An Introduction to Industrial Compressed Air Systems

Course No: M03-042

Credit: 3 PDH

Elie Tawil, P.E., LEED AP

Continuing Education and Development, Inc. 22 Stonewall Court Woodcliff Lake, NJ 076 77

P: (877) 322-5800 [email protected]

SECTION 1: INTRODUCTION TO COMPRESSED AIR SYSTEMS

This section of the sourcebook is intended for readers Components of an Industrial who want to gain an understanding of the basics of industrial compressed air systems. A glossary of basic Compressed Air System terminology is included in Appendix A for users A compressor is a machine that is used to increase the unfamiliar with the terms used in this chapter. pressure of a gas. The earliest compressors were bellows, used by blacksmiths to intensify the heat in Compressed air is widely used throughout their furnaces. The first industrial compressors were manufacturing industries and is often considered the simple, reciprocating piston-driven machines powered “fourth utility” at many facilities. Almost every by a water wheel. industrial plant, from a small machine shop to an immense pulp and paper mill, has some type of A modern industrial compressed air system is composed compressed air system. In many cases, the compressed of several major sub-systems. Major sub-systems air system is so vital that the facility cannot operate include the compressor, prime mover, controls, without it. Plant air compressor systems can vary in size treatment equipment and accessories, and the from a small unit of 5 horsepower (hp) to huge systems distribution system. The compressor is the mechanical with more than 50,000 hp. device that takes in ambient air and increases its pressure. The prime mover powers the compressor. In many industrial facilities, air compressors use more Controls serve to regulate the amount of compressed electricity than any other type of equipment. air being produced. The treatment equipment removes Inefficiencies in compressed air systems can be contaminants from the compressed air, and accessories significant. Energy savings from system improvements keep the system operating properly. Distribution can be substantial, resulting in thousands, or even systems are analogous to wiring in the electrical hundreds of thousands of dollars of potential annual world—they transport compressed air to where it is savings, depending on use. A properly managed needed. Compressed air storage can also serve to compressed air system can save energy, reduce improve system performance and efficiency. Figure 1.1 maintenance, decrease downtime, increase production shows a representative industrial compressed air throughput, and improve product quality system and its components.

Compressed air systems consist of a supply side and a demand side. The supply side includes compressors and Compressor Types air treatment (dryers and filters). The demand side Many modern industrial air compressors are sold includes distribution and end-use equipment. A “packaged” with the compressor, drive motor, and properly managed supply side will result in clean, many of the accessories mounted on a frame for ease of appropriately dried, stable air being delivered at the installation. Provision for movement by forklift is appropriate pressure in a dependable, cost-effective common. Larger packages may require the use of an manner. A properly managed demand side minimizes overhead crane. An enclosure may be included for wasted air and uses compressed air for appropriate sound attenuation and aesthetics. applications. Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system and how the two interact.

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 3 INTRODUCTION TO COMPRESSED AIR SYSTEMS

Air Air Inlet Dryer Air Filter Pressure-Flow Receiver Filter Controller Compressor Package Distribution Enclosure System

Moisture Separator

Pneumatic Tool Aftercooler and Lubricant Cooler Motor Control Panel Compressor Air End Filter, Regulator Lubricant/Air Condensate Lubricator Separator Drain

Figure 1.1 Components of a Typical Industrial Compressed Air System

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 4 INTRODUCTION TO COMPRESSED AIR SYSTEMS As shown in Figure 1.2, there are two basic compressor Single-Acting, Reciprocating Air Compressors types: positive-displacement and dynamic. In the This type of compressor is characterized by its positive-displacement type, a given quantity of air or “automotive” type piston driven through a connecting gas is trapped in a compression chamber and the rod from the crankshaft. Compression takes place on volume which it occupies is mechanically reduced, the top side of the piston on each revolution of the causing a corresponding rise in pressure prior to crankshaft. Single-acting, reciprocating air compressors discharge. At constant speed, the air flow remains are generally air-cooled. These may be single- stage, essentially constant with variations in discharge usually rated at discharge pressures from 25 to 125 pressure. Dynamic compressors impart velocity energy pounds per square inch gauge (psig), or two-stage, to continuously flowing air or gas by means of impellers usually rated at discharge pressures from 125 psig to rotating at very high speeds. The velocity energy is 175 psig or higher. changed into pressure energy both by the impellers and

the discharge volutes or diffusers. In the centrifugal- The most common air compressor in the fractional and type dynamic compressors, the shape of the impeller single-digit hp sizes is the air-cooled, reciprocating air blades determines the relationship between air flow compressor. Single-acting reciprocating compressors and the pressure (or head) generated. above 30 hp are much less common. Two-stage and

