JP3375906B2 - Control method and device for helium liquefaction device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method and apparatus for a helium liquefaction apparatus, and more particularly to a cryogenic device using the helium liquefaction apparatus, which is suitable for use in general helium liquefaction apparatus using a refrigerator. It is possible to secure safety,
The present invention relates to a control method and device for a helium liquefier.

[0002]

2. Description of the Related Art Low temperature systems requiring a liquefying temperature of helium of 4.2 K or less include a recondensing type and a type maintaining a helium liquefying cycle.

In the former method, liquid helium purchased from a vendor is inserted into a device to be cooled, and the liquid helium which is being evaporated is recondensed by a built-in small refrigerator to maintain the level of liquid helium with respect to the device. It is used for small systems such as medical diagnostic equipment such as MRI.

On the other hand, the latter is a small, medium or large plant type, and is equipped with a liquefaction device for producing liquid helium by itself. This liquefaction device has a problem that helium undergoes a phase change and its volume changes. That is, all the pipes constituting the system are made of metal, and the total internal volume is almost constant in any case. Also, the pressure is constant due to the system configuration. On the other hand, in the gas state, helium follows the Boyle-Charles law in terms of pressure,
Since the temperature and volume change, the volume is changed according to the temperature change. Further, when changing from the gas phase to the liquid phase, the volume becomes 1/700. On the other hand, since the system must maintain the pressure, the helium gas must be constantly supplied from outside the system. These operations require manpower, and if they cannot be automated, they are often forced to work day and night.

FIG. 1 shows the overall construction of an example of a conventional helium liquefaction apparatus. In this helium liquefier, a compressor 12 for pressurizing low-pressure helium gas supplied via a low-pressure line 10, and a high-pressure helium gas pressurized by the compressor 12 and supplied via a high-pressure line 20. Of the cryostat 30 in which the object to be cooled (for example, the superconducting magnet 35 shown in FIG. 3) is built in by making the helium cooled by the refrigerator 22 into mist. A Joule-Thomson valve (referred to as a JT valve) 28 supplied from a liquid injection pipe 32 to the lower portion, and a liquid storage portion 3 of the cryostat 30.
A return valve (referred to as an RT valve) 36 for returning the helium gas stored in 4 and warmed by heat exchange with an object to be cooled to the refrigerator 22 again, and the RT valve 3 during precooling.
6 and the refrigerator 22 to return the helium gas directly to the compressor 12 by the low pressure line 10 via a precooling valve 38 closed after the precooling and an intermediate pressure line 42 connected to a valve stand 44. A buffer tank (also referred to as a medium pressure tank) 40 for replenishing the helium gas reduced by the liquefaction and recovering the excess helium gas by the action of the valve stand 44.
And a helium gas purifier 52 for purifying the helium gas supplied from the high pressure line 20 on the outlet side of the compressor 12 or the helium gas cylinder 50 and supplying the helium gas to the buffer tank 40.

As shown in detail in FIG. 2, the refrigerator 22 has a plurality of (three in the figure) heat exchangers 24a, 24b, 24c connected in cascade and having different temperatures.
And turbines 26a and 26b provided between them.

At the inlet of the cryostat 30, as shown in detail in FIG. 3, a transfer tube 31 for guiding low temperature gas to the cryostat 30 without heat exchange,
A gate valve 33 for partitioning the transfer tube 31 from the outside air when the transfer tube 31 is pulled out from the cryostat 30 is provided.

The pre-cooling valve 34 is composed of, for example, a manual sluice valve connected in parallel with each other and a back pressure adjusting valve which is opened when the pressure exceeds a predetermined value, for example, 0.3 kgf / cm ² G. ing.

In the valve stand 44, the pressure in the low pressure line 10 is a predetermined value, for example, 0.1 kgf / cm ² G
When the pressure in the low pressure line 10 decreases due to liquefaction of helium gas,
An on-off valve for replenishing the low-pressure line 10 with helium gas supplied from the buffer tank 40 through the medium-pressure line 42, and conversely, the liquid helium in the cryostat 30 excessively evaporates due to the load and the pressure of the low-pressure line 10 is increased. And an on-off valve for returning excess gas to the buffer tank 40 via the medium pressure line 42 when the pressure rises.

