When applied as a low-stage (booster) compressor, several economies can be achieved in the design of the screw compressor. A majority of low-stage compressors serve low-temperature applications in the food industry, including the freezing of food and in refrigerating spaces storing frozen food. There are three operating pressures in two-stage systems, namely at the low-temperature evaporators, at the intermediate-temperature evaporators or the subcooler/desuperheater, and at the condensers. Of these three pressures, the only one expected to vary appreciably during operation is the condensing temperature because of changes in the ambient conditions. The suction and discharge temperatures of the lowstage compressor are likely to remain fairly constant, so the low-stage compressor can be selected to be optimum for the given pressure ratio. The capacity control of the high-stage compressor, for example, normally uses the suction pressure of that compressor to regulate the slide valve. Because its discharge and suction pressures do not vary much, the variable νi feature is unnecessary, so the cost of this device and its controlling mechanism can be eliminated.
For capacity control, the slide valve offers a means of quite precise control usually activated by the suction pressure. When the booster compressor experiences a reduction in the refrigeration capacity, it is not so crucial to reduce the compressor capacity. When serving a food freezer or a frozen food refrigerated space, it is not usually detrimental that the evaporating temperature drops when the load falls off. In fact, the freezing time is shortened in that process, or in the case of a refrigerated storage space, the thermal capacity of the products is so great that little change in temperature results.
Several low-cost approaches to capacity control of low-stage compressors are capacity reduction with simple plug valves that open as needed to vent gas that has been trapped in the cavities back to the suction chamber. The usual application of these valves is to provide a choice of 100%, 75% or 50% capacity. The control of these valves is done with low-cost electromechanical relays, and microprocessor control is not needed. Two methods of still lower first cost, although less efficient, are the installation of a throttling valve in the suction line or the controlled return of discharge gas to the suction line in what is called hot-gas bypass. A further use of hot-gas bypass is to be able to start the compressor unloaded, a capability offered to the high-stage compressor through the use of its slide-valve. Still another option to address low loads, for example, during off duty in a food production facility is to arrange the piping in such a way that one compressor operates single-stage during low-production periods.
Since screw compressors can operate, although with reduced efficiency, against high pressure ratios, they can function with no damage from -40°C (-40° F) evaporating to 35°C (95°F) discharge in single-stage operation, for example. Screw booster compressors offer a further economy if the oil is cooled externally in contrast to internal cooling as with the direct injection of refrigerant. Figure 5.32 shows that the heat liberated by the oil in a heat exchanger cooled by the thermosyphon process or by an external liquid cooler passes directly to the condenser. When directly injected refrigerant cools the oil, this heat passes on to the subcooler/desuperheater where the high-stage
compressor must pump additional refrigerant in order to transfer this heat to the condenser.