Refrigeration Cycle: Oil and Compressors

Because of their rotating components, all compressors require lubrication. Early designs of reciprocating compressors utilized a “splash system” in which the “throw” on the crankshaft would dip into an oil pool in the crankcase and splash it over the moving parts as the shaft rotated. Today, reciprocating compressors used in industrial refrigeration utilize “internal force feed” lubrication in which oil is forced through machined passages in the compressor to lubricate specific components such as bearings, connecting rods, cylinder walls, and piston pins. Since the oil passages are small, reciprocating compressors require an oil pump that is typically driven by the compressor shaft. 

Screw compressors require significantly more oil than reciprocating compressors. Because the oil absorbs a significant amount of the heat of compression, the temperature of the oil tends to increase when a compressor is running. For this reason, screw compressors are designed with an oil cooling system. There are three primary types of screw compressor oil cooling systems.

1) Liquid Injection Oil Cooling

Direct “liquid injection” is a widely used oil cooling method for screw compressors. This method is employed by injecting a small amount of liquid refrigerant from the high pressure receiver into the screw at an early stage of compression. Prior to injection, the high pressure liquid passes through an expansion valve to lower the temperature of the liquid. Historically, a thermostatic expansion valve, or TXV, was the control valve of choice, but in recent years, motorized expansion valves have been employed with excellent results. 

The downside of liquid injection oil cooling is that it reduces the overall capacity of the compressor since some of the volume is taken up by liquid. Additionally, liquid is generally incompressible, so the direct injection of liquid into the screw moderately increases the wear and tear on the compressor. 

Liquid injection is attractive because it is the least expensive to implement and does not require any special heat exchangers for heat removal. 

2) Thermosyphon Oil Cooling

“Thermosyphon oil cooling” is accomplished by supplying saturated liquid ammonia at the condensing pressure to an external heat exchanger mounted adjacent to the screw compressor. Typically, the liquid is supplied from a dedicated vessel called a “thermosyphon vessel or receiver”. The heat exchanger exchanger has two circuits. The first circuit receives hot lubricating oil from the screw compressor. This oil is cooled by the warm ammonia supplied from the thermosyphon vessel. As the oil transfers heat into the liquid ammonia, vapor is formed which is returned to the thermosyphon vessel. Reduced temperature oil exits the heat exchanger and is returned to the screw compressor. 

Thermosyphon oil cooling does not impose the same penalties on the system as liquid injection oil cooling. It, therefore, is known as being a more efficient means of oil cooling. The first cost of employing thermosyphon oil cooling, however, can be high because of the extra system components that are required. 

3) Water Oil Cooling

Water oil cooling is most similar to thermosyphon oil cooling in that each screw compressor will have a dedicated heat exchanger for oil cooling. Water cooled compressors, however, will utilize water as the cooling medium instead of ammonia. As the water absorbs heat from the oil, its temperature will rise. As a result, water cooling systems require a cooling tower to reject the heat absorbed in the water to the atmosphere. Sometimes, a dedicated portion of the condenser is utilized for this purpose.