Further, although not shown, an oil separator (referred to as ORS) for removing oil used for compressing the helium gas is provided on the outlet side of the compressor 12.

When such a helium liquefaction apparatus is stopped for a long period of time due to maintenance work or the like, the temperature of the superconducting magnet 35 rises to near room temperature, so that it is first precooled (referred to as precooling) to about 15K. In this pre-cooling process, the refrigerator 22 is contaminated by the released gases from various organic substances that form the superconducting magnet, and the refrigerating capacity for storing liquid helium in the cryostat 30 cannot be obtained. Therefore, the cryostat 3 in which the object to be cooled is liquid helium is stored.
When liquid is stored in the liquid storage unit 34 of 0 and the object to be cooled is cooled,
First, the helium gas in the low pressure line 10 is compressed by the compressor 12 in the high pressure line 20, for example, 8.2.
Pressurize to kgf / cm ² G. The pressurized helium gas is gradually cooled while passing through the refrigerator 22 as shown in FIG. 2 while undergoing a heat exchange process by the multistage heat exchangers 26a to 26c. Finally, by passing through the JT valve 28, cryogenic liquid helium having a temperature of 4.4 K is generated like a mist. The liquid mist-like helium discharged from the JT valve 28 is transferred to the transfer tube 3
1 to be cooled (superconducting magnet 3
It is guided to the cryostat 30 in which 5) is stored. The liquid helium guided to the lower portion of the cryostat 30 by the liquid injection pipe 32 and discharged is gradually cooled while exchanging heat with the object to be cooled. Specifically, for example, the lower coil of the superconducting magnet 35 is cooled, and the upper coil of the superconducting magnet 35 is cooled through the connecting portion 30C of the cryostat. At this time, the helium gas warmed by heat exchange with the object to be cooled returns to the low pressure line 10 again through the precooling valve 38, is compressed again by the compressor 12, and is guided to the refrigerator 22. In this state, for example, 80% of the flow rate of helium sent from the compressor 12 to the load side flows through the precooling valve 38, and the remaining 20% flows through the RT valve 36.

When such a process is repeated for a long time (for example, about 10 days) and the temperature of the entire system approaches the liquid helium temperature, the temperature difference between the superconducting magnet 35 and the heat exchanger 24c becomes small. Therefore, even if the pre-cooling valve 38 is closed and the RT valve 36 is opened to the maximum, the temperature of the heat exchanger 24c does not rise and liquid can be stored in the object to be cooled (cooling to a state in which liquid can be stored). Is called precooling).

When the pre-cooling is completed and the temperature of the entire system becomes close to the liquid helium temperature and the liquid can be stored in the object to be cooled, the pre-cooling valve 38 is gradually closed manually to adjust the return flow rate to the RT valve 36. When the number is increased, the refrigerator 22 is cooled by the low temperature helium supplied via the RT valve 36, and the high-pressure helium gas supplied to the JT valve 28 is further cooled. The cooling efficiency of the temperature of the gas is rapidly increased, and mist-like liquid helium is generated from the JT valve, and the liquid helium is stored in the cryostat 30. In this state, for example, several% is accumulated as liquid, and the remaining 90% or more passes through the RT valve 36 and returns to the refrigerator 22.

The open / closed states of the precooling valve 38 and the RT valve 36 during precooling and liquefaction are shown in FIG.

When liquefaction starts, the volume of helium gas at 0 ° C. and 1 atm is about 700 times the volume of liquid helium at 4.2 K and 1 atm, so that the gas in the buffer tank 40 is low pressure through the intermediate pressure line 42. As the pressure is supplied to the line 10, the pressure in the buffer tank 40 decreases.

Normally, when the pressure in the buffer tank 40 is less than 1 atm, there is no gas to be sent out from the buffer tank, and the motor and the like that compose the compressor 12 are easily damaged. The interlock to stop works.

The refrigerator 22 includes a heat exchanger 24 having different temperatures.
Since it is composed of a to 24c, once the refrigerator 22
When the gas is stopped and then restarted, the impure gas adhering to each heat exchanger adheres to the heat exchanger one step below, thereby lowering the liquefaction efficiency. Therefore, it takes labor and time to raise the temperature of the system and clean the gas in order to return to the steady state again.

