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The diffuser is a component which effects a reduction in flow velocity and corresponding increase in static pressure whilst causing as small a loss as possible (see also Pressure loss). 

A diffuser's distinguishing characteristic is a flow path with increasing cross-sectional area in the direction of flow, in a closed duct or channel. In centrifugal pump technology, diffusers are frequently used on the discharge side of ring-section and volute casing pumps (see Pump casing), they are also used in multistage centrifugal pumps and as a component in piping runs.

As flow separation at boundary layers is a major risk in the region of the diffuser, defined diffuser angles must not be exceeded. The critical diffuser angle (taper angle) of circular-section diffusers predominantly used in centrifugal pump technology is between 8º and 10º. In the event that diffuser elements are fitted to deal with vortex flow or heavy throttling downstream of the diffuser, these angles can be exceeded without the risk of flow separation.

A diffuser can also be used to connect pipe sections of different nominal diameters. In this case, the diffuser selected must be of adequate length so that the critical diffuser angle is not exceeded and additional pipe friction losses and pulsating flow separation are prevented. Under certain circumstances, a sudden transition from a small to a large pipe cross-section (e. g. a Carnot diffuser) may prove more advantageous than installing a diffuser, both for cost and hydraulic reasons.

The opposite of a diffuser is a nozzle (e. g. standard nozzle or bellmouth), also known as a confuser.



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