Operation Characteristics of the Helium Refrigerator for the Large Helical Device with a Dummy Load Apparatus

Abstract

The helium refrigerator/liquefier operation for the Large Helical Device (LHD) with a dummy load apparatus (DLA) has been conducted under four different refrigeration conditions: no heat load mode; precooling mode; steady state mode and extra heat input mode. The stable operation of each mode, which simulates the refrigeration requirements for the LHD, was successfully demonstrated under semi-automatic control of the refrigerator. To establish the sequential control of the system, historical and real time trends of PID controlled electro-pneumatic actuators for valves and its corresponding thermal-hydraulic behavior of helium gas in the refrigeration system were collected as a data base. According to the operating experiences, the instabilities in the refrigeration system were observed. These instability mechanisms are considered to be quite complex: they are assumed to be coupled and/or combinations of heat exchangers in the refrigerator with approximately 90 to 100 m long supercritical helium hydraulic paths. To suppress these instabilities, adjustments of PID controlled valves and modifications of pressure control for the compressors were conducted. The paper describes operation characteristics of the helium refrigerator/liquefier with a DLA. The instabilities in the refrigeration system are also discussed.