Logarithmic Mean Temperature Difference in heat exchangers employing ammonia-water mixtures

Abstract

The temperature of zeotropic mixtures does not remain constant throughout a heat exchanger. Furthermore, Zeotropes often exhibit a nonlinear temperature-enthalpy relationship. These factors contradict some of the assumptions made in deriving the log mean temperature difference (LMTD), which is used in computing the size of a heat exchanger (UA). In this paper an ammonia-water mixture in a counter-flow heat exchanger will be examined as an example of a zeotropic mixture. The actual (UA) obtained via numerical analysis will be compared to the (UA) found using (LMTD), and representative error scales shall be developed for this purpose.

Keywords: Zeotropic mixtures, Ammonia-water, Log mean temperature difference, Heat exchanger size, Error scales, Energy equation solver.