JPH05223381A - Cryogenic helium cooling system - Google Patents

Abstract

PURPOSE:To provide a highly effectual and reliable system without disposing a main turbine on the secondary side of a shielded line in a cryogenic helium freezing system including low temperature helium for a thermal shield. CONSTITUTION:A shielding line 31 is taken from the middle of main turbines 13a and 13b connected in series, and expansion turbines 13c, 13d are disposed on the secondary side of a thermal shield 32 to produce cold. In response to the temperature of the shielding line, there is estimated a position of a smaller temperature difference between it and a low pressure line to open any of valves 15a, 15b, 15c for joining of the same with the low pressure line. A variable capacity type expansion turbine is employed for the expansion turbine to maintain the efficiency in response to operation conditions of the shielding line.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cryogenic helium cooling system used in a cryogenic field such as a superconducting magnet.

[0002]

2. Description of the Related Art Cryogenic cryogenic helium of 4.4K level is used for refrigeration of superconducting devices, but it is generally more economical to reduce the thermal load of cryogenic level as much as possible. Level heat shields are often installed. This is because cheaper liquid nitrogen can be used at the 80K level. However, it is dangerous to use nitrogen, which is easily activated, in a device exposed to a large amount of radiation. Therefore, it is necessary to use helium as a heat shield refrigerant.

An example of a conventional technique will be described with reference to FIG.

In FIG. 2, 1 is a helium compressor, 2 is a high pressure helium inlet line, 3 is a low pressure helium line, 1
0 is a cold box, 11a to 11f are heat exchangers, 1
2 is an expansion turbine inlet valve, 13a and 13b are expansion turbines, 14 is a JT valve, 17a to 17d are cryogenic refrigerant transfer pipes, 30 is a cooled body, 32 is a heat shield around the cooled body, and 33 is a cooled body. ， It is a cryostat with a built-in heat shield.

Next, the operation of the conventional example configured as described above will be described. Helium compressor 1 16-18a
The helium gas compressed to tm is supplied from the high-pressure helium inlet line 2 to the cold box 10 and exchanges heat with the low-pressure helium in the first heat exchanger 11a.
After being cooled by the third heat exchangers 11b and 11c, it branches into a shield line and a liquefaction line. The helium branched to the shield line passes through the cryogenic refrigerant transfer pipe 17c, cools the heat shield 32 in the cryostat 33, passes through the cryogenic refrigerant transfer pipe 17d, and returns to the cold box 10. During this time, helium causes a pressure drop in the heat shield, for example 13-15 atm. Further, the helium gas passes through the expansion turbine inlet valve 12 and undergoes adiabatic expansion work in the first expansion turbine 13a, so that the temperature of the helium gas drops and enters the fourth heat exchanger 11d. The helium discharged from the fourth heat exchanger 11d performs adiabatic expansion work to a low pressure in the second expansion turbine 13b, drops in temperature, and joins the low pressure line. The expansion turbines 13a and 13b are main turbines that are essential for producing cryogenic helium.

On the other hand, the helium branched to the liquefaction line is
After being cooled by the fourth to sixth heat exchangers 11d to 11f,
It is adiabatically expanded to about 1 atm by the JT valve 14, partially liquefied and supplied to the cryostat 33 by the cryogenic refrigerant transfer pipe 17a to cool the cooled object 30. The cryogenic helium gas that has cooled the object to be cooled passes through the cryogenic refrigerant transfer pipe 17b, returns to the cold box 10, and returns to the suction side of the helium compressor 1 through the low pressure line.

Regarding such a helium refrigeration system, see, for example, Advances in Cryogenic Engineering, Volume 31 (1986), pp. 635 to 645 (Advances in Cryogenic Engi.
Neering, Vol.31 (1986), PP635-645).

[0008]

In the above-mentioned prior art, since the entire amount flowing through the main turbine flows through the shield line, pressure loss may occur more than necessary, and the operating condition of the shield line directly affects the main turbine. There is a defect that.

The present invention provides a highly reliable cryogenic helium cooling system which has good energy efficiency in its entirety including the shield line, has little influence on the main turbine even if the operating condition of the shield line changes, and enables stable operation. The purpose is to provide a system.

[0010]

To achieve the above object, a shield line is installed from the middle of the main turbines installed in series, and an expansion turbine is installed on the secondary side of the heat shield. By improving the efficiency, and by providing a variable capacity expansion turbine, the capacity can be changed according to the operating status of the shield line.
The confluence point of the shield line and the low pressure line can be switched according to the temperature of the shield line to maintain and improve efficiency, and because the main turbine is not installed on the secondary side of the shield line, operation of the shield line It is designed to enable stable operation even when the situation changes.

[0011]

The heat shield absorbs the heat load by the sensible heat due to the temperature rise of the low temperature helium, and when the inlet temperature is constant, it is desirable to change the flow rate with respect to the change of the heat load. Further, the pressure and temperature on the secondary side of the heat shield fluctuate depending on the operation mode due to the operating conditions of the heat shield, fluctuations in heat load, and the like. On the other hand, efficient operation of the main turbine is essential in connection with the production of cryogenic helium. By not installing the main turbine in the wake of the shield line, the influence of the operating condition of the shield line on the main turbine is small, and the main turbine can be operated efficiently and stably.

Further, in an expansion turbine having a fixed capacity, the air volume is determined mainly by the inlet pressure and temperature conditions,
Operation is not always efficient with respect to changes in inlet pressure and temperature conditions, but in variable capacity expansion turbines, the efficiency can be improved by adjusting the passage area of the nozzle with respect to changes in inlet conditions. It is possible to maintain high. This makes it possible to maintain high efficiency of the entire system.

