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Chapter: Mechanical and Electrical - Thermal Engineering - Air Compressors

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Rotary vane compressors and Multistage compression

Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor.

ROTARY VANE COMPRESSORS

 

Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor.

Ø The rotor is mounted offset in a larger housing that is either circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades.


 

MULTISTAGE COMPRESSION:

 

Multistage compression refers to the compression process completed in more than one stage i.e., a part of compression occurs in one cylinder and subsequently compressed air is sent to subsequent cylinders for further compression. In case it is desired to increase the compression ratio of compressor then multi-stage compression becomes inevitable. If we look at the expression for volumetric efficiency then it shows that the volumetric efficiency decreases with increase in pressure ratio. This aspect can also be explained using p-V representation shown in Figure.


A multi-stage compressor is one in which there are several cylinders of different diameters. The intake of air in the first stage gets compressed and then it is passed over a cooler to achieve a temperature very close to ambient air. This cooled air is passed to the intermediate stage where it is again getting compressed and heated. This air is again passed over a cooler to achieve a temperature as close to ambient as possible. Then this compressed air is passed to the final or the third stage of the air compressor where it is compressed to the required pressure and delivered to the air receiver after cooling sufficiently in an after-cooler.

 

Advantages of Multi-stage compression:

 

The work done in compressing the air is reduced, thus power can be saved

 

2. Prevents mechanical problems as the air temperature is controlled

 

3. The suction and delivery valves remain in cleaner condition as the temperature and vaporization of lubricating oil is less

 

4.    The machine is smaller and better balanced

 

5. Effects from moisture can be handled better, by draining at each stage

 

6.         Compression approaches near isothermal

 

7.  Compression ratio at each stage is lower when compared to a single-stage machine


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