Reciprocating Air Compressor

Reciprocating Air Compressor

Prepared byProf. Jagdish S. Talpada

151903 - Fluid Power Engineering

prepared by Prof. Jagdish S. Talpada

Lecture - 1


Reciprocating/Positive Displacement Compressors

Gas compression has been one of the anchor points of the industrial revolution, beginning with low pressure air supply for iron and steel refining, through higherpressure air supply for drilling and plant operating equipment, to high pressures asrequired for chemical synthesis, storage and pipeline deliveries of fuel gases. Thepositive displacement compressors in use today can trace their ancestry back tothe original pumping machines invented by James Watts, or the bellows andblowers of blacksmiths.Piston type compressors have a solid position in this field: the technology ismature (more than a century of development), the fabrication process is straightforward, and the equipment is extremely scalable, ranging from miniatureemergency tire inflation pumps to compressors of 10,000 horsepower or more.These latter are particularly used in the chemical process and gas transmissionindustries. There the requirements for high reliability, extreme range inthroughput volume, and flexibility in operating pressures make an excellent fit forreciprocating piston compressors. This module describes the operatingcharacteristics of various positive displacement compressors and develops thetheory, basic calculations and rudiments of control for the piston typereciprocating compression process. While some references are to the gascompression and transmission industry, the same equipment construction andcontrol methods are used in process compressors for industries such aspetrochemicals and chemical synthesis.


PartsPiston (Reciprocating)The reciprocating piston compressor is the most widely used equipment for gasservice. The basic design consists of a piston in a cylinder with pressure actuatedcheck valves to control suction and discharge flow through the cylinder. Standardpractice is to have the piston driven by a rod passing through a packing case toseal against pressure leaks. With this double acting design, gas can becompressed on both sides of the piston. The basic design is more than a hundredyears old, and is well developed. The throughput and loading can be adjusted byspeed variation, addition of clearance to the cylinders, deactivating cylinders toreduce displacement or active control of valve closing, which effectively givesvariable control of displacement. Efficiencies of this type of compressor can bemore than 85 percent for conversion of horsepower input to pressure rise.VaneA vane compressor consists of a cylindrical chamber with a rotating paddle wheeltype drum mounted off center in the chamber. As the drum rotates, the slidingpaddle wheel vanes section off volumes, which decrease in volume as they movetoward discharge. A suction port is machined into the area where the chambershave the highest volume, and a discharge port is located where the chambers havethe smallest volume. Gas enters at the higher volume and is compressed anddischarged at the minimum volume. This type of compressor will tolerate moredirt than a reciprocating unit, and is often used for natural gas production services.The maximum differential is limited by the strength of the paddle wheel seals, sothese units are not applicable for high pressures and differentials.


Blower (Rotary)In this compressor, two intermeshing elements rotate in an ellipsoidal chamberwith intake and exhaust ports on opposite sides. As they rotate, gas is trapped inspaces formed between the chamber and moved to the opposite side of thechamber, where it is delivered to the discharge. This action is similar to the vanecompressor, but is even more tolerant of liquids and dirt. For high pressure ratios,oil may be injected into the suction to improve the seal of the rotors and removesome of the heat of compression.Screw(Rotary)The operation of a screw compressor is similar to the blower, except that thecompression chambers are formed between two intermeshed elements similar toworm gears or screw threads. This compressor also requires oil injection forsealing and cooling. It is designed for high pressure ratios but is usually limitedto discharge pressures below 250 Psig.


Cylinder and EndsThe compressor cylinder is a casting or forging designed to safely contain somemaximum working pressure. It is machined to hold compressor valves and todirect gas flow to and from the cylinder cavity. In combination with the cylinderends, it must contain the gas pressure, while having sufficiently large gas flowpassages so there are minimal pressure drops due to gas flow. The cylinder andends may also have water passages to stabilize temperature and dimensionalchanges. All these requirements involve compromises between size, strength, andflow passage size (efficiency). Compressor cylinders are designed for someoperating range and service. If conditions change, they may not perform reliablyor efficiently. As an example, a cylinder for gas transmission has large flowpassages and valve areas for efficiency at high gas volumes and low pressureratios, and will not function at high ratios. Similarly, a process cylinder may be aforging with small passages, giving higher strength but low efficiency.Piston/RingsThe compressor piston converts the energy/work supplied by the engine, applyingit to the gas to raise its pressure. The piston must be strong enough to withstandthe pressures and forces applied, but still be as light as possible, to minimizereciprocating weights and the resulting shaking forces. The compressor rings sealgas pressure to avoid leaking from one side of the piston to the other. The pistonmay also be fitted with a rider band, which is a low friction material to keep themetal piston from contacting the bore of the cylinder and causing scuffing andwear. Material for the rings and rider bands is selected to give long life andminimal wear with the typical pressures and gas composition of the compressor.While this is generally a low friction thermoplastic type material, rings may bemade of bronze or other materials when temperatures are a problem.


