Book - School of Science and Technology

Absolute pressure MPa532 Refrigeration: water chillers and heat pumps1086543210.80.60.50.40.30.2Sub-cooledliquid–20°C0°CC′ C D′ D40°C60°C34°C4°CLiquid andvapourB′ABCritical pointSuperheatedvapourA–B follows lineof constant entropyConstant temperaturelines0.1–40°C050 100 150 200 250 300Enthalpy kJ/kg above sat. liq. at –40°CCooling produced WorkinputSaturated liquidlineFigure 19.2 Pressure±enthalpy (total heat) diagram for Refrigerant 134aSaturated vapourlineC±D this is the pressure drop through the expansion device without any change inenthalpy (adiabatic).D±A represents vaporisation to a dry saturated state in the evaporator, latent heat ±represented by increasing enthalpy ± being drawn from the water, the air orother medium being cooled. This is the cooling effect.If the condenser were arranged to `sub-cool' the liquid, say to point C 0 , each unit massof refrigerant in circulation would produce a greater cooling effect (D 0 A) and the cyclewould be more efficient in consequence.The ratio of the cooling effect (as D±A) to the energy input (as A±B), in terms ofenthalpy, is known as the coefficient of performance. The smaller the range of pressure(and hence temperature) over which the cycle operates, the less will be the energyexpended for a given cooling effect. Hence, for economy in running, it is desirable todesign for:. Evaporator temperatures as high as is consistent with other considerations (such asdew-point temperature in an air-conditioning application).. Condenser temperatures as low as possible. When cooling is to be atmospheric,weather records will decide the safe minimum level to be assumed. Maximum coolingis generally required in the hottest summer weather when the condensing arrangementsare least efficient and caution is thus necessary in selecting an appropriatetemperature.. Should the reader wish to pursue this matter further, a selection of charts depicting theproperties of refrigerants in common use is provided in the Guide Section B14.