A centrifugal compressor

 A centrifugal or axial-flow compressor is normally engineered to operate at or near its highest efficiency at the design flow rate. The operating range for a centrifugal running at fixed speed is typically from 100% to about 50% of the design flow. The low end of this range is set by the surge point, where operation becomes unstable due to flow oscillation and vibration. The operating range for an axial running at fixed speed is much narrower, since the surge point is usually 85 to 95% of design capacity. A centrifugal's range of operation can be increased using speed variation, or adjustable inlet vanes. An axial's operating range is extended by providing speed variation and a means of adjusting the stator blade angle.

axial-flow compressor

 

In general, the head versus capacity curve for a centrifugal compressor operating at fixed speed is quite flat. At minimum throughput (the surge point), the total head is typically only 105 to 115% of the head at design throughput. The corresponding curve for an axial is much steeper. Although an axial machine operates over a significantly narrower capacity range, its head at the surge point is typically 120 to 130% of design. With its steeper head- capacity curve, the capacity of an axial is much less affected by discharge pressure variations than a centrifugal.  

The performance of centrifugal compressors can usually be approximated by the "fan laws." The volumetric flow rate varies approximately with rotating speed, the head with the square of speed and the power input with the cube of speed.  

Axial compressors are sometimes placed in series with centrifugals, with the axial stages at the inlet end. In addition, axial and centrifugal impellers are also provided in the same casing. This permits large gas volumes to be compressed by the inlet, axial portion of the unit, until the gas acfm is within the centrifugal operating range.  Both centrifugal and axial-flow compressors generally require less maintenance than reciprocating machines. Except for special situations, a single centrifugal or axial- flow unit is normally used, rather than providing a spare, or two half-capacity machines in parallel. When a single centrifugal or axial-flow compressor is used for a critical, continuous service, it is good practice to stock a spare rotor assembly.