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Second-law performance of heat exchangers for waste heat recovery
Energy, 35 (5): 1936 - 1945 (2010)
DOI: 10.1016/j.energy.2010.01.007
Аннотация
Exergy change rate in an ideal gas flow or an incompressible flow can be divided into a thermal exergy change rate and a mechanical exergy loss rate. The mechanical exergy loss rates in the two flows were generalized using a pressure-drop factor. For heat exchangers using in waste heat recovery, the consumed mechanical exergy is usually more valuable than the recovered thermal exergy. A weighing factor was proposed to modify the pressure-drop factor. An exergy recovery index (etaII) was defined and it was expressed as a function of effectiveness (var epsilon), ratio of modified heat capacity rates (C*), hot stream-to-dead-state temperature ratio, cold stream-to-dead-state temperature ratio and modified overall pressure-drop factor. This etaII-var epsilon relation can be used to find the etaII value of a heat exchanger with any flow arrangement. The etaII-Ntu and etaII-Ntuh relations of cross-flow heat exchanger with both fluids unmixed were established respectively. The former provides a minimum Ntu design principle and the latter provides a minimum Ntuh design principle. A numerical example showed that, at a fixed heat capacity rate of the hot stream, the heat exchanger size yielded by the minimum Ntuh principle is smaller than that yielded by the minimum Ntu principle.
