Figure 5.7 showed that a given compressor with a fixed volume ratio is most efficient at one ratio of discharge-to-suction pressures. In a typical two-stage industrial refrigeration plant the high-stage compressor receives suction vapor from the intermediate pressure and discharges to the condensing pressure. The low-stage compressor pumps between the pressure of the low-temperature evaporators and the intermediate pressure. The only pressure of these three that is likely to vary is the condensing pressure due to changes in ambient conditions. The low-stage compressor can be selected with a built-in volume ratio as close as possible to that imposed by the combination of evaporating and intermediate pressures. The built-in volume ratio is determined at the time the compressor is manufactured by positioning the point at which discharge begins. For a high built-in volume ratio, for example, the discharge port is uncovered late in the compression in contrast to a machine with a low built-in volume ratio.
The high-stage compressor in a two-stage system or the compressor in a single stage system experiences variations in the compression ratio. A goal is to apply some device that permits the volume ratio of the compressor to change as the imposed conditions change. The ideal performance would be that shown in Figure 5.16 where the compression efficiency developed rides along the peaks of the various volume ratios. This device and process is called variable built-in volume ratio or simply variable vi.3,4,5 Variable vi devices function in conjunction with the slide valve that controls the capacity, as illustrated in Figure 5.17.
The variable vi device of Figure 5.17 consists of two parts which can move independently. In Figure 5.17a the two parts have no gap between them, so no refrigerant vapor vents back to the suction and the compressor operates at full capacity. The discharge port is uncovered when the cavities of the rotors move past the right end of the right member. If the vi is to be increased but full capacity maintained, both parts move to the right, as in Figure 5.17b. At this position the discharge is delayed so that the pressure in the cavities builds up more before discharging. If the high value of vi is to be maintained, but the capacity reduced, the left member backs off which vents some vapor back to the suction, as in Figure 5.17c.
The motion of the two members requires a complex control, and there are limitations in achieving the desired vi when the capacity must also be reduced. If the capacity has been reduced by as much as 50%, the variable vi portion of the control may no longer be able to meet its requirements.