multi-stage designs include inter-stage air-cooling to Positive Displacement Compressors reduce air temperatures to the second stage for These compressors are available in two types: improved efficiency and durability. reciprocating and rotary. Reciprocating compressors work like bicycle pumps with the exception that an Pistons used in single-acting compressors are of the electric motor replaces the human energy source. A “automotive” or “full skirt” design, the underside of the piston, driven through a crankshaft and connecting rod piston being exposed to the crankcase. Lubricated by an electric motor, reduces the volume in the cylinder versions have a combination of compression and occupied by the air or gas, compressing it to a higher lubricant-control piston rings, which seal the pressure. Single- acting compressors have a compression chamber, control the lubricant to the compression stroke in only one direction, while double- compression chamber, and act (in some designs) as acting units provide a compression stroke as the piston support for piston movement on the cylinder walls. moves in each direction. Large, industrial reciprocating air compressors are double-acting and water-cooled. Lubricant-free, or non-lube designs, do not allow Multi-stage, double- acting compressors are the most lubricant in the compression chamber and use pistons efficient compressors available, and are typically larger, of self-lubricating materials or use heat resistant, non- noisier, and more costly than comparable rotary units. metallic guides and piston rings that are self- Reciprocating compressors are available in sizes from lubricating. Some designs incorporate a distance piece less than 1 hp to more than 600 hp. or crosshead to isolate the crankcase from the compression chamber. Rotary compressors have gained popularity and are now the “workhorse” of American industry. They are Lubricant-less (or oil-less) designs have piston most commonly used in sizes from about 10 to 500 hp. arrangements similar to lubricant-free versions, but do The most common type of rotary compressor is the not have lubricant in the crankcase. Generally these helical-twin, screw-type (also known as rotary screw or have greased pre-packed crankshafts and connecting helical-lobe). Male and female rotors mesh, trapping rod bearings. air, and reducing the volume of the air along the rotors to the air discharge point. Rotary screw compressors have low initial cost, compact size, low weight, and are easy to maintain. Rotary screw compressors may be air- or water-cooled. Less common rotary compressors include sliding-vane, liquid-ring, and scroll-type.

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 5 INTRODUCTION TO COMPRESSED AIR SYSTEMS

Figure 1.2 Compressor Family Tree

Cooling. Single-acting air compressors have different arrangements for removing the heat of compression. Double-Acting, Reciprocating Air Compressors Air-cooled versions have external finning for heat Double-acting reciprocating compressors use both sides dissipation on the cylinder, cylinder head, and in some of the piston for air compression, almost doubling the cases, the external heat exchanger. Air is drawn or capacity for a given cylinder size. A piston rod is blown across the fins and the compressor crankcase by attached to the piston at one end and to a crosshead at a fan, which may be the spokes of the drive the other end. The crosshead ensures that the piston pulley/flywheel. travels concentrically within the cylinder. These

compressors may be single- or multi-stage, depending Liquid-cooled compressors have jacketed cylinders, on discharge pressure and hp size. These are generally heads and heat exchangers, through which liquid available in larger horsepower sizes. coolant is circulated to dissipate the heat of compression. Water, or an ethylene glycol mixture to Cooling. Double-acting air compressors generally have prevent freezing, may be employed. cooling water jackets around the cylinder body and in

the cylinder head. This, combined with their relatively Drives. The most common drive arrangement is a belt slow speed of operation and water-cooled intercooling, drive from an electric motor. The compressor sheave results in excellent compression efficiency. also acts as a flywheel to limit torque pulsations and its spokes often are used for cooling air circulation. Belt Lubrication. Cylinder lubrication is generally by means drives allow a great degree of flexibility in obtaining the of a pressure or vacuum-feed cylinder lubricator, with a desired speed of rotation. feed rate of several drops per minute, depending on

cylinder size and piston speed and as specified by the Flange-mounted, or direct-coupled motor drives manufacturer. Lubricant-free versions also are available provide compactness and minimum drive maintenance. with polytetrafluorethylene (PTFE) or similar materials Belts and couplings must be properly shielded for safety for pistons, riders, and compression rings. A distance and to meet Occupational Safety & Health piece is provided between the crankcase and the Administration (OSHA) requirements. cylinder(s) to ensure that no part of the piston rod, IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 6 INTRODUCTION TO COMPRESSED AIR SYSTEMS which enters the lubricated crankcase, can enter the Lubrication. The generic term “lubricant” has been lubricant-free cylinder area. used instead of oil. The lubricant may be a hydrocarbon product, but most compressors now use cleaner and Balance. Single- and two-cylinder compressors of this longer life synthetic lubricants, including diesters, type generally require a substantial foundation due to polyglycols, polyalphaolefins, polyol esters, and silicone- unbalanced reciprocating forces. based lubricants. These newer products are suitable for a wider range of temperatures. Drives. Below 200 hp, belt drives and flange-mounted induction motors are normally used. For motors larger A mixture of compressed air and injected lubricant than 300 hp, flange-mounted, synchronous motors are leaves the air end and is passed to a sump/separator sometimes used with a 1.0 power factor or 0.8 leading where the lubricant is removed from the compressed power factor to provide power factor correction to off- air. Directional and velocity changes are used to set other induction-type electrical loads. separate most of the liquid. The remaining aerosols in the compressed air, then are separated by means of an Lubricant-Injected Rotary Screw Compressors air/lubricant separator or a coalescing filter, resulting in only a few parts per million (ppm) of lubricant carry- The lubricant-injected rotary screw compressor over (usually in the range of 2 to 5 ppm). A minimum powered by an electric motor has become a dominant pressure device, often combined with a discharge check type of industrial compressor for a wide variety of valve, prevents excessive velocities through the applications. separator element until a normal system pressure is

achieved at start-up. Most lubricant-injected rotary Compression Principle. The lubricant-injected, rotary- screw compressor packages use the air pressure in the screw compressor consists of two intermeshing rotors lubricant sump/separator, after the discharge of the air in a machined housing (Rotor Housing) having an inlet end, to circulate the lubricant through a filter and port at one end and a discharge port at the other. The cooler prior to reinjection to the compression chamber. male rotor has lobes formed helically along its length Some designs may use a lubricant pump. while the female rotor has corresponding helical grooves or flutes. The number of helical lobes and Multi-stage compressors. Multi-stage compressors grooves may vary in otherwise similar designs. have different manufacturing designs. Multiple stages

