable stators, a drive ring adjusts the orientation of vanes simultaneously.

Put them to workIndustry finds many uses for compressed air, including air-driven tools, assembly line actuators and drives, powering mold presses, injection molding, process machinery, material transfer, painting, cleaning, blowing, dehydration, vacuum packing and cooling.

Small- to medium-sized plants probably use reciprocat-ing, rotary-screw and rotary-vane compressors. Laboratories that require oil-free air might opt for oilless rotary-vane or oilless reciprocating compressors. Plants with high air vol-ume requirements will favor rotary-screw, centrifugal and axial compressors. Dry rotary-screw, centrifugal and axial compressors can provide high volumes of oil-free air. Many larger-capacity applications for centrifugal and axial com-pressors are found in industries where a process consumes air. This includes air used for combustion, blast furnaces,

sewage treatment, compressed-air energy storage, air separa-tion plants and ammonia production.

No seizing allowedAir compressors can use any of several lubricants: petro-leum-based oil, petroleum oil, synthetic blends and com-pletely synthetic lubricant. Lubricant selection depends on the compressor type, service and air quality requirements. Some plants require a lube that is USDA-approved for H-1 application (lubricants with incidental food contact).

The compressor lubrication system is dependent upon compressed air quality requirements. Lubricant-free recip-rocating compressors don’t allow lubricant within the com-pression chamber. These compressors have heat-resistant, self-lubricating pistons, riders and rings. A distance piece between the crankcase and cylinders prevents crankcase oil from entering the compression chamber. Oilless recip-rocating compressors are similar, but without lubricant in the crankcase.

Rotary-screw compressor options are lubricant-injected, dry or water-injected. Lubricant-injected units use the oil to seal the space between the rotating screws, to remove heat and to lubricate the rotors and bearings. Dry-type screw compressors need no lubricant for sealing purposes, operate at higher speeds and provide oil-free air. Water-injected types use water to seal compression chambers’ internal clearance and to remove heat. The lubricated bear-ings and gears in both dry and water-injected types are isolated from the compression chamber. Centrifugal and axial compressors use pressure-lubricated bearings and drive gears. Shaft seals isolate the bearings from the com-pression chamber so that centrifugal and axial compressors can provide oil-free air.

Prime movers for driving air compressors include electric motors, turbines (steam and gas), natural gas, diesel and gaso-line engines, in constant-speed or variable-speed varieties. Fuel for engines and turbines includes natural gas, landfill gas and sewage treatment gas. Steam turbine drives can be used, particularly if the waste heat from some exothermic chemical process can produce steam.


www.dresser-rand.com/default.aspwww.compair.com.au/pages/compservindustvane.htmlwww.knowpressure.orgwww.gardnerdenver.comwww.gepower.com/prod_serv/products/compressors/en/index.htmwww.manturbo.com/en/index.php

Books, journals and magazines“Improving Compressed Air System Performance: A Sourcebook for Industry,” Compressed Air Challenge, U.S. Department of Energy, 2005.

Van den Braembussche, Veress, Arpad, “Inverse Design and Optimization of a Return Channel for a Multistage Centrifugal Compressor,” ASME Journal of Fluids Engineering, September, 2004.

Hanlon, Paul, C., “Compressor Handbook,” McGraw-Hill, 2001.

“Compressed Air Systems: The Fourth Utility,” David McCulloch, Energy User News, August 2000.

“Developments in Industrial Compressors and Their Systems-European Conference,” Proceedings of the Institution of Mechanical Engineers, IMechE, 1994.

Gresch, M. Theodore, “Compressor Performance,” Butterworth-Heinemann, 1991.

Material for self-study

Compressor type Technology development

Reciprocating Valves, rings, packing-type seals, capacity control and lubricants.

Screw Reductions in internal leakage, increased bearing life, seals, capacity control and lubricants.

Centrifugal/axial Further extension of stable operating range through improvements in impeller, blade, stator, inlet and diffuser aerodynamics and materials; labyrinth seals and bearings.

table 1



The crystal ballAir compressors are a mature technology characterized by incremental improvements in specific components and subcomponents. Technology trends are strongly driven by user requirements and by a manufacturer’s desire to increase market share. A manufacturer might focus on a single com-pressor type and work to expand its capabilities or might offer a range of compressor types covering the entire market. Some manufacturers might focus on niche industrial markets (manufacturing, oil and gas, chemical) or by performance

range (very high pressure, very high flow rate, and so on). The competition is a complex mix; key factors are flow capac-ity, output pressure capability, air quality, efficiency and, of course, price.

Manufacturers hold topics of current research closely, but they are logically driven by the characteristics of each com-pressor type. Table 1 shows the projected areas of technology development by compressor type. Computers have been an enabling technology in compressor development and applica-tion. Finite-element analysis finds application in positive-dis-placement and dynamic compressor technology development. Computational fluid dynamic software applied in 2-D and 3-D analysis is providing impetus to continuous improve-ments in dynamic compressor performance. Computer-based controls are standard on many compressor systems and often include network and Internet capabilities.

This thumbnail sketch of compressor technology should provide a useful jumping-off point for those who would like to learn more about compressors. The sidebar lists a number of books, journals and Web sites for your reference. p

Ben J. Sliwinski owns Research Associates, Urbana, Ill. Contact him at [email protected] and (217) 367-2270.

Figures: Research Associates and Ingersoll-Rand Co.

The compressor lubrication system is dependent on compressed-air

quality requirements.

May 2005www.PLANTSERVICES.com28

ManageMentCompressed Air

May 2005 www.PLANTSERVICES.com 29

The easiest but potentially the most expensive way to improve your compressed air operations is to hire someone to do a compressed air audit. A team of experienced, professional compressed air wizards will visit your plant, spend

days measuring pressures and examining your system, and give you a list of all the things that are wrong. In many cases, along with repairing leaks, poor piping and other relatively minor problems, they’ll advise you spend tens of thousands of dol-lars on new compressors and controls.

OnThe

hunTThe Top 10 TargeTs of a

compressed air audiT

By Rich Merritt, senior technical editor

And they’ll be right. Following the recommendations of an audit usually pays for itself in a short time by saving those tens of thousands of dollars in operating expenses.

You are likely to benefit from such an audit, but it may make sense to know what the audit team is likely to find so you can identify the typical problems yourself. To that end, we asked some top compressor manufacturers and service companies to tell us what they usually find. Listed below are their 10 most typical, highest-payback audit items.

One caveat, though: Fixing these before doing a full sys-tem audit can make it more difficult to justify the higher-cost improvements. “Often, many parts of a system upgrade that improve the quality, reliability and repeatability of the system are financed in conjunction with the reductions in waste,” says Mark Krisa, air audit manager at Plant Air Technology (www.plantair.com). “Energy reductions associated with your efforts cannot be incorporated into future return on invest-ment projects.”

In other words, if you pick all the low-hanging fruit and do an audit, the payback on the investment won’t be as high. It’s a numbers game, but it might be important in determin-ing who pays what, and whose budget it comes out of. The numbers game may decide which you do first: the professional audit or your own list of low-cost repairs.

1. Plug away at leaks“One of the most common problems is leaks,” says Wayne Perry, technical director, Kaeser Compressors (www.kaeser.com). “Studies indicate that as much as 35% of the compressed air produced in the market today is wasted to leaks, and everyone has leaks.” Identifying and correcting them might save not only the purchase price of a compressor, but reduce the amount of energy needed to run the compressor.

“It has been our experience that plants that have no for-mal, monitored, disciplined, compressed air leak-manage-ment program will have a cumulative leak level equal to 30% to 50% of the total air demand,” adds henry van Ormer, engineer and owner of Air Power uSA (www.airpoweru-sainc.com). Every 8 cfm to 12 cfm leak can cost you $800 to $1,200 per year.

Van Ormer suggests setting up a short-term leak inspec-tion program so that every sector of the plant is inspected once each quarter to identify and repair leaks. “Inspections should be conducted with a high-quality ultrasonic leak loca-tor during production and nonproduction,” he recommends. “A record should be kept of all findings, corrective measures and overall results.”

Afterward, he suggests setting up programs to monitor the air flow to each department and making each department

responsible for identifying its air usage as a measurable part of the expense for that area.

If you get rid of leaks, you might cause other problems. “Elimination of waste, such as leakage and artificial demand, may result in reduced loading on compressors that are not equipped to turn down efficiently,” says Mike Bakalyar, manager, enhanced services, Gardner Denver (www.gardnerdenver.com). Dynamic efficiency may actually degrade, resulting in very little positive effect on energy usage (Table 1). Waste has been reduced, but the cost recovery shifts to compressor controls.

“The campaign to reduce leaks must be complemented with configuration and control improvements that will

allow the air generation to turn down with the reduced demand,” he says.

2. Take down overpressurizationExcessive pressure increases leaks and wastes money. “Some end users will actually increase pressure in an attempt to compensate for capacity issues,” Perry says. “In fact, increas-ing pressure will have the opposite effect on air flow and often exacerbate the problem. There is also a propor-tional relationship between pressure and power consumption – for every 10 psi in excess pressure there is a 5% increase in power cost.”

Norm Fischer, president, Centrifu-gal Equipment Service (www.cescon-trols.com), says too-high pressure will amplify problems, not solve them. “The easy answer to many system problems is to jack up the pressure. Unfortu-nately, the leaks will leak more, and unregulated users will waste more air and more energy.”

Lowering the pressure may solve problems. “Lower system pressures mean less mass required, therefore fewer compressors running,” Fischer adds. “Compressors are usually more efficient when run at lower pressures.”

But you have to convince produc-tion, Fischer says. “Often, the greatest struggle is gaining the confidence of the production people that the system is reliable enough to respond when required, so they will lower the pres-sure requirement closer to the actual design requirement.”

3. Zero in on air requirementsOften, production overestimates the

amount of air it needs. “If production is allowed to define their own compressed air requirements based on as much as they want whenever they want it at any pressure, the system will never operate efficiently,” Krisa says.

Dave Booth, systems specialist at Sullair (www.sullair.com), agrees. “The entire paradigm under which the compressed air system operates must change,” he advises. “We must shift from the principal goal of maintaining a minimum pressure and that higher pressure is OK to the goal of maintain-ing a consistent and stable pressure. Plants must change their focus from ‘maintaining air supply’ to ‘supplying air to meet demand.’ More air and more pressure is simply more cost.”

Van Ormer says, “More often than not, it is one process that needs a cer-tain minimum pressure. These claims should always be reviewed. In one audit, the rest of the plant could run on 80 psi but the compressed air system had to run at 98 psi because the grinding area — with only 20% of the demand — required it. Testing revealed that the actual inlet pressure to the tool was 63 psig at load. In other words, we had a 35 psig pressure loss from the header to the tool. Further tests indicated that the optimum inlet pressure for these particular tools was 75 psig.” The plant installed a larger feed line and a regula-tor to deliver full flow to the grinders at feed pressure. The header pressure was lowered to 85 psi. Results after 18 months showed that tool repair went down for the grinders, production increased by 30% and total air demand fell from 1,600 to 1,400 cfm. Total cost for the regulator, piping changes and adding quick disconnects on nine grinders was $1,362. Annual electrical savings are about $18,000 per year.

In those cases where you have a small area that actually needs high pressure, van Ormer suggests setting up a sec-ondary, smaller, high-pressure unit or an appropriate booster, rather than drive the entire plant system at the higher pressure. “Expecting the supply system to support a black hole is not a realistic design criterion,” Krisa adds.



Table 1. Demand can affect efficiencyLeaking system Tight system Tight plus controls

Process demand* 1,500 cfm 1,200 cfm

Demand reduction 300 cfm (20%)

Power 259 kW 243.5 kW 206.9 kW

Dynamic efficiency 5.8 cfm/kW 4.9 cfm/kW 5.8 cfm/kW

Annual energy cost* $108,780 $102,270 $86,898

Net savings $6,510 (6%) $21,882 (20%)

*Example system at 90 psi and $0.05/kWh

Smooth it outFlatline compressed air header pressure to stabilize performance of pneumatic equipment.

Not for knuckleheadsGet a competent, unbiased compressed air system audit.

Baselining a compressed air sys-temSave thousands with simple measure-ments, a pencil and paper, a calculator and a telephone.

Energy conservation effort saves Conbraco $860,000Keys were correcting compressed air leaks, power factor and billing errors.

www.plantservices.com/0505_extras

More on coMpressed air audits at plantservices.coM

4. Eliminate Mr. TeeOne of the simplest fixes in a compressed air system is to replace tee connections with directional angle entry connec-tions. In a piping system where a feed line of compressed air is trying to feed into another air line, the turbulence caused by a 90° entry often causes a 3 psi to 5 psi pressure loss. Such a loss can cost you $1,200 per year at every one of those tees.

“More important, the back pressure sends a false unload signal to the controls, causing premature unloading or extra compres-sors to be on line,” van Ormer says. “Using a 30° to 45° direc-tional angle entry instead of a tee will eliminate this pressure loss. The extra cost of the directional entry is usually negligible.”

Even worse is a dead-head tee connection, where com-pressed air enters at opposite ends of the tee, causing extreme turbulence. “In one instance, the pressure loss was almost 10 psi,” van Ormer says. “This is 300 hp worth of air, or about $12,000 annual power cost.” To avoid such a situation, he suggests using two directional angle con-nections spaced so the incoming air does not cause such turbulence.

5. Set sights on bad pipingConvoluted piping, piping restrictions, old pipes and incor-rect pipe sizes often lead to pressure loss. In a well-laid-out system, the interconnecting piping from the compressed air supply to the process and header distribution piping should produce no pressure loss. In many cases, it is easy to simply replace a section of pipe to gain efficiency.

Booth looks at it more simply. “If you cannot walk up to your compressed air piping system and in a brief glance obviously figure out how the air gets from the compressors through the contaminant removal system and to the plant and then on to the points of use, you probably have a problem,” he says. Look at your piping. Is it logical? Does it make sense? Would you install it that way?

“Piping is a major consideration, especially in older facilities or shops that have grown and expanded,” Perry says. “Cast-iron piping will rust over time, releasing rust and scale into the compressed air and creating buildups at various points in the system. This not only degrades air quality, but reduces the effective internal diameter of the pipe and obstructs air flow creating unwanted pressure drops and velocity problems.”

Measuring pressure loss in piping sections will identify the worst culprits. If you find a severe pressure drop through some convoluted sections, or determine that the pipe is too small, the cost of changing the pipe often pays back quickly. “Upgrading to copper or aluminum piping provides excellent value for money and ideal delivery characteristics,” Perry says. “When upgrading, ensure that the physical pip-ing diameter is sized to deliver the required air flow with minimum pressure drop.”

Interconnecting piping between two or more compressors often needs attention. “This is the piping area where we find the most opportunities for improvement,” van Ormer says,

“particularly in systems installed after the late 1970s. Older systems were put in more carefully.”

6. Blow away obsolete restrictionsClogged filter elements, forgotten manual drain traps and neglected separator cartridges can cause significant drops in pressure and negatively impact capacity and reliability, not to mention creating air-quality issues.

In one example, a pet food plant was running a 150-hp rotary screw that produced 750 cfm. “The discharge pressure was 120 psi, and actual pressure at packaging was 90 psi,” van Ormer says. “Investigation of the main header from the compressor room to process found an old, forgotten inline filter full of rust and scale. The filter was removed, the dis-charge pressure was reduced to 100 psig, and this produced an annual electrical energy savings of $6,570.”

7. Spot small-caliber storagePerry says insuff icient storage is a common problem. “Across the board in manufacturing and processing, the value of an appropriately sized air receiver and appropriate compressed air piping is underestimated,” he says. “These tanks provide a first stage of moisture separation to help maintain compressed air quality. However, their primary function is storing and delivering compressed air to help meet periods of peak demand and to prevent excessive compressor cycling.”

All air systems will do better with storage between the user and the process. The amount of effective storage for any system is where the operating control band is equal-ized by the back pressure in the system. In one example, a 280-hp, two-step controlled, lubricant-cooled rotary screw compressor was running 24 hours per day, seven days a week at a relatively level load of 70% flow. The unit had very low storage capacity and would unload, idle for 15 to 25 seconds, then reload.

The bleed-down time for this unit was one minute to reach full unloaded power. “The unit did not stay off long enough to reach the low power point and spend time there,” van Ormer explains. “Correcting the effective storage to almost 10 gals per cfm created a two-minute idle allowing full blowdown to the low idle input power and a full one-minute run at this low power before reloading. This resulted in an annual electricity cost reduction of more than $14,000.”

8. Shoot down inappropriate useUnregulated use of compressed air, and using compressed air for inappropriate purposes, wastes a lot of energy. Con-sidering that it costs eight times as much to use air as it does to use electricity, you may want to reevaluate unregulated air-powered cabinet coolers, blow-offs, vacuum generators, mechanical pumps, air motors and hoists, vibrators, aeration, spraying and a host of other equipment.

Compressed air is readily available in a plant, and the cost of using it is not always understood. “Therefore, when a need



was identified, air was usually the easy answer,” Fischer says. “Sometimes it’s even used for cooling people at workstations, blowing dust, or to power vortex-type coolers and air to keep food clean.”

“Open blow, refrigeration and vortex cooling may all be replaceable with heat tube cabinet coolers with a potential savings of 3.5 kW to 4 kW each on a 30- x 24- x 12-in. aver-age cabinet,” van Ormer says. “The initial cost is usually in the $700 to $750 range with a potential resultant power savings of $1,000 to $2,000 per year each.”

He also suggests using venturi air amplifier nozzles or air inducers whenever possible, which will reduce blow-off compressed air by 50% or more.

9. Pump away at pumpsAir-operated diaphragm pumps tolerate aggressive conditions relatively well and can run dry, which makes them a favorite with plant personnel. But is an air-operated pump the best solution? Electric motor-driven diaphragm pumps are read-ily available, and may work just as well. A 2-in., air-operated diaphragm pump, pumping water at 130 gpm, will use 25 hp worth of compressed air at a cost of $9,947 per year. A 3-hp electric pump may well do the same, at an energy cost of $1,989 per year.

If air-operated pumps must be used, consider adding con-trols to shut the pumps off when they are not needed. Pumps waste the most air when they are pumping nothing. Also, check to see if the pump is running at the lowest possible pres-sure. Simple controls can increase pressure when needed.

10. Get a clean shotPoor air quality adversely affects overall plant operations. What you want is air that is clean and dry, and that requires maintain-ing the filters, separators and driers. Neglecting recommended maintenance can let oil get into the plant air and cause produc-tion problems from dripping tools to fisheyes in paint systems.

Poor maintenance also affects efficiency. Van Ormer says they did an audit and found three 150-hp compressors with 9.5 psi inlet pressure instead of the normal 14.2 psi. This reduced the effective output from 725 cfm to 501 cfm, or a 31% loss. The plant had to run all three compressors at full load to supply the 1,400 cfm demand. Investigation discovered dirty and restrictive inlet conditions. Correct-ing the problem resulted in almost $45,000 per year in energy savings.

Change air/oil separators, filters and other components at the optimum time, not when they clog up and cause a pressure loss.

Bring in the big guns“Most of the lasting benefits and big opportunities identi-fied in air audits are really common-sense solutions,” Booth says. “Most involve simple maintenance issues, misappli-cations and general problems caused by neglect and not fully understanding the consequences of mismanaging a compressed air system.”

But many plants can benefit from more sophisticated analysis by professional auditors who might recommend, for example, changes to the control system. “The most com-mon problem identified in complete air system audits is the improper application — or at worst, the complete lack — of compressor controls,” Perry says.

The pros cite symptoms such as compressors fighting each other, too many compressors running, compressors running “ just in case” they might be needed, and fluctuat-ing plant pressures.

