Screw Compressor Capacity Control

Capacity is controlled in screw compressors through variable compressor displacement modulation. This is accomplished by varying the speed, by using a moveable slide valve mounted near the suction area of the compressor casing, or both. With slide valve control, at part-load, the slide valve produces a slot that delays the point at which the suction entraps the vapor and compression begins. This causes a reduction in swept volume, and thus, compressor throughput. Units are also available with stepped unloading.

Typically, a capacity controller will monitor chilled water temperature. Under decreasing load (i.e., increased return water temperature), a valve, actuated by a hydraulic or gas piston and cylinder assembly located in the compressor or by a positioning motor, will reduce the working length of the rotors and provide stepless capacity modulation down to about 10% of full load. While somewhat less efficient at full load than comparable capacity centrifugal compressors, their positive displacement characteristics allow screw compressors to unload more efficiently than centrifugal compressors, and, therefore, operate with relatively superior part-load performance. As with reciprocating compressors, reducing speed reduces displacement and, therefore, capacity and power requirement.

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