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

Figure 4.30: Pneumatically-activated Compressor Unloader 
Now try 
Exercise 3
Diaphragm 
Operating 
rod 
Suction port 
Control air inlet 
Control air 
pressure 
Suction valve 
Fingers 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 39 of 90
4.2 
Rotary Compressors and Blowers 
At the time of writing this module there are no rotary compressors in the ADGAS 
plant. They are described briefly here as they are quite common in other places. 
Rotary compressors with low compression ratios are often called blowers.
The three most common rotary compressors are in that part of the classification chart 
repeated in Figure 4.31. 
They are all very similar in appearance and operation to the pumps that have the same 
name. 
Figure 4.31: Common Rotary Compressors 
Rotary 
Lobe 
Vane 
Screw 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 40 of 90
4.2.1 Lobe 
Compressors 
Lobe compressors are also called Roots blowers. Two or more lobes rotate inside a 
housing. There are very small clearances between the lobes, and between lobes and 
housing. Figure 4.32 shows a two-lobe blower with the rotor directions indicated. 
The spaces between lobes and housing get bigger as they pass the suction port. This 
reduces the pressure inside and gas flows in. As the gas is carried around to the 
discharge port, the spaces get smaller, the gas pressure rises and it flows out. 
One rotor is turned by the drive motor. This rotor drives the other through 
synchronising gears that keep both rotors turning together, maintaining minimum 
clearance between them. You can see this gear drive in Figure 4.33. 
Lobes 
Rotors