are used either for improved efficiency at a given Air flowing in through the inlet port fills the spaces pressure or to achieve higher pressures. between the lobes on each rotor. Rotation then causes the air to be trapped between the lobes and the rotor Cooling. The temperature of the lubricant injected into housing as the inter-lobe spaces pass beyond the inlet the compression chamber is generally controlled port. As rotation continues, a lobe on one rotor rolls directly or indirectly to a minimum temperature by into a groove on the other rotor and the point of controlling the discharge temperature. A thermostatic intermeshing moves progressively along the axial length bypass valve allows some or all of the lubricant being of the rotors. This reduces the space occupied by the circulated to bypass the lubricant cooler to maintain the air, resulting in increased pressure. Compression desired temperature over a wide range of ambient continues until the inter-lobe spaces are exposed to the temperatures. discharge port when the compressed air is discharged.

Generally, suitable lubricant temperature and viscosity Lubricant is injected into the compression chamber are required for proper lubrication, sealing, and to avoid during compression and serves three basic functions: condensation in the lubricant sump. It also is necessary 1) lubrication for the intermeshing rotors and to avoid excessive temperatures, which could result in a associated bearings; 2) removal of the heat caused by breakdown of the lubricant and reduced life. compression; and 3) seals the clearances between the meshing rotors and between rotors and rotor housing. In addition to lubricant cooling, an aftercooler is used to

cool the discharged air and a moisture separator with an automatic drain trap removes the condensate. In the

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 7 INTRODUCTION TO COMPRESSED AIR SYSTEMS majority of applications, air-cooled, radiator-type lubricant-free rotary screw compressors have a range lubricants and air coolers are employed and provide the from 25 to 4,000 hp or 90 to 20,000 cfm. Single-stage opportunity for heat recovery from the compression units operate up to 50 psig, while two-stage can process for facility heating. In water-cooled designs, generally achieve up to 150 psig. water-cooled heat exchangers with water control valves also are available on most rotary screw compressor In the water-injected type, similar timing gear packages. construction may sometimes be used, but water is injected into the compression chamber to act as a seal Single-stage, lubricant-injected, rotary screw in internal clearances and to remove the heat of compressor packages are available from 3 to 900 hp, or compression. This may allow pressures in the 100 to 8 to 5000 cubic feet per minute (cfm), with discharge 150 psig range to be accomplished with only one stage. pressures from 50 to 250 psig. Two-stage versions can The injected water, together with condensed moisture reduce specific power and some can achieve discharge from the atmosphere, is removed from the discharged pressures up to 500 psig. Lubricant-injected, rotary compressed air by a conventional moisture separation screw vacuum pumps also are available from 80 to device. Similar to the lubricant-injected type, lubricant- 3,100 inlet cfm and vacuum to 29.7 inches Hg. free rotary screw compressors generally are packaged Lubricant-injected, rotary-vane compressors are a less with all necessary accessories. common type of rotary compressor and are available in a limited size range. Lubrication. Lubricant-free rotary screw compressors utilize lubricant for bearings and gears, which are Air/Lubricant Separators. The air/lubricant separator in isolated from the compression chamber. The lubricant a lubricant-cooled, rotary screw compressor generally also may be used for stator jacket cooling in air-cooled starts with a 2 to 3 psid pressure drop at full-load when units. Typically, a lubricant pump is directly driven from new. Higher pressure differentials cause the a shaft in the gearbox, assuring lubricant flow compressor motor to consume more energy and immediately at start-up and during run-down in the possibly exceed the service factor. Maintenance event of power failure. A lubricant filter, typically with manuals usually suggest changing them when there is a 10 micron rating, protects bearings, gears, and the 10 to 12 psid pressure drop across the separator. In lubricant pump from damage. many cases it may make sense to make an earlier separator replacement, especially if electricity prices Cooling. The cooling system for the dry-type, lubricant- are high. free rotary screw compressor normally consists of an air cooler after each stage and a lubricant cooler. These Lubricant-Free Rotary Screw Compressors may be water-cooled or air-cooled, radiator-type. Some older two-stage designs also employ an additional heat The principle of compression in lubricant-free rotary exchanger to cool a small portion of the compressed air screw compressors is similar to that of the lubricant- for recycling to the compressor inlet during the injected rotary screw compressors but, without unloaded period. lubricant being introduced into the compression chamber. Two distinct types are available: the dry type Dynamic Compressors and the water-injected type. Dynamic compressors can be categorized as centrifugal In the dry type, the intermeshing rotors are not allowed or axial. The centrifugal-type is the most common and is to touch and their relative positions are maintained by widely used for industrial compressed air. Each means of lubricated timing gears external to the impeller, rotating at high speed, imparts radial flow to compression chamber. Since there is no injected fluid to the air or gas, which then passes through a volute or remove the heat of compression, most designs use two diffuser. Some large manufacturing plants use stages of compression with an intercooler between the centrifugal compressors for both base loading and stages and an aftercooler after the second stage. The trimming, while in other facilities, plants use other lack of a sealing fluid also requires higher rotation compressor types to accommodate demand load swings speeds than for the lubricant-injected type. Dry-type,

