Figure 9.8.1(e) shows a two-stage h

Figure 9.8.1(e) shows a two-stage hermetic centrifugal compressor. The total pressure rise in acentrifugal compressor, often called head lift, in psi, is due to the conversion of the velocity pressureinto static pressure. Although the compression ratio Rcom of a single-stage centrifugal compressor usingR-123 and R-22 seldom exceeds 4, two or three impellers connected in series satisfy most of therequirements in comfort systems.Because of the high head lift to raise the evaporating pressure to condensing pressure, the dischargevelocity at the exit of the second-stage impeller approaches the acoustic velocity of saturated vapor vacof R-123, 420 ft/sec at atmospheric pressure and a temperature of 80°F. Centrifugal compressors needhigh peripheral velocity and rotating speeds (up to 50,000 rpm) to produce such a discharge velocity. Itis not economical to manufacture small centrifugal compressors. The available refrigeration capacity forcentrifugal compressors ranges from 100 to 10,000 tons. Centrifugal compressors have higher volumeflow per unit refrigeration capacity output than positive displacement compressors. Centrifugal compressors are efficient and reliable. Their volumetric efficiency almost equals 1. At design conditions, theirηisen may reach 0.83, and it drops to 0.6 during part-load operation. They are the most widely usedrefrigeration compressors in large air-conditioning systems.