Those problems are more difficult to find and fix than the leaks, inappropriate equipment and rusty pipe problems described here. Unless you are a compressed air wizard yourself, you may need an audit to tell you what’s wrong with your system controls and overall design. p



Several of the compressor wizards we interviewed prefaced their list of audit items with a note bemoaning the sad state of affairs in most plants. Mark Krisa, air audit manager at Plant Air Technology (www.plantair.com), said it best: “The common issue that exists in almost every facility is the attitude toward acceptance of responsibility for the problems. If the intention is to correct the problems, the organization as a whole has to take responsibility for the problems.”

Krisa says that in many plants the production department determines compres-sor requirements. “This is classically based on keeping production happy,” he says. “If the operating goal for the compressed air system is to keep production from complaining, then production has no involvement in resolving problems, only in creating them.” Production may overestimate its needs for compressed air, misuse it and misapply equipment.

“Without formal changes to how the system is approached and an assignment of responsibility, the system will ultimately return to the initial state of operation, regardless of what efforts are made to purchase equipment to make the system better,” Krisa advises.

Dave Booth, systems specialist at Sullair (www.sullair.com), says it’s a lack of understanding. “Most plants really do not understand what it truly costs them to operate their system and what effects it can and does have on their overall production process and quality,” he laments. “If you don’t know what it costs or how it operates, how can you even begin to consider evaluating savings or other potential improvements and changes?”

Lack of ownership

S

Compressed air is too costly to use as a prime mover. Consider the fact that the price of 100-psig air is in the range of 18 to 32 cents per 1,000 standard cubic

feet of free air. In the automobile industry, compressed air is a significant part of the energy cost, ranging from 10% in component plants to as much as 40% in stamping plants. In a typical Ford plant, this can represent anywhere from several hundred thousand dollars to well over a million dollars per year.

One way to reduce this cost is by applying best practices and a systems approach to improve com-pressed air system efficiency. Analyzing the case from only the supply side limits the opportunities for improvement. Focus on air user demands because that is what drives system requirements. Concentrating on proper end-use application, design, operation and maintenance ensures higher operating efficiency, lower cost and reduced production losses. Review these aspects of your current air system:• Consider electro-technology conversion.

• Align supply side with demand side.• Reduce system pressure.• Improve maintenance.• Eliminate inappropriate uses.• Think in terms of life-cycle cost.

Electrotechnology conversionThe history of compressed air in the auto industry goes back to Henry Ford’s day. Then, it was a byproduct of electricity

July 2005www.PLANTSERVICES.com38

ReliabilityCompressors

queezingmoney

outof

thinair

It pays to apply best practices and a systems approach to your compressed air network

By Joe Ghislain

July 2005 www.PLANTSERVICES.com 39

production: waste steam from the generator’s turbines pow-ered the steam engine-driven compressors that produced compressed air. Electricity was in its infancy and couldn’t yet duplicate what could be accomplished with compressed air. But times have changed.

Electricity now produces compressed air, and it can take 8 hp of input power to deliver only 1 hp of work where com-pressed air is being used. At that rate, it’s obvious that it can be more economical to use electricity to drive mixers, dryers and blowers. Even direct-current nut runners are replacing air tools, not just because of the energy efficiency, but because of increased quality by being able to tie torque feedback to the line operation. The advances in electrotechnology now offer many efficient options for replacing compressed air applications.

Aligning supply with demandSystem demand drives the supply requirements in any com-pressed air system. You need to know the true air demands and how to fulfill them using proper compressor operation (number and total horsepower, duration, pressure and flow). Because the system is dynamic, it requires monitoring and controlling both the compressors and air users.

First, develop a pressure profile that quantifies system demand characteristics. Take pressure readings after the main supply components, at the beginning and end of the main distribution system and at several points of use. Spread your readings out over a period of time to establish the high, low and average system demand. The pressure variation you document indicates how the system and compressor react to the demands.

The adage, “If you can’t measure it, you can’t manage it,” applies to establishing your baseline. While temperature and dew point are useful air system measurements, the key metrics are pressure, rate of air flow and electrical consumption. This trio helps to determine the cost, monitor system operation and establish a baseline for evaluating future modifications.

Determine real-time air system efficiency using the flow rate (cfm free air) and power (kW). Let system size, compo-nent location and estimated air flow range determine the flow meter type and its location. Get your electrical consumption by calculating kW or from a kWh meter. For smaller systems, use voltage and current readings and apply the motor power factor to estimate power consumption. Convert your kW/cfm reading to cost by applying your electrical rate. Converting compressed air usage into dollars puts the system operation and improvements into terms that everyone can understand.

Apply controls to the compressors and other supply-side components and to air users that have the greatest effect on the system. The type of compressor control and operation depends on compressor type and system dynamics.

Control of an individual compressor requires consideration of demand variation and control of air users to minimize their effect on the system. Operate a minimum number of compressors necessary to base load (operate at full capacity), and use only one trim compressor to track the overall vary-ing load. If you have multiple compressors of the same type,

use sequencing controls to run all but one at full capacity. These sequencers not only control trim compressor turn-down, but also will start and stop compressors according to system demand.

For systems with multiple compressor types, it may be beneficial to separate the control for each type. Sophisticated sequencing controllers and global systems now available can control more than one compressor type. When using these con-trol schemes, don’t ignore compressor type. For example, rotary compressors with modulating, or load/unload, capacity control should be run fully loaded; variable-speed rotary compressors should be used only for trim; and centrifugal units have rela-tively efficient but limited, reduced capacity modulation.

Primary and secondary storage also can help align supply with demand by minimizing the effects that air users have on the system. Air receivers are vessels that store air that’s needed to meet peak demand events with minimal effect on changes in pressure. Primary storage, located close to the compressors, reacts to any system event. Secondary storage, located close to an end use, minimizes the effect that a local high-volume, low time-duration event has on the upstream system.

In conjunction with storage, an application that requires a narrow pressure band can be equipped with a pressure/flow controller that monitors downstream pressure and reacts quickly to maintain line pressure stability.

As you can see, proper control and monitoring aligns air supply with demand. The correct control system must be able to handle a compressed air system that is almost always dynamic. If your production process or operating schedule changes, verify your baseline numbers again to ensure the change hasn’t degraded your system dynamic.

Pressure reductionCompressed air systems often operate at excessive output pressure to compensate for pressure fluctuations caused by changes in end use (high intermittent volume). Operating at elevated pressure increases the rate of air leaks, air con-sumption at users and energy consumption. The benefits of reducing your supply pressure follow the same logic that applies to pressure drop, except in reverse: every 2 psi increase in pressure costs an additional 1% in power. For example, running a 100-hp compressor at 80 psi rather than 100 psi saves approximately $3,500 per year at 5 cents per kWh. Operating a compressed air system at the lowest possible pressure is well worth the effort.

Often, only a small number of end uses require high pressure. These need to be addressed individually. Sometimes the need for high pressure is merely a perception that entered plant lore when someone once said, “We have problems with the equipment if it drops below this pressure.” Question everything.

Any number of things can cause problems, including pressure drop and swings in the line feeding the equipment. If you suspect perception-based needs, address the cause. If a user truly requires high pressure, either modify the equipment or isolate it.

Because modification is equipment-specific, it can’t be addressed in this article, but there are techniques to iso-


late high-pressure loads. Air boosters or intensifiers can be used for intermittent loads. Booster compressors or separate, smaller compressors can be used for continuous or high-duty-cycle loads. Finally, if several loads require high pressure, it may be possible to separate them from the main system and supply them from one compressor, thus allowing the main system to be run at a lower pressure.

MaintenanceProper supply-side and demand-side maintenance is critical to efficient operation. Often, system maintenance is considered a necessary evil, one of the first cuts to hit the budget, but it may be the wrong place to start.

On the supply side, pressure drops across dryers and filters can have adverse effects on system operation. The concept that “2 psi costs 1% in power” applies, so it’s critical to change

filters and maintain dryers to minimize pressure drops. When ignored, inlet air filters will load up and reduce compressor capacity and efficiency. A good air filter guideline is that a pres-sure drop of 4 in. WC is equal to 1% of compressor capacity.

Air leaks are the biggest maintenance loss in any system. The Department of Energy suggests that a “tight” system still has a 10% leak rate. It’s common to find industrial com-pressed air systems with 20% to 30% leakage. Air leaks cause efficiency losses in several areas.

The obvious one is the leak itself. At 5 cents per kWh, the equivalent of a quarter-inch hole burns $8,382 per year. The additional rate of flow for compressed air leaks decreases system pressure. The resulting artificial load requires the system to operate at elevated pressure and can even prompt the purchase and running of more unneeded compressor capacity. Air leaks cause supply side equipment to cycle too often, thus increasing maintenance and reducing equipment life. The only way to reduce these effects is to implement an aggressive and ongoing air leak program that identifies and fixes air leaks.

Inappropriate usesCompressed air isn’t always the most appropriate energy source. Many times it’s used because it’s convenient, but this is a costly convenience. Blowing, drying and sparging are examples in which air may have been selected because it was easy or was a quick fix for a production problem. Blowing and drying are usually done at excessive pressure, which often can be reduced drastically by regulating it and using high-efficiency nozzles. Low-pressure electric blowers are a viable option.

Cooling workers and cabinets are two other examples of incorrect compressed air use. Purchasing a fan or a cooling unit

can provide a payback in less than a year, perhaps within several months. Vacuum generation, diaphragm pumps and vacuum venturis also are applications that you should review.

Think life-cycle costWhile this may be a basic concept, it’s often overlooked. System design and the equipment purchased to implement it determine 80% to 90% of the ultimate operational costs. Total life cycle cost and benefits must be weighed carefully before selecting the most cost-effective option, not only for the compressed air supply system but also for the end uses. Where is the sense in making air compressor purchases based on first cost, while the unit’s life-cycle cost is less than 10% hardware and more than 80% energy?

Rarely is pressure drop a consideration when purchasing or designing equipment and systems, yet the pressure drop across dryers, filters and piping systems has a dramatic effect on energy costs. You’d be wise to analyze the incremental cost of increasing hardware size to reduce the pressure drop. Often, the incremen-tal cost is small compared to the ongoing energy cost.

Specify air users that operate at the lowest possible pres-sure. I know of one instance where two identical large presses were purchased for two locations. One plant specified a 60-psi operating pressure, the other let the supplier dictate the operating pressure. The result was a press operating at 60 psi and the second operating at 80 psi. The difference in operat-ing cost was more than $300,000 per year. This illustrates that using life-cycle cost to drive design, specification and purchasing is critical to efficient long-term operation.

Concentrating on proper application, design, operation and maintenance ensures the highest operating efficiency and lowest cost. It improves energy efficiency while reduc-ing production losses. Reducing compressed air costs, like reducing any energy cost, has a direct effect on the bottom line. Making compressed air systems more efficient reduces costs and makes a company more competitive. p

Joe Ghislain is business strategy manager for vehicle operations at Ford Motor Company in Dearborn, Mich. Contact him at [email protected] or (313) 594-2695.

The Compressed Air Challenge (CAC) is a national collaborative of public and private organizations dedicated to increasing the understanding and improving efficiency of compressed air systems within U.S. industry. The CAC offers Fundamentals of Com-pressed Air Systems and Advanced Management of Compressed Air Systems training, and in cooperation with the US Department of Energy, the Compressed Air Systems Sourcebook for Industry as well as the Qualified AIRMaster+ Specialist training. CAC has built a reputation for being a reliable resource for cost-effective solutions and unbiased information, including the recent publica-tion Best Practices for Compressed Air Systems, a comprehensive and detailed reference for plant personnel. For more information about CAC training and publications call (301) 751-0115 or visit www.compressedairchallenge.org.



While temperature and dew point are useful air system measurements,

the key metrics are pressure, flow and electrical consumption.

August 2005 www.PLANTSERVICES.com 43

efficiencyCompressors

COMPRESSED AIR IS FOR PUSHING, NOT PULLING

Improve energy efficiency by restructuring vacuum generators

By Dan Bott

V acuum generators powered by compressed air represent one of the most inefficient uses of that valuable utility. Behind

every quiet, vibration-free, low-cost, environmentally friendly venturi vac-uum pump is an expensive, energy-con-suming, large-footprint air compressor. In many applications, electric motor-driven vacuum pumps can achieve the same performance as vacuum generators while using one-fourth to one-tenth the energy. In fact, replacing compressed air vacuum generators might be one of the last methods remaining for increasing production energy efficiency and taking overworked air compressors off-line.

The simple mechanismVenturi-style vacuum pumps, also called vacuum generators or compressed air ejectors, produce vacuum by passing high-velocity compressed air through a venturi or nozzle. Performance depends on the nozzle’s shape and size, com-pressed air pressure and flow, and the desired vacuum level. The greater the vacuum being maintained, the lower

the f low of induced air. Other motive f luids include steam, vapor, water and other liquids.

Compressed air vacuum generators are common in industry. Palletizers, material-handling systems, pick-and-place operations, drum-type vacuum cleaners and packaging applications are just a few examples. Each genera-tor is mounted in close proximity to the point of use, with supply tubing con-necting the vacuum device to a central compressed air system.

Vacuum generators are reliable, com-pact, lightweight and quiet. They have no moving parts and can be mounted directly on production machinery. Their mainte-nance requirements are minimal. They’re available in aluminum, plastic and corro-sion-resistant construction for harsh appli-cations. Replacement or repair is simple and requires no special tools or training.

Air versus electricityA vacuum generator, by itself, is equiv-

alent to an engineless automobile. Neither makes any noise nor requires maintenance. Neither has an operating cost. The drawback, of course, is that neither does any useful work. Without an air compressor “engine” operating under the equipment room “hood,” the vacuum generator can do no work. One can’t evaluate a vacuum generator with-out accounting for the air compressor in the calculation. An objective evaluation compares the relative efficiencies of elec-tric-driven vacuum pumps and vacuum generator-compressor combinations.

Vacuum generator literature uses two key terms: induced airflow and air consumption. Induced airflow is the air being evacuated from inside the vacuum system. Air consumption refers to the compressed air the vacuum generator requires. These flows combine and dis-charge through an exhaust port.

A vacuum pump driven by an electric motor, on the other hand, uses a varying rotational swept volume to produce a suc-

tion that induces flow from inside the vacuum system. The rotor compresses the induced flow and discharges it to an exhaust port. Motor-driven vacuum pumps consume no compressed air.

Analyzing the questionProduction demands dictate vacuum pump size and serve as the basis for evaluation. Continuous vacuum applications seek to maintain a fixed vacuum level. In cyclic applications, on the other hand, a chamber at atmospheric pressure is evacuated to a target vacuum level for a period of time and then vented. Given the two application types and the two vacuum technologies, which pairing is most energy efficient?

The answer lies first in determining the cost of compressed air and how much induced flow a vacuum generator develops. Table 1 illustrates a representative continuous application with 20 vacuum generators. The values represent the average performance of typical industrial units. Each generator requires 20 scfm of motive air to induce a vacuum flow that is a function of vacuum level.

Nearly every vacuum generator application uses 100-plus psig air from the central compressed-air system and regulates it down to the recommended 30 psig to 90 psig for the venturi. Rarely is low-pressure air generated specifically for these applications. This regulation in itself is a major source of inefficiency.

A typical compressed-air system produces no more than 4 scfm output for every input horsepower. While a standalone air compressor is more efficient, losses through ancillary equip-ment, headers and the partial loading of compressors reduce overall system efficiency. So, we need nearly 100 compressor horsepower to drive the 20 vacuum generators.

Table 2 highlights typical performance ratings for an elec-tric motor-driven vacuum pump that is equivalent to the 20 vacuum generators. Rotary lobe blowers are for vacuum levels below 15 in. HgV, and rotary vane vacuum pumps are used for higher vacuum levels.

The data in the tables reveal that the vacuum generator system requires nearly 100 compressor horsepower while the motor-driven vacuum pump system needs 15 hp. For any level of vacuum, an electric motor-driven vacuum pump is at least 6.5 times more efficient than a compressed-air vacuum generator.

The real kicker is that, in many cases, pressurized air flows through the vacuum generator even when no vacuum is needed. Most vacuum generator installations have built-in shutoff valves to avoid this situation, but bypassed or defective valves add significant waste.

Up and down repeatedlyVacuum generator specifications typically include a table showing pumpdown time needed to achieve a targeted vac-uum in a volume of 1 cu. ft. This pump selection information is used for applications requiring vacuum pickup or parts movement in production machinery. Locating the vacuum generator close to the point of use reduces the volume of piping to be evacuated. Smaller chamber volumes result in faster cycling.

The next example highlights a cyclic application with 20 use points, each consuming 30 scfm of compressed air. The total air demand is 600 scfm, which represents about 150 com-pressor horsepower. Figure 1 shows the horsepower required to pumpdown a volume of 20 cu. ft. Pumpdown times range from fractions of a second for 5 in. HgV to more than 20 seconds for 27 in. HgV. Many production applications require shorter pumpdown times, but this chart is intended to illustrate the relative efficiencies of each vacuum pump technology over a wide range of conditions.

The disparity between vacuum generator and electric motor-driven pumps energy efficiency in cyclic applications is quite remarkable. A closer look at cyclic applications reveals that dur-ing an average evacuate/hold/release cycle, compressed air might be used only one-third of the time. In the absence of functional shutoff valves, compressed air flows needlessly during two-thirds of the cycle.

Even with shutoff valves in place, energy comparisons between motor-driven vacuum pumps and vacuum generators are still valid. Motor-driven vacuum pumps can be downsized to meet the actual application requirement. Figure 1 illus-trates the apples-to-apples comparison when both technolo-gies are operating at full load.

The differences in efficiency are alarming for both continu-ous flow and closed applications. In addition, the efficiency gap widens as the required vacuum level increases. The bot-tom line is that from an energy perspective, compressed-air vacuum generators are not environmentally friendly.

August 2005www.PLANTSERVICES.com44


Table 2: Electricity doing the work

Vacuum (in. Hg)

Approx hp req’d

Induced flow (scfm)

Induced flow (acfm)

5 10 540 648

10 10 220 330

15 15 140 280

20 10 60 180

25 7.5 20 120

Typical rotary vacuum pump efficiencies; acfm = scfm * P1/P2

Table 1: Pulling a loadVacuum (in. Hg)

Air consumption (scfm)

Induced flow (scfm)

Induced flow (acfm)

5 400 540 648

10 400 220 330

15 400 140 280

20 400 60 180

25 400 20 120

System of 20 typical venturi vacuum pumps; acfm = scfm * P1/P2 (absolute)

August 2005 www.PLANTSERVICES.com 45

Restructure itVacuum generator popularity is derived from its low capital cost. OEMs favor f irst cost over operating cost. Regardless, retrofitting each venturi vacuum pump with a dedicated electric motor-driven vacuum pump would be cumbersome.

The alternative is a central vacuum system. Like compressed air, vacuum can be generated at a central location and distributed through a network of headers and drops. Unlike compressed air,

vacuum supply piping can be made of light, flexible, inexpensive and easy-to-install plastic.

Installing a duplex vacuum pumping station to provide 100% backup can put reliability issues to rest. If the lead pump needs servicing, the backup pump takes over automatically. Duplex vacuum pump packages with alarms, automatic sequencing, PLC interfacing and manual overrides are standard products. They can be installed in the same location as the existing “extra” air compressor.

A central vacuum system retains the advantages of indi-vidual compressed-air vacuum generators. There’s no pump or motor noise at the point of use. Vacuum tubing takes up about the same space as compressed-air supply tubing. There are no heat problems or oil mist. The servicing schedule for an electric motor-driven vacuum pump is usually identical to that of an air compressor.

Heed the numbersEconomic evaluation is straightforward. First, determine the cost of compressed air and the total air consumption for the vacuum generators. Amortize the cost of maintenance, floor space, repairs and the like, and add it to the base electric cost. In addition, determining the total air leakage in the compressed air system is sometimes sufficient to initiate a leak repair program for the entire site.

