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SECTION 1: INTRODUCTION TO COMPRESSED AIR SYSTEMSThis section of the sourcebook is intended for readerswho want to gain an understanding of the basics ofindustrial compressed air systems. A glossary of basicterminology is included in Appendix A for usersunfamiliar with the terms used in this chapter.Compressed air is widely used throughoutmanufacturing industries and is often considered the“fourth utility” at many facilities. Almost everyindustrial plant, from a small machine shop to animmense pulp and paper mill, has some type ofcompressed air system. In many cases, the compressedair system is so vital that the facility cannot operatewithout it. Plant air compressor systems can vary in sizefrom a small unit of 5 horsepower (hp) to huge systemswith more than 50,000 hp.In many industrial facilities, air compressors use moreelectricity than any other type of equipment.Inefficiencies in compressed air systems can besignificant. Energy savings from system improvementscan be substantial, resulting in thousands, or evenhundreds of thousands of dollars of potential annualsavings, depending on use. A properly managedcompressed air system can save energy, reducemaintenance, decrease downtime, increase productionthroughput, and improve product qualityCompressed air systems consist of a supply side and ademand side. The supply side includes compressors andair treatment (dryers and filters). The demand sideincludes distribution and end-use equipment. Aproperly managed supply side will result in clean,appropriately dried, stable air being delivered at theappropriate pressure in a dependable, cost-effectivemanner. A properly managed demand side minimizeswasted air and uses compressed air for appropriateapplications. Improving and maintaining peakcompressed air system performance requiresaddressing both the supply and demand sides of thesystem and how the two interact.Components of an IndustrialCompressed Air SystemA compressor is a machine that is used to increase thepressure of a gas. The earliest compressors werebellows, used by blacksmiths to intensify the heat intheir furnaces. The first industrial compressors weresimple, reciprocating piston-driven machines poweredby a water wheel.A modern industrial compressed air system is composedof several major sub-systems. Major sub-systemsinclude the compressor, prime mover, controls,treatment equipment and accessories, and thedistribution system. The compressor is the mechanicaldevice that takes in ambient air and increases itspressure. The prime mover powers the compressor.Controls serve to regulate the amount of compressedair being produced. The treatment equipment removescontaminants from the compressed air, and accessorieskeep the system operating properly. Distributionsystems are analogous to wiring in the electricalworld—they transport compressed air to where it isneeded. Compressed air storage can also serve toimprove system performance and efficiency. Figure 1.1shows a representative industrial compressed airsystem and its components.Compressor TypesMany modern industrial air compressors are sold“packaged” with the compressor, drive motor, andmany of the accessories mounted on a frame for ease ofinstallation. Provision for movement by forklift iscommon. Larger packages may require the use of anoverhead crane. An enclosure may be included forsound attenuation and aesthetics.IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY 3

INTRODUCTION TO COMPRESSED AIR SYSTEMSCompressorPackageEnclosureDryerAir InletFilterAir nd LubricantCoolerMotorControlPanelCompressorAir EndLubricant/AirSeparatorCondensateDrainFilter, RegulatorLubricatorFigure 1.1 Components of a Typical Industrial Compressed Air SystemIMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY 4

INTRODUCTION TO COMPRESSED AIR SYSTEMSAs shown in Figure 1.2, there are two basic compressortypes: positive-displacement and dynamic. In thepositive-displacement type, a given quantity of air orgas is trapped in a compression chamber and thevolume which it occupies is mechanically reduced,causing a corresponding rise in pressure prior todischarge. At constant speed, the air flow remainsessentially constant with variations in dischargepressure. Dynamic compressors impart velocity energyto continuously flowing air or gas by means of impellersrotating at very high speeds. The velocity energy ischanged into pressure energy both by the impellers andthe discharge volutes or diffusers. In the centrifugaltype dynamic compressors, the shape of the impellerblades determines the relationship between air flowand the pressure (or head) generated.Positive Displacement CompressorsThese compressors are available in two types:reciprocating and rotary. Reciprocating compressorswork like bicycle pumps with the exception that anelectric motor replaces the human energy source. Apiston, driven through a crankshaft and connecting rodby an electric motor, reduces the volume in the cylinderoccupied by the air or gas, compressing it to a higherpressure. Single- acting compressors have acompression stroke in only one direction, while doubleacting units provide a compression stroke as the pistonmoves in each direction. Large, industrial reciprocatingair compressors are double-acting and water-cooled.Multi-stage, double- acting compressors are the mostefficient compressors available, and are typically larger,noisier, and more costly than comparable rotary units.Reciprocating compressors are available in sizes fromless than 1 hp to more than 600 hp.Rotary compressors have gained popularity and arenow the “workhorse” of American industry. They aremost commonly used in sizes from about 10 to 500 hp.The most common type of rotary compressor is thehelical-twin, screw-type (also known as rotary screw orhelical-lobe). Male and female rotors mesh, trappingair, and reducing the volume of the air along the rotorsto the air discharge point. Rotary screw compressorshave low initial cost, compact size, low weight, and areeasy to maintain. Rotary screw compressors may be airor water-cooled. Less common rotary compressorsinclude sliding-vane, liquid-ring, and scroll-type.Single-Acting, Reciprocating Air CompressorsThis type of compressor is characterized by its“automotive” type piston driven through a connectingrod from the crankshaft. Compression takes place onthe top side of the piston on each revolution of thecrankshaft. Single-acting, reciprocating air compressorsare generally air-cooled. These may be single- stage,usually rated at discharge pressures from 25 to 125pounds per square inch gauge (psig), or two-stage,usually rated at discharge pressures from 125 psig to175 psig or higher.The most common air compressor in the fractional andsingle-digit hp sizes is the air-cooled, reciprocating aircompressor. Single-acting reciprocating compressorsabove 30 hp are much less common. Two-stage andmulti-stage designs include inter-stage air-cooling toreduce air temperatures to the second stage forimproved efficiency and durability.Pistons used in single-acting compressors are of the“automotive” or “full skirt” design, the underside of thepiston being exposed to the crankcase. Lubricatedversions have a combination of compression andlubricant-control piston rings, which seal thecompression chamber, control the lubricant to thecompression chamber, and act (in some designs) assupport for piston movement on the cylinder walls.Lubricant-free, or non-lube designs, do not allowlubricant in the compression chamber and use pistonsof self-lubricating materials or use heat resistant, nonmetallic guides and piston rings that are selflubricating. Some designs incorporate a distance pieceor crosshead to isolate the crankcase from thecompression chamber.Lubricant-less (or oil-less) designs have pistonarrangements similar to lubricant-free versions, but donot have lubricant in the crankcase. Generally thesehave greased pre-packed crankshafts and connectingrod bearings.IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY 5

