Condensation heat transfer for zeotropic mixtures of propane and n-butane on single tubes and in tube bundles

This work presents experimental condensation heat transfer coefficients α_cond for different binary zeotropic mixtures consisting of propane and n-butane on horizontal single tubes and in tube bundles measured with a setup specifically modified for this purpose. To study the influence of the condensation tube characteristics on α_cond, smooth, finned, and high-performance condensation tubes were investigated. The finned tubes include a systematic variation in the fin density and fin height. Besides pure copper, also a copper-nickel alloy was studied as tube material. During the condensation experiments, the vapor composition was measured continuously with the help of Raman spectroscopy, whose application for mixture condensation in a heat exchanger is presented in this work for the first time.

The results show that α_cond for zeotropic mixtures is significantly smaller than that for the pure substances because of the formation of a so-called diffusion film caused by the preferential condensation of the high-boiling mixture component. In tube bundle experiments, on the other hand, the impinging condensate from upper tube rows obviously break this diffusion film up, resulting in a significant increase of up to 670 % in α_cond with increasing irrigation mass flow rate.