Experimental investigation on the thermal stability of some new zero ODP refrigerants

Five zero ODP (ozone depletion potential) hydro-fluorocarbon refrigerants (HFC-23, HFC-143a, HFC-227ea, HFC-236fa, HFC-245fa) were tested to define their maximum usable temperature and their thermal degradation threshold. Pyrolysis is detected (a) as a pressure change at constant temperature and volume; (b) as a departure of the vapour pressure curve of the heated fluid from that of the original substance. Visual inspection of the vessel walls and fluid chemical analysis complement the method. The minimum detectable degradation rate is believed to be less than 1% in 50 h. All the fluids exhibit a variable, but excellent thermal stability up to the following temperatures at which no decomposition was observable in 50–100 h: 425 °C for HFC-227ea, 400 °C for HFC-23 and HFC-236fa, 350 °C for HFC-143a and 300 °C for HFC-245fa. Clear degradation signs were observed at temperatures 25–50 °C higher. Most of the fluids heated up to their thermal stability threshold exhibited an induction period of 5–50 h in which no decomposition was detectable but after which an observable degradation started. For a given fluid such period decreases at increasing temperatures. The use of fluids in a cyclic process in which the working medium permanence at the top temperature is very brief could take advantage of this behaviour with a reduction in degradation rates or with an increase in the limiting temperature. The influence of the decomposition products on the functionality of a thermodynamic power cycle was investigated by means of an appropriate computer code. The working fluid was assumed to be a binary mixture with 1 to 3% concentration of a light decomposition product of the methane series. Chemical species such as CH4 and CF4 with a critical temperature much lower than that of the base fluid strongly affect the cycle configuration. On the contrary species with critical temperatures closer to that of the base fluid such as CH3F, CH2F2 or CHF3 influence only marginally the cycle performance. In general a small concentration of decomposition products in the working medium is likely to be acceptable without noticeable drawbacks.