FIG. 5 shows an example of the temperature change state of the object to be cooled (for example, the upper coil of the superconducting magnet 35) during the operation of the apparatus.
During the first 5 hours, the temperature is rising due to another work. During recooling (5 to 28 hours), the helium return gas is flowing through the RT valve 36 and the precooling valve 38. In this state, the temperature of the coil is saturated and the temperature does not drop below 12 degrees K. After 28 hours, by closing the pre-cooling valve 38, the efficiency of the refrigerator 22 is increased and the temperature is lowered to around 4.2 degrees K.

[0019]

As described above, once the system cannot supply liquid helium, the refrigerator 22 cannot be stopped until the liquefaction work is completed. Therefore, during this time, there is a problem in that a person must monitor without fail and perform gas replenishment work to the buffer tank 40.

In order to improve such a problem, it is conceivable to increase the capacity of the buffer tank 40.
There is a problem with the appearance of the system and the installation space. on the other hand,
It is possible to increase the pressure of the buffer tank 40, but if it exceeds 1 MPa, there is a problem that the control is troublesome due to the regulation of high pressure gas.

Further, if an operation error or forgetting to tighten the joints of valves and pipes in the work after maintenance, a slight amount of gas escapes to the outside, and the compressor 12 may be stopped without knowing it. You can Usually, when the pressure in the buffer tank 40 falls below a certain set value, an alarm is issued or an automatic gas supply system or the like is used to supplement the trouble. This method is effective for a relatively large system, but has a problem that the cost is high for a small or medium scale system.

The present invention has been made to solve the above-mentioned problems of the prior art. By stopping the compressor without increasing the capacity of the buffer tank or increasing the pressure, and without impairing the safety of the system, It is an issue to prevent efficiency reduction.

[0023]

The present invention relates to a compressor for pressurizing low-pressure helium gas, a refrigerator for cooling the helium gas pressurized by the compressor to a liquid helium temperature, and the refrigerator. The helium gas that is lightly received by the mist is converted into a mist and supplied to the cryostat for cooling the object to be cooled, and the helium gas stored in the cryostat and heated by heat exchange with the object to be cooled. To the refrigerator, a return valve for returning to the refrigerator, a return valve and a refrigerator for precooling, and a precooling valve for returning the helium gas to the compressor, which is closed after precooling, and reduced by liquefaction. A helium liquefaction device equipped with a buffer tank for replenishing helium gas and recovering excess helium gas In the above, when the pressure of the buffer tank is lowered, a part of the helium gas in the cryostat is directly returned to the compressor without passing through the return valve and the refrigerator, and the above-mentioned problems are solved. It is a thing.

Further, in the same helium liquefaction device, by providing a bypass valve for bypassing the precooling valve and returning the helium gas in the cryostat to the compressor when the pressure in the buffer tank is lowered, This is a solution to the above problem.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

In this embodiment, in the helium liquefaction apparatus similar to the conventional one shown in FIG. 1, as shown in FIG. 6, a pressure gauge 60 for detecting the pressure in the buffer tank 40 and the pressure gauge 60 are used for detection. When the pressure in the buffer tank 40 drops, the pre-cooling valve 38 is bypassed to allow the helium gas in the cryostat 30 to flow into the RT valve 36 and the refrigerator 2.
A bypass valve 62 is provided for returning directly to the compressor 12 without going through 2.

The operation of this embodiment will be described below.

The helium gas is liquefied to reduce the amount of return gas, and the buffer tank 4 detected by the pressure gauge 60
A pressure of 0 has a certain allowable value, for example 1.8 kgf / cm ²
When the temperature becomes equal to or lower than G, the bypass valve 62 is opened and the pre-cooling state is set. Therefore, if the amount of gas (liquid) released from the JT valve 28 is the same, the gas flow rate to the bypass valve 62 side increases, so the return gas to the refrigerator 22 decreases and the refrigerator 22 cools too much. Therefore, the liquefaction efficiency decreases. Then, the helium accumulated in the cryostat 30 evaporates and is collected in the buffer tank 40. As a result, when the pressure in the buffer tank 40 becomes equal to or higher than the allowable value, the bypass valve 62 is closed again and liquefaction starts, so that an equilibrium state can be maintained.

Therefore, since the pressure in the buffer tank 40 does not decrease, unless the interlock (for example, power failure) of the compressor 12 itself occurs, the trouble of stopping the compressor 12 and restarting the refrigerating system associated therewith is prevented. be able to.

