Document ZnZQrZ23x83X4ELXOOg7bJDeO

910 CHAPTER 36 1957 Guide Complex Refrigeration Cycles The preceding sections have dealt only with refrigeration systems in which there is but one evaporator; compression, is accomplished through but a single stage, and expansion proceeds through a single expansion valve. In large systems or in low temperature systems in which the compression ratio is high, the compression process can be carried out in stages, with the refrigerant passing through several cylinders arranged for operation in series. The thermodynamic advantage of such compound compression arises from the fact that intercoolers can be placed between the stages of compression to extract heat from the vapor, and thereby cause the overall compression process to approach more closely the ideal condition of iso thermal compression. Essentially, such intercoolers serve the same pur pose as a cooling jacket, but with greater effectiveness because of the more satisfactory heat transfer conditions. In the simple saturation cycle the saturated liquid entering the expansion valve commences to vaporize as soon as its pressure starts to drop. The Refrigeration 911 The Air Cycle System Fundamentally, the air cycle1 is essentially the same as the vapor cycle. Compression is accomplished by a reciprocating or centrifugal compressor, and, since there is no change of phase of the refrigerant upon expansion, an air cooler replaces the condenser, and a refrigerator, the evaporator. Although some cooling would result from, the expansion of the gas through an ordinary expansion valve, a much greater drop in air temperature is accomplished if the expansion is controlled to approach the isentropic by replacing the valve with an expansion engine or turbine. Furthermore, the work recovered by such an expansion engine can be utilized to supply part of the work of compression or to drive other devices. Fig. 5. Pressure-Enthalpy Diagram fob Refrigeration Cycle with Subcooling and Superheating vapor produced during the expansion process has no further use, in terms of refrigerating effect, since it has already picked up its latent heat of vaporization as a result of heat which it has extracted from the unvaporized residue. Thus the instant such vapor forms, its usefulness is at an end, and to allow such material to undergo a further drop in pressure is uneconomical. With compound compression, there is at least one intermediate pressure at which flash vapor can be extracted. In such cases several expansion valves can be utilized with all of the refrigerant from the condenser passed through a first expansion valve to the higher suction pressure, and the flash vapor then extracted and returned to the condenser through the high compression stage. The remaining refrigerant can then pass through a second expansion valve where the pressure is dropped to that corresponding to the low-pressure evaporator. The number of expansion valves is limited by the number of stages of compression. Further cycle complications may arise if more than one evaporator is to be operated with a single compressor, and particularly, if the pressures in these evaporators are to differ. The most common solution is to operate the compressor at the suction pressure of the lowest pressure evaporator, and to equip all other evaporators with back-pressure regulating valves or throttling devices between the evaporator and the compressor suction. The Steam Jet System The steam jet system, under certain circumstances, is desirable for use m air conditioning.* Steam supplies directly the power used for com pressing the refrigerant, thus eliminating the losses connected with other methods of supplying energy. As the compression ratio between the evaporator and condenser under normal circumstances is large, the mechan ical efficiency of the equipment is somewhat lower than that of the positive mechanical type compressor. The condensing water requirements are considerably greater, as both the refrigerant and the impelling steam must be condensed. The steam jet system functions on the principle that water under high vacuum will vaporize at low temperatures. Steam jet boosters or com pressors of the type commonly used in power plants for various processes, will produce the necessary low absolute pressure to cause evaporation of the water. A diagrammatic representation of a typical steam ejector water cooling system is shown in Fig. 6. The figures correspond to an average repre sentative system. The water to be cooled enters the evaporator and is cooled to a temperature corresponding to the vacuum maintained. Be-