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 8 INTRODUCTION TO COMPRESSED AIR SYSTEMS while the dynamic centrifugal compressors handle the given discharge pressure increases as the ambient base load. temperature decreases, provided the motor can support the increased power requirements. Axial compressors consist of a rotor with multiple rows of blades and a matching rotor housing with rows of Typically, a capacity control system is provided with the stationary vanes. The rotating blades impart energy to compressor to maintain the desired capacity and to the air, primarily in an axial plane. The stationary vanes operate within the motor horsepower limits. The then act as a diffuser to convert the residual kinetic control system regulates the air flow by means of an energy into potential energy by increasing the pressure. inlet throttle valve or inlet guide vanes. The minimal This type of compressor is restricted to very high flow throttle point is established to prevent flow reversal capacities. Mixed flow compressors have impellers and thru an impeller/diffuser stage, which is also known as rotors which combine the characteristics of both axial surge. Control systems either unload the compressor or and centrifugal compressors. blow off the excess air to atmosphere to avoid surge, which could result in excessive vibration and potential Centrifugal Air Compressors damage to the compressor. Given adequate storage, some manufacturers will operate the compressor A centrifugal air compressor has a continuously flowing controls in a load/unload mode at lower flow air stream which has kinetic energy, imparted to it by an conditions. impeller, or impellers, which rotate at speeds that can

exceed 50,000 revolutions per minute (rpm). Centrifugal air compressors range from around 200 to Approximately one half of the energy is developed in more than 100,000 cfm, but the more common air the impeller with the other half achieved by converting compressors are from 1,200 to 5,000 cfm and with the velocity energy to pressure energy as the air discharge pressures up to 125 psig. These may have velocity is reduced in a diffuser and volute. several impellers in line on a single shaft or with

separate impellers integrally geared. The most common centrifugal air compressor is one with two to four stages for pressures in the 100 to 150 Centrifugal air compressors provide lubricant-free air psig range. A water-cooled intercooler and separator delivery as there is no lubricant in the compression between each stage returns the air temperature to chambers. Lubrication for speed increasing gears and approximately ambient temperature and removes the special high-speed shaft bearings is kept away from condensed moisture before entering the next stage. An the compression chambers by means of shaft seals, aftercooler cools the air from the final stage and a which may also have air purge and vent connections. moisture separator with an automatic drain removes the moisture prior to air delivery to distribution. Air- Centrifugal air compressors are high-speed rotating cooled centrifugal compressors are available within a machines and as such, shaft vibration monitoring is limited size range. mandated to record operational trends and protect the

equipment. Automatic control of the compressors is The inherent characteristic of centrifugal air typical and has been greatly improved by the use of compressors is that as system pressure decreases, the microprocessors, which monitor the pressure, capacity, compressor’s flow capacity increases. The steepness of temperature characteristics and main-drive motor the pressure head/capacity curve is dependent upon current draw. It is important that the manufacturer’s the design of the impellers and diffusers. recommended maintenance procedures be followed

and that certain maintenance procedures be carried out Most standard industrial centrifugal air compressor by qualified staff. This is particularly true of attempts to packages are designed for an ambient temperature of remove an impeller from its shaft, since special 95°F and near sea level barometer pressure. The procedures and tools may be involved. dynamic nature of the centrifugal compressor results in

the pressure head generated by each impeller Lubrication and Lubrication Systems. Centrifugal increasing as the air density increases. The compressor compressors use a pressure lubrication system for mass flow and actual cubic feet per minute capacity at a bearings and drive gears. The main lubricant pump may

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 9 INTRODUCTION TO COMPRESSED AIR SYSTEMS be driven from the gearbox input shaft with an electric Standby or emergency compressors may also be engine- motor-driven auxiliary lubricant pump for pre- driven to allow operation in the event of a loss of lubrication prior to start-up and for post-lubrication electrical power. Maintenance costs for engine-driven during a cool down period. A water-cooled lubricant systems are significantly higher than those that use cooler is also included. electric motors.

