O

Diseharge Vapor

Fig. 37-21 Diagram of Impeller and Volute Casing Showing Suction Vapor Inlet. Source: Carrier Corp.

operation of centrifugal vapor compressors is limited by the surge (or pulsation) point that occurs at reduced volume flow rates. Surging is a condition in which the machine rapidly alternates between shut-off and a near maximum flow rate. Capacities below the surge point are said to be in the unstable range.

Surges occur as load decreases beyond a given point, at which the pressure in the system is higher than at the compressor outlet. This causes flow to stop momentarily. Refrigerant flow continues to leave the casing of the compressor and system pressure decreases to a point at which the compressor again begins to deliver refrigerant vapor. However, the discharge pressure does not build up fast enough to deliver the near full-load volume of refrigerant. The surge is thus characterized by a rapid decrease in flow rate and is marked by a change in noise level of the compressor. This cycle repeats until the load increases sufficiently for the compressor to exit the surge region or until the head decreases.

Fig. 37-22 Prerotation Vanes Used with Large-Capacity, Multi-stage Centrifugal Compressor. Source: York International

Figure 37-23 shows the intersection of the surge and system requirement line for a given compressor characteristic, expressed in feet of head versus volume. To avoid surging under a given set of operating conditions, a certain minimum volume flow through the impeller must be maintained. If there is no partial load control, the surge point is typically about one-half to two-thirds of full-load capacity. In controlling centrifugal compressors under part load, the focus is on widening the stable operating range of the impeller as efficiently as possible.

Figure 37-24 provides representative performance

Fig. 37-23 Example Surge Line for Centrifugal Compressor. Source: Carrier Corp.
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