With current costs identified, evaluate proposed costs to determine if switching to vacuum pump technology is jus-tified. Don’t forget inlet filtration for those rotary vacuum pump technologies that require it. Life expectancy for some rotary technologies is closely related to the efficiency and care of the inlet filtration system.

Focus on the system with the lowest energy and mainte-nance cost for the required production throughput. Com-pute the payback period if a new vacuum pump is involved. The annual cost for a 100-hp air compressor, including costs for cooling, air treatment, maintenance, depreciation and the like, exceeds $50,000 at $0.06/kWh. A typical 15-hp electric motor-driven vacuum pump, on the other hand, has an annual operating cost of $7,700.

Many applications use hundreds of compressor horse-power to generate vacuum. Replacing these systems with dedicated electric vacuum pumps can save thousands of dollars annually. How vacuum is generated is irrelevant to the production process, as long as vacuum is at the required level when needed.

Not every application is a candidate for electric vacuum pump replacement. But, if demands on the compressed-air system are suggesting the need for an additional compressor, it’s worthwhile to investigate alternate vacuum technology. The effort can result in significant cost and energy reductions. p

Dan Bott is owner of Dan Bott Consulting, an independent indus-trial vacuum and compressed-air system auditor in Loxley, Ala. Contact him at (251) 960-1026 or at www.danbottconsulting.com.

What at first appears to be a winning way to produce vacuum turns out to be a technology with

inadequate performance.

No more imput than necessary

150

35 40 4030 25 20

Vac. generator hp

Rotary vac. hp

160

140

120

100

80

60

40

20

150 150 150 150 150

5“ 10“ 15“Inches of mercury vacuum

20“ 25“ 27“

Hor

sep

ower

Figure 1. Closed-system pumpdown energy requirements to evacuate 20 cu. ft.


October 2005 www.PLANTSERVICES.com 41

Most compressed air systems are relatively mod-est at first, but grow and develop as production, inappropriate uses and leaks increase over time. Even assuming that air leaks and inappropriate

uses have been investigated and reduced, adding production equipment raises demand for compressed air. Some equipment may need a different operating pressure, and the required air quality may change. These modifications represent potential problems for the plant engineer who wonders:• What is the required volumetric flow rate and the pressure

of compressed air, and what size of air compressor should be added?

• What type of compressor and control system offers greatest reliability and lowest life-cycle operating costs?

• Where should the compressor be located?• Is sufficient power, ventilation and cooling capacity available?• What type of system capacity controls would be best?• Is primary compressed air storage sufficient?• Is distribution piping adequate?• Is secondary compressed air storage sufficient?

• Is the current compressed air quality satisfactory?• Does this project require professional help?

Compressor sizeThe current average and peak compressed air flow rates, in cubic feet per minute (cfm), should have been established before considering any proposed additions. The rated output of the existing compressor(s) also should have been estab-lished. The specifications for the proposed equipment that needs additional compressed air should state the required flow rate, pressure and air quality. This information provides the new total flow rate. Deducting the rated output of the existing compressor(s) gives the additional air flow required from a new compressor.

This calculation ignores potential leakage and increased inappropriate compressed air use. Also, it doesn’t account for differences in the frequency of operation of each piece of production machinery. Individual consumption peaks might not occur simultaneously. Nevertheless, you’ll need to deter-mine the average and peak flow rates (Table 1).

By David M. McCulloch and Bill Scales, P.E.

COMPRESSORSReliability


Planning air SyStEM uPgraDES

10Ten steps to

successful system improvements

Don’t add a fudge factor to the required air f low when calculating the rated capacity for the additional air compressor(s) because it could result in the new unit(s) operating at less than full capacity most of the time, rob-bing eff iciency. Should you decide to oversize the addi-tional compressor by 20% or more, select the compressor that combines the best full-load and part-load economy to minimize the operating cost over the full range of the compressed air requirements.

Another important consideration is standby capacity in case of compressor malfunction or needed repair. This may

require at least one additional compressor. Conventional wisdom says that three 50%-capacity compressors are bet-ter than two 100% compressors, because this provides more flexibility without sacrificing system reliability, particularly during periods of reduced consumption. It’s also beneficial to operate the smallest total compressor horsepower, par-ticularly for periods of reduced capacity requirements, such as a second- or third-shift operation. These factors, com-bined with reliable compressor service, are keys to main-taining energy and production efficiencies and profitable outcomes.

Compressor typeEach type of compressor has its advantages, disadvantages and preferred range of capacity and pressure. Table 2 pro-vides a simple method for comparing different compressor types. Life-cycle cost analysis always is recommended, and should include specified maintenance.

Compressor cooling is a major consideration. If water-cooled, the important issues include availability and quality of cooling water, disposal or recirculation, possible treat-ment and overall cost. If compressors are air-cooled, adequate room ventilation is essential. Heat recovery also is a potential opportunity.

Compressor size and type determines the electrical power requirements. Additional ancil lary equipment may require a different voltage and current. Consider, too, the availability of the required electrical supply and its support equipment. Ensure that proper circuit protection is provided for the added electrical load.

Location, location, locationMany plants have a compressor room and, in some cases, that room is shared with other equipment. Several factors must be considered if a compressor is to be added. These include

the need for a foundation, space for maintenance activities, space for drying and filtration equipment, room ventilation to handle the added heat release and the sound level.

It might make sense to install the new compressor in a dif-ferent location, perhaps closer to the point of greatest demand or at the application requiring the highest pressure.

Capacity controlsThere are several types of capacity control for individual com-pressors, sequencing controls for multiple compressors and pressure and flow controls for compressed air systems. The correct selection of each determines system efficiency over the anticipated operating ranges.

Two rules for achieving optimum efficiency are (1) only the number of compressors needed to maintain the required system pressure should be in operation at any given time, and (2) all but one, a trim compressor, should be running at full capacity and pressure. The trim compressor should have an efficient capacity-control mode. If it’s a reciprocating com-

pressor, this could be unloading in a series of capacity steps. For a rotary compressor, variable-speed control or variable displacement is most efficient.

Storage and pipingThe size and location of the primary air receiver affects the efficiency of your capacity control. Efficient system control may require changes in air storage volume, but adding receiver

October 2005www.PLANTSERVICES.com42


Don’t add a fudge factor to the required

air flow when calculating the rated capacity for the additional air

compressor.

Standby capacity in case of compressor malfunction or needed

repair may require an additional compressor.

A typical compressed air system

Figure 1. Compressor efficiency is a function of pipe geometry, leaks, operating pressure and other factors.

October 2005 www.PLANTSERVICES.com 43

volume won’t compensate for insufficient compressor capacity or inadequate distribution piping.

Many compressed air distribution systems originate at a primary air receiver in the compressor room where dis-tribution piping moves the air throughout the plant. As production increases, another one or more buildings might be erected, but the supply of compressed air still passes through the original distribution piping. If the piping isn’t

adequate for the increased demand, the result can be exces-sive pressure losses and increased energy consumption.

Intermittent high-volume demand can cause severe dynamic pressure fluctuations in the entire compressed air system that can upset manufacturing processes. Many fluc-tuations can be softened with an appropriately sized and located secondary air receiver that can provide enough air to satisfy the intermittent demand without compromising the pressure in the main system.

Air qualityEach piece of production equipment requires compressed air at a given flow rate, pressure and air quality. These con-siderations may vary significantly. A cardinal rule is to avoid drying and filtering compressed air any more than is needed for the specific application. Going overboard can result in increased pressure losses and energy consumption. Consider the idea of satisfying the major compressed air requirements centrally and supplementing these requirements locally, where needed.

Many industrial applications can be served well with a pressure dewpoint of 35°F to 38°F, which can be achieved with a refrigerated dryer. Standard regenerative desiccant dryers can drop the pressure dewpoint to –40°F, and more specialized dryers can bring it down to –100°F. Dry the air only to the requirements of the end users or to meet local


Table 1. Basic demand worksheet

End user identity

Minimum flow (cfm)

Average flow (cfm)

Peak flow (cfm)

Cycle time (seconds)

#1

#2

#3

#4

#5

#6

Totals

Note: In some cases, the minimum flow rate may be very low or zero (cycle time - off) until an intermittent operation (demand event) occurs, when there is a large demand (peak flow rate) for a time (cycle time – on). The combination of these determines the average rate of flow. End users having a constant demand should be tabulated by the average flow rate. Peak flow events may require additional primary storage and secondary storage.

Table 2. Simple matrix for comparing compressor types1

Compressor characteristic2

Compressor type

Two-stage, double- acting reciprocating Lubricant-injected screw Lubricant-free screw Centrifugal

Size and weight 3 1 2 2

Compact size and complete package 3 1 1-2 1-2

Can be located close to points of use 4 2-3 2-3 3

Maintenance cost 3 2 2 1

Foundation requirements 4 1 1 1-2

Reduced capacity efficiency3 1-2 1-4 1-3 1-3

Lubricant-free air delivery - lube/lube-free 4/1 2 1 1

Lubricant carryover - lube/lube-free 4/1 3 1 1

Lubricant changes or makeup - lube/lube-free 4/1 3 1 1

First cost, including installation 4 1 2 2

Full-load operating cost, kW/100 cfm4 15 to 16 16 to 19 18 to 22 16 to 20

1These evaluations are general in nature and might not cover specific features of a given compressor type or manufacturer. They’re intended to provide a general guide for comparing compressors. It’s important to evaluate each point in any comparison of quoted equipment. Other factors to be considered include relative size and cost, warranty and service.2Each compressor type is rated from 1 to 4. Key: 1 = very good; 2 = good; 3 = fair; 4 = poor.3Refer to the section on compressor controls. It’s important to compare kW/100 cfm at all reduced capacities.4Operating costs are based on full capacity at a discharge pressure of 100 psig; a full-load motor efficiency of 92% and 0.746 kW/bhp.

ambient conditions.Improving your air quality also requires filters to remove

particulates and might require coalescing and adsorption fil-

ters to remove liquids and other contaminants. These added filters will result in increased pressure losses and maintenance requirements.

Outside helpIn most cases, seeking professional help is a good idea. Equip-ment distributors with good local service capabilities can be helpful. An alternative approach is hiring an independent compressed air consultant to provide a product-neutral opin-ion or solution. p

Bill Scales, P.E. owns Scales Air Compressor Corp. in Carle Place, N.Y. Contact him at [email protected] and (516) 248-9096 ext. 611. David M. McCulloch owns Mac Consulting Services in Shalimar, Fla. Contact him at [email protected] and (850) 651-4540.

The Compressed Air Challenge (CAC) is a national collabora-tive of public and private organizations dedicated to increasing the understanding and improving efficiency of compressed air systems within U.S. industry. The CAC offers Fundamentals of Compressed Air Systems and Advanced Management of Com-pressed Air Systems training, and in cooperation with the U.S. Department of Energy, the Compressed Air Systems Sourcebook for Industry as well as the Qualified AIRMaster+ Specialist train-ing. CAC has built a reputation for being a reliable resource for cost-effective solutions and unbiased information, including the recent publication Best Practices for Compressed Air Systems, a comprehensive and detailed reference for plant personnel. For more information about CAC training and publications call (301) 751-0115 or visit www.compressedairchallenge.org. The authors of this article, David McCulloch and Bill Scales were also the authors of Best Practices for Compressed Air Systems.

October 2005www.PLANTSERVICES.com44


The size and location of the primary air receiver affects the efficiency of

your capacity control.

January 2006 www.PLANTSERVICES.com 41

ir Liquide Large Industries U.S. LP is part of Air Liq-uide Group, which produces industrial and medical gases and is headquartered in Paris. In the U.S., Air Liquide

maintains more than 125 production facilities and 700 customer installations spread across some difficult to reach geographies. Before 2002, The U.S. company used a legacy vibration program that was inconsistent in its application of technology and wasn’t producing the desired results.

Late in 2002, Air Liquide partnered with Rockwell Automa-tion to provide vibration analysis services to 32 plants on a trial basis. The program expanded quickly in early 2003 to include vibration monitoring at 107 primary production facilities. In August 2004, Air Liquide recognized the need to expand its predictive maintenance (PdM) program to include oil and infra-red analysis, and again partnered with Rockwell.

Transitioning from legacy systemsAir Liquide’s needs and aggressive long-term strategy didn’t leave time for incremental continuous improvement. The key program objectives included:• Transition to state-of-the-art information solutions• Understand how reliability affects profitability• Recognize the need for uniformity of predictive technologies• Leverage technology in geographically challenging areas• Analyze results and setting goals for improvement

Before 2000, maintenance and reliability functions were decen-tralized as was the responsibility for approximately 100 plant sites throughout the U.S. Since that time, Air Liquide in the U.S. has centralized these functions under a new maintenance department and regional reliability centers. The new department deployed a maintenance management process, a computerized maintenance management system (CMMS) and preventive maintenance pro-grams. With these systems in place, the department turned its attention to its predictive maintenance programs.

The sites used vibration, infrared and oil condition moni-toring, but because no corporate standard existed for applying predictive technologies. Applications were inconsistent and couldn’t be integrated. Similar data was taken at different frequencies with different tools and at different locations for like equipment. Data and reports varied in format and detail, and information couldn’t be compared and analyzed.

Because a company-wide CMMS didn’t exist, predictive find-ings couldn’t be linked to traceable work orders. Compliance of corrective actions versus predictive findings was unknown.

Several plants eventually used common vibration service provider, but the program had problems. The contractor owned the data it collected. While the contractor provided some stan-dard reports, Air Liquide had to pay for ad hoc analysis it could have performed itself. But, more importantly, the contractor had only one office in the far southeast corner of the U.S. Given

reliabilityCompressors

Controlling the Compressors

A

how Air liquide integrated a successful predictive maintenance program

By mark e. lawrence, p.e., Cmrp, and george F. hofer

Air Liquide’s vast geography in the U.S., more than 80% of the costs the contractor charged were incurred for travel.

By 2001, the infrared scanning program was probably the closest to being national. Electrical standards had been devel-oped and applied, and an internal resource was used for data collection and report writing. While the program was effective, it was used primarily for electrical devices and didn’t include any applications to identify process, fixed equipment or rotating equipment problems. And given Air Liquide’s geographic dis-persal and the travel it required, having a single resource dedi-cated to the program didn’t seem viable in the long term.

Although it was implemented at several sites, oil condition monitoring was probably the least used of the technologies. There seemed to be significant potential benefits to increasing its use.

Customer relationships and profitabilityMany of Air Liquide’s products are commodities. This places a premium on our unit availability and equipment reliability. Not only must there be robust maintenance programs to ensure high reliability, but also a way to see problems far in advance. That’s why predictive maintenance programs play such a large role in Air Liquide’s reliability strategies, which play a significant role in its business strategy.

Before the 1990s, Air Liquide was primarily an air separation company that produced oxygen, liquid argon and both liquid and gaseous nitrogen. These air separation units could store large quantities of liquid nitrogen and oxygen to sustain several days of downtime. The primary strategy was to reduce our mean time to repair. This meant focusing on stocking spare parts and having resources ready to react.

Air Liquide also operates four large cogeneration units near Houston to provide electricity and steam to customers in the area. Because there’s no effective way to store backup quantities of these products, Air Liquide adopted a conserva-tive maintenance strategy that required significant costs and downtime to ensure predictable uptime.

In the late 1990s, Air Liquide expanded its business into hydrogen, a product that can’t be stored easily in quantities needed to ride through any significant downtime. The stan-dard maintenance strategy didn’t apply because even small trips caused huge problems. Major breakdowns could be costly. For hydrogen, the conservative maintenance strategy required too much planned downtime to be feasible. The strategy had to provide interruption-free production with optimum downtime for planned maintenance.

In the meantime many customers had already embarked on their own journey to reliability improvement. As they eliminated reliability problems, the reliability of their gas, steam and electricity suppliers gained greater visibility. They demanded more from suppliers. A world-class predictive maintenance program was becoming an important element of our improvement strategy.

Unity of predictive technologiesImplementing a world-class, best-practice PdM program

begins with identifying the required measurement vari-ables and appropriate technology for capturing reliability data. For Air Liquide, the nature of the equipment monitored, predominant failure modes and mean time between failures necessitated using vibration, oil and thermographic technologies. Failure modes, MTBF data and industry best practices dictated a monthly interval for vibration data collection, quarterly for oil analysis, and annually for infrared thermography scans.

Before August 2004, the Air Liquide PdM program consisted of a decentralized approach to vibration monitoring, oil analysis and thermography. Neither a unified nor integrated approach was used in managing the data. Reports and recommendations weren’t linked to the CMMS or return on investment data.

Improvements started late in August 2004 have demonstrated an integrated PdM program through the following steps:• Standardized reporting process and information flow• Centralized and access through a Web interface• Centralized PdM technologies, reports and analysis• Recommendations and reporting linked to CMMS data• An interface for live-time, closed-loop progress measurement

Standardizing the reporting process and information flow involved establishing a natural link between the reports and recommendations submitted for vibration, oil and thermography. This also included switching oil and thermography programs to a single-source provider managed at the corporate level.

Rockwell Automation provided a common platform for inte-grating vibration and oil data, and partnered with Predictive Service Corp. to provide infrared thermography. A common reporting platform was used as a report-generation tool installed on each field service engineer’s personal computer.

Because infrared thermography was provided on an annual basis and generally only used for electrical components, this technology was linked via the Web interface only. Rotating equipment is added to the thermography scan when excep-tions are noted in vibration and oil.

Centralizing the data storage into a common server platform enabled Air Liquide to leverage the CMMS database, PdM software database for reporting vibration, oil and thermographic

January 2006www.PLANTSERVICES.com42

Where the effort went

Figure 1. Distribution of PdM work orders for a single year.

August '04 - August '05 PdM Data

VibrationOilIRIR Compliance

50%

17%

30%

3%


January 2006 www.PLANTSERVICES.com 43

scans. A Web interface formed a dynamic link among the three data repositories.

When lab results are reported, oil condition data is added to the PdM database and incorporated into the overall rec-ommendations made regarding asset health. Viscosity, wear particle analysis (WPA), analytical ferrography and other diagnostics are reported along with vibration data for com-plete machine condition status.

Since January 2005, we identified more than 148 cases of viscosity breakdown or improper lubrication. In at least five cases, WPA revealed significant particulate counts in conjunction with increased vibration measurements. Fol-low-up samples verify that proper lubrication was restored and machinery repairs have been made. A direct link to the recommendation and repair action taken enables these to be tracked and linked to PdM program performance.

Linking recommendations from the three PdM technolo-gies with the computerized maintenance management system established the basis for measuring PdM program success. Once completed and properly distinguished, key PdM program per-formance indicators are tracked and measured on a real-time interface. ROI data is linked directly to individual facilities, by zone, by business class, and even summarized as a whole for Air Liquide senior management.

A Web interface gives senior management a “status at a glance” indicator, requiring properly classifying work orders and entering financial data into each action taken from PdM recommenda-tions. Leveraging our CMMS capabilities, each PdM work order was classified using the following critical components:• Work class: “PDM” denotes any work order initiated as a direct

result of a PdM program recommendation• Activity type: Denotes predictive technology used to identify

problem using one of four tags: “Predictive, Vib,” “Predictive, Oil,” “Predictive, IR” or “Predictive, Elec”

• Actual repair cost: Installation and repair cost of repair required

• Estimated savings: The reliability engineer’s evaluation of problem reported and potential cost savings averted by avoid-ing catastrophic failure

• Failure class: Type of machinery affected: motor, com-pressor, etc.

