Handbook of air conditioning and refrigeration / Shan K

13.16 CHAPTER THIRTEEN
pressure rise of a centrifugal compressor, in psi (kPa). For a variable-speed centrifugal compressor,
surge first develops at a point of zero slope along the curve that characterizes compressor speed n
versus refrigeration load Q ref, in tons (kW) (the n-Q ref curve). This point is determined by assuming
a rotating speed, in rpm, that is lower than the minimum possible speed at a required pressure lift
and volume flow of the compressor.
A three-stage centrifugal compressor has a significantly smaller surging region than a two-stage
or a single-stage centrifugal compressor. A single-stage centrifugal compressor using inlet vanes to
control its capacity has a comparatively smaller surging region than a variable-speed compressor.
Surging may damage the components of the compressor. A centrifugal compressor should never
be operated within the surge region on a compressor map.
Centrifugal Compressor Map at Constant Speed
A compressor map of a single-stage centrifugal compressor operated at constant speed is presented
in Fig. 13.10a. The abscissa is the percentage of design volume flow of refrigerant at compressor
suction 100 V˙
suc /V˙
suc, d, or load ratio
LR � Q ref
Q ref, d
FIGURE 13.10 Compressor map of a single-stage centrifugal compressor: (a) constant speed, inlet vane modulation;
(b) variable speed. (Adapted with permission from ASHRAE Handbook 1988, Equipment.)
(13.2)