Principles of naval engineering - Historic Naval Ships Association

Chapter 19. -REFRIGERATION AND AIR CONDITIONING PLANTSFigure 19-5.— Evaporator tubing.47.93is subcooled slightly below its boiling point atthis pressure to ensure that it will not flashinto vapor.A water-cooled condenser for an R-12 refrigerationsystem is shown in figure 19-7.Circulating water is obtained through a branchconnection from the firemain or by means of anindividual pump taking suction from the sea.A water regulating valve (not shown) is usuallyinstalled to control the flow of cooling waterthrough the condenser. The purge connectionshown in figure 19-6 is on the refrigerant side;it is used to remove air and other noncondensablegases that are lighter than the R-12 vapor.Most condensers used in naval refrigerationplants are water cooled. However, some smallunits have air-cooled condensers. These consistof tubing with external fins to increase the heattransfer surface. Most air-cooled condensershave fans to ensure positive circulation of airaround the condenser tubes.RECEIVER.— The receiver,shown in figure19-8, acts as a temporary storage space andsurge tank for the liquid refrigerant which flowsfrom the condenser. The receiver also servesas a vapor seal to prevent the entrance of vaporinto the liquid line to the thermostatic expansionvalve.ACCESSORIES AND CONTROLS .-In additionto the five major components just described, arefrigeration system requires a number of controlsand accessories. The most important ofthese are discussed briefly in the followingparagraphs.A dehydrator (or dryer ) is placed in theliquid refrigerant line between the receiverand the thermostatic expansion valve. In olderinstallations, such as the one shown in figure19-2, bypass valves allow the dehydrator to becut in or out of the system. In newer installations,the dehydrator is installed in the liquidrefrigerant line without any bypass arrangement.A refrigerant dehydrator is shown infigure 19-9.A solenoid valve is installed in the liquidline leading to each evaporator, Figure 19-10shows a solenoid valve and the thermostaticcontrol switch that operates it. The thermo-static control switch is connected by long flexiblecapillary tubing to a thermal bulb whichis located in the refrigerated space. Whenthe temperature in the refrigerated space dropsto the desired point, the thermal bulb causesthe thermostatic control switch to open, therebyclosing the solenoid valve and shutting off allflow of liquid refrigerant to the thermostaticexpansion valve. When the temperature in therefrigerated space rises above the desired point,the thermostatic control switch closes, thesolenoid valve opens, and liquid refrigerantonce again flows to the thermostatic expansionvalve.The solenoid valve and its related thermostaticcontrol switch serve to maintain theproper temperature in the refrigerated space.However, we may wonder why the solenoidvalve is necessary, since the thermostaticexpansion valve controls the amount of refrigerantadmitted to the evaporator. Actually, thesolenoid valve is not necessary in systemshaving only one evaporator. In systems havingmore than one evaporator, where there is widevariation in load, the solenoid valve providesthe additional control required to prevent spacesfrom becoming too cold at light loads.In addition to the solenoid valve installedin the line to each evaporator, a large refrigerationplant usually has a main liquid linesolenoid valve installed just after the receiver.If the compressor stops for any reason exceptnormal suction pressure control, the main liquidline solenoid valve closes and prevents liquidrefrigerant from flooding the evaporator andflowing to the compressor suction. Great damageto the compressor can result if liquid isallowed to enter the compressor suction.Whenever several refrigerated spaces ofvarying temperatures are to be maintainedby one compressor, an evaporator pressure479