• Problem code: Detailed definition of problem component; e.g., coupling failureDeveloping the interface for monitoring real-time progress of

maintenance work orders and PdM recommendations involved leveraging the existing CMMS database backbone and the cen-tralized database storage architecture. Partnering with Predictive Services, Rockwell Automation designed and developed a Web interface capable of supplying a PdM Web management tool for tracking the closed-loop PdM process. The interface links, tracks and reports progress of any maintenance action initiated from the PdM program technologies. This tool enables senior management to track program KPIs, maintenance activity bottlenecks and overall program effectiveness quickly and efficiently.

Fundamentally, the PdM program’s integration and unity estab-lished a direct link between maintenance repair recommendation and maintenance action taken. It permits measuring and track-ing financial data, metrics and program success to provide ROI. Unless a direct correlation is established between maintenance action taken and recommended repair, program success can’t be measured fully. An overwhelming majority of PdM programs fail because they lack this tie, and work orders and repairs are made without regard to the PdM recommendation.

Leveraging technologyManaging maintenance activities on a national level is a chal-lenge many large organizations face. Geography and disparity of equipment implementation and plant design makes stock-ing of spares and planning maintenance activities difficult. Many OEMs, suppliers and key contractors have equipment in the field that will ultimately require maintenance.

Implementing a system capable of tracking manufacturer type and reliability information simultaneously enabled Air Liquide to isolate problematic equipment manufacturers. This information can be used in supply-management negotiations and, more importantly, designing and engineering new plant construction. By leveraging the Web-based technology and interface, any reliability center manager, reliability engineer or maintenance technician can search and sort reliability problems by manufacturer type, equipment type or installed locations. We can do this at the plant level, by zone, business class or summarized for Air Liquide overall. Leveraging this data dur-ing contract negotiations can save Air Liquide a significant amount of time, money and effort.

Linking this interface to data in the CMMS database and PdM software database also enables better management of reported problems. Before visiting any Air Liquide facility, a preventive maintenance work order initiates data collec-tion in the CMMS. Any recommended repair or follow-up work generated as a result of the PdM visit is then classified as a PdM work order type, linked to the PM work order


Confirming the work

Figure 2. Tracking work order compliance ensures that the feedback loop is working.

and distinguished with the appropriate activity type and problem.

Because follow-up work orders are linked to the original PM visit, reports reveal if any follow-up work was com-pleted. This enables better tracking of problems throughout the approval process, enables reliability engineers to budget appropriately and empowers the PdM engineer with the results of closing the loop.

The bidirectional gateway and exchange of data between the PdM software database and the CMMS database is another example of leverag-ing technology to substantiate program success. There is now a direct correla-tion between vibration data and work completed. The dynamic link enables up-to-date status information and the resultant cause/effect on newly acquired PdM data.

Analyzing results and setting goalsAligning the PdM program vision statement with monthly tracking met-rics (KPIs) establishes an overall indica-tor of the relationship among Rockwell Automation, Air Liquide and contract performance. KPIs are reviewed in the form of a monthly compliance report submitted to supply management and reviewed with the senior maintenance and reliability team. Because KPIs are reviewed monthly, PdM program success is tracked and recorded to jus-tify program savings and ROI data. An example of the monthly compli-ance data and KPIs for Air Liquide include:• Sites visited• Samples taken• PdM saves

• Warranty claims• Customer care issues• Program costs and payment informa-

tionBecause each technology is a unique

component of the overall PdM pro-gram, and work orders can be classi-fied as such, further detail and tracking of individual PdM saves can be mea-sured on a monthly and year-to-date basis. Figure 1 represents a breakdown of PdM activity by type for the period from August 2004 through August 2005 (one year of implementation).

Also, we track and measure work order compliance to ensure the program derives actionable repairs from the PdM program recommendations. This vari-able is a measure of the number of new problems reported versus work orders initiated. Tracked by month, Figure 2 shows an example of one reliability zone.

Future goals include tracking and monitoring turbine eff iciency and machine performance as well as refining ROI data. One capability currently being implemented is the addition of pressure, flow and temperature measurements to the PdM program vibration routes. The PdM software uses these variables to calculate thermal and mechanical effi-ciencies. Trending can then be used to predict expander replacements based on efficiency savings.

The partnership between Rockwell Automation and Air Liquide pro-vides benefit to both Air Liquide and its customers. For starters, the nearly 2,000 interventions before equipment breakdown have avoided countless unit shutdowns. This isn’t only a benefit to our customers. It saves Air Liquide considerable costs by attacking prob-lems while they’re still relatively small.

However, this is just the start of what can be done with better information.

One can now look across common equipment and determine which OEM provides equipment with the lowest levels of vibration. Couple this with work order and reliability data from the CMMS, and we can provide quantita-tive information about what equipment to buy to improve MTBR.

We’re getting to the point where, armed with data and information that gets to the root of our problems, we can call in an OEM to discuss equipment issues. Instead of anecdotal opinions driving the discussions, the information from our systems now allow us to focus on improving long-term reliability.

Any engineer, specialist or expert, anywhere in the world, with access to our Internet site, can look at data and infor-mation on any piece of equipment in the U.S. and can help us troubleshoot prob-lems at even the most remote sites. p

Mark E. Lawrence, P.E., CMRP, is director of maintenance and reliability at AirLiquide Large Industries U.S. LP, Houston. Contact him at [email protected] and (713) 624-8585. George F. Hofer is corporate program manager at Rockwell Automa-tion, Houston. Contact him at [email protected] and (713) 402-2288.

January 2006www.PLANTSERVICES.com44


Equivalent annual cost — “Repair or replace”Air system design — “Keep it simple”Air system upgrades — “Planning air system upgrades” Valving for compressed air — “Shutoff and special duty valves”Optimum receiver location — “The compressed air receiver: the endless question”

For more, search www.plantservices.com using the keywords database, interface and vibration

More at www.plantservices.coM/this Month

February 2006www.PLANTSERVICES.com58

CategorySubcategory

A lmost every compressed air system uses flex hose to make the final connection to production machin-ery. Proper selection and application of this air hose

and the quick disconnects is critical to achieving optimum performance.

Compressed air system audits often uncover signif i-cant opportunity for savings at such locations. Typi-

cally, total system pressure is unnecessarily high to off-set pressure drops in small-diameter hose and incorrect quick disconnects.

The most important sizing data for any process is the air flow and minimum pressure required at the tool entry. If you don’t know these data, it’s easy for system analysts to measure them on-site. In areas where the pressure or flow are critical to productivity or quality, economical mass flowmeters and pressure gauges can be rigged for continuous machine monitoring.

Working by handAir-driven tools can illustrate the effect of hose and con-nector selection on productivity and quality. Most air tools are designed for a hose feed pressure of 90 psig. The tool designer really sizes for full flow at about 80 psig for opti-mum performance. Depending on the tool, pressure signifi-cantly higher than 90 psig may not increase performance, but lower pressure certainly will reduce it. In many cases, out-of-range air pressure can damage tools and reduce the time between rebuilds.

Standard impact tools, screwdrivers, grinders, chippers and banders prefer a constant 80 psig to 90 psig inlet pres-sure. The phrase “at rest pressure” has no meaning. Table 1, abstracted from selected air tool technical data sheets, clearly shows the general magnitude of performance loss at low pressure. At 70 psig, most tools will still oper-ate, but below rating. At 60 psig, performance is seriously degraded and probably will be unacceptable. Operating below 60 psig isn’t really a viable option. However, unless specifically stated, no tool is designed for inlet pressure greater than 100 psig. Table 2 shows the approximate performance losses at various inlet pressures in 1-hp to 3-hp vane motor grinders and sanders. The power drops

Hosing down your losses How to specify cost-effective air hoses and connectors By Hank van Ormer

Never select air hose unless you know the air flow and

hose length the tool requires.

February 2006 www.PLANTSERVICES.com 59

may preclude effective job performance. Along with the loss in power, which is most important, there’s also a loss in speed. Both factors affect productivity.

Beware of 3/8-in. hoseNever select air hose unless you know the air flow and hose length the tool requires. The most common hose siz-es for plant use range from 3/8 in. to 3/4 in. and handle 300 psig. Hose choice is often left to the operator, who usually wants 3/8-in. hose, regardless of application, because:

• 3/8-in. hose appears to be the lightest and easiest to handle.• A 50-ft. length of 3/8-in. or 1/2-in. hose weighs about

13 lbs., depending on grade but a 50-ft. length of 3/4-in. hose weighs 22 lbs.

• The operator might not be trained regarding the hose size required to run the tool.

A 3/8-in. hose isn’t a viable supply hose for industrial tools. The smallest size you should use is 1/2 in. Table 3

refers to premium black industrial air hose. The data leads us to specific conclusions:

• 1/2-in. hose in 50 ft. lengths is suitable only for 1 hp or smaller tools (approximately 30 cfm/hp).

• 3/4-in. hose is acceptable for 2 hp to 3 hp (60 scfm to 90 scfm), depending on the length of run.

• For runs greater than 50 ft., use larger hose or pipe, supported on the walls or ground as required, to elimi-nate pressure drop.

• For more comfort and easier operation, adding an 8-ft. to 10-ft. whip hose to the larger 3/4-in. or 1-in. main line will have minimal effect on performance, but still gives the operator the feel of a lighter hose.

• Don’t use any more hose than necessary. Coiling the extra just adds pressure drop. Cut the hose to the proper length and install fittings.

Don’t forget about OSHA safety requirements. Going from a 3/8-in. to 1/2-in. hose still allows personnel to handle

Table 1. Performance data for air-operated tools

Typical vane air motor performance at various inlet pressures (actual results will vary by manufacturer and model)

Inlet air pressure (psig) 1/2 hp 3/4 hp 1 hp 11/2 hp 2 hp 3 hp

60

rpm at max load 8,500 5,809 3,810 5,550 3,730 3,900

Max hp 0.35 0.47 0.765 0.927 1.74 2.32

scfm at max hp 20 20.1 27.5 30 51 67

Max torque ft-lb 0.36 0.88 1.67 1.67 3.7 5.0

70

rpm at max load 9,000 6,184 4,060 5,900 3,975 4,160

Max hp 0.41 0.58 0.95 1.15 2.16 2.88

scfm at max hp 21 53 32 35 60 78

Max torque ft-lb 0.42 1.0 1.95 1.95 4.3 5.8

80

rpm at max load 9,500 6,429 4,250 6,190 4,160 4,350

Max hp 0.5 0.69 1.13 1.38 2.58 3.44

scfm at max hp 22 27 36 40 68 89

Max torque ft-lb 0.5 1.2 2.2 2.2 4.9 6.7

90

rpm at max load 10,000 6,700 4,400 6,400 4,300 4,500

Max hp 0.6 0.8 1.4 1.5 3.0 4.0

scfm at max hp 24 30 39 42 76 100

Max torque ft-lb 0.55 1.3 2.5 2.5 5.5 7.5

100

rpm at max load 10,500 6,888 4,520 6,580 4,415 4,630

Max hp 0.6 0.9 1.5 1.8 3.4 4.6

scfm at max hp 26 33 45 50 85 111

Max torque ft-lb 0.6 1.4 2.8 2.8 6.1 8.3

performanceCompressors


smaller hose without the mandatory automatic air shutoff valve or safety velocity fuse. These fuses are an excellent safety device when applied correctly. Refer to U.S. Depart-

ment of Labor, Occupational Safety and Health Adminis-tration — Power Operated Tools 1926.302, page 2, para-graph 1926.302(b)(7), which mandates a safety velocity fuse on all hoses larger than 1/2 in. inside diameter.

A real-world exampleMore often than not, a process requires some minimum pressure. Trace these so-called requirements to their ori-gin to determine if they are actual OEM specifications or simply an operator’s perception.

A recent client was running the plant headers at 100 psig to 110 psig because a critical hand-tool grinding process was believed to require 98 psig to run correctly. Therefore, they reasoned, the system should run at 98 psig or more.

When you hear things like this, dig for more informa-tion. If the system header pressure falls below 98 psig, the grinders probably don’t work well. Production per-sonnel probably don’t know the actual pressure at the tool or how much air the tool uses. The rest of the plant could have run at 80 psig, but it operated at 98 psig because the grinding area supposedly required it. Grinding account-ed for only 20% of the demand, so 80% of the plant was supplied with air at a much higher pressure than needed. We didn’t calculate how much the higher pressure was costing, but intuition says it amounts to thousands of dollars a year.

Testing with a needle gauge at full operation revealed that the actual inlet pressure to the tool was 63 psig at load, but the header pressure stayed at 98 psig. In other words, there was a 35-psig pressure drop between the header and each grinder. Further testing revealed that the grinders only needed 75 psig for optimum performance.

The operators argued that they found the recommend-ed 3/4-in. hose to be too heavy, so they used 3/8-in. hose instead. The smaller hose restricted the air flow, which

produced a substantial pressure drop. Furthermore, the 3/8-in. hose used standard quick disconnects, which add-ed their own 23-psi pressure drop.

We changed the standard 3/8-in. quick disconnects to in-dustrial quick disconnects costing only $2.50 per pair — a whopping $5 per station — to reduce the pressure drop to

5 psig. Then, we replaced the 3/8-in. hose with 1-in. pipe routed to the base of the work stations at a cost of $30 each. Next, we installed a regulator that delivered full flow to the grinders at 75 psig with 80-psig feed pressure. Finally, we reduced the header pressure to 85 psig. About 18 months later, grinder repair costs had decreased and production throughput increased by 30% with the addition of more equipment. The cost of materials to implement these chang-es was $1,362 for nine grinders. Even with the production increase and new equipment, the average total air demand fell from 1,600 to 1,400 cfm.

The key to this success was monitoring the workstation inlet pressure while simultaneously monitoring header pressure. If the header pressure stays steady, and the pro-cess inlet pressure falls, then the restriction is in the feed line from the header to the process.

Break down the connectionThis case study demonstrates that small hose represented only 12 psid while the quick disconnects represented 23 psid. Often, but not always, a quick disconnect is the best answer for overall productivity. But, size the quick dis-connect for the maximum expected flow and the allow-able pressure loss. Read the manufacturer’s performance data sheet.

• Never select by connection size — select by accept-able performance at specified flow and entry pressure.

• If you want to use the same quick disconnect every-where for flexibility, do it. But, size them for the single largest flow demand at the lowest expected pressure.

Table 2. Off-design performance

Design pressure Actual pressure Performance loss

100 psig 90 psig 7% to 17%

90 psig 80 psig 7% to 16%

80 psig 70 psig 17%

70 psig 60 psig 20%

100 psig 60 psig 50%

90 psig 60 psig 39%

80 psig 60 psig 33%

Table 3. Pressure drops at flow rates

Tool size (hp) Flow (cfm)

Pressure drop per 50 ft. (psi)

1/2 in. hose 3/4 in. hose

1 30 2.4 0.4

2 60 14.9 2.2

3 90 41 4.6

(Assumes 90 psig supply, does not include fittings)

If the header pressure stays steady, and the process inlet pressure falls,

then the restriction is in the feed line from the header to the process.


February 2006 www.PLANTSERVICES.com 61

• Remember that each feed has at least two quick dis-connects.

• Use quick disconnects that shut off the flow when disconnected to eliminate potential hose whipping.

• Consider ISO 4414 exhaust-type quick disconnects that bleed off the air trapped inside the connection to eliminate blasting compressed air onto the operator at disconnect. It’s easier to uncouple a depressurized fitting.

• Quick disconnects should have proper safety latches, wires and keepers or be of a design that won’t open when dragged over the ground, floor or machinery.

Seek tested performance curvesDon’t assume that because couplers appear similar the per-formance is similar. Review the performance curves or, even better, measure the pressure loss at specific flows. On a recent audit to help select the proper disconnect for a major tool op-eration, we tested the pressure drop on two specific types of quick disconnect — a lock-ring coupler with a ball-check nipple versus an exhaust-type coupler with a standard nipple. Both had 11/4 in. diameter coupler bodies and ports sizes of 3/8 in., 1/2 in. and 3/4 in. The 3/8-in. nipple on the lock-ring type coupler didn’t have a ball check to shut off the air. The 1/2-in. and 3/4-in. units did. The exhaust-type couplers had the full shutoff and exhaust to allow disconnect at zero pressure.

Figure 1 shows the three sets of performance curves that reflect the measured pressured drop of each quick disconnect at various flows and inlet pressures. The results will probably vary by manufacturer. The key is to optimize performance by investigating.

We found a significant pressure drop difference between the 1/2-in. quick disconnects. The exhaust coupler could work in an acceptable manner from less than 30 cfm to as much as 60 cfm and still maintain 100 psig inlet with 80 psig to the tool or 90 psig inlet with 70 psig to the tool.

• The 1/2-in. lock ring/ball check nipple quick disconnect

appears acceptable at 30 cfm but probably won’t be accept-able at 60 cfm.

• The 3/4-in. quick disconnects are closer in perfor-mance, but the lock ring/ball check type introduces 30% to 40% more pressure drop.

• The 3/8-in. lock ring/ball check nipple quick disconnect tested didn’t have the ball check valve in the nipple, which accounts for its lower pressure drop compared to the 1/2-in. lock ring coupler, which did. This, of course, means that the safety feature to control potential hose whip isn’t incorpo-rated into the 3/8-in. lock ring set.

This test data isn’t intended to recommend one disconnect over another. For the particular application investigated, with many grinders and impact tools using between 60 scfm and 90 scfm, the exhaust-type quick disconnect exhibited the best overall performance and economics. On a differ-ent application, testing may well dictate another choice. The important point is to select quick disconnects, hose and pipe with diligence and attention to detail. Although dis-connects are a relatively inexpensive piece of equipment, if misapplied, they can be costly. p

Hank van Ormer is owner of AirPower USA, Pickerington, Ohio. Con-tact him at [email protected] and (740) 862-4112.

Lock-ring type with ball-check nipple• Push the lock-ring coupler to connect. Turn the lock ring about

20º to disconnect. This feature prevents accidental discon-nects.

• Nipple with ball check seals the air in the hose or tool con-nected to the nipple to eliminate blowback and possible un-controlled hose whip.

• The disconnect will be made under some pressure with variable flow dependent on the installation.

• Flow check-type nipples are more expensive than a standard industrial interchange nipple, which will work in many manu-facturer’s couplers.

Exhaust-type• These quick disconnects use a common standard industrial in-

terchange nipple. When comparing cost, it’s important to con-sider that in many operations, there are usually three or four nipples for every coupling.

• Exhaust-type couplings are push-to-connect, exhaust-style action with a self-locking sleeve to guard against accidental disconnection.

• To connect, push the nipple into the coupler. The locking sleeve slides forward automatically to lock the nipple in place. No air flows through the coupling at this point. Rotate the valve sleeve to open flow and engage the sleeve-lock mechanism.

• To disconnect, rotate the valve sleeve in the other direction to shut off the air flow and vent downstream air to atmosphere. The lock-ing sleeve can then be retracted and the nipple removed.

• The valve sleeve acts as an integral shutoff valve that allows connect and disconnect at zero pressure. The valve sleeve is operated independently of the locking sleeve. When the sleeve is moved to stop air flow, it automatically vents downstream pressure so disconnect can be performed at zero pressure.

• Exhaust couplers eliminate the need for flow-check nipples and still meet safety issues by connecting and disconnecting at zero pressure.

How quick disconnects work

Sizing and selecting FRLs — “Operation peak performance”Baselining — “Baselining the compressed air system”Proper line sizes — “The secret is in the pipe”Equivalent annual cost — “Repair or replace”

For more, search www.plantservices.com using the keywords compressor, performance or pressure.

