Principles of naval engineering - Historic Naval Ships Association

Chapter 19. -REFRIGERATION AND AIR CONDITIONING PLANTSThe refrigerant leaves the evaporator as alow pressure superheated vapor, having absorbedheat and thus cooled the space. Theremainder of the cycle is concerned withdisposingof this heat and getting the refrigerant backinto a liquid state so that it can again vaporizein the evaporator and thus again absorb heat.The low pressure superheated vapor is drawnout of the evaporator to the suction side of thecompressor. The compressor is the unit whichkeeps the refrigerant circulating through thesystem. In the compressor cylinders, the refrigerantis compressed from a low pressurevapor to a high pressure vapor, and its temperaturerises accordingly.The high pressure R-12 vapor is dischargedfrom the compressor to the condenser. Here therefrigerant condenses, giving up its superheat,its latent heat of vaporization, and its heat ofcompression to the cooling sea water whichflows through the condenser tubes. The refrigerant,still at high pressure, is now a liquidagain.From the condenser, the refrigerant flowsinto a receiver, which serves as a storage placefor the liquid refrigerant. From the receiver,the refrigerant goes to the thermostatic expansionvalve and the cycle begins again.From this brief summary of an R-12 vaporcompressionrefrigeration system, it may beseen that the cycle is indeed one in which heatis "pumped uphill" as a result of the arrangementswhich cause the refrigerant to go throughsuccessive phases of expansion, evaporation,compression, and condensation.Major ComponentsThe major components of a shipboard R-12refrigeration plant are shown diagrammaticallyin figure 19-2. The primary parts of the systemare the thermostatic expansion valve, the evaporator,the compressor, the condenser, and thereceiver. Additional equipment required to completethe plant includes piping, pressure gages,thermometers, various types of control switchesand control valves, strainers, relief valves, sightflow indicators, dehydrators, and charging connections.Figure 19-3 shows most of the componentson the high pressure side of an R-12system, as actually installed aboard ship.In the following discussion of the major componentsof an R-12 system, we will treat thesystem as though it had only one evaporator,one compressor, and one condenser. As may beseen from figure 19-2, however, a shipboardrefrigeration system may (and, indeed, usuallydoes) include more than one evaporator and mayinclude additional compressor and condenserunits to provide operational flexibility and toprotect against loss of refrigerating capacity.THERMOSTATIC EXPANSION VALVE .-Thethermostatic expansion valve, shown in figure19-4, is essentially a reducing valve between thehigh pressure side and the low pressure side ofthe system. The valve is designed to proportionthe rate at which the refrigerant enters the coolingcoil to the rate of evaporation of the liquidrefrigerant in the coil; the amount depends, ofcourse, on the amount of heat being removedfrom the refrigerated space.A thermal bulb for the thermostatic expansionvalve is clamped to the cooling coil, nearthe outlet. The bulb contains R-12. Control tubingconnects the bulb with the area above thediaphragm in the thermostatic expansion valve.When the temperature at the bulb rises, the R-12expands and transmits a pressure to the diaphragm;this causes the diaphragm to be moveddownward, thus opening the valve and allowingmore refrigerant to enter the cooling coil.When the temperature at the bulb falls, thepressure above the diaphragm is decreasedand the valve tends to close. Thus the temperaturenear the evaporator outlet controlsthe operation of the thermostatic expansionvalve.EVAPORATOR.— The evaporator consists ofa coil of copper tubing installed in the space tobe refrigerated. Figure 19-5 shows some of thistubing. The liquid R-12 enters the tubing at avery much reduced pressure and the boilingpoint is therefore very much lowered. In passingthrough the expansion valve, going from thehigh pressure side of the system to the lowpressure side, some of the refrigerant boils andvaporizes because of the reduced pressure andsome of the remaining liquid refrigerant isthereby cooled to its boiling point. Then, as therefrigerant passes through the evaporator, theheat flowing to the evaporator from the surroundingair causes the rest of the liquid refrigerantto boil and vaporize.After the refrigerant has absorbed its latentheat of vaporization and all the liquid has beenvaporized, the refrigerant continues to absorbheat until it has acquired about 10° F of superheat.The amount of superheat is determined bythe amount of liquid refrigerant admitted to the475