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

2 Compressor 
Basics 
The principle of operation of a compressor is the same as that for a pump. The 
compressor driver supplies kinetic energy that the compressor converts to pressure 
energy. Some energy is wasted because of fluid friction; this turns kinetic energy to 
heat energy. Like all other machines, compressors efficiency is always less than 
100%. 
2.1 Pressure 
Units 
The units used to measure gas pressure are the same as those used for liquids. The 
basic SI unit is the newton/metre squared (N/m
2
), or, to use its other name, the pascal 
(Pa). The most common unit of gas pressure used on the ADGAS plant is the bar but 
the pound/square inch (psi) is also still used. 
1bar = 100000 or 10
5
Pa (or 10
5
N/m
2
) 
1bar = 14.5038psi 
The only pressure unit not commonly used for gas is metres of head. Head is only 
used for liquids because it is convenient to describe pressure by the height the liquid is 
pumped through. 
Gas pressure may be given as 
• absolute pressure (bara and psia)—pressure measured above that of a perfect 
vacuum and used mainly for pressure calculations 
• gauge pressure (barg and psig)—pressure measured above atmospheric 
pressure; the unit shown on most pressure gauges on the plant 
• a pressure difference or pressure differential (bar and psi) measured as the 
change in pressure between to parts of a process

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 8 of 90
2.2 
Effects of Pressure on a Gas
The main difference between liquids and gases is that: 
• liquids are incompressible—pressure has almost no effect on a liquid’s volume 
• gases are compressible—pressure has a great effect on the volume of a gas 
There is a law that describes how a change of pressure changes the volume of a gas.
You met this law in the Physics module in the Basic Course. It is called Boyle’s Law 
and it says something very simple: as long as the temperature of the gas does not 
change, increasing the pressure on a gas always decreases its volume. This is 
probably obvious: a gas is squashed by pressure. 
If you reduce the pressure on a gas the opposite happens. Figure 2.1 shows these 
changes. 
• if you double the pressure, the volume is halved (Fig. 2.1(a) to (b)) 
• if you halve the pressure, the volume doubles (Fig. 2.1(a) to (c))