More at www.plantservices.coM/tHisMontH


Flow

(SCF

M)

Pressure drop (PSID)

190180170160150140130120110100

908070605040302010

00 5 10 15 20 25

**

**

* * * * *

*

¾" Parker EZ 12HB@90 cfm 2.5 psid@65 cfm 1 psid@30 cfm Neg

�⁄�" Hansen B/C

¾" Hansen 12 HB@90 cfm 3.9 psid@60 cfm 2.5 psid@30 cfm 1 psid

½" Parker EZ 8HB@90 cfm 11 psid@60 cfm 5 psid@30 cfm <2 psid

½" Hansen 8HB@90 cfm 22.5 psid@60 cfm 10 psid@30 cfm 3 psid

Flow

(SCF

M)



190180170160150140130120110100

908070605040302010

00 5 10 15 20 25

**

** * * * * *

*

Flow

(SCF

M)

190180170160150140130120110100

908070605040302010

00 5 10 15 20 25

**

** * * * * *

*

¾" Parker EZ 12HB@90 cfm 3.4 psid@60 cfm 1.4 psid@30 cfm <1 psid

¾" Hansen 12 HB@90 cfm 5.4 psid@60 cfm 3 psid@30 cfm 1.5 psid

½" Parker EZ 8HB@90 cfm 14.5 psid@60 cfm 6.2 psid@30 cfm 2.8 psid

½" Hansen 8HB@90 cfm >30 psid@60 cfm 11 psid@30 cfm 4 psid

�⁄�" Hansen HB

¾" Parker EZ 12HB@90 cfm 4 psid@60 cfm 2 psid@30 cfm <1 psid

¾" Hansen 12 HB@90 cfm 5.5 psid@60 cfm 3.2 psid@30 cfm 1.5 psid

½" Parker EZ 8HB@90 cfm 16 psid@60 cfm 6.5 psid@30 cfm 2.8 psid

½" Hansen 8HB@90 cfm >30 psid@60 cfm 12 psid@30 cfm 4.4 psid

�⁄�" Hansen 6HB

Figure 1. These measured performance characteristics of several quick disconnects show flow as a function of pressure drop.



Measure before you act

R egenerative desiccant dryers are used in com-pressed air systems that require dew points to be below the minimum that refrigerated dryers

can produce (generally 40°F). Three types of regenera-tive desiccant dryers are widely used throughout industry: heatless, heated and blower purge.

The following discussion doesn’t address heat-of-com-pression (HOC) desiccant dryers, even though they require the least amount of energy to operate. The use of HOC dry-ers is limited to lubricant-free compressors.

Many plants require air quality that only regenerative desiccant dryers can produce. Unfortunately, in too many cases, the decision about which type of regenerative dryer to purchase is based on initial capital cost alone. This decision basis ignores the cost of energy that will be required to oper-ate the dryer. Including energy cost can alter the economics of a purchase decision dramatically.

Regenerative desiccant dryers use a desiccant medium to re-move moisture from the compressed air stream. Wet air passes directly through the desiccant medium, which then adsorbs

the moisture. The desiccant medium has a finite capacity for adsorbing moisture before it must be dried out, or regenerated. To do this, the tower containing saturated desiccant medium is depressurized and the accumulated water is driven off using purge air, heat or a combination of both. Desiccant dryers are generally of a twin-tower construction, with each tower con-taining its own desiccant bed. This allows one bed to dry com-pressed air as the other undergoes regeneration.

The energy cost associated with operating a desiccant dryer depends primarily on how the desiccant is regenerated — us-ing purge air, heat or a combination of both. Desiccant dry-ers are categorized by their method of regeneration, the three primary types of which are:• Heatless, which uses only compressed air as a purge.• Heated, which uses both heat and compressed air to purge

moisture.• Blower purge, which uses air from an external blower,

heat and minimal compressed air.The energy cost to operate each of these dryers depends

on the amount of purge air used, the heater size and the blower motor size.

Heatless desiccant dryerHeatless desiccant dryers use treated, unheated compressed air to regenerate the desiccant bed. This dryer is the most ex-pensive to operate because of the large amount of compressed air that’s consumed for purging during the regeneration cycle. Typically, 15% of the rated flow capacity of the desiccant dryer is consumed as purge air. This is a significant amount of expen-

www.PLANTSERVICES.com

The energy cost associated with operating a desiccant dryer depends primarily on the means by which the

desiccant is regenerated.

hung out

to dryhung out

to dryDon’t getDon’t get

Insight into the economics of

operating regenerative desiccant dryers

By Noel Corral and Andrew Sheaffer

April 2006 55


sive compressed air to be used in this manner. In many facilities that operate heatless units, the air dryers are the largest single user of compressed air at the facility. Although the cost to oper-ate this type of dryer is high, the heatless type is often selected because it has the lowest initial cost of any desiccant dryer.

Heated desiccant dryerGenerally, heated desiccant dryers have the lowest energy operating cost of the three types. A heating element supple-ments the drying action of the purge air. The heater can be mounted internally to heat the desiccant bed directly or ex-ternally to heat the purge air that’s blown through the bed.

Internal heating elements shorten the life cycle of desic-cant beds and may cause dew point spikes. Therefore, they are not recommended and are not covered here. Like heat-less dryers, the purge is compressed air supplied from the system. However, the amount of air required is reduced be-cause the heat supplements the air’s drying action.

The amount of purge air an externally heated dryer requires varies by manufacturer, but is typically about 7% of the dryer’s rated flow capacity. The combination of heat and purge air is more energy efficient than purge air alone, yielding lower energy costs.

Blower purge desiccant dryerLike heated-type desiccant dryers, blower purge-type dryers supplement the drying action of purge air with heat. In a blow-er purge desiccant dryer, however, the purge air is ambient air supplied by an electric blower. The typical blower supplies the entire purge air load, which is typically 20% of the dryer’s rated flow. Depending on the dryer design, blower purge-type dryers may still consume a small amount of compressed air that cools the bed at the end of the regeneration cycle. Blower purge-type dryers are generally more expensive to operate than the heated type, but less expensive to operate than the heatless variety.

Determining your own energy costsSo, we know that heatless desiccant dyers are the most ex-pensive to operate in terms of energy costs, heated dryers are the least expensive, and blower purge dryers are somewhere in between. But, how can plant management quantify these costs so they can be used to make an informed decision when purchasing a desiccant dryer? Table 1 shows the appropriate equations you can use to calculate the energy cost to operate each type of desiccant dryer.

These equations can be used with dryer manufacturer specifications to determine the energy cost to operate the unit. The equations, with generalized manufacturer dryer specifications, were used to develop Figure 1, which sum-marizes the energy cost associated with the operation of heatless, heated and blower purge desiccant dryers.

The heatless desiccant dryer is the most expensive type to operate at any capacity, while the heated type is the least expensive to operate at any capacity. The magnitude of dif-ference between the heated and blower purge type is fairly


HeatlessEC =

RDC x 15% x 0.746 x t x $/kWh

CGE

Heated

HC =(375-T ) x RDC x 7%

3,160

EC =RDC x 7% x 0.746

CGE+ (0.75 x HC ))( x t x $/kWh

BlowerPurge EC = + (0.75 x HC )

HPb x 0.746

etab )( x t x $/kWh

HC =(375-T ) x RDC x 20%

3,160

EC = Annual energy cost to operate ($)

HC = Electrical capacity of heater (kW)

RDC = Rated dryer capacity (cfm)

t = Annual operating time (hrs)

$/kWh = Utility rate ($/kWh)

CGE = Compressor generation e�ciency (cfm/hp)

T = Temperature of regeneration air (°F)

HPb = Blower horsepower (hp)

etab = Blower motor e�ciency (%)




Heatless

EC =RDC x 15% x 0.746 x t x $/kWh

CGE

Heated

HC =(375-T ) x RDC x 7%

3,160

EC =RDC x 7% x 0.746

CGE+ (0.75 x HC ))( x t x $/kWh






CGE = Compressor generation e�ciency (cfm/hp)


HPb = Blower horsepower (hp)

etab = Blower motor e�ciency (%)

Blower purge

HC =(375-T ) x RDC x 20%

3,160

EC = + (0.75 x HC )HPB x 0.746 )( x t x $/kWh

Table 1. Use these formulae to calculate your own operating costs.






CGE = Compressor generation efficiency (cfm/hp)


HPB = Blower horsepower (hp)

ηB = Blower motor efficiency (%)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

55,000

60,000

5,0004,0003,0002,0001,000500

Tally the cost

Figure 1. Annual operating cost of desiccant dryers. Data assumes 8,760 operating hours per year, $0.05/kWh utility rate, compressor generation efficiency of 4.5 cfm/hp and compressed air and ambient air temperature of 100°F.

April 200656

ηB

small, indicating that users should base any decisions be-tween these two types on other factors (initial cost, mainte-nance costs, excess capacity for purge air, etc.)

What about controls?Manufacturers of desiccant dryers provide optional con-trols that can reduce the dryer’s operating cost. The most widely used control is the dew point demand controller. It uses a probe that continuously monitors the compressed air leaving the drying tower. It’s configured to delay regenera-tion until the exit air reaches the user’s maximum allow-able dew point. The controller bases regeneration on dryer

performance instead of a timed cycle, thereby reducing the number of regeneration cycles. Controls such as these can yield 30% to 80% in energy savings, depending on the maximum allowable dew point. Savings achievable using these controls must be calculated on a case-by-case basis.

Controls reduce cycling frequency, resulting in lower over-all maintenance and longer desiccant and valve life. Although the cost of controls increase the dryer’s purchase price, the benefits they provide can pay back the initial cost quickly.

Desiccant air dryers are a proven and effective technology that can provide extremely dry air. However, they can be ex-pensive to operate in terms of energy costs, and every plant professional who needs a desiccant dryer should be aware of these costs before going out for bids. Using the equations laid out here, you can determine what your energy costs will be and make informed decisions about what type of desic-cant dryer is best for your application. p

Andrew Sheaffer and Noel Corral are research engineers at the En-ergy Resources Center at the University of Illinois at Chicago. Con-tact Corral at [email protected] and (312) 413-8546. Contact Sheaffer at [email protected] and (312) 413-3615.

Figures: Energy Resources Center



Air system design — “Keep it simple” Dew point monitors — “Fog in the pipeline” System engineering — “Engineered air” Treatment and storage — “Compressed air quality” HOC calcs — “What does Mother Nature say about cooling hot air?”

For more, search www.plantservices.com using the keywords dryer, CAC and desiccant.

More resources at www.PlantServices.com/this month

April 2006 57

Compressed air isn’t free. You’d think that would be obvious to

maintenance and operations personnel in plants where some of the largest motors are

harnessed to compressors, howling and yammer-ing away 24/7, sucking up, in many cases, the larg-est percentage of an increasingly outrageous utility

bill. Wasting upwards of 30% of that energy should probably be illegal, but it’s not, and apparently most folks

are oblivious.“If water or oil are leaking, you know it,” says Len Bishop,

manager, Draw Professional Services (www.drawproser-vices.com). “If gas is leaking, you smell it. You can see steam. But compressed air? It’s not a safety issue until someone

complains that it’s too loud. So unless it’s knocking your hat off, you ignore it.”

Statistics from the Compressed Air Challenge (CAC) and DOE are confirmed by the compressed air system experts: The average facility has 30% to 35% leakage if it hasn’t taken any recent action. And a survey by the

Office of Industrial Technologies says 57% of facilities have taken no ac-tion during the past two years.

“Air is a clear fluid that doesn’t make a mess,” says David Booth, system specialist, Sullair (www.sullair.com). “It can leak forever

and not directly affect anything

but cost. So, unfortunately, leaks are a big component that no one is willing to fix permanently.”

Even a ¹/₁₆-in. diameter leak can cost big bucks (Figure 1). But you needn’t get out a caliper. “If you can hear it without an ultrasonic leak detector, it’s at least 8 cfm to 10 cfm at $300/year per cfm,” says Bishop. “It may not be a safety issue, but it’s costing you money.”

The excess cost goes beyond wasted energy. Leaks lead to other plant problems:• Fluctuating system pressure: Inconsistent or faulty performance

of air tools and other air-operated and powered equipment.• Excess compressor capacity: higher than necessary equip-

ment and maintenance costs. “Not only are you wasting en-ergy, you’re likely to be mismatching the compressor,” says John Bartos, vice president of engineering and new product development, Cooper Compression (www.coopercameron.com). “You buy based on calculated demand, and leaks can really affect the actual performance.”

• Excess load on supply equipment: Increased maintenance costs, decreased service life.

• Thwarting other system efficiency efforts: It’s impossible to optimize system pressure and compressor control schemes with excessive leaks. “In cases where the total leak load ex-ceeds 10%, the artificial demand created by leaks must be addressed in order to obtain an accurate air demand profile for the plant,” says Wayne Perry, technical director, Kaeser Compressors (www.kaeser.com). “Only then can other rec-ommendations and improvements take place.”


By Paul Studebaker, CMRP, Editor in Chief


• Wet air: While letting air out, leaks let moisture in.“Valvesondrain legs are left crackedopenbecause toomuch water is coming into the equipment,” says ScottStroup, president, Airometrix Manufacturing (www.ai-rometrix.com).“Butunlesstheairdryerismalfunction-ing,theairisleavingitata-40°dewpoint.Where’sthewatercomingfrom?Itcomesinattheairleaks.”It’s counterintuitive,butFick’sLawexplainswhywater

migrates from high-humidity outside air into dry com-pressedaireventhoughairiscomingouttheleak.“It’shardand for some people, impossible to believe, but it’s true,”Stroup says. “Is your water problem bigger when the hu-midityishighorit’sraining?That’swhy.”

Sowithallthesepotentialbenefits,whyis itthat,asJanZuercher,director,air systems,QuincyCompressor (www.quincycompressor.com),says,“Mostpeopledoverylittleleakmanagement—amajoritydoitinfrequentlyornotatall.”

Someplantsareignorant,morefeeltheycan’tdevotetimeandefforttoleakmanagement,andplentyseeitaspointless:fixingsomeleaksmakestheothersleakmore,andyou’rebacktozero(seesidebar,“Exerciseinfutility?”).Butunderstand-ingwheremostleaksoccur,efficientwaystodetectthem,andtheireffectsonsystempressureandperformancecanhelpyouimplementanenergy-savingleakmanagementprogramthat’ssimple,efficient,rewardingand,darewesay,almostfun.

Modi operandiBeforeyoustartshoppingfor leakdetectionequipmentorget bogged down seeking management support, it’s help-ful to know where many of the largest leaks are likely tobe found (Table 1). Drawing on his extensive experience,Stroupprovidesthisranking:

1.Hosesandhosefittings:Onhoses,thefitting-to-hoseconnections are most likely due to improper clamping orworkingloose.“Nextcomesthehoseitself,cutorgashed,”Stroupsays,“thenthequick-disconnects.”

2. Pressure regulators and filter/regulator/lubricators(FRLs):Thestemsonthefilterbowlsleakorareleftopen,O-ringsleakatthebowl-to-housingconnection,filterbowlsarecracked,andontheregulatorsthemselves,O-rings,gaskets,andthepipingconnectionsbetweenelementsoftenleak.

3.Plastictubing:“Therearesomanyofthem,thesheernumber adds up,” Stroup says. “The leaks are mainly atpush-to-connectfittings.”

Zuercheragreesthattubingconnectorsareacommoncauseofleaks,butsays,“They’retypicallytoosmalltobeworthrepairing.Fixthelargestleaksfirsttogetthebiggestbangforthebuck.”

4.Headeranddistributionpiping:“Weldedsteelandsol-deredcopperpipearegenerallypretty tight,”Stroup says.“Threadedpipeismorelikelytoleak.”

5.Leakswithinequipment:Insomeplants,theseareamajorconcern.“Asmallholetherecangoundetectedandcauseapressuredropthatleadstoanequipmentmalfunc-tion,”saysStroup.“Peopleendupreplacingacylinderorso-

Figure 1: At 24/7 operation and $.08/kWh, even a small leak with an equivalent diameter of ¹/₁₆ in. steals more than $700 per year.

Grand larceny

$13,867

$11,427

$31,200

$25,733

$55,333

$45,733

$720

$865

80 psig100 psig

$60,100

$50,100

$40,100

$30,100

$20,100

$10,100

$1001/2

3/81/4

1/16 Leak size, inches

Cost

CouplingsHosesTubingFittingsPipe jointsQuctsFilter/regulator/lubricator (FRL) units

Condensate trapsValvesFlangesPackingsThread sealantsPoint-of-use devicesOpen condensate trapsOpen shut-off valves

Table 1: Common culprits

ManageMentCompressed Air Systems

0

100

200

300

400

500

600

700

800

900

1000

0

100

200

300

400

500

600

700

800

900

1000

5 10 15 20 25 30 35 40

Compressed air leakage (cfm)

Rolling average

Figure 2: Even solid progress in reducing leaks over time will be uneven, as shown by this chart from a wood-products plant.

Tough cookies

Weeks


“Everybody knows they have leaks and most have a lot of them,” says Jan Zuercher, director, air systems, Quincy Compressor (www.quincycompressor.com). “It’s a bit of a vicious circle. As you fix leaks, pressure rises and the remaining leaks leak more. A lot of plants reach the point where they say, ‘What’s the use?’ Most maintenance staffs are running very lean. They’re focused on keeping the plant running. Who has the time?”

It’s true that most calculations of potential savings from leak repairs are optimistic, and for two reasons:

1. Estimates assume that the system is 100% efficient: saved cfm are converted directly to less horsepower consumed in the compressor room. “In reality, this is never the case, and it’s usu-ally not even close,” says Robert Horneman, marketing manag-er, Industrial Air Solutions, Ingersoll Rand Industrial Technolo-gies (www.irco.com).

For example, reducing the air load on a modulating compres-sor will cause the inlet valve to close more, drawing a vacuum under the valve. “A modulating machine at 70% load will still pull 93% of its full-load horsepower,” Horneman says. “Similarly a centrifugal compressor can only back down so far depending on its surge points before it starts to blow off. If it’s close to its surge point and you fix leaks, it will simply blow off air and you have gained nothing. You need a good handle on the air system as a whole and how each part affects it, especially how the sys-tem reacts to part loading.”

2. Estimates assume that fixing a leak has no effect on the rest of the system. In practice, fixing a leak often causes pressure to rise in the vicinity of the repair. This may not be all bad -- ac-cording to Scott Stroup, president, Airometrix Manufacturing (www.airometrix.com), “Most of our customers would do back-flips for five more psi.”

But higher pressure means an exponential increase in flow through unregulated uses, including leaks. “This will very quickly return the leak load to original levels,” Horneman says.

The solution is to tightly control the demand pressure so the decrease in leak load will not increase the local pressure, and to control the compressor output so the decreased demand will be reflected in online horsepower. Pressure can be controlled with an expander valve between supply and demand. “This al-lows you to fix leaks without affecting the rest of the system by holding pressure within two psi,” says Horneman. “This valve can react to dynamic demand changes quicker than the com-pressor controls, allowing the system to respond to events with stored compressed air rather than compressor horsepower.

“I am not advocating throwing a piece of machinery into any system for a resolution,” Horneman says. “Compressed air systems are unique and components must be understood in their relation to the whole before an effective solution can be identified. A detailed analysis of the system is essential to avoid improper allocation of capital.”

ExErcisE in futility?


lenoid valve trying to fix an intermittent equipment prob-lem that’s actually caused by an undetected leak.”

For example, a packaging machine wouldn’t work proper-ly when other equipment in the area was running. Mechan-ics replaced a solenoid valve with no effect. “When we got next to it, we could hear a leak,” Stroup says. “We crawled under it and found where a chain had worn a hole in an air line. Fixing that fixed the problem with the machine.”

Pneumatic systems inside panels often share a common, ducted exhaust, masking the presence of leaks in the normal airflow. An internal failure of a solenoid valve can blow a lot of undetected air, whether the machine’s running or not. “We also often find missing faceplates and uncapped lines in solenoid cabinets,” Stroup adds.

