## DYNAMIC COMPRESSORS – RATIO OF COMPRESSION

While the difference between suction and discharge pressures denotes the work done on a gas system, the ratio of absolute discharge pressure to absolute suction pressure is known as compression ratio. When a gas is compressed, part of the energy input or work done is converted into heat and friction losses. The ratio of compression, R, is the relation between the absolute discharge pressure and the absolute suction pressure. If P2 is the discharge pressure and P 1 is the suction pressure, then the compression ratio R = P 2/P1. This means the compression ratio denotes how many times the discharge pressure is greater than the suction pressure. In determining compression ratio, only absolute pressures must be used. To get absolute pressure, add the atmospheric pressure to gage pressure.

For example, compressor discharge pressure = 300 psig: Absolute discharge pressure = 300 + 14.7 = 314.7 psia.

Example:

Air is compressed to 100 psig using an air compressor. What is the com-

pression ratio of the compressor?

Air enters at atmospheric pressure. Therefore, P1 = 14.7 psia

Discharge pressure = 100 psig = 100 + 14.7 = 114.7 psia

Therefore, compression ratio R = 114.7/14.7 = 7.81

Example (metric units):

Air is compressed to 6 kg/cm2g using an air compressor. What is the com-

pression ratio of the compressor?

Air enters at atmospheric pressure. Therefore, P1 = 1.033 kg/cm2a

Discharge pressure = 6 kg/cm2 g. Absolute pressure = 6 + 1.033 = 7.033

Compression ratio R = 7.033/1.033 = 6.81

Note how the pressures have been converted into absolute pressures.