Exchanger performance behavior through irreversibility analysis for 1-2 TEMA G heat exchangers

The objective of this paper is to illustrate, discuss, and explain the interrelationship between the temperature difference irreversibility and heat exchanger effectiveness to clarify the performance trends of exchangers with some complex flow arrangements. This is because there is no physical explanation provided for the following results presented by Shah and Skiepko (ASME J. Heat Transfer, 126, pp. 994-1002, 2004): the heat exchanger effectiveness can be maximum, having an intermediate value or minimum at the maximum irreversibility operating point depending upon the flow arrangement of two fluids; similarly, the heat exchanger effectiveness can be minimum or maximum at the minimum irreversibility point. The analysis of such complex performance behavior is presented in this paper with an example of overall parallelflow and counterflow 1-2 TEMA G exchangers. This is accomplished by the decomposition of complex flow arrangements into simple subexchangers, and then the overall irreversibility trends for the exchangers are explained by irreversibilities produced due to temperature difference and fluid mixing in component subexchangers. It is shown for 1-2 TEMA G exchangers that the temperature difference irreversibility for a pure parallelflow subexchanger passes through a maximum at finite value of NTU1, and then approaches 0 when NTU1 -> infinity. On the contrary, the irreversibility for a pure counterflow subexchanger attains a minimum value at finite NTU1 and then increases with NTU1 and approaches 0 when NTU1 -> infinity for 1-2 TEMA G exchangers. This is because the temperatures at the inlet of the subexchangers are variable and dependent on the exit temperatures from the preceding subexchangers. Detailed exchanger effectiveness and temperature ratios are presented as a function of NTU1 for the explanation.