Figure 4.19 shows some adiabatic discharge temperatures that would occur with ammonia and R-22 were the compressions ideal (frictionless) and with no transfer of heat. The actual discharge temperatures would be higher than those shown because of inefficiencies of the compressor if negligible heat is lost to the ambient. Because the cylinders and heads are hot, there is natural convection of heat to air, but particularly in the case of ammonia, more intensive cooling is needed. It is standard, then, for ammonia compressors to be equipped with water-cooled heads, thereby keeping valves cooler to prolong their life and preventing the breakdown of oil at high temperatures. Sometimes R – 22 compressors are equipped with water-cooled heads. Manufacturers recommend that discharge temperatures not exceed a temperature of approximately 135°C (275°F).
A modest provision that must be planned by the designer and provided by the builder of the plant is a small supply of cooling water to the compressor. The flow rate of water is usually regulated by a control valve that maintains an outlet cooling water temperature of approximately 45° C (113°F). Typical figures for the flow rate of cooling water are 0.7 kg/s of cooling water for each 100 kW of refrigeration (1 gpm per 25 tons of refrigeration). With an assumed temperature rise of 15°C (29°F) with those cooling water flow rates, ammonia leaves the compressor at a temperature 16°C (29° F) lower than it would have if there had been no cooling of the head.
Two precautions to follow to avoid condensation of refrigerant in the head of the cylinder are: (1) ensure that the temperature of the cooling water never falls below the condensing temperature, and (2) terminate the supply of cooling water when the compressor is not in operation.