Abu Dhabi Gas Liquefaction Company Ltd Job Training Mechanical Technician Course Module 9 Compressors

2.3 Compression 
Ratio 
The compression ratio of a compressor tells us how much the pressure of a gas 
increase as it travels from the suction to the discharge of the compressor. 
2.4 Capacity 
The capacity of a compressor is the volume flow rate of gas through the compressor. 
This is similar to pump capacity, which is volume flow rate of liquid through a pump.
Pump capacity has the SI units of litres/second (l/s)
Compressor capacity is usually given in the SI units of metres cubed/hour (m
3
/h).
1m
3
= 1000litre 
Because gas volume changes as its pressure and temperature change, the volume flow 
rate must be given for the gas at a fixed pressure and temperature. If not, you would 
get different capacity values at suction and discharge of the same compressor.
ADGAS uses the standard flow rate measured in normal metres cubed/hour (Nm
3
/h) 
to measure compressor capacity. Normal means that the flow rate is calculated after 
converting the gas volume to what it would be if it was at atmospheric pressure 
(1.1013bara) and at a temperature of 15
o
C. 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 11 of 90
2.5 
Parallel and Series Connections 
Compressors can be connected in two ways: parallel and series, just like pumps. 
Parallel connection increases the capacity because the gas flowing through each 
compressor is added together. 
Two identical compressors connected in parallel: 
• give the same pressure increase as a single compressor 
• give double the flow rate (capacity) of a single compressor 
Figure 2.3 shows two compressors in parallel. 
Notice the symbol used for a compressor. It gets smaller in the direction of flow.
This is because the gas volume gets smaller as it passes through the compressor. 
The suction ports of compressors in parallel are connected to a suction header.
Discharge ports are connected to the discharge header. These headers may be 
replaced by snubbers (see