The determination of mixed refrigerant (MR) and the experimental study on the Joule-Thomson cryocooler for liquid nitrogen temperature with this MR are described in this thesis.
First, the thermodynamic cycle design was considered to develop a cryocooler by using a compressor of domestic air-conditioning unit. The target cooling performance of the designed cryocooler is 5 W around 70 K with less than 5 kJ/kg enthalpy rise. The systematic approach of selecting a proper refrigerant among 20 different kinds of mixture for such cryogenic temperature was introduced in detail. The main components of the cryocooler are compressor, evaporator, oil separator, after-cooler, counterflow heat exchanger, and J-T expansion device. Due to the limitation of the compressor operation range, the temperature after the compression was limited below 117℃ (390 K) and the temperature before compression was restricted above 5℃ (278 K). 20 atm of discharging pressure (high pressure) and less than 3 atm suction pressure (low pressure) were the design conditions. The inlet temperature of a counterflow heat exchanger in the high pressure side was about 300 K. The proper composition of the mixed refrigerant for the designed J-T cryocooler was 15 % mol of $N_2$, 30 % mol of $CH_4$, 30 % mol of $C_2H_6$, 10 % mol of $C_3H_8$ and 15 % mol of $i-C_4H_{10}$.
To validate the designed MR, J-T refrigeration experiment was preformed with a single stage air-conditioning compressor. Mixed refrigerant was charged into the system from the highest NBP(Normal Boiling Point) gas to the lowest one in order. Oil mist in the MR stream could be eliminated completely through the glass microfiber filter. In case of a traditional J-T refrigeration system (without bypass) with the mixed refrigerant, there occurred a fatal damage in a single stage compressor that had been designed for R22. Two main causes to this failure were considered in this thesis. First, the pressure ratio in the compressor was extremely increased at the beginning of the experiment if the pressure ratio was kept higher than 6 in the J-T expansion device. Second, the outlet temperature in compressor was higher than its limited temperature (117℃). For a stable operation of the compressor, a portion of the MR was by-passed at the inlet of the heat exchanger and transferred directly to the suction of the compressor in the modified system. The pressure ratio in J-T expansion device was gradually decreased due to condensation of the MR at low temperature. Therefore, the buffer volume at the suction was also used to increase the MR gas density in the system after the transient cool-down period. From the experiment with this modified system, the refrigeration could reach the lowest temperature of -152℃ without cooling load, and about -150℃ with 5 W of cooling load.
