INTRODUCTION TO PLANT AIR

WHAT IS PLANT AIR

Basically plant air is a centralized source of compressed air that is available throughout the plant.

COMPRESSED AIR

Compressed air is used in many operations and processes on the plant, it is also used to operate pneumatic tools, equipment, and to meet instrumentation needs throughout the plant. Air can be compressed in several different ways and supplied at varying pressures and degrees of filtration depending on its use.

COMPRESSED AIR

A typical system consists of a compressor with inlet air filter, a safety relief valve, aftercooler, air/oil separator, air receiver with safety relief valve, prefilter, air dryer, afterfilter, oil/water traps, and oil/water separator.

COMPRESSED AIR

Typical industrial compressed air system

COMPRESSED AIR

A modern industrial compressed air system is composed of several major sub-systems and many sub-components. Major sub-systems include the compressor, prime mover, controls, treatment equipment and accessories, and the distribution system

COMPRESSED AIR A modern industrial compressed air system is composed of several

major sub-systems and many sub-components. Major sub-systems include the compressor, prime mover, controls, treatment equipment and accessories, and the distribution system.

The compressor is the mechanical device that takes in ambient air and increases its pressure.

The prime mover powers the compressor. Controls serve to regulate the amount of compressed air being

produced. The treatment equipment removes contaminants from the

compressed air, and accessories keep the system operating properly.

Distribution systems are analogous to wiring in the electrical world they transport compressed air to where it is needed.

Compressed air storage can also serve to improve system performance and efficiency.

COMPRESSED AIR The end-use instruments, tools, and processes

dictate the best choice for the supply side of the compressed air system.

Another measure that affects the supply and quality of your compressed air system is using outside air as the intake air for your compressor.

Compressor selection is arguably the most important component in a compressed air system and can dramatically influence equipment, maintenance, and energy costs of the system.

AIR QUALITY

The downstream components and processes also drive the requirements for filters, after-coolers, and dryers. Also, there may be a requirement for applications that cannot tolerate oil, like food processing, to use oil free compressors like centrifugal or oil-free rotary screw compressors.

AIR QUALITY Coalescing filters should be placed where condensed

moisture and oil may be present and need to be removed. Coalescing oil filters should be used upstream from a desiccant type dryer to remove oil that will contaminate the desiccant material. Particulate filters should be placed on the intake of the compressor and downstream from equipment that may introduce particulate into the compressed air stream like compressors and dryers.

Maintenance of filters is critical when operating an efficient system. A clogged filter increases the flow resistance causing increased pressure drops and consuming additional energy.

AIR QUALITY Pressure gages or sensors should be placed upstream

and downstream of the filters to determine when a filter element requires cleaning or replacement. The pressure drop of 6-10 psi indicates the need for a filter element to be replaced or cleaned.

The best options for dryers are determined by the pressure dew point requirements for your processes. The most common air dryers are refrigerated type. This type of dryer will provide pressure dew points as low as 33-39F, which is adequate for most industrial process and pneumatic tools. Cycling refrigerated air dryers are the best choice for 40F dew point air.

COMPRESSORS

Reciprocating, rotary screw, and centrifugal compressors are the most common types of compressors used in industrial and commercial air systems.

RECIPROCATING COMPRESSORS

Reciprocating compressors work like bicycle pumps. A piston, driven by an electric motor, through a crankshaft and connecting rod, reduces the air volume in the cylinder, compressing it to a higher pressure.

Single-acting compressors have a compression stroke in only one direction, while double-acting units provide a compression stroke as the piston moves in each direction. Typically large industrial reciprocating air compressors are double-acting.

Single acting reciprocating compressor

RECIPROCATING COMPRESSORS

Reciprocating compressors are most always multi-stage compressors. Multi-stage compressors compress air part way, cool the air, and compress it further with a second stage and possibly a third or fourth stage depending on the final pressure requirement.

Multi-stage double-acting compressors are the most efficient compressors available, and are typically larger, noisier, and have a higher purchase price and maintenance costs than rotary units of comparable capacity. Reciprocating compressors are available in sizes from less than 1 hp to more than 600 hp.

Double acting reciprocating compressor

RECIPROCATING COMPRESSORS

Reciprocating compressors control air output by unloading cylinders. The pistons operate against very little air-pressure resistance in this mode and therefore, very little energy is wasted. For this reason, reciprocating compressors are typically efficient at part-load operation.

ROTARY COMPRESSORS Rotary compressors are the

most commonly purchased today. They are most commonly used in sizes from about 30-200 hp.

