Engineers who expect the appearance of the logarithmic-mean-temperature difference or complex heat-exchanger effectiveness factors to be encountered in selecting and analyzing the performance of evaporators will be pleased to know of a simplification. The streamlined process is made possible by the fact that the temperature of one of the fluids (the refrigerant) remains constant through the evaporator. The temperature distributions with a constant refrigerant temperature are shown in Fig. 6.7.
The rate of heat transfer is the product of UA and the log-mean-temperature difference,
Equation 6.12 shows that for a given evaporator operating with a constant flow rate that the term Factor is constant and the rate of heat transfer q is proportional to the temperature difference between the temperatures of the entering fluid and the refrigerant. In their catalogs, then, manufacturers only need to present the term Factor for each evaporator and each fluid flow rate.