A computational model to predict the performance of flooded refrigerant evaporators

A theoretical model was developed to predict the performance of flooded refrigerant evaporators (FRE). The model is general and can be applied to any refrigerant. It incorporates theoretical relations to predict the refrigerant shell-side boiling heat transfer coefficient and pressure drop across the tube bundle. The model accounts for the effect of refrigerant pressure drop across the tube bundle on the performance of FRE. The model adopts an incremental iterative procedure to perform row-by-row calculations over specified incremental tube lengths for each water-side pass. A modified form of Chen model is used to estimate the shell-side two-phase heat transfer coefficient.;The results from the program were compared with experimental data obtained from manufacturers of commercial FRE operating with R-11. It underpredicts the heat duty 4.0 to 6.0% for various FRE with specified fluid conditions, tube bundle geometry and water pass arrangement. The model was extended further, to predict the performance of FRE operating with low pressure refrigerant R-11 and "alternate" R-123, and high pressure refrigerants R-12, "alternate" refrigerant R-134a and R-22 with integral fin tube as well as "enhanced" tube bundles. Parametric studies were conducted for FRE operating with four refrigerants--R-11, R-123, R-12 and R-134a. The effects of bundle aspect ratio and entering flash gas on the performance of FRE were studied. Conclusions were drawn regarding the possible mechanism of heat transfer on the shell-side. Results are presented to show the local variation of shell-side heat transfer coefficients and pressure drop. Performance evaluation analyses were conducted for three different design criteria. The compressor power requirements for various refrigeration cycles operating with the four refrigerants and different FRE were obtained. In computing the compressor power requirements the condenser temperature and compressor efficiency were fixed. The possible costs involved as a result of substituting refrigerant R-11 by "alternate" refrigerant R-123 and R-12 by "alternate" refrigerant R-134a in the refrigeration cycles are discussed.