DESIGN OF OILFREE ALL TURBO-TYPE HELIUM REFRIGERATOR

We have designed a high efficient, compact 300 W oilfree all turbo-type helium refrigerator capable of long maintenance free continuous operation. The main compressor at ambient temperature which is driven by a helium gas turbine attached to the same shaft, is supported by magnetic bearings, and, has an inlet and outlet pressure of 0.35 MPa and 0.7 MPa. Four turbo expanders are employed, including one at 80 K because LN(2) is not used in this system. Each expander drives a cold compressor, accompanying a generator brake by use of an induction motor. This provides very easy control. Variable nozzles are adopted to the expanders, and the number of valves are diminished as much as possible to reduce line pressure loss. Concerning a heat exchanger, we have examined a micro tube laminar flow heat exchanger which is compact and enables high efficiency at low temperature. This heat exchanger can reduce pressure loss greatly.