Section 4.2 in the chapter on reciprocating compressors addressed the shift in the market in the past several decades from reciprocating to screw compressors. Since these two types of compressors are virtually the only types used in industrial refrigeration, the roles of the two compressors are interrelated. From the time in the early 1970s when the screw compressor appeared in the refrigeration market, the screw compressor has captured the major share of the industrial refrigeration practice, at least from the standpoint of refrigeration capacity. The screw compressor has proven to be a reliable machine usually capable of 50,000 hours or more of operation between overhauls.
The limitation of pressure ratio of 8 or 9 confronted by the reciprocating compressor does not apply to the screw type, which is capable of performing adequately up to pressure ratios of 20 or more. While the capability of working against a high pressure ratio is available, the benefits of multistage compression usually lead to the choice of multiple compressors and multistage compression. Even in a multistage plant, however, there may be periods of light refrigeration load where it is advantageous to abandon the savings of multistage compression and operate one screw compressor in a single-stage mode from the low evaporating pressure to the condensing pressure.
The maximum pressure sustainable by screw compressors is about 2400 kPa (350 psia), and the compressor can operate against a pressure difference of approximately 1700 kPa (250 psi). The minimum practical suction pressure is of the order of 17 kPa (2.5 psia).
The typical first-cost comparison between the screw and reciprocating compressor is shown in Figure 5.34. A single reciprocating compressor will normally cost less than a single screw compressor. The maximum pumping rate of reciprocating compressors is of the order of 0.33 to 0.47 m3/s (700 to 1000 cfm), so for higher refrigeration loads two reciprocating compressors are required. At that point and at higher refrigeration capacities the screw compressor is lower in first cost.
A controversial subject is the tolerance of screw compressors to slugging with liquid refrigerant. An enthusiastic promoter of screw compressors might assert that the screw compressor is unaffected by liquid slugging. It may be true that the screw compressor is less sensitive to slugging in comparison to the reciprocating type, but a slug of liquid refrigerant may vaporize and interrupt the flow of injected oil resulting in scored rotors. One engineer recommends an inspection of the machine following any known instance of slugging.
The size and capacity range of the screw compressor extends from the small mini-screw machines to large units driven by motors of 1000 kW or more. The mini-screw unit is available in refrigerating capacities as low as about 100 kW (28 tons of refrigeration). Due to its small-diameter rotors the mini-screw is usually not as efficient as the larger-size units. The mini-screw of one manufacturer operating with -6.7°C (20°F) evaporating temperature and 35°C (95°F) condensing temperature develops a coefficient of performance of 3.94. At corresponding evaporating and condensing temperature, a large compressor exhibits a COP of 4.10, thus, about 4% better efficiency.