HFC Mixtures as Replacement for HCFC-22 in Large-Scale Heat Pumps and Chillers

Doktorsavhandling, 2000

Replacement of the conventional refrigerant HCFC-22 (R22) is one of the urgent issues to be solved within the scheduled time-frame of the international regulation of the HCFCs. Since there exists no pure fluid that can replace R22 in the same general way as HFC-134a (R134a) is a replacement for CFC-12, HFC mixtures are suggested as possible substitutes. The main aim of this work has been to find good alternatives to R22 in two different applications. For this purpose a comprehensive simulation program was developed. Together with methods for technical and economic optimization, the program can be used to find the optimal design for a specific working fluid ("new design situation") as well as the optimal working fluid for a specific plant ("drop-in situation"). As one application, the replacement of R22 with the zeotropic HFC mixture R407C in a liquid chiller, equipped with a dry-expansion evaporator and a condenser with shell-side condensation, was investigated, both experimentally and theoretically. Full-scale measurements show an evaporator performance of R407C similar to that of R22, except at very low temperatures. The simulation model could predict the change in evaporation heat transfer within 10%. Changes in design parameters, such as the optimal number of evaporator tubes for a given area, or the distribution of heat exchanger area between the evaporator and the suction gas heat exchanger for a given cost, could increase the performance by only 0.5% and 3%, respectively. The condenser design should, however, be made differently for a zeotropic mixture, since the measurements show a decrease in overall heat transfer coefficient of up to 70%. This decrease was not predicted by the simulation program, implying that the calculation model must consider not only the mass transfer resistance, but also the degree of condensate mixing. The other application studied deals with heat pumps used for district heating. In these heat pumps, with condensation temperatures up to 80ºC, the only commercially available, non-flammable alternative to replace R22 is R134a. Its use could, however, result in a decrease of heating capacity by up to 35%. Screening and simulations show that there are HFC mixtures, such as 25%R32 + 75%R134a, and 30%R143a + 25%R125 + 45%R134a (by mole), which can offer an increase in capacity of 10-20% compared to R134a. There is, however, a corresponding decrease in COP of 5-15%. The importance of considering all limiting parameters of the system when comparing the performance of different working fluids was shown.