Principles of naval engineering - Historic Naval Ships Association

Chapter 19. -REFRIGERATION AND AIR CONDITIONING PLANTSefficiency of a power cycle. The thermal efficiencyof a power cycle is given by the equationthermal efficiency=work outputheat inputSince thermal efficiency is a function of absolutetemperature alone in the Carnot cycle,the equation may also be given asT - T^sthermal efficiency =where Tg is the absolute temperature at the heatsource and Tj- is the absolute temperature at theheat receiver.For the refrigeration cycle, the coefficient ofperformance is given by the equationcoefficient of performancerrefrigerating effectwork inputwhich, as in the power cycle, can be shown to be afunction of absolute temperature alone.THE R-12 PLANTThe refrigeration system most commonlyused in the Navy utilizes R-12 as the refrigerant.3 Chemically, R-12 is dichlorodifluoromethane(CCL2F2). The boiling point of R-12 isso low that the subtance cannot exist as a liquidunless it is confined and put underpressure; forexample, R-12 boils at -21 "F at atmosphericpressure, at 0' F at 9.17 psig, at 50 'F at 46.69psig, and at 100 °F at 116.9 psig. Because of itslow boiling point, R-12 is well suited for use inrefrigeration systems designed for only moderatepressures. It also has the advantage ofbeing practically nontoxic, nonflammable, nonexplosive,and noncorrosive; and it does notpoison or contaminate foods.The R-12 refrigeration system is classifiedas a mechanical system of the vapor-compressiontype. It is a mechanical system becausethe energy input is in the form of mechanicalenergy (work). It is a vapor-compression systembecause compression of the vaporized refrigerantis the process which allows the refrigerantIn accordance with recent policy, refrigerants usedin the Navy are no longer identified by trade names.Instead, they are identified by the letter R followed bythe appropriate number, or else they are identifiedsimply as "refrigerants." For example, the refrigerantformerly known as "Freon 12" is now identifiedeither as R-12 or simply as a refrigerant.to discharge heat at a relatively high temperature.The R-12 CycleThe basic cycle of an R-12 refrigerationcycle is shown shcematically in figure 19-1. Asan introduction to the system, it will be helpfulto trace the refrigerant through the entire cycle,noting especially the points at which the refrigerantchanges from liquid to vapor and fromvapor to liquid, and noting also the concomitantflow of heat in one direction or another.As shown in figure 19-1, the cycle has twopressure sides. The low pressure side extendsfrom the orifice of the thermostatic expansionvalve up to and including the intake side of thecompressor cylinders. The high pressure sideextends from the discharge side of the compressorto the thermostatic expansion valve.The condensing and evaporating pressures andtemperatures indicated in figure 19-1 are notstandard for all refrigeration plants, sincepressuresand temperatures are established as partof the design of any refrigeration system. Itshould be noted, also, that the pressures andtemperatures shown in figure 19-1 are theoreticalrather than actual values, even for thisparticular system. If the system were in actualoperation, the pressures and temperatures wouldvary slightly because they are dependent uponthe temperature of the coolingwater enteringthecondenser, the amount of heat absorbed by therefrigerant in the evaporator, and other factors.Liquid R-12 enters the thermostatic expansionvalve at high pressure, from thehighpressureside of the system. The refrigerant leavesthe outlet of the expansion valve at a much lowerpressure and enters the low pressure side of thesystem. Because of the relatively low pressure,the liquid refrigerant begins to boil and to flashinto vapor.thermostatic expansion valve, theFrom therefrigerant passes into the cooling coil (evaporator).The boiling point of the refrigerant underthe low pressure in the evaporator is extremelylow— much lower than the temperature of thespaces in which the cooling coil is installed.As the liquid boils and vaporizes, it picks up itslatent heat of vaporization from the surroundings,thereby cooling the space. The refrigerantcontinues to absorb heat until all the liquid hasbeen vaporized andthe vapor has become slightlysuperheated. As a rule, the amount of superheatis about 10° F.473