Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGVALVE STEMPORTDIAPHRAGMSPRINGCHAMBER^OUTLETADJUSTMENTINLETPRESSURE ANDFLOW PATHSFigure 19-4.— Thermostatic expansion valve.168.1evaporator; and this, in turn, is controlled bythe spring adjustment of the thermostatic expansionvalve. About 10°Fof superheat is considereddesirable because it increases the efficiency ofthe plant and because it ensures the evaporationof all liquid, thus preventing liquid carryover intothe compressor.COMPRESSOR.— In a vapor- compression refrigerationsystem, the compressor is the unitthat pumps heat "uphill" from the cold side tothe hot side of the system.The heat absorbed by the refrigerant in theevaporator must be removed before the refrigerantcan again absorb latent heat in the evaporator.The only way in which the vaporizedrefrigerant can be made to give up the latentheat of vaporization that it absorbed in theevaporator is by condensation. In view of therelatively high temperature of the availablecooling medium (sea water), the only way tomake the vapor condense is by first compressingit.The vapor drawn into the compressor is atvery low pressure and very low temperature. Inthe compressor, both the pressure and the temperatureare raised. Since an increase in pressurecauses a proportional rise in temperature,and since the condensation point of a vapor isdetermined by the pressure, raising the pressureof the vaporized refrigerant provides a condensationtemperature high enoughtopermittheuseof sea water as a cooling and condensing medium.In other words, the compressor raises the pressureof the vaporized refrigerant sufficientlyhigh to permit heat transfer and condensationto take place in the condenser.In addition to this primary function, thecompressor also serves to keep the refrigerantcirculating and to maintain the required pressuredifferential between the high pressure side andthe low pressure side of the system.Many different types of compressors are usedin refrigeration systems. Figure 19-6 shows amotor-driven, single-acting, two-cylinder reciprocatingcompressor of a type commonly usedin naval shipboard refrigeration plants.CONDENSER.— The compressor dischargesthe high pressure, high temperature refrigerantvapor to the condenser, where it flows a round thetubes through which sea water is being pumped.As the vapor gives up its superheat to the circulatingsea water, the temperature of the vapordrops to the condensation point. As soon as thetemperature of the vapor drops to its condensingpoint at the existing pressure, the vapor condensesand in the process gives up the latentheat of vaporization that it picked up in theevaporator. The refrigerant, now in liquid form.478