RR-6 The Second Law Efficiency of Process Heat Exchangers

K. J. Farrell

An efficiency is defined for process heat exchangers using the second law of thermodynamics. This parameter accounts for the unavoidable sources of irreversibility in a real heat exchanger—e.g., heat transfer over a finite temperature difference and flow friction. Its definition is based on the destruction of flow availability, which is a function of the mass flow rate of the streams, the duty, the dead state temperature, and the entropy. Currently, heat exchanger design approaches, including those in Xchanger Suite®, implement steady-state conservation laws only for mass and energy (the first law of thermodynamics). This report presents examples of second law efficiency analysis for various exchanger types. The more comprehensive second law analysis highlights the shortcomings of traditional design methods like effectiveness-NTU which may lead designers to high effectiveness exchangers with significant irreversibilities and that may not meet performance objectives. Moreover, the report demonstrates how second law analysis facilitates the calculation of the environmental impact (i.e., the carbon footprint) of the operation of a process heat exchanger.