The diffuser is the important part of the compressor (centrifugal compressor) which surrounds the impeller and provide diverging passages for air flow thus increasing the air pressure. The air coming out from the diffuser is collected in the casing and taken out from the outlet of the compressor.
The air leaving the impeller enters the diffuser where its velocity is reduced by providing more cross-sectional area for the air flow. Part of the Kinetic energy of the air is converted into pressure energy and pressure of the air is further increased.
Nearly half of the total pressure size is achieved in the impeller and remaining half in the diffuser.
In the diffuser the kinetic energy is converted into pressure energy with the result that there is a further rise in static pressure.