Reprinted May, 1997 Page 1 Reference No. 004

Condensed water is still a majorcause of downtime in compressedair systems. Air quality which wasgood enough for the old hand-controlled valves or wide-toleranceair tools is just not dry enough whenused in today’s sophisticated airsystems. Water causes rust, pitting,blockages, and freeze-ups, withresultant component failure andproduct rejection.

The only way to prevent con-densation of water in air lines is tolower the dew point of the air in thesystem. This can be done byinstalling an air dryer after thecompressor.

♦ ♦ ♦

Charles Henderson is marketingdirector of Henderson EngineeringCo., Inc. in Sandwich, Illinois.His company developed and holdsthe patent on the heat-of-compression dryer.

It is less expensive to own andoperate an air dryer than it is to livewith the problems it can prevent.Drying compressed air doesn’tcost…it saves.

Condensate source

Some knowledge of the reasonwater vapor condenses easily incompressed air systems is helpfulbefore investigating the differenttypes of available air dryers.

The table shows the amount ofliquid water which could becondensed from a 1000 scfm systemduring a 24-hour period at varyingambient temperatures and saturationlevels. For example, air at 100°Fand 100% relative humidity (rh)contains 488 gallons of water, whileair at 0°F and 100% relativehumidity contains only 11 gallons.

The capacity of air to hold watervapor increases with increasing

A primer forA primer forA primer forA primer forair dryer selectionair dryer selectionair dryer selectionair dryer selectionEnergy and maintenance costs are key factors inchoosing these important componentsC. Henderson

HYDRAULICS&PNEUMATICS

NOVEMBER 1980

ByHENDERSON ENGINEERING CO., INC.

95 North Main Street • Sandwich, Illinois 60548815-786-9471 ♦ 800-544-4379 ♦ Fax: 815-786-6117

www.saharahenderson.com

EXTRACTED FROM

Table: gallons of water carried intoa 1000 scfm system in 24 hours bya compressor operating at 100 psi.

Ambient airtemperature degrees F

Relative humidity

60% 80% 100%

110 386 515 644100 292 390 48890 219 292 36080 162 216 27070 118 155 19460 85 113 14150 60 80 10040 42 56 7030 28 38 4720 18 24 3010 11 15 190 7 9 11

-10 4 5 7-20 2 3 4

Reprinted Ma

temperaturecharge air fThe heat-othe air temdegrees. Tis 2 to 10%temperaturemuch moredry.

But most aiair directly reasons ofcomponentAir must be

Compresso

Here’s an compressorpressure of80°F and 1of free airholds 11 gr

The first stakes in 9 into 3 cub(approxima

AIR PRODUCTSSAHARA

HENDERSON

For Desert-Dry Air…

y, 1997 Page 2 Reference No. 004

. For this reason, dis-rom a compressor is dry.f-compression has raisedperature several hundredypical relative humidity. Because of its high

, the air could hold water vapor and still be

r systems cannot use hotfrom the compressor for personnel safety and temperature limitations. cooled prior to use.

r example

example with a 2-stage designed for a discharge 120 psi. Intake air is at00% rh. Each cubic foot under these conditionsains of water.

tage of the compressorscfm and compresses itic feet of air at 30 psitely ¼ the final dis-

charge pressure). Air temperature is300°F. Each cubic foot of air nowcontains 33 grains of water, butactually could hold much morebecause the relative humidity isonly 2%.

Before the air enters the secondstage, it will be cooled to 105°F bythe intercooler. At this temperatureand pressure, each cubic foot canhold only 7 B grains of water. Theremainder condenses in theintercooler. The compressor isactually taking water out of the air.However, the air now is saturated.

In the second stage, pressure rises to120 psi and temperature goes backup to 300°F. Each cubic foot of airnow contains 23 grains of water, butrelative humidity is only 5%because of the elevated temperature.

When the aftercooler brings thetemperature down to 100°F, eachcubic foot can hold only 2 ¼ grains.The remainder condenses into liquidwater.

Although 2 ¼ grains doesn’t soundlike much, with a 1000 scfm systemoperating around the clock for ayear, 2251 gallons could be drainedfrom the aftercooler, enough to fill asmall swimming pool.

Condensation

As long as the saturated compressedair leaving the aftercooler stays at100°F, no water vapor will con-dense. But if the air cools only 20degrees to 80°F, 876 gallons ofwater per year will condense in thepiping system.

If the system supplies air tools orspray guns, another problem sur-faces…adiabatic expansion. As ex-haust air expands to atmosphericpressure, it cools approximately 10degrees. More liquid water willdrip from air tools or ruin paintjobs.

Because actual plant ambient tem-peratures often are even lower thanthis, it’s no surprise that most com-

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pressed air distribution systems areplagued with liquid water.

Aftercoolers and separators

Every compressed air system shouldhave an aftercooler and separator.The aftercooler is the first stage ofmoisture removal because here thebulk of potential condensate is takenout of the air. But remember, com-pressed air leaving the aftercooler issaturated at a relatively high tem-perature. The separator is a filterand can remove only liquid waterfrom the air. By definition, a dryeris a device to lower the dew point ofair. Therefore, although aftercool-ers and separators are importantcomponents in an air system,neither of them is a dryer.