ValvesCompressor valves are simply fast acting check valves with a low pressure drop.They must be optimized to balance the opposing demands for long operating lifeand minimal pressure drop/flow losses. They may also have special features suchas center ports to allow cylinder unloading.The compressor valve is possibly the most critical component when determiningthe requirements for a compressor service. The flow area is sensitive, as too smallan area will give low efficiency, but too large an area can result in valve flutterand early failure. Similarly, valve components must be designed for the expectedpressure and temperature conditions.Valves have been designed with many configurations, particularly in the sealingelements. These have progressed through steel, Bakelite, glass filled Teflon orNylon, and high strength plastics. The most popular designs for sealing elementsare ring shaped strips, mushroom shaped poppets, and straight channel strips. The design of compressor valves includes a number of variations to accommodatecylinder flow and unloading requirements. The simplest is a single deck valve,shown on the left above, where gas flows into passages in one face, across thesealing elements, and out through passages in the back face of the valve.A modification of this design is to have a large opening in the center of the valve.This allows adding a cylinder deactivator or clearance volume to the cylinder.This added feature comes at the expense of reduced flow area and efficiency. Tocompensate for this, two valves may be assembled together with a flow passagethrough the center. This double deck valve design has improved flow area andefficiency. This type of valve can only be used in a cylinder designed to acceptits increased height.


PackingThe compressor packing is a series of pressure containing rings located in thecrank end of a double acting compressor cylinder. These seal against the pistonrod and prevent leakage, so that the cylinder can compress gas on both sides ofthe piston. Again, as with compressor rings, the packing material is selected toprovide best life and sealing with expected conditions. The packing is generallypressure lubricated, and may have coolant flow to remove friction heat. There arealso various specialty types to reduce gas leakage around the rod. This may beimportant when compressing highly flammable or toxic gases. It is alsobecoming more important in reducing gas leakage and emission of “greenhousegases”.Clearance UnloadersIn many applications, the volume of gas to be delivered may change based oneither gas supply or process demands. Also, varying pressure conditions canchange the load on the driver, requiring load control. This may be accomplishedby speed variation, deactivating cylinders or cylinder ends, or by varying cylinderclearance. This last option is highly preferred, as speed control may have alimited range, and deactivating cylinders or ends can cause mechanical shaking oracoustic pulsations. Clearance unloaders allow varying throughput and load withminimal loss of efficiency. Unloaders are not actually a part of a compressor, butare included on many installations, to give load and throughput control. This maybe done by volumes cast into the cylinder or heads, with a valve to close thepassageway. Other options are valve cap pockets and head end variable pockets.Added clearance may have a simple handwheel to control its operation, or mayhave pneumatic actuators, which allow automatic operation.


Distance Piece Compartment(s)A distance piece section may be placed between the crosshead and cylinder

toprevent leakage of gas from the compressor packing entering the compressorcrankcase. At the crosshead end, an oil seal around the compressor rod

preventsoil from migrating to the cylinder, and gas from entering the crankcase. Thisdistance piece is normally vented to remove any gas which leaks from thepacking. In cases of explosive or toxic gases there may be two distance pieces

inseries, to assure containment of the gases.


Lecture- - 2


Definition of TermsSingle and Double Acting CompressorA Single Acting piston compresses gas on only one face, either by designor by deactivating valves on one side of a double acting cylinderDouble Acting – Piston compresses gas alternately on both faces.Connecting RodA compressor element connecting the crankshaft to the compressor pistonor crosshead. The connecting rod converts the rotation of the crankshaftinto linear motion to drive the compressor piston.CrossheadA crosshead is a sliding component at the outer end of the connecting rod,which converts the eccentric motion of the connecting rod to pure linear,eliminating side forces on the compressor piston.


