J.M. Corberán

Modelling of plate finned tube evaporators and condensers working with R134A

Abstract A model to predict the behaviour of finned tube evaporators and condensers working with R134a has been developed. For modelling of the refrigerant phase change, evaporation or condensation, the heat transfer and the pressure drop for the two-phase flow have to be calculated. Therefore, a number of correlations, the most recommended ones in the reviewed literature, have been analysed and compared. The results of this comparison are presented for the evaporation and condensation heat transfer coefficients and for the evaporator frictional pressure drop. Once the correlations have been implemented in the model and compared with the experimental results, the ones that work best for the studied heat exchangers have been ultimately selected. The experimental study to validate the model has been carried out in a small airconditioning unit with cross-flow air-refrigerant type heat exchangers. The results are compared with model predictions for thermal capacity, refrigerant superheat or subcooling, and tube-side pressure drop.

Error estimation of single phase effectiveness and LMTD methodologies when applied to heat exchangers with phase change

Single phase formulas based on logarithmic mean temperature difference (LMTD) or Effectiveness (ɛ-NTU) are widely and wrongly used for the thermal analysis of evaporators and condensers. Those formulas do not take into account that temperature variation during phase change is due to pressure variation and/or concentration changes when using nonazeotropic refrigerant mixtures. This paper first presents the correct evaluation of the mean temperature difference and effectiveness, for parallel and counter flow arrangements, under the hypothesis of linear temperature variation, for both evaporator and condenser cases. Then, the analytical solution is employed to evaluate the error of applying the single phase formulas of LMTD and Effectiveness to the phase change part of evaporators and condensers.