In the present embodiment, the pre-cooling valve 38 and RT are used at the time of precooling, when the liquefaction is normal, and when the buffer tank pressure is normal.
The open / closed states of the valve 36 and the bypass valve 62 are shown in FIG. 7.

In this way, it is possible to prevent the compressor 12 from stopping due to the pressure drop in the buffer tank 40 due to some factor, and to find an event without being present at that moment (for example, at night). Even afterwards, it is possible to deal with it sufficiently.

In the present embodiment, since the bypass valve 62 is provided in parallel with the precooling valve 38, the control according to the present invention is simple. It should be noted that without providing the bypass valve 62, the precooling valve 38 itself is changed to an automatic control valve, and the pressure gauge 6
The precooling valve 38 itself may be electrically controlled to be opened / closed by the output of 0. In this case, since it is not necessary to provide a separate bypass valve, the structure is simple.

[0033]

According to the present invention, when the pressure in the buffer tank is lowered, the capacity of the refrigerator is lowered and the pressure in the low pressure line is raised, so that the pressure in the buffer tank is restored.
Therefore, it is possible to prevent the compressor from stopping due to the pressure drop in the buffer tank, and while maintaining safety, monitor the buffer tank pressure to prevent the troublesome restoration work due to the sudden stop of the compressor and the sudden stop of the compressor. Is unnecessary.

[Brief description of drawings]

FIG. 1 is a pipeline diagram showing the overall configuration of a conventional helium liquefaction device.

FIG. 2 is a pipeline diagram showing a configuration of a refrigerator of the helium liquefaction device.

FIG. 3 is a sectional view showing the structure of the cryostat.

FIG. 4 is a diagram showing the open / closed state of the precooling valve and the return valve.

FIG. 5 is a diagram showing an example of a change state of the temperature of an object to be cooled during liquefaction.

FIG. 6 is a pipeline diagram showing a configuration of an embodiment of a helium liquefier according to the present invention.

FIG. 7 is a diagram showing the open / closed state of the precooling valve, the return valve, and the bypass valve.

[Explanation of symbols]

10 ... Low pressure line 12 ... Compressor 20 ... High pressure line 22 ... Refrigerator 28 ... Jules Thomson valve (JT valve) 30 ... Cryostat 34 ... Liquid storage section 36 ... Return valve (RT valve) 38 ... Precooling valve 40 ... Buffer tank (medium pressure tank) 42 ... Medium pressure line 44 ... Valve stand 60 ... Pressure gauge 62 ... Bypass valve

Claims (2)

(57) [Claims] 1. A compressor for pressurizing low-pressure helium gas, a refrigerator for cooling the helium gas pressurized by the compressor to a liquid helium temperature, and a mist for the helium gas cooled by the refrigerator. And a Joule-Thomson valve for supplying a cryostat for cooling the object to be cooled, and a helium gas stored in the cryostat and warmed by heat exchange with the object to be cooled for returning to the refrigerator again. A return valve, a pre-cooling valve for returning the helium gas to the compressor by bus-passing the return valve and the refrigerator during pre-cooling, and a helium gas reduced due to liquefaction to replenish the helium gas, resulting in excess. And a buffer tank for recovering the helium gas, wherein the buffer tank When the pressure of the click is lowered, some of the helium gas within the cryostat, not via the return valve and the refrigerator, the control method of the helium liquefier and returning directly to the compressor. 2. A compressor for pressurizing low-pressure helium gas, a refrigerator for cooling the helium gas pressurized by the compressor to a liquid helium temperature, and a mist for the helium gas cooled by the refrigerator. And a Joule-Thomson valve for supplying a cryostat for cooling the object to be cooled, and a helium gas stored in the cryostat and warmed by heat exchange with the object to be cooled for returning to the refrigerator again. A return valve, a pre-cooling valve for returning the helium gas to the compressor by bus-passing the return valve and the refrigerator during pre-cooling, and a helium gas reduced due to liquefaction to replenish the helium gas, resulting in excess. And a buffer tank for recovering the helium gas, wherein the buffer tank When the pressure of the click is lowered, the control device of the helium liquefier, characterized in that the helium gas within said cryostat bypasses the precooling valve has a bypass valve for returning to the compressor.