JP2005351613A - Cooling device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cooling device used for cooling a conductor to a superconductive temperature. <P>SOLUTION: The cooling device includes a cooling system for forming a closed path in which a refrigerant flows. The system includes a pump for allowing the refrigerant to flow, a supply line that is extended from the pump to the cooling position and is arranged at a low-temperature retainer for cooling the position, and a return line extended from the cooling position to the pump. The pump is arranged outside the low-temperature retainer. The first heat exchanger is arranged in the low-temperature retainer, and connects supply and return lines for heat-exchanging between them so that the refrigerant flowing through the supply line is cooled by the refrigerant flowing through the return line. A refrigerating machine has a cooling stage in the low-temperature retainer and is connected to the supply line at the downstream of the first heat exchanger so that the refrigerant reaching the first cooling stage can be cooled preliminarily by the first heat exchanger. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a cooling device for use, for example, in cooling an electrical conductor to a superconducting temperature. The present invention is particularly suitable for cooling electromagnets to superconducting conditions for use in NMR (nuclear magnetic resonance) and ICR (ion cyclotron resonance) experiments.

High field NMR magnets are “supercooled” to a temperature several kelvin below the atmospheric boiling point of liquid ⁴ He (4.2 K) to improve the superconductor's critical current capacity and generate a higher magnetic field. There are many cases. This is usually accomplished using a liquid ⁴ He bath in which the magnets are immersed. Bath or container of the magnet is cooled to normal ～2.2K, this temperature is slightly higher than the ⁴ superfluid transition temperature of He (T _λ = 2.17K), i.e. lambda point.

For two reasons, 2.2K is the preferred operating temperature. ^Since the specific heat capacity of ⁴ He peaks at the λ point (Figure 2), it is desirable to operate as close as possible to this lambda point to improve the temperature stability of the system. However, operating below the λ point is generally considered undesirable. This is because a proportion of the liquid becomes a zero-viscosity superfluid and flows through the smallest cracks and orifices, even against gravity, towards the higher temperature cryostat area, This is because heat leakage is caused and boil-off (so-called “super leak” phenomenon) is increased.

  In early supercooling systems, a magnet-containing vessel containing liquid He was simply pumped to a lower pressure, thus gradually evaporating the liquid bath and supercooling the magnet. It is necessary to warm up the system for a simple design, so when the tank needs to be refilled, the magnet must be turned off. To avoid this great cost and inconvenience, Roubeau et al. (Biltcliffe, Hanley, McKinnon, Roubeau) ("Operation of superconducting magnets at temperatures below 4.2 K", Cryogenics, February 1972, pages 44-47). ) Invented the lambda point refrigerator.

  More recently, as shown in FIG. 1, a “lambda point refrigerator” has been used. Referring to FIG. 1, the magnet 2 is immersed in the liquid He in the first coolant-containing container 1 at atmospheric pressure. The second coolant-containing container 3 opened to the atmosphere holds a reservoir of liquid He boiling at 4.2 K, which can be refilled at any time. It is connected to the container 1 through a quench valve 14. The liquid He is transferred from the second container 3 to the heat exchanger 5 in the first container 1 through the (optional) second heat exchanger 6 and the expansion valve 4. The heat exchanger 5 is generally a coiled loop cage immersed in the upper part of the liquid helium tank of the first container. The pressure in the loop 5 downstream of the valve 4 is typically reduced to 20-50 mbar by pumping using an external pump 13. The helium liquid passing through the valve 4 partially evaporates due to the pressure drop across the valve and is cooled by a few Kelvin. The vapor pressure dropped in the loop lowers the boiling temperature of the remaining liquid, which eventually evaporates and absorbs heat from the magnet bath and cools it through heat exchange through 5. The steam leaving the heat exchanger 5 passes through an optional second heat exchanger 6 which reduces the rate of evaporation in the valve and thus a predetermined cooling power. In order to reduce the required mass flow rate, the liquid entering the valve is precooled.

The cooling power of the lambda point refrigerator constituted by the components 4, 5 and 6 is given by the following equation.

Here, dm / dt is the total mass flow rate, H is the enthalpy, and λ is the ratio of the liquid that has instantaneously evaporated into vapor in the valve.
The components described so far are located in a cryostat 20 that includes several shields described below.

