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

5.1.2 Diffuser 
and 
Volute 
 
The purpose of the diffuser and volute is to increase gas pressure by reducing its 
speed of flow. These opposite changes of energy keep the total energy of the gas 
constant—if kinetic energy reduces, pressure energy increases. 
The diffuser does not rotate; it is fixed in the compressor casing. The spaces between 
diffuser vanes increase as they get further from the impeller. This allows a wider flow 
path for the gas and it slows down. You can see the increase of these spaces in 
Figure 5.6.
The vane angle on some compressors is adjustable to change the performance of the 
compressor. 
Figure 5.6: Diffuser
Impeller 
Diffuser 
(b) Diffuser Vanes 
(a) Impeller and Diffuser Assembled in Casing 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
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The gas enters the volute after passing through the diffuser. The flow path around the 
volute gets bigger as the gas gets closer to the discharge. This allows the gas to slow 
even more, and its pressure continues to increase. Figure 5.7 shows the gas flow path 
through the volute casing of an electric-motor driven compressor. 
Some centrifugal compressors have additional vanes at the compressor inlet. These 
can be opened or closed to control capacity. 
Figure 5.7: Volute Casing with Adjustable Entry Vanes 
Flow path of gas 
around volute 
Adjustable 
entry vanes 
(closed) 
Adjustable entry 
vane control 
mechanism 

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Personnel & Training Division
Job Training—Mechanical Technician 
Compressors/Rev. 0.0 
Page 49 of 90