Because of the high rotation speeds, some designs use a The oldest method of driving compressors is through high-pressure lubricant supply to the special bearings the use of a steam engine or turbine. In general, how- involved. The manufacturer’s recommended lubricant ever, it is not economical to use a steam engine or should be used and changed at the specified intervals. turbine unless the steam is inexpensively and readily available within the plant for use as a power source. Compressor Prime Movers Compressed Air System Controls The prime mover is the main power source providing energy to drive the compressor. The prime mover must Compressed air system controls serve to regulate the provide enough torque and power to start the pressure by matching compressor supply with system compressor, accelerate it to full speed, and keep the demand. Proper compressor control is essential to unit operating under various design conditions. This efficient operation and high performance. Because power can be provided by any one of the following compressor systems are typically sized to meet a sources: electric motors, diesel or natural gas engines, system’s maximum demand, a control system is almost steam turbines and combustion turbines. Electric always needed to reduce the output of the compressor motors are by far the most common type of prime during times of lower demand. Compressor controls are mover. typically included in the compressor package, and many manufacturers offer more than one type of control Electric motors are a widely available and economical technology. Systems with multiple compressors use means of providing reliable and efficient power to more sophisticated controls (network or system master compressors. Most compressors use standard, controls) to orchestrate compressor operation and air polyphase induction motors. In many cases, either a delivery to the system. standard- or a premium-efficient motor can be specified when purchasing a compressor or replacement motor. Network controls use the on-board compressor The incremental cost of the premium efficient motor is controls’ microprocessors linked together to form a typically recovered in a very short time from the chain of communication that makes decisions to resulting energy savings. When replacing a standard stop/start, load/unload, modulate, vary displacement, motor with a premium-efficient version, careful and vary speed. Usually, one compressor assumes the attention should be paid to performance parameters, lead with the others being subordinate to the such as full-load speed and torque. A replacement commands from this compressor. motor with performance as close as possible to the original motor should be used. Most new motors are System master controls coordinate all of the functions suitable for inverter duty applications. necessary to optimize compressed air as a utility. System master controls have many functional Diesel or natural gas engines are common compressor capabilities, including the ability to monitor and control power sources in the oil and gas industries. all components in the system, as well as trending data, Considerations such as convenience, cost, and the to enhance maintenance functions and minimize costs availability of liquid fuel and natural gas play a role in of operation. Other types of controllers, such as selecting an engine to power a compressor. Although pressure/flow controllers, may also improve the the majority of industrial compressed air systems use performance of some systems. electric motors for prime movers, in recent years there has been renewed interest in using non-electric drives, The type of control system specified for a given system such as reciprocating engines powered by natural gas, is largely determined by the types of compressors being particularly in regions with high electricity rates. used and the facility’s demand profile. If a system has a

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 10 INTRODUCTION TO COMPRESSED AIR SYSTEMS single compressor with a very steady demand, a simple shaft or by a small auxiliary electric motor. In plants control system may be appropriate. On the other hand, where good quality water is available, shell and tube a complex system with multiple compressors, varying heat exchangers generally are used. Sometimes this demand, and many types of end uses will require a heat can be recovered and used for other purposes. more sophisticated control strategy. In any case, careful consideration should be given to compressor system Intercooling. Most multi-stage compressors use control selection because it can be the most important intercoolers, which are heat exchangers that remove single factor affecting system performance and the heat of compression between the stages of efficiency. For information about efficiency and compression. Intercooling affects the overall efficiency compressor controls, see the fact sheet titled of the machine. Compressed Air System Controls in Section 2. Aftercoolers. As mechanical energy is applied to a gas Accessories for compression, the temperature of the gas increases. Aftercoolers are installed after the final stage of Accessories are the various types of equipment used to compression to reduce the air temperature. As the air treat compressed air by removing contaminants such as temperature is reduced, water vapor in the air is dirt, lubricant, and water; to keep compressed air condensed, separated, collected, and drained from the systems running smoothly; and to deliver the proper system. With cooler water temperatures, most of the pressure and quantity of air throughout the system. condensate from a compressor with intercooling is Accessories include remote aftercoolers, filters, removed in the intercooler(s), and the remainder in the separators, dryers, heat recovery equipment, aftercooler. Almost all industrial systems, except those lubricators, pressure regulators, air receivers, and that supply heated process air require aftercooling. In condensate drain traps. some systems, aftercoolers are an integral part of the compressor package, while in other systems the Air Inlet Filters. An air inlet filter protects the aftercooler is a separate piece of equipment. Some compressor from atmospheric airborne particles. systems have both. Further filtration is typically needed to protect equipment downstream of the compressor. Moisture Separators. Moisture separators are devices that remove liquids entrained in the air or gas. A Compressor Cooling. Air or gas compression generates separator generally is installed following each heat. As a result, industrial air compressors that operate intercooler or aftercooler to remove the condensed continuously generate substantial amounts of heat. moisture. This involves changes in direction and velocity Compressor units are cooled with air, water, and/or and may include impingement baffles. Lubricant- lubricant. Single-acting reciprocating compressors are injected rotary compressors have an air/lubricant typically air-cooled using a fan, which is an integral part coalescing separator immediately after the compressor of the belt-drive flywheel. Cooling air blows across discharge to separate the injected lubricant before it is finned surfaces on the outside of the compressor cooled and recirculated to the compressor. This cylinder’s cooling fins. Larger, water-cooled, double- separation must take place before cooling to prevent acting reciprocating air compressors have built-in condensed moisture from being entrained in the cooling water jackets around the cylinders and in the lubricant. cylinder heads. The temperature of the inlet water and the design and cleanliness of the cooler can affect Dryers. When air leaves an aftercooler and moisture overall system performance and efficiency. Centrifugal separator, it is typically saturated. Any further radiant compressors are generally water-cooled. cooling as it passes through the distribution piping, which may be exposed to colder temperatures, will Lubricant-injected rotary compressors use the injected cause further condensation of moisture with lubricant to remove most of the heat of compression. In detrimental effects, such as corrosion and air-cooled compressors, a radiator-type lubricant cooler contamination of point- of-use processes. This problem is used to cool the lubricant before it is reinjected. The can be avoided by the proper use of compressed air cooling fan may be driven from the main motor-drive dryers.

