Examination of Figure 5.4 suggests that it would be possible to provide an opening in the housing of the compressor to tap into a cavity during compression. The refrigerant in Cavity 5, for example, is at a pressure somewhere between
suction and discharge. Refrigerant can be supplied through this opening at an intermediate pressure, and the compressor continues the compression of all the refrigerant. This feature opens the possibility of a liquid subcooler as was first illustrated in Figure 3.24 which is reproduced as Figure 5.27. This opening, often called the side port, offers within one compressor some of the advantages of a multiple-compressor, two-stage installation. A subsystem as shown in Figure 5.27 is called an economizer.
Manufacturers of screw compressors are usually able to choose the position of the side port so that the desired intermediate pressure can be provided.
Once the position has been established, however, it is fixed and the compressor has no flexibility to maintain a constant intermediate pressure as the suction and discharge pressures change. When operating at its optimum conditions the economizer cycle provides a significant benefit with a low first-cost investment. For example, the shell-and-tube heat exchanger, including its control, shown in Figure 5.27 is a low-cost addition. When the side port is placed into service, the volume flow rate drawn in at the compressor suction is not affected, because the gas is already trapped by the time the side port is uncovered. Additional refrigeration capacity is provided, however, because the liquid flowing to the evaporators has been chilled and its enthalpy reduced. The power reqirement of the compressor will increase because of the additional gas to be compressed from the side-port pressure to the condensing pressure.
The exhileration of discovering a low-cost replacement for a two-stage system that requires an extra compressor must be tempered somewhat by the realization that the economizer cycle in its best operation is not quite as efficient as two stage. Stegmann analyzed the performance of the side port and several of his findings will be reported in the next several pages. Figure 5.28 shows the comparative coefficients of performance (COP) at various suction temperatures when operating with a condensing temperature of 35°C (95°F).
One reason for the inability of the economized system using a side port to attain the efficiency of a two-stage system is illustrated in Figure 5.29 which shows that the pressure within the cavity changes during the time that the side port is uncovered. In the early stage of admission there is an unrestrained expansion, as discussed in Section 5.3, of the side-port gas as it flows into the compressor. This unrestrained expansion consitutes a thermodynamic loss.
From an understanding of the process associated with an economizer operation using the side port, it can be inferred that the capacity of the system will increase. This increase occurs, because the enthalpy of liquid reaching the expansion valve is reduced, even though the volume flow rate at the inlet to the compressor remains unchanged. Due to the admission of additional gas during the compression process, the power requirement increases. Both of these effects are shown in Figure 5.30, which presents multiplying factors for both refrigerating capacity and power with respect to the noneconomized system. Since the factors for the increase of refrigerating capacity exceed those for the increase in power, the economized cycle shows an improvement over the noneconomized cycle.
While the use of the side port with an economizer shows performance advantages with only a moderate additional first cost, there are some limitations. The economizer cycle is most effective when the compressor is operating at full refrigeration capacity. With compressors equipped with slide valves for capacity control, the opening of the slide valve changes the pressure within the compressor at the side port. As Figure 5.31 shows, when the slide valve is in a partial capacity position, the point at which the gas is trapped in the cavity moves further to the right. Because the start of compression is delayed, the pressure in the cavity is low when the side port is first uncovered. Thus, the pressure at the side port progressively drops as the slide valve opens. One consequence of this pressure change is that the optimum intermediate pressure no longer prevails, and the improvement of the flash-gas removal by liquid subcooling diminishes. The slide valve can even move to the point where the side port uncovers at the instant the gas is trapped in the cavity. At this point the sideport pressure has dropped to the suction pressure and the economizer is completely ineffective.
The equipment shown in Figure 5.27 with the shell-and-tube liquid subcooler turns out to be one of the best means of exploiting the side port. Other potential applications include using a flash tank with the vapor drawn off by the side port. As discussed in Section 3.4 in the chapter on multistage systems, the flash tank drops the temperature of liquid more than the liquid subcooler which must operate with a temperature difference between the leaving subcooled liquid and the saturation temperature of the side-port pressure. But the drop in side-port pressure as the slide valve opens has consequences over and above the reduction in efficiency. The pressure of the liquid leaving the flash tank also drops and may not provide enough pressure to force the liquid through downstream valves.
Another potential application of the side port is to provide the suction for an intermediate-temperature evaporator. Here again there are limitations imposed by the prospect of the drop in side-port pressure. In the food industry the intermediate-temperature evaporator is often serving spaces storing unfrozen food where the drop in evaporating temperatures much below freezing could damage products. A conclusion is that the side port offers attractive possibilities, but it also has limitations.