Chapter 19. -REFRIGERATION AND AIR CONDITIONING PLANTS 



efficiency of a power cycle. The thermal ef- 

 ficiency of a power cycle is given by the equation 



thermal efficiency = 



work output 

 heat input 



Since thermal efficiency is a function of ab- 

 solute temperature alone in the Carnot cycle, 

 the equation may also be given as 



thermal efficiency = 



T - T 

 ^s r 



where Tg is the absolute temperature at the heat 

 source and Tj- is the absolute temperature at the 

 heat receiver. 



For the refrigeration cycle, the coefficient of 

 performance is given by the equation 



coefficient of performance 



refrigerating effect 

 work input 



which, as in the power cycle, can be shown to be a 

 function of absolute temperature alone. 



THE R-12 PLANT 



The refrigeration system most commonly 

 used in the Navy utilizes R-12 as the refrig- 

 erant. 3 Chemically, R-12 is dichlorodifluoro- 

 methane (CCL2F2). The boiling point of R-12 is 

 so low that the subtance cannot exist as a liquid 

 unless it is confined and put underpressure; for 

 example, R-12 boils at -21 "F at atmospheric 

 pressure, at 0' F at 9.17 psig, at 50 'F at 46.69 

 psig, and at 100 °F at 116.9 psig. Because of its 

 low boiling point, R-12 is well suited for use in 

 refrigeration systems designed for only mod- 

 erate pressures. It also has the advantage of 

 being practically nontoxic, nonflammable, non- 

 explosive, and noncorrosive; and it does not 

 poison or contaminate foods. 



The R-12 refrigeration system is classified 

 as a mechanical system of the vapor-compres- 

 sion type. It is a mechanical system because 

 the energy input is in the form of mechanical 

 energy (work). It is a vapor-compression system 

 because compression of the vaporized refrig- 

 erant is the process which allows the refrigerant 



In accordance with recent policy, refrigerants used 

 in the Navy are no longer identified by trade names. 

 Instead, they are identified by the letter R followed by 

 the appropriate number, or else they are identified 

 simply as "refrigerants." For example, the refriger- 

 ant formerly known as "Freon 12" is now identified 

 either as R-12 or simply as a refrigerant. 



to discharge heat at a relatively high tempera- 

 ture. 



The R-12 Cycle 



The basic cycle of an R-12 refrigeration 

 cycle is shown shcematically in figure 19-1. As 

 an introduction to the system, it will be helpful 

 to trace the refrigerant through the entire cycle, 

 noting especially the points at which the re- 

 frigerant changes from liquid to vapor and from 

 vapor to liquid, and noting also the concomitant 

 flow of heat in one direction or another. 



As shown in figure 19-1, the cycle has two 

 pressure sides. The low pressure side extends 

 from the orifice of the thermostatic expansion 

 valve up to and including the intake side of the 

 compressor cylinders. The high pressure side 

 extends from the discharge side of the com- 

 pressor to the thermostatic expansion valve. 

 The condensing and evaporating pressures and 

 temperatures indicated in figure 19-1 are not 

 standard for all refrigeration plants, sincepres- 

 sures and temperatures are established as part 

 of the design of any refrigeration system. It 

 should be noted, also, that the pressures and 

 temperatures shown in figure 19-1 are theoreti- 

 cal rather than actual values, even for this 

 particular system. If the system were in actual 

 operation, the pressures and temperatures would 

 vary slightly because they are dependent upon 

 the temperature of the coolingwater enteringthe 

 condenser, the amount of heat absorbed by the 

 refrigerant in the evaporator, and other factors. 



Liquid R-12 enters the thermostatic expan- 

 sion valve at high pressure, from thehighpres- 

 sure side of the system. The refrigerant leaves 

 the outlet of the expansion valve at a much lower 

 pressure and enters the low pressure side of the 

 system. Because of the relatively low pressure, 

 the liquid refrigerant begins to boil and to flash 

 into vapor. 



From the thermostatic expansion valve, the 

 refrigerant passes into the cooling coil (evapo- 

 rator). The boiling point of the refrigerant under 

 the low pressure in the evaporator is extremely 

 low— much lower than the temperature of the 

 spaces in which the cooling coil is installed. 

 As the liquid boils and vaporizes, it picks up its 

 latent heat of vaporization from the surround- 

 ings, thereby cooling the space. The refrigerant 

 continues to absorb heat until all the liquid has 

 been vaporized andthe vapor has become slightly 

 superheated. As a rule, the amount of superheat 

 is about 10° F. 



473 