  The cold steam leaving the second heat exchanger 6 passes through another heat exchanger 10 through the cryostat 20 and absorbs heat from the gas cooling shield 7 which is placed at a temperature of about 40K. Next, the heat is absorbed from the second shield 8 through the final heat exchanger 11. The shields 7 and 8 of the cryostat 20 reduce the radiant heat load on the helium vessels 1 and 3 and reduce the total boil-off. Because the outer shield 8 has the greatest radiative load, it is common for the shield 8 to have supplemental cooling from nitrogen boiling at atmospheric pressure (77K) in a vessel 8a thermally connected to the shield 8. It is. The entire container assembly is housed in a degassed container 9 to reduce losses due to conduction and convection. The magnet 2 and the inner container are typically suspended using a fiberglass rod web (not shown) to reduce the convective heat load. A bore tube (not shown) at room temperature and pressure passes through the assembly, passes through the magnet bore 22 and places the sample inside the magnet 2.

After passing through a pump 13 located outside the cryostat 20, the helium gas is exhausted to the atmosphere and either lost or collected for later reuse (reliquefied in another plant). later).
In the case of magnet quenching (release of superconducting state failure and stored magnet energy as heat), the spring-loaded safety valve 14 allows boiling helium in the first container 1 to escape to the second container 3. Therefore, let it escape into the atmosphere before dangerous overpressure conditions occur.

Roubeau et al. (Biltcliffe, Hanley, McKinnon, Roubeau) “Operation of superconducting magnets at temperatures below 4.2 K”, Cryogenics, February 1972, pages 44-47. Buckingham, M.M. J and Fairbank, W.M. M.M. Will be published in the book "Properties of Lambda Transition", "Low Temperature Physics III", 1961

  The system described above consumes large amounts of helium while solving the refill problem. The global supply of helium is limited and prices are expected to rise significantly over the next decade. Therefore, it is desirable to reduce the amount of helium used in the supercooled cryostat.

  According to the present invention, a cooling device includes a pump for flowing a coolant, a supply line arranged in the low temperature holding device for extending the pump from the pump to the cooling position and cooling the position, and A cooling system that forms a closed path through which the coolant flows, including a return line extending to an externally located pump, and supply and return so that the coolant flowing through the supply line is cooled by the coolant flowing through the return line A first heat exchanger disposed in the low temperature holding device for connecting the lines and enabling heat exchange therebetween, and a cooling stage in the low temperature holding device, and reaching the first cooling stage A refrigerator connected to the supply line downstream of the first heat exchanger such that the coolant is precooled by the first heat exchanger.

  The inventor has provided a solution to the above-mentioned problems by providing a cooling system that forms a closed path through which a coolant such as He flows. This has the added advantage of avoiding the need for refilling of the cooling system and requiring only minor changes to the existing cooling system to implement the present invention. This solution involves the use of a refrigerator having at least one cooling stage that includes a first heat exchanger for precooling the coolant before it reaches the first cooling stage. Inclusion helps the cooling stage. This reduces the power requirement of the first cooling stage to the extent that a conventional refrigerator such as a pulse tube refrigerator can be used. Generally, the cooling system includes a lambda point refrigerator placed in a cooling position, while this cooling position can be located in an auxiliary coolant containing container. Alternatively, the item to be cooled could be connected directly to the closed path of the cooling system.

In some cases, a single first heat exchanger may be sufficient, but especially when cooling to higher temperatures, the device preferably allows the coolant flowing through the supply line to be cooled by the coolant flowing through the return line. A second heat exchanger disposed in the cryostat that connects the supply and return lines to be cooled, the second heat exchanger comprising a coolant flow along the supply line; Upstream of the first heat exchanger relative to the direction.
The use of a second heat exchanger further generates power requirements for the refrigerator as it allows additional precooling to be achieved. Of course, further heat exchangers could be provided if necessary.

In some cases, a single stage refrigerator can be used, but in a preferred embodiment, the refrigerator has an additional cooling stage that is higher in temperature than one cooling stage, and this additional cooling The stage is located in the cryostat and is connected to the supply line to cool the supply line at a position upstream of the first heat exchanger.
In addition or alternatively, the refrigerator has an additional cooling stage having a higher temperature than one cooling stage, which is disposed in the cryostat and is arranged in the cryostat. Connected to the shield to cool the shield.

