TYPES OF AIR COMPRESSOR AND WORKING PRINCIPLE WITH DIAGRAM !

RECIPROCATING AIR COMPRESSOR

Reciprocating air compressors are used for producing high pressure air. Atomspheric air is drawn into a cylinder during suction stroke of piston and is compressed by the piston during the return stroke; the piston driven by power from a external source. If air is admitted to one side of the piston being only, the compressor piston and is compressed by the piston during the return stroke; the piston driven by power from a external source. If air is admitted to one side of the piston being only, the compressor is called single acting. A double-acting compressor admits air to each side of the piston alternately. Whilst one face of the piston is performing the suction stroke, air on the other face is compressed.

Use of compressed air delivered by the compressor

(i) To driver a compressed air engine.

(ii) For producing an air blast for work-shop.

(iii) For spraying the fuel into eylinder of the diesel engine.

Air compressors are also used for cooling large buildings. Fresh air is drawn into the compressor and compressed adiabatically, which causes a large increase in temperature. High-pressure air leaving the compressor is cooled by means of cold water. This cool high pressure air is then partially expanded adiabatically in another cylinder, which reduces its temperature to much below the surrounding atmosphere, the cold air is then circulated through the ducts of building.

Sequence of operations represented on the diagram:

(i) Operation 4-1: Volume of air V1 aspirated into the compressor at pressure p1 and temperature T1.

(ii) Operation 1-2 : air compressed according to the law pV^n = C from pressure p1 to p2.

(iii) Operation 2-3 : Compressed air of volume V2 and at pressure p2 with temperature T2 delivered from the compressor.

TWO- STAGE COMPRESSOR

Let compression follow the law, pV^n = constant, and intercooling be incomplete, and so that point d has not reached the isothermal,

Let p1 and V1 represents condition of air entering low pressure cylinder,p2 and V2 represent condition of the air entering high pressure cylinder and p3 be the final pressure of the air.

Total work required to drive the compressor per cycle will be the sum of work required in low pressure and high pressure cylinders.

Types of Air compressor and working principle with Diagram !

MULTISTAGE COMPRESSOR

In multistage compressors, air is compressed in several stages instead of full compression being performed in single cylinder. In principle, it is equivalent to number of compresaors in series, the air passing from one cylinder to the next, the pressure increasing in each cylinder.

●Cycle 8156 is single stage cnnpressor.

●Cycles 8147 und 7456 are tow stage compressor without interenoling between cylinders.

●Cyeles 8147 ond 7236 are that a tow-stage compressor with perfect intercooling between cylinders.

Perfect intercooling

It means that after the initial compression in the L.P. cylinder, with its consequent temperature air is cooled in an intercoolers buck to its original temperatur, i.e. T2= T1, in which case point 2 lies on isothermal through point 1.

Condition for maximum efficiency

If inital pressure p1 and final presaure p3 are fixed, then best value of intermediate pressure p2 thus found will denote at what pressure to exhaust the first stage, so that work done on the compressor will be minimum.

p1, V1 and term n-1/n is a constant

Since for maximum efficiency, pressure ratio in each cycle is same,therefore

p2/p1 = p3/p2

For maximum efficiency, intermediate pressure is geometric mean of the initial and final pressure ;

p2 = √ p1p3

CENTRIFUGAL COMPRESSORS

These in its simplest form consists of a number of curved vanes fitted symetrically. The rotor rotates in an airtight volute casing with inlet and outlet points. The casing for the compressor is so designed that kinetic energy of the air is converted into pressure energy before it leaves the casing.

AXIAL FLOW COMPRESSOR

In axial flow compressors, gus essentially flows parallel to the axis. It consists of a number of rotating blades are fixed on a rotating drum and stator blades are fixed on cusing in alternate rows.

Eech stage consists of one moving row of blades and one row of fixed blades. The enthalpy and pressure of rises as it passes through rotating blades. This happens at the expense of a reduction in relative velocity, the absolute velocity of the gas increaseing along the axis of rotor due to work input. This increase in kinetic energy is partly converted into pressure energy as the air passes through diverging fixed blades. These fixed blades help the gas to reach next set of moving blades for further compression. The blades are generally made of aerofoil section to reduce losses caused by tarbulence and boundary separation.In axial flow compressors, the drum with rotor blades,rotates inside a casing with a fixed or stator blades.

-: Fiza Engineering :-

Writer - Sk Najmul. ( munna )