Prepared By : Nauman Khan

09PWMEC2835Department Of

Mechanical Engineering

UET Peshawar

Introduction of Air compressors Types of Air compressors Single acting and Multiple acting

compressors P-V diagrams of Reciprocating compressor

( single and double) Equations related to Reciprocating

compressor Applications of the Air compressor

Air Compressors are machines that compress air or gas.

Compression is achieved through the reduction of the volume that the gas (or air) occupies. As a side effect of the minimization of volume, the temperature of air or gas increases.

Atmospheric air is drawn into a cylinder during the suction stroke of the piston and is compressed by the piston during the return stroke , the piston being driven by power from an external source.

1) Positive Displacement (PD) : It works by trapping a specific volume of gas and forcing it into a smaller volume

Two types of Positive Displacement are Reciprocating compressorRotary compressor

2) Centrifugal : Operate by accelerating the gas and converting the kinetic energy to pressure

Two types of centrifugal areCentrifugalAxial

Reciprocating compressors are equipped with a crankshaft, which drives the pistons.

They are commonly found in versions that produce 5 to 30 HP. However, larger ones used for industrial purposes can produce up to 1,000 HP.

It may be single acting or double acting reciprocating compressor.

Reciprocating compressors are often used with air reservoirs to provide compressed air for industrial and civil duties driving air tools etc.

If air is admitted to one side of the piston only then it is single acting compressor.

It has single cylinder . One revolution gives you single discharge of

air.

A double acting compressor admits air to each side of the piston alternately.

In one revolution it gives double discharge of air.

During the suction stroke the air at the other end of piston is get compressed.

Operated by controlled valves.

• The figure shows a hypothetical indicator diagram for a single stage -single acting reciprocating compressor.

• a -1... Air is drawn into the cylinder on the suction stroke

1 -2... The suction valve is closed and air is compressed according to the law Pvn = c

2 -b... The delivery valve opens and air is delivered under pressure

b -a... The delivery valve closes and the suction valve opens

In multistage compressor there are multi compressors

i.e. LPC and HPC. Low pressure is produced in LPC and for

high pressure it is sent to the HPC. Intercooler are present between the

multistage compressor to low the temperature of air

A practical single stage compressor cylinder will have a small clearance at the end of the stroke.  

This clearance will have a significant effect on the work done per cycle.

If Effective stroke = Actual stroke then efficiency =100%

In other words, no clearance volume exists. This is practically impossible

because some clearance space is required otherwise piston hits on the cylinder head as it travels. Also expansion of piston occurs as it travels and very little clearance may cause the same problem.

Also increased clearance space reduces compressor efficiency and increase its running hours. So clearance volume must be kept around a reasonable value as instructed by the manufacturer

Reciprocating compressors are generally cooled with air or water.

In water cooled compressors, freshwater is circulated through jackets that are built into the walls of the cylinders and cylinder heads.

Cooling is done to compress the air isothermally in order to save the work done.

Inter cooler are used for cooling purposes.

For single acting reciprocating compressor

or W= (n/n-1)*P1v1{(P2/P1)n-1/n -1}

For 2 stage reciprocating compressor W= (n/n-1)*P1v1{(P2/P1)n-1/n +(P3/P2)n-1/n -2}

The conditions for minimum work has been found which is

P2 = (P1 P3)1/2

For minimum work, conclusion is Inter cooling should be completed. Pressure ratio in each stage should be same. Intermediate pressure is the geometric mean

of suction pressure and delivery pressure. Work done in each stage is same.

Actually there is clearance volume in the cylinder of the compressor so due to this clearance volume the work equation become

W= (n/n-1)*(Va-Vd) P1{(P2/P1)n-1/n -1}

Where (Va-Vd)= induced volume

The volumetric efficiency become ηv= 1-vc/vs{(P2/P1) 1/n -1}