In the most preferred embodiment, the additional cooling stage cools both the coolant and the shield in the supply line.
If a second heat exchanger is provided, this is preferably arranged upstream of one cooling stage of the refrigerator with respect to the direction of coolant flow along the supply line.
In general, the coolant flowing through the closed path of the cooling system includes He, but other coolants could be used depending on the temperature required at the cooling location. For example, nitrogen is an alternative.

The refrigerator is generally a mechanical refrigerator that is powered by electricity, such as a pulse tube refrigerator, since vibration problems are minimal. However, it will be appreciated that any cooler that provides a cold cooling stage and consumes a coolant (such as ⁴ He) can be used. Thus, alternatives to pulse tube refrigerators include Stirling, Gifford-McMahon, Joule-Thomson refrigerators, dilution refrigerators, and the like.

As mentioned above, the cooling device can be used to cool various objects, for example, the necessary conductors in NMR, MRI and ICR where superconducting magnets are required. Particularly suitable for cooling to conditions. In these cases, the magnet will generally form a bore at room temperature, and the surrounding container will be shaped to allow remote access to the bore.
An example of the cooling device according to the present invention will be described below with reference to the accompanying drawings.

In the following description, the same reference numerals are given to the components of the apparatus shown in FIG. 3 which are the same as those shown in FIG. 1, and details are not described again.
In the example of FIG. 3, closed cooling formed by a supply line 26 extending from the pump 13 through the pump filter 13a in the cryostat to the lambda refrigerator 4-6 and a return line 28 returning from the lambda refrigerator to the pump 13. A system is provided. The supply line 26 is opened into the second container 3 and connected to the second stage of a two-stage pulse tube refrigerator (PTR) 24. This second stage 16 recondenses the helium vapor boiling in the vessel 3 and condenses the helium supplied along the supply line 26 as well. It typically consumes tens to hundreds of mW of power.

  Prior to reaching the second stage 16 of the PTR, the supply line extends through the “first” heat exchanger 17, which connects the supply line 26 with the return line 28. The heat exchanger 17 cools the helium supplied along the supply line 26 before the cold return helium in the return line 28 reaches the first cooling stage. This reduces the cooling power required in the first cooling stage 15. This first heat exchanger 17 is particularly important to keep the power requirement for cooling the second stage 16 of the PTR 24 below about 1 W (the current PTR technology limit at 4.2 K).

The “second” heat exchanger 19 is provided upstream of the heat exchanger 17 with respect to the supply line. The heat exchanger 19 allows further heat exchange between the supply and return lines 26 and 28 to further precool the helium in the supply line.
The “third” heat exchanger 18 is connected between the supply line 26 and the shield 8. The shield 8 is connected to the first stage 15 of the PTR 24, which is used to cool the outer shield 8 to about 40K, which requires about 30W for a typical large NMR magnet system. To do. With these connections, the first stage 15 of the PTR 24 cools both the shield 8 and the helium in the supply line 26.

Heat exchangers 17 and 19 use the enthalpy of the cold gas exiting the lambda point refrigerator used in the prior art system to cool shields 7 and 8. The heat exchanger 18 applies a slight heat load (several watts) to the first stage 15 of the PTR 24.
Current PTR technology is limited to the cooling power of the second stage 16 of about 1 W at 4.2K. Without heat exchanger 17, it would not be possible to recondense the entire helium boil-off from the lambda refrigerator in a typical conventional high field NMR magnet cryostat. By replacing the enthalpy of the warm gas with the refrigerator exhaust in the heat exchanger 17 (and preferably also 19), the problem is solved.
Thus, it will be appreciated that the present invention provides a zero boil-off (ZBO) system that does not consume helium in normal operation, and thus provides the great benefit of not requiring destructive and expensive refilling.

The effect of the “first” heat exchanger 17 can also be seen from the following analysis.
Assuming that the first stage 15 cools the vaporized helium to about 45K, in the absence of the heat exchanger 17, the cooling power required for the second stage 16 should be as follows:
Q ′ = n ′ · (H _{T = 45K} −H _{T = 4.2} ) + n ′ · L = 3.5 · 10 ⁻³ • (932-87) + 3.5 · 10 ⁻³ • 83 = 3 + 0.3≈3 .3 watts where L = 83 J / Mol (latent heat of helium), n ′ = flow rate of helium, H _T = enthalpy of helium.

