The two major categories of screw compressors are twin screw and single screw. The twin-screw compressor is widely used and has many years of operating experience. It occupies a position alongside reciprocating and centrifugal types as a standard choice of refrigeration compressors. The single-screw type, described in Section 5.22, is becoming well established because of the efforts of several manufacturers. The twin- screw will simply be referred to in this chapter as the screw compressor.
The invention and evolution of the screw compressor bears a heavy Swedish imprint through a succession of firms beginning about the turn of the century with the company of the Ljungstrom brothers—a name that became associated later with the Ljungstrom air preheater for power plants. In 1913 the brothers organized a subsidiary, Svenska Turbinfabriks Aktiebolaget Ljungstrom, also known by the acronym STAL. Following some successes and reverses, the Ljungstrom brothers resigned from the company in the 1920s and a new chief engineer, Alf Lysholm, was appointed, who provided the firm with several inventions, including that of the screw compressor.
The early screw compressors were fraught with many deficiencies in design and operation which had to be solved one by one. In 1951 the name of AB Ljungstroms Angturbin was changed to Svenska Rotor Maskiner AB (SRM). Up until this time screw compressors were equipped with synchronizing gears and operated dry, but during the 1950s the practice of injecting oil began and this development gave the screw compressor new impetus. Used primarily for air compressors initially, development work on the application of screw compressors to refrigerants began in the 1950s. The improvement of the rotor profiles to provide ease of manufacture and efficient performance has been an ongoing emphasis in the screw compressor development. By now about a million air compressors and nearly 100,000 refrigerant screw compressors have been manufactured.
Section 4.2 in the previous chapter on reciprocating compressors presented a picture of the competitive situation between reciprocating and screw compressors. The conclusion of that discussion was that during the past several decades the screw compressor has gained much of the compressor market in industrial refrigeration, particularly in large-capacity units. Of the refrigeration capacity installed each year the screw compressor serves more of this capacity than does the reciprocating type, so the principles, applications, and procedures described in this chapter are especially important. Screw compressors are available in volume capacity ranges from about 0.05 to 1.5 m3/s (100 to 3300 cfm), driven by motors ranging in output from 25 to 1250 kW, and operating at usual speeds of 3550 rpm (2950 rpm with 50-Hz power).
This chapter describes the screw compressor and explains how it works. The performance of the basic compressor is first explored, particularly as it encounters changes in evaporating and condensing temperatures. Capacity regulation of a screw compressor is typically achieved through the use of a special valve which provides continuous-capacity modulation over a wide range. Screw compressors are basically constant-volume-ratio machines, the implications of which will be explored. Oil is injected in screw compressors for sealing the spaces between the lobes, and this oil must subsequently be separated and cooled. End users of refrigeration plants usually buy screw compressors incorporated in packages that include the necessary auxiliaries, which will be described. The chapter concludes with an explanation of the single-screw compressor.