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 11 INTRODUCTION TO COMPRESSED AIR SYSTEMS equipped with a pre-cooler/reheater that reheats Atmospheric air contains moisture. The higher the air the dried compressed air with an air-to-air heat temperature, the more moisture the air is capable of exchanger using the hot incoming air. This lowers holding. The term “relative humidity” is commonly used the temperature of the incoming air before it to describe the moisture content. When the air contains passes through the refrigerant or thermal mass-to- all the moisture possible under the prevailing air heat exchanger, reducing the heat load on the conditions, it is called “saturated.” Air at 80 percent refrigerant system. Reheating the dried air relative humidity would contain 80 percent of the prevents condensation on the outside of the maximum possible moisture content. compressed air piping in warm humid environments. The refrigerated dryer lowers the When air is cooled, it will reach a temperature at which dew point of the air to the approximate the amount of moisture present can no longer be temperature of the air exiting the refrigerant contained in a vapor state and some of the moisture evaporator. To avoid freezing, the evaporator will condense and drop out. The temperature at which temperature should not go below 32°F. Allowing the moisture condenses is called the dew point. In for separator efficiency, a pressure dew point of general, reducing the temperature of saturated between 35 and 40°F can usually be obtained. compressed air by 20°F will reduce the moisture • Cycling dryers cool compressed air indirectly content by approximately 50 percent. through a thermal storage medium (heat sink,

thermal mass, chilled media, etc.) while non-cycling When air is compressed and occupies a smaller volume, dryers directly cool compressed air in a refrigerant it can no longer contain all of the moisture possible at to air heat exchanger. Refrigerant-type cycling atmospheric conditions. Again, some of the moisture dryers are controlled with one or two thermostats will drop out as liquid condensate. The result of both of to shut off the refrigerant compressor, saving these situations is a difference between the dew point energy when it is not needed, and a thermal at atmospheric conditions and the dew point at higher storage medium (sometimes referred to as heat pressures. Drying compressed air beyond the required sink, chilled media or thermal mass) prevents rapid pressure dew point will result in unnecessary energy cycling of the refrigerant compressor(s). Powdered and costs. metal, glycol and water, sand, steel, and aluminum

have all been used as this thermal storage medium. Different types of compressed air dryers have different The ideal characteristics of this medium would be operating characteristics and degrees of dew point high specific heat (effective storage), high suppression. Dryer ratings usually are based on coefficient of heat transfer (easy transfer of stored standard dryer inlet conditions, commonly referred to cooling), corrosion protected, and low cost. The as “the three 100s.” That is, 100 psig, 100°F (inlet quantity of medium required is determined by the compressed air temperature), and 100°F ambient temperature band of the controlling thermostat(s) temperature. Deviations from these conditions will and the refrigerant capacity to be stored. affect the capacity of a dryer. An increase in inlet temperature or a decrease in inlet pressure will reduce • Refrigerant-type, non-cycling dryers cool the air in the dryer’s rated capacity. Most manufacturers provide a refrigerant-to-air heat exchanger. The cooling correction factors for this. effect is from the evaporation of a liquid refrigerant causing moisture in the air to condense. The most common types of dryers are discussed below. The moisture then is removed and drained by a separator and drain. The temperature of the air • The refrigerant dryer is the most commonly used leaving the refrigerant evaporator is controlled by a dryer in the industry, having relatively low initial hot gas bypass valve and operating costs. Refrigerant-type air dryers • Regenerative-desiccant-type dryers use a porous (cycling and non-cycling) are not recommended for desiccant that adsorbs the moisture by collecting it operation in sub-freezing ambient temperatures. in its myriad pores, allowing large quantities of The moisture in the compressed air can freeze and water to be retained by a relatively small quantity damage the dryer. Most refrigerated dryers are

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 12 INTRODUCTION TO COMPRESSED AIR SYSTEMS of desiccant. Desiccant types include silica gel, air for regeneration of the desiccant bed. The activated alumina, and molecular sieves. Use only second air stream is the remainder of the air the type specified by the manufacturer. In some discharged from the air compressor after it passes cases, more than one desiccant type can be used through the air aftercooler. This air passes through for special drying applications. In most of these the drying section of the dryer’s rotating desiccant cases, a larger particle size (1/4 inch or more) is bed, where it is dried. The hot air, after being used used as a buffer zone at the inlet, while a smaller for regeneration, passes through a regeneration particle size desiccant (1/8 to 1/4 inch) is used for cooler before being combined with the main air final drying. Where very low dewpoints are stream by means of an ejector nozzle before required, molecular sieve desiccant is added as the entering the dryer. final drying agent. The twin-tower, heat-of-compression dryer Normally, the desiccant is contained in two operation is similar to other twin-tower, heat- separate towers. Compressed air to be dried flows activated, regenerative-desiccant dryers. The through one tower, while the desiccant in the difference is that the desiccant in the saturated other is being regenerated. Regeneration is tower is regenerated by means of the heat of accomplished by reducing the pressure in the compression in all of the hot air leaving the tower and passing previously dried purge air discharge of the air compressor. The total air flow through the desiccant bed. The purge air may also then passes through the air aftercooler before be heated, either within the dryer or externally, to entering the drying tower. Towers are cycled as for reduce the amount of purge air required. Purge air other regenerative-desiccant dryers. may also be supplied by a blower. Dryers of this The heat-of-compression dryers require air from type normally have a built-in regeneration cycle, the compressor at a sufficiently high temperature which can be based upon time, dew point, or a to accomplish regeneration. For this reason, it is combination of the two. used almost exclusively with centrifugal or • Deliquescent-type dryers use a drying medium that lubricant-free rotary screw compressors. absorbs, rather than adsorbs, the moisture in the • Membrane technology dryers have advanced compressed air. This means that the desiccant considerably in recent years. Membranes medium is used up as it changes from solid to liquid commonly are used for gas separation, such as in and cannot be regenerated. The most common nitrogen production for food storage and other deliquescent chemicals for compressed air drying applications. The structure of the membrane allows are salts of sodium, potassium, calcium, and those molecules of certain gases (such as oxygen) to pass with a urea base. Various compounds of these have through (permeate) a semi-permeable membrane been developed and sold under a variety of trade faster than others (such as nitrogen), leaving a names. concentration of the desired gas (nitrogen) at the • Heat-of-compression dryers are regenerative- outlet of the generator. When used as a dryer in a desiccant dryers that use the heat generated compressed air system, specially designed during compression to accomplish desiccant membranes allow water vapor (a gas) to pass regeneration, so they can be considered as heat through the membrane pores faster than the other reactivated. There are two types: the single-vessel gases (air) reducing the amount of water vapor in and the twin-tower. the air stream at the outlet of the membrane dryer, suppressing the dew point. The dew point achieved The single-vessel, heat-of-compression dryer is usually 40°F, but lower dew points to –40°F can provides continuous drying with no cycling or be achieved at the expense of additional purge air switching of towers. This is accomplished with a loss. rotating desiccant drum in a single pressure vessel

