The mixed refrigerant (MR) Joule-Thomson (JT) refrigeration cycle was designed for cooling HTS (High Temperature Superconductor) cable since the JT refrigeration cycle satisfies the demand of large cooling capacity, simple structure, and low capital cost. The refrigeration cycle was designed as two-stage cascade system to increase the system efficiency. The pre-cooling stage cools down to 100 K with nitrogen-hydrocarbon MR, and a neon-nitrogen mixture was used in the main cycle to decrease the cooling temperature down to 64 K. The low temperature cycle is to operate with a neon-nitrogen mixture, and the required com-pression ratio is 24.1 when the suction pressure is 100 kPa. This high compression ratio of 24.1 with heat of compression gases is a challenging issue which cannot be readily provided in existing compression systems. To operate such refrigeration system it is necessary to study the compressor.
An innovative system is developed with commercial oil-lubricated air-conditioning compressors. It is the combination of 2-stage rotary compressors in series originally designed for R410A. The compressors are modified to accommodate effective inter-cooling at every stage to decrease the discharge gas temperature.
This 2-stage rotary compressor is applied to an inverter system, to control the frequency of each compressor. The P-V diagram is made by using the each compression stage’s pressure and temperature measurement, this is performed to check the insufficient compression of the compression system, and it is presented to the experimental method of finding an optimum compression process by converting the frequency of each com-pressor.
Additionally, fine-grade three-stage filters are installed at the last stage to avoid a clogging problem from oil mist and moisture at low temperature sections. The present compression system is specifically demonstrated with a neon-nitrogen MR JT cryocooler. The operating pressure ratio is satisfactory to meet the design specifi-cations, and the recorded no-load minimum temperature is 64 K. The commercial air-conditioning compres-sors are successfully applied to the modified high-compression ratio MR JT cryocooler which can be easily constructed with off-the-shelf components.
초전도케이블용 극저온 Joule-Thomson 냉동기는 대용량 및 구조가 간단하며 저비용으로 구현이되며,이러한 장점을 활용한 혼합냉매용 Joule-Thomson 냉동기가 설계되었다. 설계된 냉동사이클은 시스템의 효율을 증대하기 위해 2단 cascade 설계되었으며, 예냉사이클의 경우 질소-탄화물로 구성되어 있으며 100K 냉각되게 설계되었으며, 본 사이클로 구성된 사이클의 경우 네온-질소 혼합물로 구성되었으며 64K 온도까지 냉각하게 설계되었다.네온-질소 혼합물을 적용한 저온 사이클의 압축비는 24.1이며 흡입압은 100kPa 요구된다. 네온질소혼합물을 24.1의 압축비로 압축할 경우 압축가스온도는 도전적인 과제이며 이를 해결하기 위해 에어컨용 R410A 2단 로터리압축기를 활용하여 4단 압축 시스템을 구성하였다. 로터리 2단 압축기를 수정하여 각단에서 압축후 토출가스온도를 냉각후 압축하는 구성으로 재조합하였다. 2단 로터리 압축기는 가변속압축기로 각각의 압축기의 주파수를 변환하는 시스템으로 구성되어 있다. 각각의 압축단의 온도와 압력을 측정함으로써 P-V 선도를 만들수 있었으며, 이를 활용하여 압축시스템의 부족압축 부분을 확인하였으며, 각각의 압축기의 운전주파수를 변환함으로써 최적의 압축과정을 찾았다. 추가적으로 오일이 적용된 압축기의 유순환율은 3단계 필터를 적용함으로써 저온사이클에서 Clogging을 해결할 수 있었다. 압축시스템은 네온-질소혼합물을 적용하여 구현이 되며, 작동압력은 설계압을 만족하고, 극저온 냉동기를 적용하여 진행할 경우 최저온도를 64K을 구현하였다.