Wrist Pin/Crosshead PinThe wrist or crosshead pin connects the outer end of a connecting rod toeither a single acting, trunk type piston (wrist pin) or to a crosshead(crosshead pin)Compressor Rod/Piston RodA cylindrical rod which connects the compressor piston to a crosshead,normally passing through a packing case to seal compression pressure intothe cylinderCompressor PistonA reciprocating component, normally fitted with piston rings whichchanges the volume of a cylinder, providing compression. It may be asimple trunk type piston directly connected to the connecting rod, ordouble acting, driven by a compressor rod.Compressor RingsCompressor rings provide a seal between the compressor piston andcylinder wall, preventing gas leakage either into or out of the cylindervolume.Rider Rings and Rider BandsRider rings or bands are normally provided on a double acting piston toprevent contact of the piston with the cylinder wall. Rider rings/bands arenormally made of carbon filled Teflon or other low friction material.


Compressor PackingCompressor packing is used in a double acting cylinder to seal around thecompressor rod, preventing gas leakage from the cylinder. Packing isnormally a series of segmented metallic rings, assembled and held in theend of the cylinder by the packing case.Compressor ValvesCompressor valves are high speed check valves, controlling flow of gasinto the cylinder (suction valve) or out of the cylinder (discharge valve).They are designed for minimal pressure loss and maximum reliabilityCylinder Clearance (Mechanical)Clearance must be provided at the end of the piston stroke to avoid contactbetween the piston face and the compressor cylinder head. This clearanceis expressed in linear measurement (inches or mm.).Cylinder Clearance (Volume)Volumetric clearance is space left at the end of a piston stroke, both due tomechanical clearance and volumes above suction and discharge valves toallow for good gas flow. Clearance may also be added for control ofthroughput volume and/or load control (unloaders or clearance pockets).


Compression RatioCompression ratio is the measure of increase in pressure across acompressor cylinder. It is determined by dividing the discharge pressureby suction pressure (both pressures must be absolute rather than

gauge)Pressure – Absolute and GaugeGauge pressure is the value which would be measured by a gaugecalibrated to indicate zero pressure when exposed to atmosphere.Absolute pressure is pressure which would be read from a gaugecalibrated to read zero when exposed to complete vacuum. Normallyabsolute pressure is gauge pressure + 14.73 PSI.


Lecture - 3


Cycle EventsIn a reciprocating compressor, the process follows four main events –compression, discharge, re-expansion and intake. The first two are accomplishedas the piston moves forward, reducing cylinder volume, while the second takesplace as the piston moves back down the cylinder.For a more complete picture, assume starting the cycle with the compressor at thebottom of its stroke, with maximum cylinder volume. The cylinder is full of gasat suction pressure, and both suction and discharge valves are closed by gaspressure. As the piston moves forward, the cylinder volume decreases andpressure rises. When the cylinder pressure rises slightly above dischargepressure, the discharge valve opens and gas is pushed into the discharge piping forthe rest of the stroke. At top center, the discharge valve closes. As there must beclearance between the piston face and cylinder head to prevent parts hitting eachother, some volume of gas is trapped in the cylinder at discharge pressure. As thepiston moves back down the cylinder, this gas re-expands until it reaches suctionpressure. At this point, the suction valve opens and a fresh charge of gas flows


Volumetric EfficiencyAs noted above, the cylinder does not bring gas in through the entire piston travel.The percentage of stroke the suction valve is open, compared to the entire strokeis called “volumetric efficiency”. If there were no clearance (volume) left whenthe piston completed its compression stroke, then cylinder pressure wouldimmediately drop to suction pressure as the piston returned, giving 100 percentvolumetric efficiency.Thus, the cylinder displacement would be equal to the volume delivered with eachstroke. However, due to gas re-expansion, the suction valve opening is delayed.This delay becomes greater when the cylinder pressure ratio increases or theclearance volume increases. Thus, the cylinder delivers a reduced volume to thedischarge condition.The pictures below illustrate this effect, with the picture on left showing effect ofincreasing clearance, and on right the effect of increasing pressure ratio. At highpressure ratios, or with large amounts of clearance, the valve opening may bedelayed to the point that the valve does not open, and no gas flows through thecylinder. This condition is called zero volumetric efficiency, and can causeserious cylinder heating problems.In normal operation, friction of rings on the cylinder creates heat which is carriedaway with the gas being compressed. Since at zero volumetric efficiency, no gasis entering or leaving the cylinder, all friction heating effects are contained withinthe cylinder, causing an uncontrolled temperature rise. As the hot gas iscontained within the cylinder, normal temperature detection in the discharge linewill not be effective.


