Although small compressors may be able to achieve adequate lubrication of the moving parts by splash lubrication, virtually all reciprocating compressors used in industrial refrigeration practice are provided with forced lubrication. A positive-displacement pump draws oil from the crankcase and delivers the oil to bearings, cylinder walls, and to the shaft seal on many compressors. Most pumps are driven off the compressor shaft and some are nonreversible, which fixes the required direction of compressor rotation.
Several auxiliaries to the compressor lubrication system are the oil cooler, a crankcase heater, a separator to remove oil from the discharge gas, and safety cutouts. Particularly on large compressors, the oil is passed through a
watercooled heat exchanger that cools the oil. The rate of water flow required for cooling is of the order of 10 liters/min (several gallons per minute). Another guideline is to set the water-flow rate such that the leaving water temperature is about 45°C (113°F), and then rely on the compressor manufacturer to have provided a cooler large enough to maintain a satisfactory oil temperature with this flow rate. A typical oil temperature during operation is 50°C (122°F).
Crankcase heaters automatically come into service during compressor shutdown. If the oil is permitted to become cool during shutdown, the refrigerant—particularly the halocarbons—will dissolve in the oil. Upon startup, the refrigerant boils off, causing oil foaming and possible oil carryout from the compressor.
The type of oil separator traditionally found in the discharge line of reciprocating compressor is a small vessel using abrupt changes of direction of the oil-laden refrigerant to separate oil droplets that then periodically are returned to the compressor crankcase. The oil concentration leaving this type of separator is in the range of 50 to 80 ppm. When oil-injected screw compressors appeared they required a much more efficient separator, so the coalescing type, which is discussed more thoroughly in Chapter 5, was developed. The coalescing separator reduces the oil concentration to approximately 5 ppm. This type of separator is also now available for reciprocating compressors and is quite widely used on new installations.
Typical safety cutouts associated with the oil system are those that shut off the compressor if a high oil temperature or a low oil pressure occur. The oil pressure cutout usually senses the pressure differential across the pump, which typically must be higher than about 100 kPa (15 psi). The cutout could be set to shut down the compressor after a 90-second duration of low pressure. This time delay permits the compressor a time interval to build up the oil pressure on startup.