Although Chapter 5, Screw Compressors, explains more thoroughly the features and operation of screw compressors, an intriguing situation surrounding intercooling of these machines will be addressed here. Screw compressors operate with oil injected into the refrigerant during compression. This oil absorbs some of the heat of compression and leaves the compressor with the refrigerant at a moderately high temperature. After separation from the refrigerant, as in Figure 3.13, the oil is cooled in a heat exchanger before it returns for injection into the compressor. In one concept of oil cooling, the heat from the heat exchanger flows directly to the condenser. Figure 3.13 applies to the low-stage compressor whose discharge gas is to be intercooled.
Figure 3.13 shows that the refrigerant leaves the compressor at a temperature of approximately 60°C (140°F) instead of the 110 to 120°C (230 to 250°F) typical of a reciprocating ammonia compressor. Furthermore, when the popular thermosiphon type of oil cooler is used, the heat removed from the oil passes directly to the condenser without requiring any refrigerant to vaporize and cool it in the intercooler. The efficiency of the intercooling process is thereby improved. Continued desuperheating with refrigerant to a temperature approaching the saturation temperature at the intermediate pressure is, however, still desirable. This continued desuperheating is advisable because directing 60°C (140°F) vapor to the high-stage compressor would result in unacceptably high oil and refrigerant temperatures leaving the high-stage compressor, particularly for ammonia. Also, the benefits of intercooling from 60°C (140°F) are still waiting to be achieved.