Thermodynamic Efficiency of Heat Exchange Devices

Abstract

The continued rise in the cost of energy 'has it imperative to augment the usual heat flow analyses for power plants, refineries, chemical plants and other energy intensive industries by adding analyses of available energy flow and device irreversibilities. The reclamation of what was formerly 'waste heat' by using additional, or more efficient, equipment has become not only economically feasible, but sometimes essential.

A thermodynamic efficiency based on the second law of thermodynamics is defined for heat exchange devices. The efficiency can be simply written in terms of the mean absolute temperature levels of the two fluids exchanging heat, and an appropriate environment temperature. It is also shown that for a given ratio of hot to cold inlet temperatures, the efficiency and effectiveness for particular heat exchange configurations are related. Conclusions regarding the effect of stream temperatures on the efficiency of various types of exchangers are made. The concept is applied to typical heat exchange cases to demonstrate its applicability and sensitivity.