抄録
Centrifugal helium compressors were developed to improve efficiency and reliability of helium refrigerators. These comprssors are operated almost below the temperature of liquid nitrogen. The rotor of the compressor is supported by helium lubricated gas bearings. A controlled attractive magnetic bearing is adoptted as auxiliary.
Two compressors were manufactured. The 1st. trial one was made simply for experiment to study the possibility of low-temperature helium compressors. The 2nd. trial one was made taking practical use into consideration in respect to flow rate and compression ratio.
This paper describes mainly the 2nd. trial compressor. Its typical specifications are as follows: type: two-stages centrifugal compressor with intercooler, flow rate: 75g/s, total compression ratio excluding the pressure loss in intercooler: 1.69, inlet helium temperature: 80K, rotational speed: 700rps (42, 000rpm), motor output: 10.2kW, type of bearings: helium lubricated internal tilting pad radial bearings and a spiral grooved thrust bearing with a controlled attractive magnetic bearing.
The compressor was installed in a vacuum chamber for cold test, and the performance was examined in a closed loop. Items to be measured are rotational speed, electric input power, flow rate, pressure and temperature of helium gas at each inlet and outlet ports, vibration of the rotor and the current of the magnetic bearing. The results showed that the compressor excelled the expected values in performance. The following is the principal performance at the rated rotational speed, total compression ratio at the flow rate of 74.8g/s: 1.76, adiavatic efficiency: 1st. stage; 71%, 2nd. stage; 73%, total efficiency: 62%. The rotor supported by the bearing above mentioned operated stably up to 44, 000rpm. The controlled attractive magnetic bearing gave an adequate clearance to the thrust gas bearing in start and stop condition, so that the rotor was able to start many times without any damage to the gas bearings. The compressor has operated for a long time.
