Book - School of Science and Technology

Mechanical refrigeration 531GasCompressorGasCoolingeffectEvaporatorCondenserRejectedheatGas/liquidLowLiquidExpansiondeviceHigh pressureFigure 19.1 Principle of vapour compression cycle. A compressor to apply pressure to the refrigeration medium.. A condenser to receive the compressed gas and liquefy it, the latent heat being takenaway from the circuit by some external means. One method is to cool the condenserwith a water circulation which may then pass to a cooling tower for re-use. Alternatively,the condenser may be cooled by an air current.. An expansion device by which the pressure of the liquid is reduced.. An evaporator in which the medium re-evaporates, extracting heat from whateversurrounds it, e.g. from cooling water or air in an air-conditioning plant or from brinewhere temperatures below the freezing point of water are needed.Refrigeration cycleThe vapour compression cycle may best be considered on a pressure±enthalpy (totalheat) diagram, as Figure 19.2, which is drawn for the fluid Refrigerant 134a (see later text).Inside the curved envelope, the medium exists as a mixture of vapour and liquid and theincrease in enthalpy from left to right on any pressure line within the envelope representsan increase in latent heat. Further, within the envelope, lines of equal temperature(isotherms) are horizontal. Outside the envelope to the left, the medium exists as a liquidbelow its saturation temperature and there the isotherms are almost vertical. To the rightof the saturated vapour curve, the medium exists in the form of a superheated vapour andthe isotherms curve downwards. The critical point is that at which latent heat ceases toexist: it is not possible to liquify a gas by pressure alone if it is above the criticaltemperature.On the diagram, the refrigeration cycle is represented by the outline A±B±C±D, thecomponents being:A±B here the gas is compressed causing a rise in pressure and enthalpy which equalsthe energy put into the gas by the compressor, all in the superheat region. Thistakes place at constant entropy.B B 0 represents cooling of the superheated gas in the condenser down to the saturatedvapour temperature.B 0 C here latent heat is removed, also in the condenser, and the gas is condensed toliquid.