Book - School of Science and Technology

558 Refrigeration: water chillers and heat pumpsrespect to be aware whether the consumption of all appropriate auxiliaries is included inthe calculation.It must be remembered of course that, in comparison with a cooling application, theenergy in driving the compressor is an asset as far as a heat pump is concerned; theequation being transposed algebraically:Coolingevaporator capacity ˆ condenser capacitycompressor powerHeatingcondenser capacity ˆ evaporator capacity ‡ compressor powerIt will be appreciated that it is practicable for the same equipment to have a dual function,providing cooling in summer and heating in winter. Such a unit is described in Chapter 14and the operating principle illustrated in Figure 14.26. One technical reservation whichmust be appreciated is that, in designing the components for optimum operation for thisdual function, these may not be those best suited to either the cooling or heating modealone and in consequence if the period in one mode far exceeds the other the unit shouldbe selected with a bias towards the more extended period of usage. Typically, a heat pumpoperating from a source temperature of 5 C and designed for heating only would have aCOP of about 3, whereas a reversible machine for the same conditions would have a COPof about 2.6.As may be seen from Figure 19.23, which represents the output of a reciprocating semihermeticmachine, performance varies with evaporating and condensing temperature, the830735Coefficient of performance65440455055Condensing temperature, °C32–15 –10 –5 0 +5 +10Evaporating temperature, ºCFigure 19.23 Typical heat pump performance