PRINCIPLES OF NAVAL ENGINEERING 



VALVE STEM 



DIAPHRAGM 



SPRING 

 CHAMBER 



PORT 



^OUTLET 



INLET 

 ADJUSTMENT 



PRESSURE AND 

 FLOW PATHS 



168.1 



Figure 19-4.— Thermostatic expansion valve. 



evaporator; and this, in turn, is controlled by 

 the spring adjustment of the thermostatic expan- 

 sion valve. About 10°Fof superheat is considered 

 desirable because it increases the efficiency of 

 the plant and because it ensures the evaporation 

 of all liquid, thus preventing liquid carryover into 

 the compressor. 



COMPRESSOR.— In a vapor- compression re- 

 frigeration system, the compressor is the unit 

 that pumps heat "uphill" from the cold side to 

 the hot side of the system. 



The heat absorbed by the refrigerant in the 

 evaporator must be removed before the refrig- 

 erant can again absorb latent heat in the evapo- 

 rator. The only way in which the vaporized 

 refrigerant can be made to give up the latent 

 heat of vaporization that it absorbed in the 

 evaporator is by condensation. In view of the 

 relatively high temperature of the available 

 cooling medium (sea water), the only way to 

 make the vapor condense is by first compress- 

 ing it. 



The vapor drawn into the compressor is at 

 very low pressure and very low temperature. In 

 the compressor, both the pressure and the tem- 

 perature are raised. Since an increase in pres- 

 sure causes a proportional rise in temperature, 

 and since the condensation point of a vapor is 

 determined by the pressure, raising the pressure 



of the vaporized refrigerant provides a conden- 

 sation temperature high enoughtopermittheuse 

 of sea water as a cooling and condensing medium. 

 In other words, the compressor raises the pres- 

 sure of the vaporized refrigerant sufficiently 

 high to permit heat transfer and condensation 

 to take place in the condenser. 



In addition to this primary function, the 

 compressor also serves to keep the refrigerant 

 circulating and to maintain the required pressure 

 differential between the high pressure side and 

 the low pressure side of the system. 



Many different types of compressors are used 

 in refrigeration systems. Figure 19-6 shows a 

 motor-driven, single-acting, two-cylinder re- 

 ciprocating compressor of a type commonly used 

 in naval shipboard refrigeration plants. 



CONDENSER.— The compressor discharges 

 the high pressure, high temperature refrigerant 

 vapor to the condenser, where it flows a round the 

 tubes through which sea water is being pumped. 

 As the vapor gives up its superheat to the circu- 

 lating sea water, the temperature of the vapor 

 drops to the condensation point. As soon as the 

 temperature of the vapor drops to its condensing 

 point at the existing pressure, the vapor con- 

 denses and in the process gives up the latent 

 heat of vaporization that it picked up in the 

 evaporator. The refrigerant, now in liquid form. 



478 