In general, mixing helium having different temperatures is thermodynamically accompanied by loss. It is desirable that the temperatures of both the low-pressure line and the shield line at the confluence point are exactly the same, but as a suboptimal measure, the confluence point is switched depending on the temperature of the shield line so that they merge at a relatively small temperature difference. Therefore, it is possible to maintain the efficiency of the system satisfactorily with respect to changes in the operating condition of the shield line.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG.

In FIG. 1, the helium compressor 1 has 16-
The helium gas compressed to 18 atm is supplied from the high-pressure helium inlet line 2 to the cold box 10, exchanges heat with the low-pressure helium in the first heat exchanger 11a, and is further cooled in the second heat exchanger 11b. , Expansion turbine line and liquefaction line. The helium branched to the expansion turbine line passes through the expansion turbine inlet valve 12 and
The helium in the expansion turbine line, which has dropped in temperature by performing adiabatic expansion work in the expansion turbine 13a, enters the fourth heat exchanger 11d, is further cooled, and is low in pressure in the second expansion turbine 13b. Adiabatic expansion work is performed until the temperature drops, and it joins the low pressure line.

The helium branched to the shield line passes through the cryogenic refrigerant transfer pipe 17c, cools the heat shield 32 in the cryostat 33, passes through the cryogenic refrigerant transfer pipe 17d, and reaches the cold box 10. Further, the temperature of the helium gas drops due to the adiabatic expansion work of the third and fourth expansion turbines 13c and 13d, and the converging point of the low pressure line 3 where the temperature is relatively close is selected in accordance with the temperature, and the valve 13a , The valve 13b, and the valve 13c to join the low-pressure line.

On the other hand, the helium branched to the liquefaction line is
After being cooled by the third to sixth heat exchangers 11c to 11f,
It is adiabatically expanded to about 1 atm by the JT valve 14, partially liquefied, and supplied to the cryostat 33 by the cryogenic refrigerant transfer pipe 17a to cool the cooled object 30. The cryogenic helium gas that has cooled the object to be cooled returns to the cold box through the cryogenic refrigerant transfer pipe 17b and the low pressure line to the helium compressor 1.
Return to the suction side.

Generally, depressurizing helium with a valve is
Compared with adiabatic expansion by an expansion turbine to generate cold, the loss is thermodynamically, but in the present embodiment, although there is a pressure loss in the heat shield part, the main turbine 1
It can be said that cold is effectively generated by the expansion turbines 13c and 13d installed on the shield lines 3a and 13b.

According to this embodiment, since the main turbines 13a and 13b are not installed on the secondary side of the shield line 31, it is possible to operate the main turbine stably even with respect to the operating condition of the shield line. effective.

Further, the second expansion turbine 13b, the third and third
By using the variable capacity type as the fourth expansion turbines 13c and 13d, the capacity of the helium expansion turbine can be changed and operated according to the pressure and temperature of the shield line. This has the effect of maintaining the efficiency of the helium expansion turbine high.

Further, since the confluence point of the shield line and the low pressure line can be switched depending on the temperature of the shield line, there is an effect that gas mixture having a large temperature difference can be prevented and efficiency can be maintained high.

In an embodiment, a helium expansion turbine,
Although the liquid helium container and the like are installed in the cold box, it goes without saying that even if the liquid helium container is separately installed outside the cold box, the same functions and effects are provided.

Although each helium expansion turbine is composed of one unit, the same function can be obtained even if a plurality of expansion turbines are installed or arranged in series or in parallel according to the expansion ratio and flow rate.・ It goes without saying that it has an effect.

[0024]

According to the present invention, since the main turbine is not installed on the secondary side of the shield line, there is an effect that the main turbine can be stably operated even in the operating condition of the shield line. .. By using a variable capacity expansion turbine as the helium expansion turbine, the capacity of the helium expansion turbine can be changed according to the operating condition of the shield line, and therefore the efficiency of the helium expansion turbine can be maintained high. Further, since the confluence point of the shield line and the low pressure line can be switched according to the temperature of the shield line, there is an effect that gas mixing with a large temperature difference can be prevented and efficiency can be maintained high.

[Brief description of drawings]

FIG. 1 is a flow chart of an embodiment of the present invention.

FIG. 2 is a flow chart of a conventional example.

[Explanation of symbols]

1 ... Helium compressor, 10 ... Cold box, 11 ...
Heat exchanger, 13 ... Expansion turbine, 17 ... Cryogenic refrigerant transfer pipe, 30 ... Cooled object, 31 ... Shield line, 32 ... Heat shield, 33 ... Cryostat.

Claims (3)

[Claims] 1. A compressor for compressing helium gas, a cold box having a heat exchanger and an expander for generating cold,
In a cryogenic helium cooling system having a shield line cooled by low temperature helium and further comprising a cooled object cooled by cryogenic helium, a shield line is provided from the middle of a plurality of expansion turbines installed in series. The cryogenic helium cooling system is characterized in that an expansion turbine is installed on the secondary side of the heat shield of the shield line and merges with the low pressure line guided to the suction side of the compressor. 2. A helium expansion turbine installed on the wake side of a plurality of expansion turbines installed in series, which branches off the shield line, and / or a helium expansion installed on the secondary side of the heat shield of the shield line. The cryogenic helium cooling system according to claim 1, wherein a variable capacity expansion turbine is used as the turbine. 3. The cryogenic temperature according to claim 1, wherein the confluence of the shield line and the low-pressure line guided to the suction side of the compressor can be switched to a plurality of positions according to the temperature of the shield line. Helium cooling system.