Heat Transfer and Flow Characteristics of R-22, R-321R-125 and R-134a in Smooth and Micro-Fin Tubes

Local in-tube heat transfer coefficient and pressure drop data have been collected for R-22 and two potential replacements, R-134a and R-32/R-125, in both smooth and micro-fin tubes. The smooth tube data indicates that the heat transfer coefficient of R-32/R-125 is as much as 50% higher than that of R-22 for similar conditions, while the pressure drop is 30% less. R-134a generally shows a weaker performance than R-22 in terms of higher pressure drop while exhibiting similar heat transfer performance. The micro-fin tube data indicates that the most significant gains in heat transfer are realized at low mass fluxes were the microfin tube seems to promote tube wall wetting. The study of return bend pressure drop indicates that the pressure drop due to return bends may be as much as 30% of the total pressure drop in a typical serpentine evaporator. A computer model has been developed and used to simulate the evaporator component in order to compare the three refrigerants. R- 32/R-125 outperforms R-22 in the simulation while R-134a shows the weakest performance. The computer model is also used to perform an optimization of evaporator tube length and diameter. A clear optimum is difficult to ascertain from the model, but the optimum tube length is clearly a strong function of pressure drop which increases very rapidly as the tube diameter is decreased. Therefore, a slight undersizing of the tube diameter requires a much longer tube, so a conservative design of oversizing the tube diameter is preferable.Air Conditioning and Refrigeration Center Project 0