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INTRODUCTION Practical HTS applications such as a power cable, a current limiter, a transformer and a motor have been studied actively [1-4]. These applications require a cooling system that has cooling power from 2 kW to 10 kW at the temperature of 70 K and is able to achieve long maintenance interval [5]. Most of conventional cryocoolers, such as Stirling, Gifford- McMahon (G-M) and pulse tube cryocoolers, have cooling capacity from several W up to 1 kW, and first two coolers have a reciprocating displacer as well as reciprocating compressor. Consequently, they need to have frequent maintenance [6]. On the other hand, the refrigerator with a turbo-expander and a turbo-compressor is able to obtain long maintenance interval because they can employ non-contact bearings such as gas bearings and magnetic bearings. So, the development of reverse-Brayton cycle neon refrigerator of 2 kW at 65 K with a turbo-expander and a turbo-compressor is in progress. In order to keep the operating condition of turbo-expander stable, the boiling temperature of working fluid must be lower than the cooling temperature of HTS applications. Since the cooling temperature of the refrigerator is 65 K, neon, helium and hydrogen can satisfy the requirement of the refrigerator. Neon gas was chosen as working fluid for the refrigerator because the rotational speed of the turbo-expander is lower in the case of the larger molecular weight gas; molecular weight of neon is the largest of these gases [7]. A prototype neon refrigerator was initially designed and fabricated, which had a cooling power of 2 kW at the temperature of 70 K in 2007. A conventional reciprocating compressor was applied in this prototype refrigerator because there was not an adequate neon turbo-compressor. Therefore, the refrigerator can not obtain long maintenance interval. If a turbo-compressor were adopted for the refrigerator, the refrigerator could achieve high efficiency, maintenance-free and downsizing. Now, the development of a high performance neon turbo-compressor with magnetic bearings is underway. However, it is significantly difficult to make the efficiency of the turbo-compressor higher in this process pressure of 2 MPa ~ 1 MPa, because the calculated blade height of the compressor impeller becomes extremely small and the clearance leakage loss in the impeller is much larger. Therefore, the process pressure of the refrigerator was changed from 2 MPa ~ 1 MPa to 1 MPa ~ 0.5 MPa. And a new turbo-expander with active magnetic bearings for new process pressure was tested for the prototype refrigerator. TURBO-EXPANDER DESIGN Foil type dynamic gas bearings and a full-shrouded, closed type impeller were employed for the turbo-expander in the prototype refrigerator. Dynamic gas bearings have the advantage of keeping the rotating shaft non-contacting except for startup and shutdown [8]. The closed type impeller has generally higher efficiency than the open type impeller. The impeller was made by precision casting, but there were many defective impellers in the casting process, and also the manufacturing cost of complex shaped casting mold for the closed impeller was relatively expensive. On the other hand, active magnetic bearings and open type impeller were integrated to the new turbo-expander. One of the advantages of the magnetic bearings is that there is no contact between the bearing surface and the shaft even in case of startup and shutdown. 896PDF Image | NEON CRYOGENIC TURBO-EXPANDER WITH MAGNETIC BEARINGS
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