The most common device for achieving a variation in refrigerating capacity with a screw compressor is the slide valve, as illustrated in Figure 5.11. The slide valve is cradled between the rotors and consists of two members, one fixed and the other movable. The compressor develops full capacity when the movable portion bears on the fixed member.
For capacity reduction the movable portion of the slide separates from the fixed portion so that some of the gas that has filled the cavity during the suction process is not compressed. Instead, as Figure 6.12 illustrates, at the beginning of volume reduction in the cavity, the gap in the slide valve permits some of the gas to vent back to the suction. The slide valve permits a smooth, continuous modulation of capacity from full to 10% of full capacity.
Even though the slide valve can provide smooth changes of capacity the method results in reduced efficiency at part load. This reduction in efficiency is shown in Figure 5.13 where the percent of full power is related to percent of full capacity2. The 45° line shown in Figure 5.13 represents the ideal where a given percentage of full capacity requires the same percentage of full power. The curves show, however, that the percentage of full power always exceeds the percentage of full capacity. Two reasons for the drop in efficiency associated with opening of the slide valve are (1) the friction of the gas. venting back to the suction, and (2) the changing of the νi of the compressor which is assumed to be properly matched to the external conditions at full load. The recommendation that plant operators draw from the data of Figure 5.13 is to operate screw compressors as close to full load as the mix of compressors allow.
The percent capacity reduction does not vary linearly with the motion of the slide valve. The precise relation varies from compressor to compressor, but the general curve is as shown in Figure 5.14. The relationship shows that small
changes of position of the slide valve at high capacity have a dominant influence on the capacity.