INTRODUCTION TO COMPRESSED AIR SYSTEMSFigure 1.2 Compressor Family TreeCooling. Single-acting air compressors have differentarrangements for removing the heat of compression.Air-cooled versions have external finning for heatdissipation on the cylinder, cylinder head, and in somecases, the external heat exchanger. Air is drawn orblown across the fins and the compressor crankcase bya fan, which may be the spokes of the drivepulley/flywheel.Liquid-cooled compressors have jacketed cylinders,heads and heat exchangers, through which liquidcoolant is circulated to dissipate the heat ofcompression. Water, or an ethylene glycol mixture toprevent freezing, may be employed.Drives. The most common drive arrangement is a beltdrive from an electric motor. The compressor sheavealso acts as a flywheel to limit torque pulsations and itsspokes often are used for cooling air circulation. Beltdrives allow a great degree of flexibility in obtaining thedesired speed of rotation.Flange-mounted, or direct-coupled motor drivesprovide compactness and minimum drive maintenance.Belts and couplings must be properly shielded for safetyand to meet Occupational Safety & HealthAdministration (OSHA) requirements.Double-Acting, Reciprocating Air CompressorsDouble-acting reciprocating compressors use both sidesof the piston for air compression, almost doubling thecapacity for a given cylinder size. A piston rod isattached to the piston at one end and to a crosshead atthe other end. The crosshead ensures that the pistontravels concentrically within the cylinder. Thesecompressors may be single- or multi-stage, dependingon discharge pressure and hp size. These are generallyavailable in larger horsepower sizes.Cooling. Double-acting air compressors generally havecooling water jackets around the cylinder body and inthe cylinder head. This, combined with their relativelyslow speed of operation and water-cooled intercooling,results in excellent compression efficiency.Lubrication. Cylinder lubrication is generally by meansof a pressure or vacuum-feed cylinder lubricator, with afeed rate of several drops per minute, depending oncylinder size and piston speed and as specified by themanufacturer. Lubricant-free versions also are availablewith polytetrafluorethylene (PTFE) or similar materialsfor pistons, riders, and compression rings. A distancepiece is provided between the crankcase and thecylinder(s) to ensure that no part of the piston rod,IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY 6

INTRODUCTION TO COMPRESSED AIR SYSTEMSwhich enters the lubricated crankcase, can enter thelubricant-free cylinder area.Balance. Single- and two-cylinder compressors of thistype generally require a substantial foundation due tounbalanced reciprocating forces.Drives. Below 200 hp, belt drives and flange-mountedinduction motors are normally used. For motors largerthan 300 hp, flange-mounted, synchronous motors aresometimes used with a 1.0 power factor or 0.8 leadingpower factor to provide power factor correction to offset other induction-type electrical loads.Lubricant-Injected Rotary Screw CompressorsThe lubricant-injected rotary screw compressorpowered by an electric motor has become a dominanttype of industrial compressor for a wide variety ofapplications.Compression Principle. The lubricant-injected, rotaryscrew compressor consists of two intermeshing rotorsin a machined housing (Rotor Housing) having an inletport at one end and a discharge port at the other. Themale rotor has lobes formed helically along its lengthwhile the female rotor has corresponding helicalgrooves or flutes. The number of helical lobes andgrooves may vary in otherwise similar designs.Air flowing in through the inlet port fills the spacesbetween the lobes on each rotor. Rotation then causesthe air to be trapped between the lobes and the rotorhousing as the inter-lobe spaces pass beyond the inletport. As rotation continues, a lobe on one rotor rollsinto a groove on the other rotor and the point ofintermeshing moves progressively along the axial lengthof the rotors. This reduces the space occupied by theair, resulting in increased pressure. Compressioncontinues until the inter-lobe spaces are exposed to thedischarge port when the compressed air is discharged.Lubricant is injected into the compression chamberduring compression and serves three basic functions:1) lubrication for the intermeshing rotors andassociated bearings; 2) removal of the heat caused bycompression; and 3) seals the clearances between themeshing rotors and between rotors and rotor housing.Lubrication. The generic term “lubricant” has beenused instead of oil. The lubricant may be a hydrocarbonproduct, but most compressors now use cleaner andlonger life synthetic lubricants, including diesters,polyglycols, polyalphaolefins, polyol esters, and siliconebased lubricants. These newer products are suitable fora wider range of temperatures.A mixture of compressed air and injected lubricantleaves the air end and is passed to a sump/separatorwhere the lubricant is removed from the co

IMPROVING COMPRESSED AIR SYSTEM PERFORMANCE: A SOURCEBOOK FOR INDUSTRY . ACKNOWLEDGEMENTS . Improving Compressed Air System Performance: A Sourcebook for Industry is a cooperative effort of the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) Advanced Manufacturing Office and the Compressed Air Challenge .