However, the cooling power of the outgoing helium stream is not required to be used to cool any radiation shield since the first stage 15 achieves cooling (unlike the prior art). Can be used for pre-cooling the incoming helium stream. The outgoing flow and the return or incoming flow circulation are the same, so a pre-cooling of the return flow that drops from 45K to about 5K can be achieved using the heat exchanger 17. As a result, the heating power required in the second stage can be reduced to about 0.3-0.45W. Such power is readily available from commercial pulse tube refrigerators.
As described above, the system shown in the drawings is used to cool various items, particularly superconducting magnets that may be used in any conventional configuration such as MRI, NMR, and ICR. Can do.

It is a schematic sectional drawing through the well-known cooling device. It is a figure which shows the heat capacity of ⁴ He in the vicinity of a lambda point. It is a figure similar to FIG. 1 which shows an example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st helium container 2 Magnet 3 2nd helium container 6, 10, 11 Heat exchanger 7, 8 Shield 13 Pump 20 Low temperature holding device

Claims (18)

Pump for flowing coolant,
A supply line arranged in a low temperature holding device for extending from the pump to a cooling position and cooling the position, and a return line extending from the cooling position to the pump arranged outside the low temperature holding device;
A cooling system that forms a closed path through which the coolant flows, and
Located in the cryostat to connect the supply and return lines and allow heat exchange therebetween so that the coolant flowing through the supply line is cooled by the coolant flowing through the return line. A first heat exchanger;
The supply line downstream of the first heat exchanger has a cooling stage in the cryostat and the coolant reaching the first cooling stage is precooled by the first heat exchanger A refrigerator connected to the
A cooling device comprising: The cooling system includes a coolant-containing container that forms part of the supply line;
The cooling stage of the refrigerator is located in the coolant-containing container,
The apparatus according to claim 1.   The coolant-containing container is also connected to an auxiliary coolant-containing container at the cooling position, whereby a portion of the coolant can flow from the coolant-containing container to the auxiliary coolant-containing container. The apparatus according to claim 2.   The apparatus according to any one of claims 1 to 3, wherein the cooling system includes a helium lambda point refrigerator disposed at the cooling position. A second heat exchanger disposed in the cryostat and connecting the supply and return lines such that the coolant flowing through the supply line is cooled by the coolant flowing through the return line;
The second heat exchanger is upstream of the first heat exchanger with respect to the direction of coolant flow along the supply line;
The apparatus according to any one of claims 1 to 4, characterized in that: The refrigerator has an additional cooling stage having a higher temperature than the one cooling stage,
The additional cooling stage is located in the cryostat and is connected to the supply line to cool the supply line at a position upstream of the first heat exchanger;
The device according to claim 1, wherein the device is a device. The refrigerator includes an additional cooling stage having a higher temperature than the one cooling stage,
The additional cooling stage is located in the cryostat and is coupled to a shield of the cryostat to cool the shield;
The device according to claim 1, wherein the device is a device.   The shield is connected to the supply line through a third heat exchanger such that the additional cooling stage of the refrigerator cools both the shield and the coolant of the supply line. An apparatus according to claim 6 or claim 7.   The apparatus of claim 8, wherein the second heat exchanger is disposed outside the shield.   10. The shield according to any one of claims 7 to 9, wherein the shield is also cooled by a second coolant contained in a second coolant-containing container attached to the shield. The device described.   The apparatus according to any one of claims 1 to 10, wherein the coolant of the cooling system comprises helium.   The apparatus according to any one of claims 1 to 11, wherein the refrigerator includes a pulse tube cryocooler.   The apparatus according to any one of claims 1 to 11, wherein the refrigerator includes Stirling, Gifford-McMahon, Joule-Thomson, or a dilution refrigerator.   The apparatus according to any one of claims 1 to 13, wherein the cooling stage of one of the refrigerators supplies a temperature of about 4K.   7. The apparatus according to claim 6, wherein the additional cooling stage of the refrigerator supplies a temperature in the range of 40-50K.   The apparatus according to any one of claims 1 to 15, further comprising a superconducting magnet disposed in the cooling position.   The apparatus of claim 16, wherein the superconducting magnet forms a room temperature bore adapted to receive a sample. A cooling device according to claim 17,
A probe for insertion into the bore having means for supporting a sample;
An NMR or ICR apparatus characterized by comprising:

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