How dry?

The major consideration in dryerselection is the degree of drynessrequired. Because the system isunder pressure, pressure dew pointdetermines the degree of dryness.(Ambient atmospheric dew point isirrelevant to the closed system.) Asa rule of thumb, a dryer shoulddeliver a pressure dew point 15degrees below the lowest ambienttemperature the air distributionsystem will encounter in service.To lower the dew point any furtherrequires unnecessarily expensiveequipment and raises operatingcosts dramatically.

Types of dryers

There are three basic types of airdryer: deliquescent, refrigerant, andregenerative.

The deliquescent dryer is simply apressure vessel filled with deliques-cent desiccant. Air enters at the topand flows down a central pipewhich contains prefilters to removedirt, liquid water, and oil. Thecleaned air then flows up

through a desiccant. The desiccantremoves water from the air, and indoing so, slowly dissolves. Anafterfilter prevents any desiccantmaterial from carrying over down-stream.

The exit dew point from a deliques-cent dryer is determined by inlet airtemperature. The dryer will lowerthe dew point 20 to 30 degreesbelow this.

With no moving parts, the deliques-cent dryer is very reliable. It con-sumes no energy. Operating costsinclude monitoring and replacingdesiccant and filter elements.

The refrigerant dryer lowers thedew point by mechanically coolingthe air, then removing thecondensed water in a separator.These dryers are rated at 35°F or50°F pressure dew points. Anylow-er dew point would freeze thecon-densate and block air flow.

The commercial refrigeration unitsused in these dryers require acontinuous flow of electricityduring operation. Other operatingcosts are minimal, except somelarge flow capacity refrigerateddryers may require a water-to-airheat exchanger.

The regenerative dryer uses adifferent form of desiccant whichcan be regenerated for repeatedreuse. The basic dryer consists oftwo pressure vessels and someautomatic valving. One vessel ortower dries air for the distributionsystem while the secondregenerates. The desiccant cannotdry air at a temperature over120°F, so an aftercooler precedingthe dryer is essential.

Regenerative-type dryers deliverdrier air then either deliquescent orrefrigerant. Pressure dew pointsrange from 0° to –100°F.

Operating costs depend on which ofthe five type of regeneration areused.

Heatless regeneration dryers useapproximately 15% of the driedcompressed air from the dryingtower to regenerate the desiccant.This air is lost to the operatingsystem, so it can be a considerableexpense at high flows.

Exhaust purge dryers take a smallerportion of the dried air (about 7%),heat it electrically, and use this airfor regeneration. Air waste is less,but heating costs must beconsidered.

Blower purge dryers heat ambientair and blow it through thedesiccant. No compressed air is lost,but blower and heater operatingcosts become a factor.

Closed system dryers produce al-most constant dew points for criticalair applications by regeneratingwith a captive air stream. Electri-city and water costs are high. Air ofthis quality is unnecessary in mostindustrial systems.

Heat-of-compression dryers bringhot air directly from the compressorinto the regenerating tower, thencool this air in an aftercooler andport it into the drying tower. Theenergy for regeneration is providedby the heat imparted to the airduring compression. No compress-ed air is lost, and no external heatersor blowers are needed, so operatingcosts are negligible.

Conclusion

Any type dryer will pay for itself inless than a year by reducing airsystem downtime and damage tocomponents. To maximize dryerpayback, carefully evaluate operat-ing costs along with first cost whenmaking a selection.

Reprinted May, 1997 Page 4 Reference No. 004

The award-winning SAHARA-PAKHEAT-OF-COMPRESSION REGENERATIVE TYPE DRYERIs a revolutionary advance in compressed air drying!

The result of nearly 10 years ofdevelopment work by HendersonEngineering Co., Inc., the SAHARA-PAK is considered by many industryobservers to be the most significantdevelopment ever made in the design ofequipment for drying compressed air.

In 1978, the patented SAHARA-PAKdryer was selected for Top Honors inthe Dryers & Evaporators category ofCHEMICAL PROCESSING magazine’scoveted Vaaler Awards.

Unlike conventional regenerative sys-tems, where air exiting the compressoris always immediately cooled down, in aSAHARA-PAK installation the hot com-pressed air is piped directly into thedryer.

Because its high temperature and lowrelative humidity enable it to hold muchmore water, it is directed into thedesiccant tower on the regenerating

cycle where it picks up the accumulatedmoisture and leaves the desiccant readyfor the next drying cycle. Now moisture-laden, the air passes through anaftercooler/separator where the bulk ofthe moisture is removed, and then intothe tower on the drying cycle where finaldrying lowers the dew point to the –40°Fto -70°F range.

By using the normally wastedheat-of-compression to accomplishregeneration, the SAHARA-PAK eithereliminates or substantially reduces allthe normal operating costs of aregenerative type dryer. No valuablecompressed air is lost, and in mostinstallations no external heat source orblower is required, thus enabling theSAHARA-PAK to produce “desert-dry”air for about the same amount ofelectricity that is used by an ordinarylight bulb!

AIR PRODUCTSSAHARA

A Division of Henderson Engineering Co., Inc.95 North Main StreetSandwich, Illinois815-786-9471800-544-4379Fax: 815-786-6117www.saharahenderson.com