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

摘要

Local in-tube heat transfer coefficient and pressure drop data have been collected forR-22 and two potential replacements, R-134a and R-32/R-125, in both smooth and micro-fintubes. The smooth tube data indicates that the heat transfer coefficient of R-32/R-125 is asmuch 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 pressuredrop while exhibiting similar heat transfer performance. The micro-fin tube data indicatesthat the most significant gains in heat transfer are realized at low mass fluxes were the microfintube seems to promote tube wall wetting. The study of return bend pressure dropindicates that the pressure drop due to return bends may be as much as 30% of the totalpressure drop in a typical serpentine evaporator. A computer model has been developed andused 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 anddiameter. A clear optimum is difficult to ascertain from the model, but the optimum tubelength is clearly a strong function of pressure drop which increases very rapidly as the tubediameter is decreased. Therefore, a slight undersizing of the tube diameter requires a muchlonger tube, so a conservative design of oversizing the tube diameter is preferable.