Don’t ignore “useful” leaks. For example, compressed air used to position paper in an envelope machine should be turned off when the machine’s not running. Add a shut-off valve interlocked with the on-off switch on the machine. A manual valve isn’t reliable because operators don’t want to worry about it and will simply leave it on.

Get on the caseDetection methods range in sophistication from listening and running hands over pipes and equipment to soap bubbles and, today’s big gun, the ultrasonic leak detector (ULD). All these tools have their place, but the most effective and least familiar is the ULD.

“Ultrasonic is a critical element,” says Zuercher. “The old-school thinking was that you had to detect leaks on nights

and weekends when the plant is quiet so you can hear the leaks, and most people don’t want to do that. With ultra-sound, you can detect leaks during normal production.”

Handheld ULDs locate leaks both in compressed air and vacuum systems, and search areas such as valve seats, drain traps, tanks and piping. USDs can detect internal leaks and leaks that are near the ceiling, outdoors or in noisy environments. “For example, a blown diaphragm on a valve in a baghouse can leak a lot of air where it is easily overlooked,” Zuercher says. “Good ultrasonic can detect leaks from over 50 feet away.”

A good ULD can find leaks quickly, but aficionados dis-agree on whether they can be used to judge leak rate. Zu-ercher says the size of a leak is proportional to the volume of the sound given off by the ultrasonic leak detector, but Ben Fried, product development and support, CTRL Systems (www.ctrlsys.com), says, “The number-one misconception about ULDs is that they can measure the intensity of a leak and correlate it to dollars lost. This is 100% false.”

Many factors change the intensity of a leak, includ-ing pressure; temperature; humidity; distance from the receiver; size, shape and location of the hole; proximity of hole to other objects; battery power of receiver; etc., Fried says. “There is no way to determine amount of cfm loss based on a decibel reading or some other relative measurement.”

In general, ULD capabilities vary, and experts have strong words about making sure the one you buy will do what you expect (see sidebar, “ULD: Pick from a lineup”).


Experts have observed a number of effective practices that can help you enlist operations and more easily implement an effec-tive leak management program:• Bereadytofixasyougo: Carry a wrench when you look for

leaks. A lot of them can be repaired on the spot.• Tagleakswithathree-parttag:Bring in one stub to make

a work order. Bring in the second stub when the leak is fixed. Have the repair checked, then bring in the rest of the tag. “Leaks can be hard to fix or come back right away,” says Da-vid Booth, system specialist, Sullair (www.sullair.com). “Use a three-part tag — find, fix, verify — and have someone check the repair and bring in the tag before you say it’s done.”

• Enlist management: “One guy takes a camera out in the plant, gets pictures, brings them into management meetings and says, ‘This is a $500 problem,’” says Booth. “He’s taken pictures of leaks with two and three leak ID tags that haven’t been repaired. Until he did this, no one took him seriously.”

• Educateoperatorsandgettheirbuy-in: “Otherwise, peo-ple will stand at a machine for years with air blowing on their knees,” Stroup says. “They’re already wearing earplugs so they aren’t even bothered by the noise. But if you approach their machine with an ultrasonic instrument and tell them what you’re doing, they’ll point out the leaks for you.”

Like safety, everyone can be aware of the cost of air leaks and be on the lookout. “The day after you do a survey, you can get a large leak,” says Jan Zuercher, director, air systems, Quincy Compressor (www.quincycompressor.com). “If the employees care and report those leaks, it can be a good part of the program. Management can provide incentives.”

One plant decided to share the profit. “They pay opera-tors $5 apiece for identifying leaks,” says Booth. “It pays off. Operators can be the world’s largest detection force.”

• Seewhichequipmentholdsup:Depending on operating conditions, there can be a lot of variation in different types and brands of regulators, FRLs, quick-disconnects, etc. Toss the ones you replace in a bucket, fill it up and look at them. Quit buying stuff that doesn’t survive. “One client picked the best quick-disconnect and replaced 10,000 all at the same time — that is an automotive plant with 3,200 employees, all using hand tools,” says Scott Stroup, president, Airome-trix Manufacturing (www.airometrix.com). “They did it three years ago and it paid off.”

• Useatrade-insystem:One plant uses a two-barrel trade-in system near the machines on the plant floor to replace leaking hose assemblies. Operators can quickly and easily eliminate a leak by tossing a leaky assembly into a red barrel and taking a refurbished one from a green barrel. “It saves a fortune in air and maintenance labor,” Stroup says.

• Addleaksurveystocompressormaintenancecontracts:“If it makes sense, divide the plant into quarters and survey one quarter with each visit,” Booth says.

• Keepeveryoneaware: Like a safety program, hang signs such as, “¼-in. leak = $8,000/year.”

Start a neighborhood watch

Table 2: Critical steps

1. Evaluation (leaks, pressures and compressor controls)2. Detection (involve operators)3. Identification (tagging)4. Tracking5. Repair6. Verification7. Evaluation


Those same experts agree that, although the learning curve is short, some training is required. “Get a demo,” says Zuercher. “Have someone come in and show you how they work.”

Though not yet a common use, increasingly sophisticated infrared imagers can detect the thermal plume of a compressed air or gas leak. “We have used this technique in the past for pinpointing leaks on certain pieces of equipment, especially when it is in an environment where you cannot enter safely,” says Stroup.

Stop repeat offendersVigilance is your best defense against a growing leak popu-lation, but a few key practices can help. First, set aside the Teflon tape when assembling threaded pipe and fittings. “Teflon tape seals the contact points, but it doesn’t fill the voids,” says Tom Buckley, application engineer, Henkel Corp. (www.loctite.com). “There’s a lot of space left for leak paths.” Sealants in the form of thick liquids or pastes fully fill the spaces in the threads.

“On compressors, our extensive testing shows Teflon tape is not an answer,” says Booth. “You need a quality sealant com-patible with the compressor fluid.”

Before the sealant has cured, pipe threads can be adjusted for alignment by loosening or tightening without compromis-ing the seal. Sealants that won’t cure while exposed to air can be applied to all the pipes and fittings in one operation before assembly. Fittings can be adjusted for several hours after as-sembly and once cured, the sealant adds mechanical strength and vibration resistance to the joint.

Fluid compatibility is an issue with the elastomers in FRLs, non-threaded pipe joints and connected equipment. “The di-ester lubricants can be extremely aggressive,” Booth says. “If you’re using a diester, be sure you’re using the correct elasto-mers and hoses.”

Though piping ranks low on the frequency list, leaks there can be the most difficult to repair. “Joints and fittings can loosen or wear over time and should be routinely checked,” says Perry. “Old or outdated piping of any material should be immediately replaced.”

While many prefer copper and stainless steel, there are several modular aluminum piping products that offer a cost-effective al-ternative. “Not only can these products be fully integrated into existing systems, they offer push-to-fit connections that elimi-nate brazing and welding joints and fittings – the most likely place piping leaks occur,” Perry adds.

Lightweight modular systems can be installed quickly without specialized skills. “Our push-to-connect sys-tem uses high-nitrile seals and is rated to 232 psi,” says Tim Deal, manager of customer service and sales, Legris Transair (www.transair.legris.com). “The seal material is the same that is commonly used in compressors so it will resist all compressor fluids. It’s a good way to replace steel or copper so it doesn’t corrode and it leak again.”



lenoid valve trying to fix an intermittent equipment prob-lem that’s actually caused by an undetected leak.”

For example, a packaging machine wouldn’t work properly when other equipment in the area was running. Mechanics replaced a solenoid valve with no effect. “When we got next to it, we could hear a leak,” Stroup says. “We crawled under it and found where a chain had worn a hole in an air line. Fixing that f ixed the problem with the machine.”




“Ultrasonic is a critical element,” says Zuercher. “The old-school thinking was that you had to detect leaks on nights and weekends when the plant is quiet so you can hear the leaks, and most people don’t want to do that. With ultra-sound, you can detect leaks during normal production.”

Handheld ULDs locate leaks both in compressed air and vac-uum systems, and search areas such as valve seats, drain traps, tanks and piping. USDs can detect internal leaks and leaks that are near the ceiling, outdoors or in noisy environments. “For example, a blown diaphragm on a valve in a baghouse can leak a lot of air where it is easily overlooked,” Zuercher says. “Good ultrasonic can detect leaks from over 50 feet away.”

A good ULD can find leaks quickly, but aficionados disagree on whether they can be used to judge leak rate. Zuercher says the size of a leak is proportional to the volume of the sound given off by the ultrasonic leak detector, but Ben Fried, product devel-opment and support, CTRL Systems (www.ctrlsys.com), says, “The number-one misconception about ULDs is that they can measure the intensity of a leak and correlate it to dollars lost. This is 100% false.”

Many factors change the intensity of a leak, including pressure; temperature; humidity; distance from the receiv-er; size, shape and location of the hole; proximity of hole to other objects; battery power of receiver; etc., Fried says. “There is no way to determine amount of cfm loss based on a decibel reading or some other relative measurement.”


Those same experts agree that, although the learning curve is short, some training is required. “Get a demo,” says Zuerch-er. “Have someone come in and show you how they work.”


Stop repeat offendersVigilance is your best defense against a growing leak pop-ulation, but a few key practices can help. First, set aside the Teflon tape when assembling threaded pipe and fittings. “Teflon tape seals the contact points, but it doesn’t fill the voids,” says Tom Buckley, application engineer, Henkel Corp. (www.loctite.com). “There’s a lot of space left for leak paths.” Sealants in the form of thick liquids or pastes fully fill the spaces in the threads.



Ben Fried, product development and support, CTRL Systems (www.ctrlsys.com) offers these suggestions for choosing an ultrasonic leak detector (ULD):

1. Sensitivity: Several ultrasonic leak detectors (ULDs) on the market offer a wide range of sensitivity. Rule of thumb: You get what you pay for.

2. Selectivity: This relates to dynamic range of a ULD. Some detectors have wide bandwidth, thereby picking up more white noise or static. Additional noise decreases the user’s ability to detect a leak.

3. Volume: Some ULDs feature adjustable sensitivity and volume. Changing the volume increases the output ampli-tude, but doesn’t improve sensitivity.

4. Price: ULDs range in price from $100 to $10,000. Gener-ally, higher-price ULDs are capable of doing condition moni-toring as well. The lowest-priced ULDs aren’t very effective in a manufacturing facility and can miss most leaks. “Ultra-sonic leak detector quality is all over the map,” adds Jan Zuercher, director, air systems, Quincy Compressor (www.quincycompressor.com). “Someone wanting to purchase a leak detector should talk to an experienced user or test a handful of different instruments in their plant.”

ULD: Pick from a LineUP

lenoid valve trying to fix an intermittent equipment problem that’s actually caused by an undetected leak.”

For example, a packaging machine wouldn’t work properly when other equipment in the area was running. Mechanics replaced a solenoid valve with no effect. “When we got next to it, we could hear a leak,” Stroup says. “We crawled under it and found where a chain had worn a hole in an air line. Fixing that f ixed the problem with the machine.”




“Ultrasonic is a critical element,” says Zuercher. “The old-school thinking was that you had to detect leaks on nights and weekends when the plant is quiet so you can hear the leaks, and most people don’t want to do that. With ultra-sound, you can detect leaks during normal production.”

Handheld ULDs locate leaks both in compressed air and vacuum systems, and search areas such as valve seats, drain traps, tanks and piping. USDs can detect internal leaks and leaks that are near the ceiling, outdoors or in noisy environments. “For example, a blown diaphragm on a valve in a baghouse can leak a lot of air where it is easily overlooked,” Zuercher says. “Good ultrasonic can detect leaks from over 50 feet away.”

A good ULD can find leaks quickly, but aficionados disagree on whether they can be used to judge leak rate. Zuercher says the size of a leak is proportional to the volume of the sound given off by the ultrasonic leak detector, but Ben Fried, product devel-opment and support, CTRL Systems (www.ctrlsys.com), says, “The number-one misconception about ULDs is that they can measure the intensity of a leak and correlate it to dollars lost. This is 100% false.”

Many factors change the intensity of a leak, including pressure; temperature; humidity; distance from the receiv-er; size, shape and location of the hole; proximity of hole to other objects; battery power of receiver; etc., Fried says.

“There is no way to determine amount of cfm loss based on a decibel reading or some other relative measurement.”


Those same experts agree that, although the learning curve is short, some training is required. “Get a demo,” says Zuerch-er. “Have someone come in and show you how they work.”


Stop repeat offendersVigilance is your best defense against a growing leak population, but a few key practices can help. First, set aside the Teflon tape when assembling threaded pipe and fittings. “Teflon tape seals the contact points, but it doesn’t fill the voids,” says Tom Buckley, application engineer, Henkel Corp. (www.loctite.com). “There’s a lot of space left for leak paths.” Sealants in the form of thick liquids or pastes fully fill the spaces in the threads.



Fluid compatibility is an issue with the elastomers in FRLs, non-threaded pipe joints and connected equipment. “The di-ester lubricants can be extremely aggressive,” Booth says. “If you’re using a diester, be sure you’re using the correct elasto-mers and hoses.”

Though piping ranks low on the frequency list, leaks there can be the most difficult to repair. “Joints and fittings can loosen or wear over time and should be routinely checked,” says Perry. “Old or outdated piping of any material should be immediately replaced.”

While many prefer copper and stainless steel, there are several modular aluminum piping products that offer a cost-effective al-ternative. “Not only can these products be fully integrated into existing systems, they offer push-to-fit connections that elimi-nate brazing and welding joints and fittings – the most likely place piping leaks occur,” Perry adds.

Lightweight modular systems can be installed quickly with-out specialized skills. “Our push-to-connect system uses high-nitrile seals and is rated to 232 psi,” says Tim Deal, manager of customer service and sales, Legris Transair (www.transair.legris.com). “The seal material is the same that is commonly used in


ManageMenTCompressed Air Systems



Make a major crackdownQuantifying and systematically controlling compressed air leakage starts and ends with periodically evaluating your system’s leak rate (Table II), preferably as a percentage of use. “Today, most plants are at 30% to 40%, and I’ve been in several that were at 50% or more,” says Stroud.

The best way to determine energy use and savings is to mea-sure electricity consumption of compressors. The Compressed Air Challenge (CAC, www.compressedairchallenge.org) offers other methods to baseline and evaluate system leakage. Once you know where you are, you can estimate potential savings and measure progress over time. “No one is going to get to zero leaks,” Stroud says, “but even in heavy industry with inaccessible equipment, you should be able to achieve 15% or less. Cleaner facilities should reach 10% and the best are at about 5%, though I know of two that made it to 3%.”

The inspirational dollar amounts shown in Figure 1 aren’t very useful for estimating real-life potential savings, as typical leaks are not perfect orifices at constant, known pressure. “We test air systems and meter the flow volumes to quantify the leaks,” says Stroup. “When we do that at different pressures, we don’t see a one-to-one correlation.”

For example, he says, the charts will show a ¼-in. hole at 100 psig leaks 104 cfm and costs about $10,000 per year, but that’s based on a perfect orifice. “A real leak is gener-ally a gash or cut or other irregular shape, so you use a ¼-in. equivalent area, but you can’t put 104 cfm through the tube, so you have to take it with a grain of salt,” Stroup says. “I usually cut it in half for a more realistic estimate.”

Ready to find and fix leaks? First, prioritize. Start in the area where the most air is used or where the most leaks are. Stroup says, “Most plants have a feel for that.”

The first time through, focus on 20 cfm leaks and ignore 1 cfm leaks. “Start with your ears and hands, then get to ultrasonic,” Stroup says. “You can’t quantify a leak with ul-trasonic or any other method, but you can estimate.”

Stroup says, initially, a large leak is more than 10 cfm, me-dium is 5 cfm to 10 cfm, and small is less than 5 cfm. “Do the large leaks first, and the medium ones if you can,” he says. “Next time around, the medium leaks will be large — reclassi-fy them and do it over. Eventually the leaks will be too small to be worth fixing. Then watch them and fix the ones that grow.”

Don’t be daunted by the results of your first survey. “You do a survey and three days later, you have 150 tags,” says Bishop, “You gag on it.” The answer, he says, is don’t try to do it all at once — do it one day a month. “After a year, issue five open work orders a week from your CMMS to find and fix a leak — and don’t come back until it’s fixed.”

Repeat the survey at regular intervals. In a high-vibration and shock facility like a sawmill, check for leaks monthly, Stroup says. In most facilities, such as food and light manu-facturing with stationary machines, quarterly is good. Clean, quiet industries like pharmaceuticals can go semi-annual, but, he warns, “Nobody should go longer than six months.”

The key is to keep at it. The benefits don’t come in one month. “You’ll fix the leaks, then fix more,” Stroup says. “For most plants it takes six months to a year to see benefits. The progress of a typical facility is shown in Figure 2.

“Ultrasonic, feel, listen, whatever you’ve got,” says Bartos. “The important thing is to do it regularly. You spend so much money on a compressor and energy, it’s going to pay off.”

Figures: Airometrix, Kaeser

The compressed air industry is taking a step that will clear the air about the performance of its equipment. For the first time, many compressed air equipment

manufacturers will submit their 50 hp to 200 hp lubricated rotary-screw compressors and 200 cfm to 1,000 cfm refrig-erated air dryers to an independent laboratory for perfor-mance testing. The Compressor Distributors Association (CDA) led the charge that moved the industry to this deci-sion. The CDA, a group of industrial air compressor dis-tributors representing different manufacturers, was formed to sponsor the Compressed Air Challenge (CAC) and to have a voice in its operations. The CDA is represented on both the CAC Board of Directors and its Product Develop-ment Committee.

After several meetings, the CDA began a series of dis-cussions about how the organization might improve the industry. The group agreed that a need existed for ac-curate and independently verified performance data on the equipment they provide to end users. The motivation is that distributors are being pressured to provide much

more accurate system analysis and recommendations than have been required in the past.

With the ability to be accurate comes an increasing need for accurate and dependable input information. The commitment to this need prompted CDA to fund an independent, blind testing program for seven manu-facturers’ 50-hp lubricated rotary-screw compressors against their published performance information. Find-ing a capable, well-equipped laboratory was a formidable task, as was securing the use of seven competitive ma-chines for the short time window available.

Test results are presented as percentage variance from the published data. With test data in hand, CDA approached the Compressed Air Challenge Board and the Compressed Air and Gas Institute (CAGI) to illustrate the need for universal testing. While there had been many discussions within the industry about independent testing, nobody seemed to be making tangible moves in that direction. CAGI members had adopted a Compressor Data Sheet to report performance data in accordance with ISO 1217, but many manufacturers hadn’t made this information readily available. After CDA’s presentations, these groups, along with Association of State Energy Research and Technol-ogy Transfer Institutions (ASSERTI), endorsed the value of outside verification of product performance.CAGI took the lead and worked with other organizations to formulate the testing program. Three years later, after a lot of coop-eration and hard work, the program is now in place.


Third-party certification will power better decisions

By Ken Byrd

Once a machine tests successfully, compressor and air dryer

manufacturers will be entitled to use the CAGI Performance Label.

July 2006 www.PLANTSERVICES.com 47

What’s being tested?The CAGI compressor performance testing includes rated capacity (cfm) at full-load operating pressure (psi), total input power at rated capacity at full operating pressure (kW) and specif ic package power (kW/100 cfm). Refrigerated air dryers are tested for f low (scfm), outlet pressure dewpoint (deg. F), pressure drop (psid), total input power (kW) and specif ic package power (kW/100 cfm).

The lab will administer the program, contact the man-ufacturer and select a specific machine to be tested. Their random selection will come either directly from manu-facturer inventory or from a distributor’s stock. After the test, the manufacturer will be advised if the machine passed or failed.

