Page 224 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 207

Figure 5.11 An axial-flow compressor.

In Figure 5.12, the characteristic curve illustrates the performance of a compres-
sor rotating at a definite speed. In addition to showing the pressure-capacity
characteristic, the curve also shows important operating limits. The most impor-
tant one is the "surge limit" or minimum-flow point below which the compressor
operation becomes unstable. If the flow rate is reduced, the pressure developed
by the compressor decreases. Then, the pressure in the discharge line becomes
greater, and the gas flows back into the compressor. As soon as the pressure in
the discharge line drops to below that developed by the compressor, the gas
again flows into the discharge line. Then, the cycle repeats. The oscillating pres-
sure and flow rate will cause audible vibrations and shocks, and could damage
the compressor blades, seals, and other components. Therefore, the compressor
requires an antisurge control system to limit the flow rate at a minimum point,
safely away from the surge limit. The surge limit usually is clearly marked, but,
if not, it should be understood that the left end of the curve terminates at the
surge limit. The lower right end of the curve usually terminates before reaching
a limiting condition referred to as the "choke limit", where the gas flows at the
speed of sound. If the curve were extrapolated as shown by the dashed line, it

Copyright © 2003 by Taylor & Francis Group LLC