The adiabatic compression efficiency ηa was defined for the reciprocating compressor in Equation 4.12:
The compression efficiency of reciprocating compressors is strongly influenced by the pressure ratio, and for a screw compressor the additional influence is that of the built-in volume ratio, as shown in Figure 5.7.
The pattern is that the efficiency reaches a peak at a certain discharge to suction pressure ratio, and the pressure ratio for optimum ηa is a function of the built-in volume ratio. The mechanisms of how the screw compressor works now are used to explain some of the trends appearing in Figure 5.7.
The ideal situation is where the pressure in the cavity during compression builds up at the instant the discharge port is uncovered to precisely the dischargeline pressure, as in Figure 5.5a. Table 5.1 predicted pressure ratios corresponding to several built-in volume ratios for ammonia and R-22 and shows that the pressure ratio is higher than the volume ratio. The efficiency curves in Figure 5.7 reach their peaks at pressure ratios slightly higher than those shown in Table 5.1 for a given volume ratio.
Two reasons for the shifts from expectations are:
1. Some cooling is performed during compression rather than taking place adiabatically, and
2. There is some leakage of refrgierant so that the ideal pressure ratio is not achieved.
Overcompression prevails to the left of the peak efficiency, and under compression to its right. The previous section commented that overcompression results in greater losses than does undercompression, and this fact is borne out by the rapid dropoff of ηa to the left of the peak efficiency.
The trends demonstrated in Figure 5.7 provide guidance for choosing the builtin volume ratio when a compressor is being selected. It is not the best strategy to select a compressor with its peak efficiency occurring at the design pressure ratio. It is preferable to choose a compressor that exhibits its peak efficiency at a pressure ratio lower than design, as shown in Figure 5.8. In industrial refrigeration systems the suction pressure usually remains nearly constant, but the condensing pressure will likely drift lower than design during most of the hours of operation. Thus, in Figure 5.8 the pressure ratio shifts downward from the design condition toward the value where the peak efficiency occurs. Furthermore, the low efficiencies experienced during overcompression are avoided.