The most common type of rotary compressor is the helical twin screw-type, also known as rotary screw or helical lobe. Meshing male and female screw-rotors rotate in opposite directions and trap air reducing the volume of the air along the rotors to the air discharge point. Rotary screw compressor

ROTARY COMPRESSOR

Rotary screw compressors come in oil-less or oil-flooded models. Oil-less compressors have the advantage of not contaminating air for sterile air requirements.

However, the oil in an oil-flooded compressor acts as a sealant between the male and female rotors and carries away heat from the inside of the compressor. This makes the oil-flooded compressor more efficient.

ROTARY COMPRESSOR Rotary screw compressors control compressed air output using one of several

different methods. Inlet valve control: This method basically starves the compressor of intake air to

reduce compressed air volume. It effectively increases the pressure that the compressor must work against. As a result this is the least efficient part-load control.

Rotor shortening: This method reduces the effective compression length of the meshing rotors, which reduces its capacity to make compressed air. Similar to reciprocating part-load control, this unloading of the screw wastes little energy and is comparatively efficient at part load.

Variable speed drive control: Slowing the positive displacement screw compressor is an efficient method of part load control. Like reciprocating compressors and rotor shortening screw compressors this method reduces compressor power in an almost direct proportion to the output capacity.

Online\offline or load\unload control: In this control sequence, the screw compressor modulates between 100% loaded and unloaded where the screws are not compressing air at all. This method is similar to unloading cylinders in a reciprocating compressor and is therefore quite efficient. However, a receiver tank or large storage vessel may be required, since the unloading portion of the cycle completely interrupts the production of compressed air.

CENTRIFUGAL COMPRESSORS Centrifugal compressors are

dynamic compressors. which raise the pressure of air by imparting velocity energy, using a rotating impeller, and converting it to pressure energy.

Centrifugal compressors are oil-free by design and have few moving parts hence decreasing maintenance requirements and costs. Centrifugal compressors are designed to handle a base or continuous load in compressed air systems because they have limited turn-down or reduced output capability.

Centrifugal air compressor skid

CENTRIFUGAL COMPRESSORS

Centrifugal compressors control output with inlet valves or inlet guide vanes similar to the throttling inlet valve control on a rotary screw compressor. This is not efficient.

Also, because of the surge problem associated with centrifugal compressors, they can only be partially throttled down to about 80 percent flow capacity with inlet valve control. When a centrifugal compressor needs to provide flow less than 80 percent capacity it will “blow-off” or vent the compressed air directly to the atmosphere or the surroundings.

CENTRIFUGAL COMPRESSORS

Running a centrifugal compressor in “blow-off” mode wastes a lot of energy. For this reason, centrifugal compressors should be base-load compressors that operate at near 100 percent capacity at all times.

MOTOR OR ENGINE

A motor or engine must provide enough power to start the compressor, accelerate it to full speed, and keep the unit operating under various design conditions.

Electric motors are by far the most common type power source.

Electric motors are a widely available and economical means of providing reliable and efficient power to compressors.

RECEIVER TANK A receiver tank is a storage tank or

accumulator for a compressed air system.

Receivers are used to provide compressed air storage capacity to meet peak demand events and help control system pressure by controlling the rate of pressure change in a system. Receivers are especially effective for systems with widely varying compressed air flow requirements. Where peaks are intermittent, a large air receiver may allow a smaller air compressor to be used and can allow the capacity control system to operate more effectively and improve system efficiency.

Air receiver

RECEIVER TANK

An air receiver after a reciprocating air compressor can provide dampening of pressure pulsations, radiant cooling, and collection of condensate. Demand-side control will optimize the benefit of the air receiver storage volume by stabilizing system header pressure and “flattening” the load peaks. Air receivers also play a crucial role in orchestrating system controls, providing the time needed to start or avoid starting standby air compressors.

AFTER COOLERS & AIR DRYERS

Most pneumatic instruments, tools, and processes cannot tolerate the hot compressed air coming from a compressor. An after-cooler is a heat exchanger immediately downstream from the compressor. It reduces the temperature of the compressed air and also condenses out some of the moisture from the compressed air.

AFTER COOLERS & AIR DRYERS

An after-cooler discharging saturated compressed air at 100F will pass along 67 gallons of water per 1,000 standard cubic feet per minute ever 24 hours.

If left in the compressed air system, this moisture can condense in the piping, pneumatic tools and instruments, causing premature damage or failure.

