The injected oil that seals the clearances in the compressor is intimately mixed with the refrigerant undergoing compression. The refrigerant vapor becomes hot during compression and transfers some heat to the oil as it passes through the compressor. The oil must be cooled before reinjection, and four of the important methods of oil cooling are: (1) direct injection of liquid refrigerant, (2) external cooling with a thermosyphon heat exchanger, (3) external cooling with cooling water or antifreeze, and (4) pumping of liquid refrigerant into the refrigerant/oil mixture as it leaves the compressor. The first two methods are the most popular and will be addressed in more detail in Sections 5.13 and 5.14, respectively. This section describes external cooling using a cooling liquid or antifreeze and the introduction of liquid refrigerant into the discharge stream.
The method of cooling oil with an external heat exchanger that rejects heat to cooling water or antifreeze, as illustrated in Figure 5.19, was the earliest, widely-used concept. In warm climates cooling water could serve the heat exchanger, and this cooling water could come from a cooling tower or a closedcircuit cooler. In the closed-circuit cooler the fluid being cooled flows through pipe coils with water spraying from the top and ambient air flowing up through the sprays and coils. Some plants simply draw water from the pan of the evaporative condenser, but this practice is not recommended. In the first place, this water is likely to collect foreign matter that could deposit on the tubes of the heat exchanger, reducing its heat-transfer effectiveness. Also, in cold climates the water in the evaporative condenser must sometimes be drained to avoid freezing, and in that situation no means of oil cooling would be available. Even with the closed-circuit cooler, in cold climates antifreeze must be circulated to avoid freezing water.
The second method8 of oil cooling, as illustrated in Figure 5.20, extracts liquid refrigerant from the receiver and delivers it into the discharge of the compressor. Evaporation of this pumped liquid cools the refrigerant/oil vapor mixture to the desired 49°C (120°F) temperature before the mixture enters the separator. A temperature sensor at some point following the injection of liquid regulates a variable-speed pump to adjust the flow rate of liquid. In this method and the method of direct injection the temperature of the oil separator operates at 49°C (120°F) in contrast to the external heat exchanger and thermosyphon systems where the operating temperature is that of typical discharge, namely, 60°C (140°F). Oil separators function slightly better when the temperature is low, because the viscosity as well as the vapor presssure of the oil are lowered.