The screw compressor is provided with oil to serve three purposes: (1) sealing of internal clearances between the two rotors and between the rotors and housing, (2) lubrication of bearings, and (3) actuation of the slide valve. The circulation and distribution of oil is illustrated in Figure 5.18. All the oil supplied to the compressor leaves with the refrigerant and flows to the oil separator. The separator removes the oil from the refrigerant and the refrigerant passes on to the intermediate stage (in the case of a low-stage compressor) or to the condenser (in the case of a high-stage compressor). The oil from the separator is warm because it has absorbed some of the heat of compression and must pass through an oil-cooling heat exchanger before distribution to the three streams serving the compressor. The means of rejecting heat from this exchanger will be discussed in the next several sections.
The type of oil separator used in screw compressor packages is called the coalescing type which will be discussed in more detail in Chapter 15, Lubrication and Oil Handling. This type of separator is much more efficient than the olderstyle inertia separators that have been traditionally, used for reciprocating compressors. A strong trend prevails now to use coalescing separators for reciprocating compressors as well. Coalescing separators are expected to pass no liquid oil, so the only oil that escapes the separator is in vapor form, which means that the oil concentration in the refrigerant leaving the separator will be of the order of 5 ppm. Dirty oil will plug the coalescing elements requiring that the element be replaced, a procedure possible by access through the handhole. Normally the coalescing element lasts for several years.
The end user of the refrigeration facility usually has neither the need nor opportunity to choose the size of the oil separator. This decision is made by the assembler of the compressor package. The size must be such that velocities are not so high that they carry liquid oil out of the separator. The critical operating condition is at low discharge temperatures when the discharge gas experiences a high mass flow rate and high specific volume, both contributing to high refrigerant velocities. Limiting velocities are in the neighborhood of 0.76 m/s (150 fpm) for ammonia and 0.38 m/s (75 fpm) for R-22. Sudden drops in pressure in the separator which would generate foam should be avoided, because coalescing separators are not capable of preventing carryover of foam.
The injection oil flow rate should be adequate to seal the internal clearances, as well as to lubricate the moving parts and to cool the gas being compressed. On the other hand, excessive oil quantities will result in undesirable hydraulic hammer. Knowledge of the rate of flow of injection oil is necessary to properly design the oil-cooling heat exchanger. Often, however, when the oil cooler is part of the package it is selected by the assembler, and the system designer and operator may not need to know the injection oil flow rate. Some compressor manufacturers publish the information in their catalogs, and the order of magnitude of oil flow rates range from about 0.065 to 0.11 L/min per kW of refrigeration (0.06 to 0.1 gpm/ton of refrigeration) for high-stage machines.