Development of a high efficiency non-flammable mixed refrigerant Joule-Thomson refrigerator for 100 K cooling temperature

Abstract

Non-flammable mixed refrigerant (MR) Joule-Thomson (JT) refrigerator is developed for the cooling temperature of 100 K. The working fluid for non-flammable MR JT refrigerator is a mixture of Nitrogen, Argon, Tetrafluoromethane (R14, $CF_4$ ) and Octafluoropropane (R218, $C_3F_8$ ). Unfortunately, non-flammable refrigerant has considerably higher freezing temperature compared to target cooling temperature of 100 K. Freezing temperatures of the working fluid are investigated before constructing the non-flammable MR JT refrigerator. Certain mixture of Argon, R14 and R218 shows remarkably low freezing temperature. Entire compositions of non-flammable MR, which obtains extremely low freezing temperature, are analyzed to achieve high COP (coefficient of performance). Simple configuration (Single MR JT refrigerator) and complex configuration (cascade MR JT refrigerator) are considered in the simulation to figure out the efficiency according to the complexity of the refrigerator. The simulation results show that the cascade MR JT refrigerator by using MR 4-9 ($N_2$ /Ar/R14/R218 = 0.2/0.3/0.3/0.2) has the maximum COP as 0.216 among the entire configurations. Tiny precooling in cascade MR JT refrigerator could dramatically enhance the COP by temperature match inside the heat exchanger. Experimental work is performed to confirm the design result of the cascade MR JT refrigerator. The lowest temperature is approximately 98.5 K and the maximum cooling capacity records 15 J/g at the 103 K, which is the normalized value with respect to the mass flow rate of the working fluid. Furthermore, concentrated pressure drop expansion (JT valve) and gradual pressure drop expansion (capillary tube) are utilized in the MR JT refrigerator. Due to the absence of controllability in the capillary tube, MR JT refrigerator cannot develop the sufficient operating condition. Thus, if the MR JT refrigerator requires sufficient degree of subcooling in front of the expansion device, then adjustable expansion device should utilize as the JT expansion part.