Abstract:

The Logarithmic Mean Temperature Difference (LMTD) method and the effectiveness-NTU method are the two important methods for design and analysis of heat exchangers. The derivation of these two methods relies on a critical assumption, i.e., the fluid specific heats are constant. Under special operating conditions where one fluid experiences condensation or evaporation at constant temperature, these two methods are still valid. In practice, however, the fluid temperature in heat exchangers will never remain constant during phase change because of pressure drop. Meanwhile, zeotropic refrigerant mixtures exhibit temperature variations even during a constant pressure phase change process. Therefore, both LMTD and effectiveness-NTU methods can introduce appreciable errors when applying to the cases in which refrigerant temperature change is not caused by heat transfer, rather than by pressure drop or temperature glide. This paper proposes modified LMTD method and effectiveness-NTU relations to remove the restriction of constant temperature phase change in the original approaches. The new methods account for the effects of pressure drop and temperature glide on the two-phase heat transfer process and make corresponding corrections based on simplifying assumptions. The new methods are applicable for both parallel-flow and counter-flow configurations, with phase change on one side. Rigorous error analyses indicate that the new approaches can substantially improve the thermal performance prediction for heat exchangers with large pressure drop and temperature glide.