Axial Compressors

These machines have around 6 to 8% better polytropic efficiency than centrifugal compressors but are highly susceptible to loss of performance due to fouling.

The selection of a more efficient axial type over centrifugal is partly driven by the duty in terms of volume flow rate and pressure ratio. Nevertheless, a number of users have found it economically attractive to retrofit axial machines to replace centrifugal machines, for instance FCC air blowers.

Axial compressors are often fitted with variable angle stator vanes, especially when operating with a motor drive at fixed speed. Though increasing initial cost, the added flexibility over the operating range of pressure ratio and flow is attractive.

As above, coating the rotor and stator can prevent or reduce fouling and maintain efficiency. Many applications use axial compressors in air service. In these cases, it is possible to monitor efficiency easily and clean when required. The poly-tropic efficiency of a compressor is determined simply by the inlet and discharge pressures and temperatures.

npoi = polytropic efficiency (multiply by 100 to get %)

k = Cp/Cv: For air use 1.4 which is good enough for the P and T of an air compressor since it is monitoring change rather than absolute value which is important.

P = absolute pressure

T = absolute temperature

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