divided into two separate air streams. One air Compressed Air Filters. Depending on the level of air stream is a portion of the hot air taken directly purity required, different levels of filtration and types of from the air compressor at its discharge, prior to filters are used. These include particulate filters to the aftercooler, and is the source of heated purge remove solid particles, coalescing filters to remove IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 13 INTRODUCTION TO COMPRESSED AIR SYSTEMS lubricant and moisture, and adsorbent filters for tastes compressor package. A pressure/flow controller is a and odors. A particulate filter is recommended after a device that serves to separate the supply side of a desiccant-type dryer to remove desiccant “fines.” A compressed air system from the demand side. Properly coalescing-type filter is recommended before a sized storage receiver capacity is needed for adequate desiccant- type dryer to prevent fouling of the desiccant pressure/flow control. Not all systems would benefit bed. Additional filtration may also be needed to meet from this type of controller. requirements for specific end uses. Air Receivers. Receivers are used to provide Compressed air filters downstream of the air compressed air storage capacity to meet peak demand compressor are generally required for the removal of events and help control system pressure by controlling contaminants, such as particulates, condensate, and the rate of pressure change in a system. Receivers are lubricant. Filtration only to the level required by each especially effective for systems with widely varying compressed air application will minimize pressure drop compressed air flow requirements. Where peaks are and resultant energy consumption. Elements should intermittent, a large air receiver may allow a smaller air also be replaced, as indicated by pressure differential, compressor to be used and can allow the capacity to minimize pressure drop and energy consumption, control system to operate more effectively and improve and should be checked at least annually. system efficiency. An air receiver after a reciprocating air compressor can provide dampening of pressure Heat Recovery. As noted earlier, compressing air pulsations, radiant cooling, and collection of generates heat. In fact, industrial-sized air compressors condensate. Demand-side control will optimize the generate a substantial amount of heat that can be benefit of the air receiver storage volume by stabilizing recovered and put to useful work. More than 80 system header pressure and “flattening” the load peaks. percent of the electrical energy going to a compressor Properly sized air receivers also play a crucial role in becomes available heat. Heat can be recovered and orchestrating system controls, providing the time used for producing hot water or hot air. See the fact needed to start, or avoid starting, standby air sheet in Section 2 titled Heat Recovery and Compressed compressors. Of course proper application and design Air Systems for more information on this energy-saving is required to optimize the utilization of air receivers. opportunity. Traps. Traps (sometimes called drains) allow the Lubrication. In lubricant-injected rotary screw removal of moisture from the compressed air system. compressors, lubricants are designed to cool, seal, and Automatic condensate traps are used to conserve lubricate moving parts for enhanced performance and energy by preventing the loss of air through open longer wear. Important considerations for compressor petcocks and valves. Poorly maintained traps can waste lubricants include proper application and compatibility a lot of compressed air. with downstream equipment, including piping, hoses, and seals. A lubricator may be installed near a point- of- There are four methods to drain condensate. use to lubricate items such as pneumatic tools. The lubricator may be combined with a filter and a pressure 1. Manual. Operators will manually open valves to regulator to make up what is commonly called a FRL discharge condensate. However, this is not (filter-regulator-lubricator). The lubricant should be that automatic, and unfortunately, too often, manual specified by the point-of-use equipment manufacturer. valves are left open to drain condensate from moisture separators, intercoolers, refrigerated Pressure/Flow Controllers. Pressure/flow controllers are dryers, and filters, allowing compressed air to system pressure controls used in conjunction with the continually escape into the atmosphere. individual compressor or system controls described 2. Level-operated mechanical traps. Float-type previously. Their primary function is to regulate system traps do not waste air when operating properly, pressure more precisely than multiple individual but they often require a great deal of compressor controls and Master system controllers. A maintenance and are prone to blockage from pressure/flow controller does not directly control a sediment in the condensate. Inverted bucket compressor, and it is generally not included as part of a