Once a machine tests successfully, compressor and air dryer manufacturers will be entitled to use the CAGI Performance Label. CAGI manufacturers whose equip-ment fails the test will have an opportunity to correct the problem. If it can’t be corrected, the manufacturer isn’t entitled to use the CAGI label. The CAGI Web site will post a notice of non-performance, thus putting the manufacturer at a significant marketing disadvantage. The program also is being offered to manufacturers that aren’t CAGI members.

What it means to youThis testing program allows users to evaluate the data pre-sented with new equipment bids and to feel more confident with purchasing decisions. In the most basic comparisons of machines, end users ask:• How much compressed air will this machine provide

compared to its capital cost?• How much energy will it consume per unit of compressed

air (cfm/bhp)?• How much will it cost to operate?

Answering these and similar questions always has been a fundamental step in the decision to purchase a compressor or dryer. In the past, end users could only rely on answers based solely on the information the man-ufacturer provided. This typically included only the full-load volume (cfm), horsepower required at full load and horsepower at an unloaded state. In sales presentations, this information most often is coupled with discussion of the control system in an attempt to evaluate the ma-chine’s eff iciency while operating at partial load, often leaving room for substantial doubt. Soon, most of these performance claims will be substantiated and reported on the CAGI Web site.

After purchase, a machine’s full-load and partial-load eff iciency has been diff icult to verify and often faded to

a point of little interest. The result has been many, if not most, systems are unmonitored and operating with unnecessary energy con-sumption. Further, system changes with air users being added or removed, pressure in-

creased or reduced, production hours increas-ing or decreasing, and evolving leaks can make dramatic changes in the energy consumed. Without a reliable baseline for comparison, it’s been diff icult to quantify the effects of these changes.

A system analyst now can easily evaluate a machine’s performance with modern data collection tools, but reliable equipment performance information is still essential. With good data, knowledgeable compressed air system analysts can verify system performance and easily determine if the system is operating efficiently. This data also can determine if system changes would lead to substantial energy savings.

An exampleConsider a plant that’s operating two 125-hp fixed-speed single-stage rotary air compressors and one 200-hp fixed-speed single-stage rotary screw air compressor. This plant operates 24 hours per day with a load that varies greatly throughout the day. The machines operate off individual controls that sense discharge pressure. Plant maintenance

personnel decide when to run the 200-hp as the lead ma-chine instead of a smaller unit.

The air system energy analysis found that during most of the day one 125-hp unit provides more than enough air to maintain pressure. For a few hours each day, the sys-tem required either both 125-hp units to operate partially loaded or the 200-hp unit to run partially loaded.

Most of the time, the plant runs two partially loaded 125-hp units, an inefficient approach given that f ixed-speed compressors are inherently inefficient when oper-ated that way. The plant considered buying a 160-hp vari-able-speed single-stage rotary air compressor to improve eff iciency. The unit would be able to handle the entire load for most of each day, varying its speed to maintain pressure. During the short periods when the 160-hp unit wouldn’t be capable of maintaining pressure, one of the 125-hp units would start and run fully loaded.

Because a variable-speed compressor is more eff icient than a partially loaded f ixed-speed machine, the 160-

Many, if not most, systems are unmonitored and operating with

unnecessary energy consumption.


hp unit would serve as a trim machine after the smaller unit loads up. Once system demand drops into the 125-hp unit’s range, the smaller f ixed-speed machine will unload and the 160-hp variable speed unit takes over to satisfy demand.

In this case, the estimated yearly system operating cost with the two 125-hp fixed-speed single-stage rotary air compressors and 200-hp fixed-speed single-stage rotary air compressor was about $68,000. Installing the 160-hp variable-speed single-stage rotary air compressor into the system could reduce power costs about $36,000 per year. Payback on the project was less than two years.

As with many things, these recommendations are only as good as the input. Accurate performance data for the existing compressors and the machine being rec-ommended are critical for the desired outcome. Before suggesting any changes, the analyst must have confi-dence in information being used for the decision. This same information continues to be helpful for verifying savings and in monitoring the effects of future changes in the system.

The CDA members are proud to have been the impetus for industry taking this step forward and continue to look for opportunities to promote and improve our industry. For further information, contact CAGI at www.cagi.org.

Ken Byrd is on the board of directors of the Compressed Air Chal-lenge, the Compressor Distributor Association as well as the North American Association of Compressor Distributors. Contact him at [email protected] and (630) 766-7900.



Controls — “Compressor capacity controls”Baseline measurements — “Vital signs”Dryers — “Controlling compressed air moisture”Energy surveys — “So, you want a compressed air energy survey?”TCO — “Look beyond the sticker price”Efficiency issue — “Load ‘em up”Dryers — “There’s more than one way to dry the air”

For more, search www.plantservices.com using the keywords dryer, survey and efficiency.

More resources at www.PlantServices.com/thismonth

The CAGI Web site will post a notice of non-performance, thus putting the manufacturer at a significant

marketing disadvantage.

or the first half of the 20th century and through the 1950s and 1960s, the predominant industrial air compressor was the positive-displacement double-

acting, water-cooled reciprocating design. The predominant control methodology allowed for three to five control load settings: 0% — 50% — 100% or 0% — 25% — 50% — 75% — 100%. These settings responded to the system’s dy-namic demand and the power consumption was quite close to the load factor, while the unloaded horsepower ran be-tween 18% and 20% of maximum.

The mechanisms that accomplished this essentially changed the geometry of the compression chamber cylin-der with a variety of devices that held a set of valves open, which allowed air to move in and out of the chamber with-

out compression. With respect to efficiency, this design generally consumed 15 kW to 16 kW per 100 CFM of compressed air. Today’s compressors and controls barely reach that level of efficiency.

Then, why have reciprocating designs fallen out of favor rela-tive to the rotary compressor that is so predominant today?

Problems with pistons A double-acting reciprocating compressor is a belt-driven cast iron water-cooled compression chamber. It generates unbalanced forces that require large concrete foundation pads. Installation is arduous. Think of an Erector set having a compressor, motor, starter, belt drive, aftercooler, receiver, condensate drain traps and dozens of minor pieces. The


August 2006

working device also requires more floor space than a rotary-screw unit. The only cooling option was water. Maintaining the valves, crossheads and main bearings required substan-tial mechanical skills lacking in today’s labor pool.

Compressed air delivery wasn’t smooth. This caused a slight but real interruption in the compressed air flow. Large receiver tanks were required to tame it into a smooth air flow. And, don’t forget the temperature gain of about 300°F across the compression cycle.

The rise of rotariesThe 1960s saw the advent of rotary compressors. From the outset, these packaged machines received a warm market re-ception. They were generally available as prewired, prepiped units. Vibration was minimal and a rotary unit could be placed on any structure that could support its weight.

Other features included zero unbalanced forces, only about 200°F heat gain, no receiver required, no startup or-deal, no run-in time, and lower installed cost. Three to five gallons of storage capacity per CFM worked for even the largest machine (Figure 1).

Rotary compressor efficiency was only 18 kW to 19 kW per 100 CFM, or about 18% worse than the reciprocating design. In those days, electricity cost only about $0.025 per kWh, erroneously considered insignificant. After all, air is free, isn’t it?

These figures apply to single-stage lubricant-flooded com-pressors. Two-stage lubricant-flooded compressors are more efficient and often pay for themselves in a reasonable time. The extra investment was justified. The operating efficiency of two-stage units is 16 kW to 17 kW per 100 CFM.

Only load/unload controls were available. At a preset pressure, the compressor unloaded and idled. Generally, unloaded horsepower is 25% of full-load horsepower.

Pay adequate attention to the timing of the load and unload cycles on lubricant-flooded rotary screw compres-sors. The cycle should allow ample time for the sump to blow down, often as long as 60 seconds or more. Each ma-chine is different. Measure blow down times individually. Short cycling can cause the lubricant to foam and dimin-ish control system life expectancy. It also risks foaming the lubricant out of the compressor inlet onto the floor.

Inlet modulation is inefficientThe load/unload approach was supplanted by modulation controls that feature a gradually closing inlet butterfly valve. The weakness in this design is miserly little power savings at part load (Figure 2). Note that at:

• 100% capacity, 100% of the power is required• 90% capacity, the power required is 97%• 80% capacity, the power required is 95%• 70% capacity, the power required is 90%• 60% capacity, the power required is 85%• 50% capacity, the power required is 83%

1 gal/cfm

10 gal/cfm

3 gal/cfm

5 gal/cfm

Percent capacity

1 gal/cfm

3 gal/cfm

5 gal/cfm

10 gal/cfm

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Load/unload and storage size

No blowdown

With blowdown

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity

Inlet valve modulation

Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity

Rotary compressor performance with variable displacement

Variable displacement

110

100

90

80

70

60

50

40

30

20

0 20 40 60 80 10010 30 50 70 90

10

0

Percent capacity

Perc

ent

kW in

pu

t p

ow

er

% kW input vs % capacity

With unloading

With stopping

Variable-speed drive

Total kW input and/or specific power over the full operating range must be analyzed for a proper comparison with other types of capacity control.

Figure 1. Input power versus capacity for a lubricant-injected rotary screw compressor using load/unload control.

On/off

1 gal/cfm

10 gal/cfm

3 gal/cfm

5 gal/cfm

Percent capacity

1 gal/cfm

3 gal/cfm

5 gal/cfm

10 gal/cfm

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90


No blowdown

With blowdown

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity


Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity



110

100

90

80

70

60

50

40

30

20

0 20 40 60 80 10010 30 50 70 90

10

0

Percent capacity

Perc

ent

kW in

pu

t p

ow

er


With unloading

With stopping



Figure 2. Input power versus capacity for a lubricant-injected rotary screw compressor controlled by inlet modulation.

Throttling

1 gal/cfm

10 gal/cfm

3 gal/cfm

5 gal/cfm

Percent capacity

1 gal/cfm

3 gal/cfm

5 gal/cfm

10 gal/cfm

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90


No blowdown

With blowdown

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity


Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity



110

100

90

80

70

60

50

40

30

20

0 20 40 60 80 10010 30 50 70 90

10

0

Percent capacity

Perc

ent

kW in

pu

t p

ow

er


With unloading

With stopping



Figure 3. Input power versus capacity for a lubricant-injected rotary screw compressor using variable-displacement control.

Flexible cylinders

49www.PLANTSERVICES.com

EFFICIENCYCompressors

• 40% capacity, the power required is 80%Zero capacity requires 25% power. For best energy effi-

ciency run multiple rotary compressors at full load and use one for trim. Operating several inlet modulation control systems at part load is a terrible waste of money.

Pushing the frontiersTechnology brought us variable-displacement machines in which the geometry of the compression chamber could change. Some machines used a spiral valve, while others used poppet valves that shorten the effective length of the compression chamber. Both approaches allow much better part-load performance (Figure 3). Note that at 100% capac-ity, 100% of the power is required, but:

• 90% capacity needs 92% power• 80% capacity needs 83% power• 70% capacity needs 78% power• 60% capacity needs 68% power• 50% capacity needs 63% power• 40% capacity needs 60% powerThe next big thing in the world of rotary compressors

was the introduction of variable-speed drives (Figure 4). As before, 100% capacity requires 100% power. But:

• 90% capacity requires 91% power• 80% capacity requires 81% power• 70% capacity requires 71% power• 60% capacity requires 61% power• 50% capacity requires 51% power• 40% capacity requires 42% power

Control for efficiencyCentrifugal compressors rely on modulation and blow-off, which generally limits turndown capacity to between 70% and 75% of total capacity. When demand falls, the control goes to blow-off mode, dumping the full capacity to atmosphere. When operating multiple machines, the same suggestions about hav-ing only one trim unit and modulating multiple units apply.

A variety of system controls for multiple compressor instal-lations are available. Single master or sequencing controls use one control to sequence or lead/lag the compressors. Multiple master or network controls allow the system to alternate, unload or turn off automatically as system demand changes. Control systems can be integrated with computer-based overall plant energy control systems that monitor HVAC, lights and other items. Pressure flow control devices are separate from com-pressor controls and are intended to deliver a relatively constant static pressure supply to the air distribution system.

The end of modulation?It appears likely that the industry will serve the rotary screw, both lubricated and dry, with less complicated, more ener-gy-efficient controls. You’ll probably see modulation elimi-nated, leaving only load/unload or variable speed as the two most energy-efficient mechanisms available today.

A major caution is in order. At full load, variable-speed units generally require more power. Thus, in multiple rota-ry-compressor installations, operate only one variable-speed unit on trim duty if you want the promised energy savings.

Other surprises are on the horizon. Consider the global-ization of the compressor business. Fifty years ago, it was inconceivable for an offshore automobile manufacturer to own half the U.S. car market. The same is happening in the compressor industry.

The entrepreneurial creativity demonstrated by the young folks coming into the market today is far beyond anything I could imagine. The Electronic Age has everything moving at supersonic speed. We ain’t seen nothin’ yet.

Henry Kemp is principal at Strategic Air Concepts in St. Petersburg, Fla. Contact him at [email protected] and (727) 867-4044.

Figures: Compressed Air Challenge


EFFICIENCYCompressors

1 gal/cfm

10 gal/cfm

3 gal/cfm

5 gal/cfm

Percent capacity

1 gal/cfm

3 gal/cfm

5 gal/cfm

10 gal/cfm

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90


No blowdown

With blowdown

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity


Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

Perc

ent k

W in

pu

t po

wer

110

100

90

80

70

60

50

40

30

200 20 40 60 80 10010 30 50 70 90

Percent capacity



110

100

90

80

70

60

50

40

30

20

0 20 40 60 80 10010 30 50 70 90

10

0

Percent capacity

Perc

ent

kW in

pu

t p

ow

er


With unloading

With stopping



Figure 4. Input power versus capacity for a lubricant-injected rotary screw compressor using variable-speed control.

Motor drives

Capacity controls — “Compressor capacity controls”

Baseline measurements — “Vital signs”

Integration issues — “Integrated compressor controls”

Smart compressors — “Air compressors and microprocessors”

Efficiency and loading — “Load ‘em up”

For more, search www.plantservices.com using the keywords compressor, modulation and load/unload.


During the past several years, the variable-speed-drive (VSD) compressor has become a frequent choice for new compressor purchases. The VSD compressor’s

popularity is partly due to rising energy prices and its effi-ciency as a trim compressor. Unfortunately, much of the VSD compressor’s popularity is a result of marketing spin. For ex-ample, sales personnel offer “free audits” during which they install a black box that might as well flash the phrase on the screen, “Buy a VSD compressor.” Often, the free audits exag-gerate paybacks by including savings the end user can achieve without purchasing a VSD compressor. The marketers also fail to provide the necessary engineering details required to maxi-mize the purported benefits. It’s time to eliminate the spin and point out both the limitations and engineering required if you are to derive the benefits of a VSD compressor.

The spinA trim compressor is a cycling compressor that matches supply capacity to fluctuating system demand, while a base-load compressor is a constant-speed compressor operating at its maximum efficiency.

Over much of its capacity range, the VSD compressor is the most efficient part-load rotary screw compressor. On the other hand, the VSD compressor isn’t a panacea for all your compressed air system problems, as the spin doctors would like you to believe. Some of what they say about VSD compressors includes:• Compressor efficiency: “You get maximum efficiency

while peaking at 100%, or only operating at the average, 50% to 70% of capacity.” In graphical form, the spin doc-tors always show a power consumption comparison of op-erating modes in a graph of percent power versus percent capacity (Figure 1).

• Energy savings: One ad claims, “Cuts energy costs up to 50%,” while another ad by the same manufacturer says, “40% reduction in energy costs possible.” Another states “Energy savings up to 35%.” Yet another manufacturer says, “Slash power use resulting in a life-cycle cost savings of 22%.”

• Power factor: The spinners claim, “No penalties for spikes or low power factor.”

• Free energy analysis: They want us to believe that a free en-ergy analysis is the same as a compressed air system audit.

www.PLANTSERVICES.comOctober 2006 45


• Turndown:SpinnerswantustobelievethateveryVSDcompressorhasaturndownof80%ormore.

• Nameplate horsepower:Theclaimedhorsepoweroftenisn’ttherequiredhorsepower.

• Constant pressure: One manufacturer claims, “Linepressures held within 1 psi,” while another says, “Linepressuresheldwithin1.5psi.”Stillanotherstates,“Highprocess stability.” This leads compressor sales personneltomakethestatement,“IfyouinstallaVSDcompressor,youwon’tneedapressure/flowcontroller.”

• Storage:BecausetheVSDcompressorrespondstosys-temeventswithonlinehorsepower,thespinnerstellus,“Alargereceiverisn’trequired.”While, in a limited perspective, one can interpret these

claimsastechnicallytrue,thespindoctorshavegoneforthemaximumrotationalspeed.Fromthispoint forward,how-ever,thediscussionaboutVSDcompressorsentersaNoSpinZone,wherewepeeloffthespintoleaveonlythefacts.

Compressor efficiencyVSDcompressorliteratureusesthepercentpowerversuspercentcapacitygraphtoshowhowmuchmoreeff icienttheVSDcompressoristhanaconstant-speedcompres-soroperatingineithermodulationorload/unloadmode.The VSD and VSD II curves (Figure 1) represent ef-ficiency curves from two manufacturers. The problemwiththisgraphisthatithidesthebenefitsofconstant-speedcompressors.

For example, at full load, a VSD compressor isn’t asefficientasthecomparableload/unloadcompressorfromthe same manufacturer because the variable-frequencydriveincreasespowerdrawby2%to4%.Toremovethespin,simplyplot thegraphusingactualaircapacity in-steadofpercentcapacity.Asyouapproach100%power,

theconstant-speedcompressorismoreefficientthantheVSDcompressor(Figure2).Inaddition,constant-speedcompressorsgenerallyhaveahighercapacity.Thisfigureshowsthattheconstant-speedcompressordelivers10%or45cfmmorecapacityatfullloadthantheVSDcompres-sor,whiletheVSDIIcompressordelivers4cfmless.

Graphingcompressorefficiency(inputkW/100cfm)versuscfm (Figure 3) properly portrays the efficiency differences.The load/unloadefficiencies assume10gal./cfmof storage,baseduponthetrimcompressor’scapacity.Thegraphshowsthatastheloadonthecompressordecreases,theVSDcom-pressorefficiencyisrelativelyflatbetweenapproximately46%and78%ofitscapacity,whiletheload/unloadefficiencyde-creasesratherquickly.Furthermore,below15%to20%ca-pacity,theVSDcompressor’sefficiencyfollowsadownwardtrend,whiletheload/unloadcompressor’sefficiency,becauseofunloadedhorsepower,remainsatahigherlevel.

The second compressor efficiency graph (Figure 4)zooms in on the crossover point, above which the con-stant-speedcompressorismoreefficient.Atfullload,theconstant-speed compressor is approximately 12% moreefficientthantheVSDcompressorandapproximately5%moreefficientthantheVSDcompressoroperatingatitsmostefficientoperatingpoint.

Energy savingsStatementssuchas“cutsenergycostsupto50%,”“40%reductioninenergycostspossible,”“energysavingsupto35%,”and“slashpoweruseandreducelife-cyclecostby22%”leadplantperson-neltobelievethatjustbyinstallingaVSDcompressor,theycanreducetheirtotalannualenergycostbyasmuchas50%.Thespindoctorsreinforcethisnotionbyofferingafreeenergyanalysisoraudit.However,unlessyouoperateonlyonecompressor,a50%reductionisn’tpossiblemerelybyinstallingaVSDcompressor.