Clearance ControlAs noted above, cylinder clearance will significantly affect throughput andhorsepower of a compressor. Some amount of volumetric clearance is built intothe cylinder to prevent the compressor cylinder from contacting the heads at theextremes of piston travel, and to provide a smooth gas flow path into and out ofthe cylinder.Beyond this, additional clearance can be introduced by providing clearancepockets or passages which open into the cylinder cavity. These have valveswhich can be opened or closed to add or remove the clearance from thecompression process. Also, some cylinders may be equipped with a variableclearance pocket on the outboard cylinder head. These have a piston positionedby a screw and hand wheel, which will add a variable amount of clearance.Work of CycleThe familiar definition of work is force times distance. In the pressure-volumecards shown above, piston movement or change in volume defines a distance. Asthe force against the piston changes as pressure increases and decreases, the areaof the card defines the work involved in the cycle.A key point to note is that for a given pressure differential, changing thevolumetric efficiency changes both the volume delivered and the work of thecycle. This is the basis for load control of compressors by changing the cylinderclearance.


Pressure RatioPressure ratio is the discharge pressure of the compressor divided by the suctionpressure. These pressures must be in absolute (Psia) rather than gauge (Psig)pressure. As most operating gauges read in Psig, atmospheric pressure must beadded. This is normally about 14 Psi.A reciprocating compressor may be able to operate at high pressure ratios, but isusually limited by other conditions, particularly temperature. A compressor’sdischarge temperature increases with pressure ratio. For example, at a pressureratio of four and a suction temperature of 60 degrees, discharge temperaturewould be about 310 degrees. This is a safe practical limit for most compressorcomponents. Consequently, pressure ratios across any single compressorcylinder rarely are allowed to exceed four to one.Temperature Rise – Ratio EffectWhen a gas is compressed, its temperature rises in proportion to the pressureratio. For low pressure ratios, the discharge temperature may be only twenty to


fifty degrees higher than suction temperature. When the pressure ratio is high,such as on storage or production service, the discharge temperature may be morethan a hundred degrees higher than the suction.This is true for all types of compressors. This temperature rise may limit theamount of pressure rise allowable across a compressor, or require specialcomponents to withstand the temperature. This temperature must be reducedbefore gas is put into underground pipelines, to prevent melting their protectivecoatings.In most cases, the discharge temperature from a compressor station must be keptbelow 1250F, requiring gas coolers at higher pressure ratios. This is particularlythe case at storage and production stations, where high pressure ratios giveextreme discharge temperatures.


Lecture - 4


Multi-StagingSharing DifferentialThe limits of operation listed above show that a reciprocating compressor has anumber of mechanical limits, most of which are related to pressure differential.Often differentials are required greater than can be accomplished with a singlestage of compression. In this case, it is necessary to have multiple stages ofcompression.This is accomplished by having a cylinder or cylinders which take gas in at a lowpressure, compress and discharge to an intermediate pressure, then repeat withadditional cylinders to take the gas to the discharge pressure. In this process,pressure differential and temperature rise across each cylinder can be controlled toa reasonable level. The gas may be cooled between stages to minimize dischargetemperatures. Normally this is done with two or more cylinders on the samecompressor unit, with gas cooling between stages.


Efficiency IncreaseWhen gas is compressed, the temperature rise effectively creates higher volume atthe discharge conditions. This requires more energy (work) for compression. Inmultiple stage compression with cooling, the temperature rise is minimized,which reduces the total work required to compress to the final discharge.Operating DifficultiesMultiple stage compression presents challenges for both design and operation. Atthe design stage, cylinders must be sized so that all stages are operating withintheir limits. In operation, the pressure balance between stages must be maintainedby following a specified unloading sequence when pressures change, or whencontrolling engine load.Mechanical failures such as leaking compressor valves or rings can cause pressureunbalance, which may put excessive differentials or temperatures on other stages.The compressor piping and pulsation bottles will also be more complex, whichwill probably require an electric analog or digital evaluation to avoid pulsation orvibration problems.

Reciprocating Air Compressor

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