혼합냉매 조성비에 따른 초저온 MR 줄-톰슨 냉동기의 성능특성

Abstract

There are ways to reach very low temperature below -60℃ such as multi stage cascade refrigeration system, auto-cascade refrigeration system and Joule-Thomson refrigeration system using mixed refrigerant. In this paper, performance change of J-T refrigeration cycle using mixed refrigerant was analyzed with respect to composition of the mixed refrigerant experimentally. The composition of the mixed refrigerant includes refrigerants with high boiling point over one sort which can be condensed by water or air at operating pressure, refrigerants with low boiling point which can reach lower evaporating temperature than target temperature and refrigerants with middle boiling point between low boiling point and high boiling point. Cycle performance is different with respect to concentration of each component, so mixed refrigerant should have appropriate composition and concentration corresponding with designed operating condition. In previous study on MR J-T refrigeration, hydrocarbon refrigerant has been applied mainly due to its high Joule-Thomson cooling capacity. The hydrocarbon refrigerant has characteristics of flammability and explosiveness, so the characteristics are regarded hindrance to apply industrial filed because of facilities necessary for explosion proof and fire prevention. In this paper, performance characteristics of mixed refrigerant applying flammable refrigerant, R-600a, was analyzed with respect to concentration of each component. Also, R-134a and R-245fa were applied in order to replace the flammable refrigerant R-600a. As results of the experiment, compressor discharge temperature decreased as compressor inlet temperature decreased when concentration of the refrigerant with high boiling point increased. However, liquid compression occurred when the concentration of the refrigerant had over 65% mol. In case of the refrigerant with middle boiling point, condensation pressure and temperature at inlet of expansion valve decreased when concentration of the refrigerant increased. In case of the refrigerant with low boiling point, increase of compressor suction and discharge temperature were presented as the concentration increased. The compressor suction temperature and the discharge temperature increased -15.5℃, 85.7℃ with 20.1% mol concentration of the refrigerant. Based on these results, it is confirmed that compressor discharge temperature decreases because dew point of the mixed refrigerant increases as the concentration of the refrigerant with high boiling point increases. Also, concentration of the refrigerant with middle boiling point effects on cooling for refrigerant at hot side, so it causes condensation pressure decreases. The concentration of the refrigerant with low boiling point influences on state of the compressor inlet and outlet directly. R-134a were applied as a refrigerant with high boiling point replacing flammable refrigerant R-600a, and experimental results are as follows. The concentration of R-134a changed from 58.0% mol to 68.1% mol, and compressor discharge temperature tended to decrease like R-600a test when the concentration increased. So evaporation temperature could reach until -75.2℃ at 65.0% mol concentration. But liquid compression was also occurred with the concentration over 65.0% mol. In order to solve the liquid compression, concentration of refrigerants with high boiling point was fixed at 58.0% mol. And then, R-245fa was applied to increase dew point of the mixed refrigerant. When concentration of R-245fa increased from 10.0% mol to 30.0% mol, compressor discharge temperature decreased from 102.4℃ to 90.4℃. Whereas, evaporator inlet temperature presented increase from -72.2℃ to -67.8℃. Consequently, performance change of the chiller was analyzed with regard to component of mixed refrigerant and concentration of each component. And the experimental data can be utilized as fundamental design data to select working fluid to MR J-T refrigeration.