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 14 INTRODUCTION TO COMPRESSED AIR SYSTEMS traps may require less maintenance but will waste consulted for allowable contamination levels. compressed air if the condensate rate is Condensate treatment should meet local codes. inadequate to maintain the liquid level (or prime) in the trap. Air Distribution Systems. The air distribution system links the various components of the compressed air 3. Electrically operated solenoid valves. The system to deliver air to the points-of-use with minimal solenoid-operated drain valve has a timing device pressure loss. The specific configuration of a that can be set to open for a specified time and at distribution system depends on the needs of the preset adjustable intervals. There are two issues individual plant, but frequently consists of an extended with using these valves. network of main lines, branch lines, valves, and air • The period during which the valve is open may hoses. The length of the network should be kept to a not be long enough for adequate drainage of minimum to reduce pressure drop. Air distribution accumulated condensate. piping should be large enough in diameter to minimize • The valve will operate even if little or no pressure drop. A loop system is generally condensate is present, resulting in the loss of recommended, with all piping sloped to accessible drop valuable compressed air. Level-operated and legs and drain points. electrically operated solenoid valves should

have strainers installed to reduce When designing an air distribution system layout, it is contaminants, which block the inlet and best to place the air compressor and its related discharge ports of these automatic devices. accessories where temperature inside the plant is the Motorized ball valves are also used with lowest (but not below freezing). A projection of future programmable timers. However, while fairly demands and tie-ins to the existing distribution system reliable, these valves can be even more wasteful should also be considered. Air leaks are an important as the duration of the valve opening is dependent issue with distribution system and are addressed in the on the valve actuator and is not adjustable. fact sheet in Section 2 titled Compressed Air System 4. Zero air-loss traps with reservoirs. There are Leaks. It is important to note that the majority of various types of zero air-loss traps. system leakage will be at the point of use and not in the • A float or level sensor operates an electric distribution piping. solenoid or ball valve and maintains the condensate level in the reservoir below the Headers should have a slight slope to allow drainage of high-level point. condensate and drop legs from the bottom side of the • A float activates a pneumatic signal to an air header should be provided to allow collection and cylinder to open a ball valve through a linkage drainage of the condensate. The direction of the slope to expel the condensate in the reservoir to the should be away from the compressor. low-level point. Piping from each compressor to the header, and the Be sure to drain and clean the reservoir often to header to points-of-use should connect to the top or prevent the accumulation of contaminants, which could side of the header to avoid being filled with condensate. foul the mechanisms of these traps. Drainage-drop legs from the bottom of the header should be installed to collect the condensate. The potential for freezing must be considered and provision made for heated drains where necessary. The relatively common practice of leaving a manual drain Uses of Compressed Air valve cracked open should not be tolerated because it Industrial facilities use compressed air for a multitude of wastes costly compressed air. operations. Almost every industrial facility has at least two compressors, and in a medium-sized plant there Contaminated condensate requires removal of lubricant may be hundreds of different uses of compressed air. before the condensate is discharged to a sewer system. It is recommended that the local sewage authority be Uses include powering pneumatic tools, packaging and automation equipment, and conveyors. Pneumatic tools

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 15 INTRODUCTION TO COMPRESSED AIR SYSTEMS tend to be smaller, lighter, and more maneuverable Many manufacturing industries also use compressed air than electric motor-driven tools. They also deliver and gas for combustion and process operations such as smooth power and are not damaged by overloading. oxidation, fractionation, cryogenics, refrigeration, Air-powered tools have the capability for infinitely filtration, dehydration, and aeration. Table 1.1 lists variable speed and torque control, and can reach a some major manufacturing industries and the tools, desired speed and torque very quickly. In addition, they conveying, and process operations requiring are often selected for safety reasons because they do compressed air. For some of these applications, not produce sparks and have low heat build-up. however, other sources of power may be more cost Although they have many advantages, pneumatic tools effective (see the fact sheet titled Potentially are much less energy-efficient than electric tools. It Inappropriate Uses of Compressed Air in Section 2). generally costs 7 to 8 times more energy for pneumatic Compressed air also plays a vital role in many non- tools to produce the same mechanical output as electric manufacturing sectors, including the transportation, tools. construction, mining, agriculture, recreation, and service industries. Examples of some of these applications are shown in Table 1.2.

Table 1.1 Industrial Sector Uses of Compressed Air

Industry Example Compressed Air Uses

Apparel Conveying, clamping, tool powering, controls and actuators, automated equipment Automotive Tool powering, stamping, control and actuators, forming, conveying Chemicals Conveying, controls and actuators Food Dehydration, bottling, controls and actuators, conveying, spraying coatings, cleaning, vacuum packing Furniture Air piston powering, tool powering, clamping, spraying, controls and actuators General Clamping, stamping, tool powering and cleaning, control and actuators Manufacturing Lumber and Wood Sawing, hoisting, clamping, pressure treatment, controls and actuators Metals Fabrication Assembly station powering, tool powering, controls and actuators, injection molding, spraying Petroleum Process gas compressing, controls and actuators Primary Metals Vacuum melting, controls and actuators, hoisting Pulp and Paper Conveying, controls and actuators Rubber and Plastics Tool powering, clamping, controls and actuators, forming, mold press powering, injection molding Stone, Clay, Conveying, blending, mixing, controls and actuators, glass blowing and molding, cooling and Glass Textiles Agitating liquids, clamping, conveying, automated equipment, controls and actuators, loom jet weaving, spinning, texturizing

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 16 INTRODUCTION TO COMPRESSED AIR SYSTEMS Table 1.2 Non-Manufacturing Sector Uses of Compressed Air

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY | 17