MANAGEMENTCompressors

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psid

93 psig 91 psig

filter tank5 psid 5 psid

dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

95 psig

104 psig

93 psig

102 psig

91 psig90 psig

Header pressurecontrolled by VSD

pressure dips to start#1 base and base #2

75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

95 psig

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psidfilter tank

5 psid 5 psid

dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

104 psig

93 psig

102 psig

91 psig90 psig

pressure rises 10 psiif below min flow

VSD cycles but pressure never drops below 95 psig setpoint

pressure rises 4 psito unload base #2


75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing

pressure pressure pressure upstream drop across downstream Flow (cfm) of dryer (psi) dryer (psi) of dryer (psi)

494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)

Pressure swing with local control signal, a 5-psi rise in pressure to the stop point and a 20-sec. blowdown time

VSD compressor with 80% turndown,

95-psi setpoint, 494-cfm

full load capacity.

Total system storage

dryer

Pressure swing within turndown range is 7.8 psi. With 120 gal. of system storage, the pressure swing below the turndown range is 34.8 psi.

Power versus capacity

% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II

Compressor efficiency curves

cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD

Variable Displacement

Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II

Average annual capacity of load/unload compressor (%)

An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%

VSD % annual savings

90%

Figure 2. To take out the spin in Figure 1, look at percent power as a function of volumetric flow.

A fair comparison

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psid

93 psig 91 psig


dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

95 psig

104 psig

93 psig

102 psig

91 psig90 psig



75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

95 psig

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psidfilter tank

5 psid 5 psid

dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

104 psig

93 psig

102 psig

91 psig90 psig





75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing


494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)




full load capacity.


dryer



% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD


Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II


An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%


90%

Figure 1. The spin doctor will show percent power as a function of percent capacity, which masks the efficiency of the constant-speed compressor.

One-sided view

October 200646

To reduce the total annual energy cost in a multi-compressor system by these large percentages, the VSD compressor supplier also must configure the system to base load each compressor except the VSD unit. Of course, you can base load all of your compressors, except one, without installing a VSD compressor. Therefore, the only actual savings attributable to the VSD com-pressor is the difference between it and the operating mode for the constant-speed trim compressor. In fact, because it’s easy and inexpensive to convert most modulating compressors to load/unload, the savings attributable to the VSD compressor is only the difference between its annual energy cost and that of a load/unload compressor, not a modulating compressor.

Figure 5 shows the annual savings percentage available by installing a 125-hp VSD compressor versus a 125-hp constant-speed compressor. Note that it’s more expensive to operate a VSD compressor near full load. The graph shows that the VSD compressor won’t provide any savings unless the load/unload compressor operates below 83% of its capacity. In addition, you can achieve most of the savings shown in the lower capacity range by replacing the oversized constant-speed compressor with a smaller unit. Generally, if you expect that the constant-speed trim compressor will operate above 80% of its capacity, it’s the more efficient choice. On the other hand, if the constant-speed trim compressor is operating below 80% of its capacity, then replacing it with a VSD compressor will provide additional savings. However, the question you must ask is, “Will installing a VSD compressor generate enough savings to provide an ac-ceptable payback?” This isn’t an easy question to answer.

For example, in a multi-compressor system, we want the VSD compressor to operate in the trim mode. When demand decreases, we must unload a base-load compressor rather than stopping the VSD compressor. To prevent the base-load compressors from short cycling, which can damage them, the VSD compressor’s turndown capacity must be equal to or greater than the capacity of the largest base-load compres-

sor that will unload. Therefore, given an 80% turndown, the VSD compressor must be a minimum of 1.25 times the size of the largest base-load compressor that will unload.

If the VSD compressor has only a 50% turndown, then it must be twice the size of the largest base-load compressor that will unload. To use a smaller VSD compressor and still pre-vent short cycling the base-load compressor, you must either provide sufficient storage or reduce the size of the base-load compressors. As you can see, the payback for a VSD compres-sor depends on its turndown and system storage volume. In ad-dition to the VSD compressor cost, you must account for other costs such as line reactance and a remote transducer.

Two clients decided to install a constant-speed trim com-pressor rather than a VSD compressor because the simple pay-



105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psid

93 psig 91 psig


dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

95 psig

104 psig

93 psig

102 psig

91 psig90 psig



75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

95 psig

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psidfilter tank

5 psid 5 psid

dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

104 psig

93 psig

102 psig

91 psig90 psig





75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing


494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)




full load capacity.


dryer



% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD


Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II


An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%


90%

Figure 3. Efficiency as a function of volumetric flow.

Low flow is inefficient

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psid

93 psig 91 psig


dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

95 psig

104 psig

93 psig

102 psig

91 psig90 psig



75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

95 psig

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psidfilter tank

5 psid 5 psid

dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

104 psig

93 psig

102 psig

91 psig90 psig





75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing


494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)




full load capacity.


dryer



% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD


Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II


An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%


90%

Figure 4. Efficiency as a function of volumetric flow (expanded detail).

Zoom in on the crossover point

October 2006 47

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psid

93 psig 91 psig


dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

95 psig

104 psig

93 psig

102 psig

91 psig90 psig



75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

95 psig

105 psigadjustable

rangePID

setpoint

always in trim

VSD-100 hp500 cfm

compressor

dryer

5 psidfilter tank

5 psid 5 psid

dryer dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig

104 psig

93 psig

102 psig

91 psig90 psig





75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing


494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)




full load capacity.


dryer



% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD


Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II


An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%


90%

Figure 5. Savings as a function of capacity.

There can be a payoff


back exceeded a 24-month requirement. In one case, one compressor manufac-turer quoted a $22,660 (74% premium) for a VSD compressor having the same horsepower rating as the constant-speed compressor, while another manufac-turer wanted a $10,785 (30% premium), but the constant-speed compressor cost $5,187 more. The second project re-quired a VSD compressor larger than the constant-speed unit. The premium for this VSD compressor was $27,720,

or 2.4 times the cost of the constant-speed compressor.

On the first project, we project-ed the VSD compressor to oper-ate at 50% capacity and provide an annual savings of approximately $7,000, giving a simple payback of 47.5 months. On the second project, operating at 70% capacity, the VSD compressor had a simple payback of 83.1 months. Both projects involved retrofitting existing systems.

When we design new systems, we often find that installing appro-priately sized constant-speed com-pressors provides the same or lower annual energy costs than a system containing a VSD compressor, and at a lower capital cost.

Part 2 of this two-part article will appear in the November 2006 issue. It will discuss power factor, energy analysis, turndown, horsepower, storage and con-trol schemes.

Chris E. Beals is president of Air System Man-agement, Inc., in Denver, Colo. Contact him at [email protected] or (303) 771-4839.

Figures: Air System Management, Inc.


October 200648

CA as a process variable — “Treat compressed air as a process variable”

Speed-regulated drives — “Drive down the cost of compressed air”

Control systems — “Control options in rotary screw compressors”

Condensate — “What does Mother Nature say about compressor condensate rates?”

Flow monitoring and zoning — “Building a case for better air”

For more, search www.plantservices.com using the keywords VFD, audit and storage.


The payback for a VSD compressor depends on its turndown and

system storage volume.

Part 1 of this two-part article appeared in the October, 2006 issue. It discussed different ways to quantify compressor efficiency and operation schemes that can lead to energy savings.

Much of the VSD compressor’s popularity is a result of marketing spin. Often, free audits ex-aggerate paybacks by including savings the end

user can achieve without purchasing a VSD compressor. The marketers also fail to provide the necessary engineering de-tails required to maximize the purported benefits. It’s time to eliminate the spin and point out the limitations and engi-neering required to derive the benefits of VSD.

Power factorThe spinners claim there are no penalties for spikes or low power factor. In the context of a compressor, this statement is true. The VFD provides a soft start similar to a Wye-Delta or electronic starter and the power factor for a VSD compressor motor is close to unity over the complete speed range. A unity power factor is a benefit and it can reduce energy costs if the VSD motor constitutes a significant por-tion of the plant’s motor load. On the other hand, if the local utility doesn’t have a power factor penalty or the VSD com-pressor constitutes only an insignificant portion of the mo-tor load, then a unity power factor offers little cost benefit.

Free energy analysesThe spinners offer free energy audits. The old adage, you get what you pay for, applies here. Many compressor manu-facturers gear free audits to get their sales personnel into a plant to sell equipment. Free air audits seldom address pres-

sure drop, piping, air quality, risk of an outage or demand-side issues. Nor do they offer any non-equipment solutions.

The whole process is automated so it produces an equip-ment quote for the sales personnel regardless of expertise or experience in the industry. When clients ask us to review these free audit reports, we generally find that the salesper-son has misinterpreted the data, the proposals are incom-plete, and plant personnel can achieve the majority of the projected savings without purchasing any compressor.

Turndown goes up and downAt a 100-psi operating pressure, many lubricated VSD com-pressors can regulate capacity from 100% down to 15% before


unw

inding the spin

(P

art 2)

November 2006 57

105 psigAdjustable

rangePID

setpoint95 psig

104 psig

93 psig

102 psig

91 psig90 psig

93 psig 91 psig

always in trim

VSD-100 hp500 cfm

compressor

Dryer

5 psidfiltertank

5 psid 5 psid

Dryer Dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig


Pressure dips to start#1 base and base #2

75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

105 psigAdjustable

rangePID

setpoint95 psig

104 psig

93 psig

102 psig

91 psig90 psig


75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing

95 psig

always in trim

VSD-100 hp500 cfm

compressor

Dryer

5 psidfiltertank

5 psid 5 psid

Dryer Dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

Pressure rises 10 psiif below min flow

Pressure rises 4 psito unload base #2


Pressure Pressure Pressure upstream drop across downstream Flow (cfm) of dryer (psi) dryer (psi) of dryer (psi)

494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)




full load capacity.


Dryer

control signal



% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD


Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II


An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%


90%

Table 1. Operating a VSD compressor outside of its turndown range can promote large pressure swings.

Turndown range


they must operate in either start-stop or load-unload mode. Spinner literature suggests that all VSD compressors have the same turndown capability, but the fact is that at 100 psi, some can only regulate output capacity from 100% down to 55%.

In addition, turndown range decreases as discharge pres-sure increases. For example, at 125 psi, the turndown can be as little as 40%, from 100% capacity to 60% capacity.

Turndown range is important because a smaller turndown decreases the savings, increases the size and frequency of pressure swings, and can result in short cycling. Or, it can increase the cost to integrate the compressor into the system properly. In summary, turndown varies significantly among compressor manufacturers, models and pressure settings.

Horsepower: the nameplate gameThe size of a VSD compressor can be confusing because spinners often play games with the motor nameplate data. For example, a motor might be a 125-hp unit with a 1.15 ser-vice factor or a 100-hp motor with a 1.4 service factor. The motor manufacturer can nameplate the motor either way. This practice has resulted in end users installing undersized electrical circuit breakers. It’s important to determine the actual power applied at full load at the operating pressure to understand efficiency and installation requirements.

Constant pressure variesThe spinners claim that VSD compressors hold pressure within 1.5 psi. The truth is that VSD compressors can do that only at their discharge over their turndown range. You

must properly integrate the unit into your system if you want to prevent larger swings.

In one example, a VSD compressor requires a signal from a pressure transducer located downstream of the cleanup equipment if it is to prevent the cleanup equipment from increasing system pressure swings. If the control signal is on the compressor, the smallest swing possible is 7.8 psi within the VSD compressor’s turndown range. To reduce the pres-sure swings, we can either oversize the dryer or just move the control signal to the downstream side of the cleanup equipment, which can reduce the maximum pressure swing, within the compressors’ turndown range, to 3.0 psi.

If you’re operating within the turndown range, storage isn’t required, except to ride through sizeable system events. As Table 1 shows, you can’t prevent larger pressure swings from occurring when the unit operates outside its turndown range or when base-load compressors load and unload.

You still need enough storageAdding sufficient storage minimizes the pressure swings that occur when the compressor operates below minimum turndown or when base-load compressors load and unload. Understanding why swings occur requires an explanation of how VSD compressors operate below minimum speed.

Consider a single-VSD-compressor system that stops when demand is less than its minimum capacity. As demand de-creases, the compressor reduces motor speed to its minimum. If demand continues to fall, the pressure rises to the stop


November 200658

105 psigAdjustable

rangePID

setpoint95 psig

104 psig

93 psig

102 psig

91 psig90 psig

93 psig 91 psig

always in trim

VSD-100 hp500 cfm

compressor

Dryer

5 psidfiltertank

5 psid 5 psid

Dryer Dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

95 psig


Pressure dips to start#1 base and base #2

75 cfm

485cfm

885cfm

max1225cfm

Flow increasing

105 psigAdjustable

rangePID

setpoint95 psig

104 psig

93 psig

102 psig

91 psig90 psig


75 cfm

445cfm

815cfm

max1225cfm

Flow decreasing

95 psig

always in trim

VSD-100 hp500 cfm

compressor

Dryer

5 psidfiltertank

5 psid 5 psid

Dryer Dryer

75 hp370 cfm

compressor

75 hp370 cfm

compressor

base 1 base 2

Pressure rises 10 psiif below min flow



Pressure Pressure Pressure upstream drop across downstream Flow (cfm) of dryer (psi) dryer (psi) of dryer (psi)

494 93.5/96.5 5.0 88.5/91.5

99 93.5/96.5 0.2 93.3/96.3

98 (with 65.2/100.0 0.05 65.1/99.9120 gal storage)




full load capacity.


Dryer

control signal



% Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0% 60%10% 20% 30% 40% 50% 70% 80% 90% 100%

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


Capacity (cfm)

100%

80%

60%

40%

20%

10%

0%0 30050 100 150 200 250 350 400 450 500

90%

70%

50%

30%

% P

ow

er (k

W)

VSD

Modulation

Load/unload

VSD II


cfm

35

31

27

23

19

17

150 30050 100 150 200 250 350 400 450 500

33

29

25

21

Effic

ien

cy (k

W/1

00 c

fm)

VSD


Load/unload

VSD II


Effic

ien

cy (k

W/1

00 c

fm)

cfm

24

23

22

21

20

19

18

17250 300 350 400 450 500

VSD


Load/unload

VSD II


An

nu

al e

ner

gy

savi

ng

s (%

)

45%

40%

35%

20%

15%

10%

5%

0%

–5%

30%

25%

77%14% 25% 40% 53% 67% 80% 86% 100%70%20%10%

0

30% 40% 50% 60% 80% 100%


90%

Figure 1. An integrated VSD compressor in a multi-compressor system (courtesy Dean Smith at [email protected]). The left sides represents increasing demand, the right side represents falling flow demand.

Optimizing the whole system


point, which is normally set 5 psi above the target pressure, and the VSD com-pressor shuts down.

Some lubricated VSD compressors blow down the sump when they stop. When the pressure drops to the target pressure, the compressor starts immedi-ately, but the VFD needs approximately 10 sec. to ramp up to minimum speed.

If the VSD compressor doesn’t blow down when it stops and pressure drops to the target pressure, a blow-down valve then opens and the sump pressure must drop to 55 psi or 60 psi before the VSD compressor starts. The blowdown timer is adjustable so the startup requires between 15 sec. and 25 sec. The size of the storage required to minimize the pressure swings is directly proportional to startup time and inversely propor-tional to turndown.

Compressor manufacturers recom-mend 1 gal. to 2 gal. of storage per cfm for VSD compressors, but this means there’s a different guideline for each compressor type. To eliminate this prob-lem, use the following rule for all rotary-screw compressor operating modes:

Given a 10-psi differential, storage must equal 5 gal. to 10 gal. per cfm of that portion of the trim compressor’s capacity operating load/unload or start/stop.

Until recently, we’ve always rec-ommended 10 gal. per cfm, but re-ceiver prices require us to consider 5 gal. per cfm, the minimum required to prevent short cycling. For example, if the 494-cfm compressor in the table were a load/unload compressor, we’d recommend 2,500 gal. to 5,000 gal. of storage. The reason is that when the compressor unloads, it unloads 100% of its capacity. However, because the compressor is a VSD compressor hav-ing an 80% turndown, it only unloads 98 cfm of the compressor’s capacity when it starts and stops, so we would recommend 500 gal. to 1,000 gal. of storage. This guideline doesn’t con-sider system events such as the failure of the largest compressor.

In a perfect system having multiple base-load rotary screw compressors

operating in load/unload mode and a VSD trim compressor, the VSD unit would never operate below its mini-mum speed and the capacity of the base-load compressors, which would be less than the turndown capacity of the VSD compressor. This design points out the importance of turn-down on VSD compressors.

To prevent the VSD compressor from starting and stopping when, at times, the VSD compressor is the only unit operating, the low load demand must exceed the minimum speed ca-pacity of the VSD compressor. This minimizes storage but increases capital costs because it requires at least one ad-ditional base-load compressor. On the other hand, if we use base-load com-pressors with a capacity larger than the turndown capacity of the VSD com-pressor, we might short cycle the base-load compressor unless we add storage.

For example, assume the base-load compressor capacity is 500 cfm and the capacity and turndown of the VSD compressor are 470 cfm and 80%, re-spectively. When the base-load com-pressor unloads, the VSD compressor can supply only 376 cfm (470 cfm x 0.8). This means that demand exceeds the online capacity by 124 cfm. Based on a minimum cycle time of 90 sec., which most rotary screw compressor manufacturers recommend to prevent short cycling, and a 5-psi differential, we must have minimum system stor-age of 2,045 gal.:

We can increase the cycle time to 3 min. by increasing the stop pressure 5 psi to 10 psi above the VSD compressor target setpoint or by doubling the storage.

You need storage for a VSD com-pressor in a system having multiple compressors if you want to prevent pressure sags during base-load unit startup. Also, storage prevents short cycling the base-load compressors, which can occur when their capacity exceeds the turndown capacity of the VSD compressor.

VSD vs. pressure/flow controllerSpin doctors often equate a VSD pres-sure control to a pressure/flow control-ler with storage. But, we must point out that a VSD compressor responds to all events using brute strength horsepower and can’t keep the next compressor from starting. On the other hand, a pressure/

flow controller with sufficient storage responds to events with stored air, which can prevent the startup of the next com-pressor while maintaining the system pressure within 1% of its setpoint.

Integrating the VSD unitFigure 1 shows how to stage multiple base-load compressors with a VSD compressor to minimize pressure fluc-


The spinners seldom mention our favorite uses for VSD

compressors.

November 2006 59

CA as a process variable — “Treat compressed air as a process variable”Speed-regulated drives — “Drive down the cost of compressed air”Control systems — “Control options in rotary screw compressors”Condensate — “What does Mother Nature say about compressor condensate rates?”Flow monitoring and zoning — “Building a case for better air”

For more, search www.plantservices.com using the keywords VFD, audit and storage.


x 14.7 x 7.48124 cfm x 90 sec./60 sec.5 psi x 2

PS0611_57_61_MGT_Cmp_Reflow.indd59 59 2/21/07 2:20:14 PM


tuations. Note that this arrangement requires:• Dedicated dryers and filters or tighter

control bands• A remote transducer• Keeping the control signal for the

base-load compressors on the package• An adjustable-range stop pressure

setpoint on the VSD compressorAs you add base-load compres-

sors, pressure variation will increase unless automation controls the base-load units. While we hope your com-pressed air system is engineered this well, the remaining question you face is, “Does the savings from us-ing a VSD compressor with all this additional equipment provide an ac-ceptable payback?”

Good uses for VSDThe spinners seldom mention our fa-vorite uses for VSD compressors: ex-tending the turndown range on cen-trifugal compressors and making more efficient use of storage located upstream of a pressure/flow controller. Using a VSD compressor with networked cen-trifugal compressors that are capable of load-sharing significantly extends the turndown range of the centrifugal units while maintaining a relatively constant pressure.

Automating the base-load com-pressors and using a VSD compressor upstream of a pressure/flow controller maximizes the available differential in the control storage, which reduces re-ceiver size, achieves more differential to support system events, or provides more startup time.

Chris E. Beals is president of Air System Man-agement, Inc., in Denver. Contact him at [email protected] and (303) 771-4839.

Figures: Air System Management, Inc.


November 200660

RELIABILITY EFFICIENCY ASSET MANAGEMENT

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