Conventionally, for the purpose of preventing the occurrence of operating noises of a compressor inside a room, a compressor unit has been provided outside the room and helium gas discharged from the compressor has been air-cooled by an air-cooling heat exchanger. However, since the cooling by the air-cooling heat exchanger installed outside the room is air-cooling, the helium gas cannot be cooled to a temperature lower than the air temperature by this air-cooling heat exchanger. Therefore, when the outdoor temperature is high in summer or in a similar case, the temperature of the helium gas supplied to the cryogenic expander is hard to be maintained at a temperature not higher than a temperature (35.degree. C., for example) at which the insulating property of a motor is assured.
In view of the above, there is proposed a cryogenic refrigerating apparatus (Japanese Patent Laid-Open Publication No. HEI 6-249148) which can cool the helium gas to a lower temperature through two-stage cooling by installing a compressor unit provided with a compressor and a first air-cooling heat exchanger outside a room and installing an intermediate unit provided with a second air-cooling heat exchanger inside the room.
In this cryogenic refrigerating apparatus which performs the two-stage cooling, as shown in FIG. 7, a compressor unit 1 is constructed of a helium gas compressor 11, a first air-cooling heat exchanger 12 comprised of, for example, a cross fin coil provided in communication with discharge side piping 21 of this compressor 11 and an oil separator 13 provided in communication with the discharge side piping 21 on the outlet side of this first air-cooling heat exchanger 12, while an intermediate unit 3 provided with a second air-cooling heat exchanger 31 comprised of, for example, a cross fin coil is provided separately from this compressor unit 1, the compressor unit 1 being installed outside a room and the intermediate unit 3 being installed inside the room.
An end portion which belongs to the discharge side piping 21 connected to the discharge side of the compressor 11 is connected to gas supply piping 41 of the intermediate unit 3, while an end portion which belongs to an intake side piping 22 connected to the intake side of the compressor 11 is connected to gas return piping 42 of the intermediate unit 3.
The gas supply piping 41 of the intermediate unit 3 is connected to a high-pressure side communication piping 51 communicated with a cryogenic expander 5, while the gas return piping 42 of the intermediate unit 3 is connected to a low-pressure side communication piping 52 communicated with the cryogenic expander 5.
Further, the second air-cooling heat exchanger 31 is connected to the gas supply piping 41, an adsorber 32 is provided in communication with the outlet side of the second air-cooling heat exchanger 31 and the second air-cooling heat exchanger 31 is provided with a fan 33.
It is to be noted that oil collected in a bottom portion of the oil separator 13 is injected into a compression element of the compressor 11 via oil injection piping 23 and oil collected to a height higher than a specified oil surface height inside the oil separator 13 is returned from the intake side piping 22 into the compressor 11 via an oil return piping 24. On the other hand, oil collected in a bottom portion inside the compressor 11 is cooled in the first air-cooling heat exchanger 12 via an oil cooling piping 25 and thereafter returned from the intake side piping 22 into the compressor 11.
Then, in the first air-cooling heat exchanger 12 of the compressor unit 1 installed outside the room, by making compressed high-temperature helium gas to exchange heat with the outdoor air to firstly cool it by the outdoor air for the achievement of the greater part of the heat radiation of the helium gas outside the room and further cooling the helium gas by the second air-cooling heat exchanger 31 of the intermediate unit 3 installed inside the room, the compressed helium gas is cooled in two steps by the outdoor air and the indoor air. This arrangement has allowed the helium gas to be cooled to a temperature not higher than a specified temperature (35.degree. C., for example) even when the outdoor temperature is high and prevented the operating noises inside the room with the compressor unit 1 installed outside the room.
In the cryogenic refrigerating apparatus constructed as above, the helium gas, of which cooling has been insufficient in the compressor unit 1, can be cooled in the intermediate unit 3. However, in regard to the cooling in the intermediate unit 3, constant cooling is consistently performed no matter whether the cooling capacity of the first air-cooling heat exchanger 12 of the compressor unit 1 depending on the outside air temperature is great or small, i.e., a fan 33 for cooling the second air-cooling heat exchanger 31 is driven to rotate consistently at a constant rotating speed so as to make the air flow constant. Therefore, when the outdoor temperature is low in winter, constant cooling is performed by the second air-cooling heat exchanger 31 regardless of the load from outside the room in spite of the fact that sufficient cooling has been performed in the first air-cooling heat exchanger 12, and this has resulted in excessive cooling and a significant change in refrigerating capacity, causing a disadvantage that a stable refrigerating operation can still not be performed.
Furthermore, although not shown in FIG. 7, the first air-cooling heat exchanger 12 of the compressor unit 1 is normally provided with an outdoor fan for cooling. When starting the refrigerating apparatus in a case where the outdoor temperature is extremely low in winter, the viscosity of the oil (mainly ether-based oil) inside the units 1 and 3 is very high. Therefore, when excessive cooling is performed by the operation of the outdoor fan at the first air-cooling heat exchanger 12, the viscosity of the oil does not reduce, and this has tended to cause a disadvantage that the units 1 and 3 do not correctly operate.
The present invention is intended to solve the aforementioned problems, and its principal object is to provide a cryogenic refrigerating apparatus capable of reducing the change in refrigerating capacity as far as possible with respect to a wide range of change in outdoor temperature and performing a stable refrigerating operation.
Another object is to allow the units to regularly operate by speedily reducing the viscosity of the oil even when the outdoor temperature becomes very low in winter.