Manufacturers of condensers provide performance data directed toward selecting equipment. By applying some fundamentals of heat transfer, a user can frequently translate catalog data to nondesign conditions. The strategy in extending catalog data to nondesign conditions is usually to compute the UA value (the product of the overall heat transfer coefficient and the heat-transfer area) and for situations where the UA remains essentially constant, apply this UA value to the new set of operating conditions. The temperature profiles are somewhat complex because of desuperheating and subcooling, as shown in Fig. 7.5a, but to approximate, assume the condensing temperature prevails throughout the condenser, as shown in Fig. 7.5b.
In the desuperheating section, the actual temperature difference between the refrigerant and cooling water is higher than the ideal, but this error is at least partially compensated for by the fact that the actual heat-transfer
coefficient for the convection process is less than during condensation. Real condensers are rarely circuited strictly for counterflow or parallel flow. When one fluid is at a constant temperature, however, the flow pattern is immaterial, and an equation comparable to the one for evaporators, Eq. 6.11, applies: