reciprocating compressor

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Transcript of reciprocating compressor





Introduction to compressorsThe machine which takes in air or any other gas at low pressure and compresses it to high pressure is called compressor. The compressor is power consuming machine in which mechanical work is converted into pressure energy of fluid. They are also considered as reversed heat engine.

Uses of compressed airTo compress the refrigerant and to create pressure difference in system.Operating pneumatic tools.To operate air motor in mines where fire risks are more.To compress air in gas turbine power plant. Spray painting.For sand blasting.Operating blast furnaces.

Classification of compressorsThe compressors are classified as follows:According to design and principle of operation:Positive displacement type compressor: In this type, air/gas is compressed by positive displacement of air by piston or rotating elements.Examples: Reciprocating compressor, Rotary compressor, etc.


Roto dynamic compressors: In this type, compression is carried out by a rotating element imparting velocity to the flowing gas and develops desired pressure, and compression is achieved by dynamic action of rotor.Examples: Centrifugal compressor

Axial flow Mixed flow

According to number of stages:Single stage compressor Low delivery pressure(10 bar)Multi stage compressor High delivery pressure(10 bar)

According to final(delivery) pressure of gasLow pressure compressors : final pressure < 10 barMedium pressure compressors : final pressure between 10 bar to 80 barHigh pressure compressors : final pressure between 80 bar to 1000 bar

According to pressure ratio:Fan : pressure ratio 1.1 and 2.3

According to quantity of air delivered (capacity):Low capacity : volume flow rate300 According to number of cylinders:Single cylinder

Multi cylinder

According to type of fluid to be compressedAir compressorGas compressorVapor compressor

According to action of piston:Single acting compressors: They are the reciprocating compressor which has piston working only in one direction. There is one suction and delivery stroke per revolution of crank.

Double acting compressors: As from its name it uses its both sides to compress the air. These types of compressors have two sets of suction/intake and delivery/exhaust valveon both sides of the piston .As the piston moves up and down, both sides of the piston is utilized in compressing the air. The intake andexhaust valveoperates corresponding to the movement of the piston or with the stroke of thecompressor. The air is compressed accordingly and delivered continuously as compared to single-acting air compressor. Here both of sides are effectively used for compressing the air.

According to methods of cooling:Water cooled air compressorsAir cooled air compressorsAccording to arrangements of cylinders:VerticalHorizontalV type

RadialW typeAccording to types of drives:Motor drivenI.C. engine drivenTurbine driven






Reciprocating compressorReciprocating compressors are widely used for compressing air and it is satisfactory for all ranges of pressures. The number of cylinders may be more than one. In single stage reciprocating compressor pressure ratio is kept 5 to 8, and the speed varies from 100 to 1500 rpm.

Reciprocating compressors are driven by electric motors, or I.C. engine. It is not driven by gas turbines because turbines have high rotational speed. The air/gas coming from reciprocating compressor is not continuous but intermittent. So the air/gas is stored in receiver from where continuous flow of air/gas can be supplied. Reciprocating compressorgenerally seen where there is requirement of high pressure and low flow. Mostly where the air is used for hand-tools, cleaning dust, small paint jobs, commercial uses, etc.

Construction and workingThe single stage, single cylinder, single acting compressors consists of piston, cylinder, cylinder head, connecting rod, crank shaft, flywheel, crank case, water jacket, suction valve and delivery valve as shown in fig.

Thecompressortakes inside successive amount of volume of air from intake valve and confined it in closed surface at that time piston moves downward with the closure of intake valve. Then there iscompressionof air by reducing its volume .Now the piston moves upward and compress the air and then displace the compressed air throughexhaust valve. And then again intake take place and cycle repeat itself.

These types ofcompressoralso called positive displacement machines. They are available in both as lubricated and oil-free.The reciprocatingcompressoris single acting when the compressing is accomplished using only one side of piston and double acting when both the sides of piston used.

Advantages:Relatively Cheap.Easy maintenanceSuitable for high pressure

Disadvantages:Sounds too much. One has to arrange a room for it or put it into isolating box.High outlet temperature of compressed air.High oil content in air piping.

Single stage reciprocating compressor :

Let us consider piston to be at top dead centre (TDC) and move towards bottom dead centre (BDC). Air gets into cylinder during this stroke and is subsequently compressed in next stroke with both inlet valve and exit valve closed. After piston reaching BDC it reverses its motion and compresses the air inducted in previous stroke. Compression is continued till the pressure of air inside becomes sufficient to cause deflection in exit valve.

Due to this piston movement from TDC to BDC suction pressure is created causing opening of inlet valve. With this opening of inlet valve and suction pressure the atmospheric air enters the cylinder. Both inlet valve and exit valves are of plate type and spring loaded so as to operate automatically as and when sufficient pressure difference is available to cause deflection in spring of valve plates to open them23

At the moment when exit valve plate gets lifted the exhaust of compressed air takes place. This piston again reaches TDC from where downward piston movement is again accompanied by suction. This is how reciprocating compressor keeps on working as flow device. Reciprocating compressor described above has suction, compression and discharge as three prominent processes getting completed in two strokes of piston or one revolution of crank shaft.

Work done:

Compression of air in compressor may be carried out following number of thermodynamic processes such as isothermal compression, polytropic compressor or adiabatic compressor. Fig. shows the thermodynamic cycle involved in compressor. Theoretical cycle is shown neglecting clearance volume but in actual cycle clearance volume cannot be negligible. Clearance volume is necessary in order to prevent collision of piston with cylinder head, accommodating valve mechanism etc.

The compressor is work consuming machine, hence work done is negative.Let, pressure of air at inlet of compressor pressure of air at outlet of compressor volume of air before compression volume of air after compression absolute temperature of air before compression absolute temperature of air after compression

Isentropic Compression


polytropic compression

Isothermal compression

If compression process is isothermal, pV = C, temperature remains constant, the change of internal energy during compression process is zero. Thus as per law of conservation of energy the entire work of compression is related to the cooling medium (surroundings). It is clear from for p-V, the area 1-2-3-4 is less than area 1-2-3-4 and area 1-2-3-4. It means that in isothermal compression no energy is wasted in heating the air or increasing the internal energy. It has been found that work required for compression is minimum when the process is isothermal.

Methods of cooling:Faster heat dissipation from inside of compressor to outside by use of fins over cylinder. Fins facilitate quick heat transfer from air being compressed to atmosphere so that temperature rises during compression can be minimized. Water jacket may be provided around compressor cylinder so that heat can be picked by cooling water circulating through water jacket. Cooling water circulation around compressor regulates rise in temperature to great extent.

The isothermal efficiency of a compressor should be close to 100% which means that actual compression should occur following a process close to isothermal process. For this the mechanism is derived to maintain constant temperature during compression process.


The water may also be injected at the end of compression process in order to cool the air being compressed. This water injection near the end of compression process requires special arrangement in compressor and also the air gets mixed with water and needs to be separated out before being used. In case of multistage compression in different compressors operating serially, the air leaving one compressor may be cooled up to ambient state or somewhat high temperature before being injected into subsequent compressor. This cooling of fluid being compressed between two consecutive compressors is called intercooling and is frequently used in case of multistage compressors.

Single stage reciprocating compressor with clearance:

With clearance volume the cycle is represented on Fig. The work done for compre