The air dryer removes this water vapor from the compressed air. Dryers are specified based on pressure dew point required.

AIR DRYERS

Air dryers can generally be categorized into one of two types:

• Refrigerated • Desiccant.

REFRIGERATED DRYERS Refrigerated dryers decrease the temperature of the air below the

dew point using a cooling coil that condenses moisture out of the air. Refrigerated air dryers can produce air with a dew point as low as 33-39F. Refrigerated dryers cannot operate below this range because the condensing water will freeze on the cooling coil of the dryer.

Refrigerated dryers are either non-cycling or cycling. The refrigerant compressor for a non-cycling dryer runs continuously regardless of the dryer load indicated by moisture content in the compressed air.

Cycling dryers use thermal storage, and cycle the refrigeration compressor on and off depending on the compressed air flow. They are ideal for systems with fluctuating airflow and moisture content. Cycling refrigerant dryers are recommended in the upper Midwest due to the widely varying humidity levels throughout the year.

DESICCANT DRYERS Desiccant dryers dry air through the process of

moisture adsorption with a desiccant material. Desiccant dryers can dry compressed air to a dew point of –40F or below. Typical desiccant materials are silica, alumina, or molecular sieve polymers. The desiccant material must be regenerated (have adsorbed moisture moisture driven off) to maintain its adsorption capabilities. There are two desiccant regeneration designs:

• Heated• Heatless

HEATLESS DESICCANT DRYERS

Heatless desiccant dryers typically use two identical drying towers, each containing a desiccant bed. While one tower is drying the compressed air the other tower is regenerating by purging a certain portion of the dried air coming from the active desiccant dryer through it. The purge air requirements can range from 10-18% of the total compressed airflow, making them very inefficient compared to other dryers.

HEATED DESICCANT DRYERS

Heated desiccant dryers can be internally or externally heated, blower purge, and heat-of-compression type dryers. Both the internally and externally heated desiccant dryers use a heater and a low-rate air purge, relative to heatless dryers, to regenerate the desiccant.

The blower purge design uses a heater and a blower instead of compressed air for regeneration. The heat-of-compression dryer, specifically designed for use with an oil-free compressor, uses the hot compressed air to regenerate the desiccant and remixes this air back into the compressed air supply yielding the lowest energy costs for desiccant type dryers.

FILTERS

Filters in a compressed air system have two functions:

• to remove particulate from the system • to remove condensed water and oil from

the system.

FILTERS

An important filter within the system is the intake filter for the compressor. This filter removes dust and other particulates from the intake air feeding the compressor. Compressors are close tolerance equipment and even small amounts of particulate can score rotors or cylinders and cause premature wear and failure.

FILTERS

Particulate filters should also be placed downstream from a desiccant dryer to capture desiccant particles.

Coalescing filters are used throughout a compressed air system to capture condensed air and oil. Coalescing filters should have automatic condensate drains

FILTERS

It is important to monitor pressure drop across filters to determine whether a filter element is in need of being replaced. Typically a pressure drop of 6-10 psi determines the need to replace a filter.

PIPING DISTRIBUTION SYSTEM

The piping portion of the compressed air system serves a number of purposes. The most obvious is to transport the air from the supply side of the system to the downstream equipment. It also provides limited storage capacity and controls velocity to and from various process parts of the system.

PIPING DISTRIBUTION SYSTEM

An efficient distribution system is designed to limit the amount of pressure drop as the air moves towards the end-use. This is accomplished through adequate pipe sizing and limiting bends, elbows, and other flow restrictions within the system. It is also important to avoid wasteful leakage in joints and couplings.

MAINTENANCE OF COMPRESSED AIR SYSTEMS

Like all electro-mechanical equipment, industrial compressed air systems require periodic maintenance to operate at peak efficiency and minimize unscheduled downtime. Inadequate maintenance can have a significant impact on energy consumption via lower compression efficiency, air leakage, or pressure variability. It can also lead to high operating temperatures, poor moisture control, and excessive contamination. Most problems are minor and can be corrected by simple adjustments, cleaning, part replacement, or the elimination of adverse conditions.

MAINTENANCE OF COMPRESSED AIR SYSTEMS

All equipment in the compressed air system should be maintained in accordance with manufacturers’ specifications. Manufacturers provide inspection, maintenance, and service schedules that should be followed strictly. In many cases, it makes sense, from efficiency and economic standpoints, to maintain equipment more frequently than the intervals recommended by manufacturers, which are primarily designed to protect equipment.