To maximize cost-effectiveness, we recommend that you analyze the entire chilled water system, in addition to exercising care in specifying the efficiency of the chiller itself. The higher the EER, the more efficient the unit. Rotary screw chillers, available in sizes ranging from 100 to 1,100 tons, dominate the market for small to midsize chillers. They are most commonly used in applications of 300 tons or less. The screw compressor is a relative newcomer to comfort conditioning, although it has long been used for air compressors and low-temperature refrigeration.
Screw compressors are positive displacement devices.
The refrigerant chamber is actively compressed to a smaller volume by the twisting motion of two meshing rotors. The single-screw compressor consists of a cylindrical main rotor positioned between identical gaterotors, as shown in Figure 1 . The screw or double helical screw compressor consists of two mating twin-grooved rotors, as shown in Figure 2 . They are up to 40 percent smaller and lighter than centrifugal chillers and are becoming popular as replacement chillers. Screw compressors operate well at partial loads and are stable down to about 10 percent capacity.