US20050229609A1 - Cooling apparatus - Google Patents
Cooling apparatus Download PDFInfo
- Publication number
- US20050229609A1 US20050229609A1 US10/960,281 US96028104A US2005229609A1 US 20050229609 A1 US20050229609 A1 US 20050229609A1 US 96028104 A US96028104 A US 96028104A US 2005229609 A1 US2005229609 A1 US 2005229609A1
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- United States
- Prior art keywords
- coolant
- heat exchanger
- refrigerator
- supply line
- cryostat
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- 238000001816 cooling Methods 0.000 title claims abstract description 68
- 239000002826 coolant Substances 0.000 claims abstract description 44
- 239000001307 helium Substances 0.000 claims description 24
- 229910052734 helium Inorganic materials 0.000 claims description 24
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 claims description 23
- 238000011144 upstream manufacturing Methods 0.000 claims description 6
- 238000010790 dilution Methods 0.000 claims description 2
- 239000012895 dilution Substances 0.000 claims description 2
- 239000000523 sample Substances 0.000 claims 4
- 230000037431 insertion Effects 0.000 claims 1
- 238000003780 insertion Methods 0.000 claims 1
- 239000007788 liquid Substances 0.000 description 13
- 238000009835 boiling Methods 0.000 description 5
- 238000005481 NMR spectroscopy Methods 0.000 description 4
- 239000007789 gas Substances 0.000 description 4
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 3
- 230000005855 radiation Effects 0.000 description 3
- 239000004020 conductor Substances 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 238000000669 high-field nuclear magnetic resonance spectroscopy Methods 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 238000010791 quenching Methods 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000001704 evaporation Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 239000011152 fibreglass Substances 0.000 description 1
- 230000005484 gravity Effects 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- QJGQUHMNIGDVPM-UHFFFAOYSA-N nitrogen group Chemical group [N] QJGQUHMNIGDVPM-UHFFFAOYSA-N 0.000 description 1
- 238000005086 pumping Methods 0.000 description 1
- 239000002887 superconductor Substances 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
Images
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
- F25B9/02—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point using Joule-Thompson effect; using vortex effect
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
- F25B2400/17—Re-condensers
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
- F25B9/10—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point with several cooling stages
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
- F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
- F25B9/14—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the cycle used, e.g. Stirling cycle
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25D—REFRIGERATORS; COLD ROOMS; ICE-BOXES; COOLING OR FREEZING APPARATUS NOT OTHERWISE PROVIDED FOR
- F25D19/00—Arrangement or mounting of refrigeration units with respect to devices or objects to be refrigerated, e.g. infrared detectors
- F25D19/006—Thermal coupling structure or interface
Definitions
- the invention relates to cooling apparatus, for example for use in cooling electrical conductors to a temperature at which they superconduct.
- the invention is particularly suited for cooling electromagnets to their superconducting condition for use in NMR (nuclear magnetic resonance) and ICR (ion cyclotron resonance) experiments.
- 2.2K is the preferred operating temperature for two reasons.
- the specific heat capacity of 4 He peaks at the ⁇ point ( FIG. 2 ), so it is desirable to operate as near the lambda point as possible to improve the temperature stability of the system.
- a “lambda point refrigerator” has been used.
- a magnet 2 is submerged in liquid He in a first coolant containing vessel 1 at atmospheric pressure.
- a second coolant containing vessel 3 which is open to atmosphere, holds a reservoir of liquid He boiling at 4.2K; this reservoir 3 may be refilled at any time. It is connected to the vessel 1 via a quench valve 14 .
- Liquid He is conveyed from the second vessel 3 to a heat exchanger 5 in the first vessel 1 via an (optional) second heat exchanger 6 and an expansion valve 4 .
- the heat exchanger 5 is typically a coiled loop tube immersed in the top of the liquid helium bath of the first vessel 1 .
- the pressure in the loop 5 is reduced by pumping using an external pump 13 , typically to 20-50 mbar.
- Helium liquid passing through the valve 4 is partially vaporized and cooled by a few Kelvin due to the pressure drop across the valve.
- the reduced vapour pressure in the loop lowers the boiling temperature of the remaining liquid, which consequently evaporates, absorbing heat from the magnet bath and cooling it via heat exchange through 5 .
- the vapour leaving the heat exchanger 5 is passed through the optional second heat exchanger 6 , which pre-cools the liquid entering the valve with the aim of reducing the fraction vaporized in the valve, and hence reducing the mass flow rate required for a given cooling power.
- cryostat 20 comprising a number of shields to be described below.
- the cold vapour leaving the second heat exchanger 6 passes up the cryostat 20 though another heat exchanger 10 , absorbing heat from a gas cooled shield 7 , which sits at a temperature at about 40K, and then through a final heat exchanger 11 , absorbing heat from the second shield 8 .
- the shields 7 , 8 of the cryostat 20 reduce the radiation heat load on the helium vessels 1 & 3 , reducing total boil-off. Because the outer shield 8 sees the largest radiation load, it is common for it to have supplementary cooling from nitrogen boiling at atmospheric pressure (77K) in a vessel 8 a thermally connected to the shield 8 .
- the entire vessel assembly is enclosed in an evacuated vessel 9 to reduce conduction and convection loss.
- the magnet 2 and inner vessels are typically suspended using a web of fibreglass rods (not shown) to reduce conduction heat load.
- a bore tube (not shown) at room temperature and pressure passes through the assembly and through the magnet bore 22 to allow samples to be placed inside the magnet 2 .
- the helium gas After passing through a pump 13 located outside the cryostat 20 the helium gas is either vented to atmosphere and lost, or collected for later re-use (after re-liquefaction in a separate plant).
- the spring-closed pop-off valve 14 allows the boiling helium in the first vessel 1 to escape to the second vessel 3 , and hence to atmosphere, before a dangerous over-pressure condition develops.
- cooling apparatus comprises a cooling system defining a closed path around which a coolant flows, the system including a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump, the pump being located externally of the cryostat; a first heat exchanger positioned within the cryostat and linking the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line; and a refrigerator having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.
- the solution involves utilizing a refrigerator having at least one cooling stage and assisting that cooling stage by including the first heat exchanger so as to precool the coolant before it reaches the first cooling stage. This reduces the power requirement of the first cooling stage to such an extent that conventional refrigerators such as pulse tube refrigerators, can be used.
- the cooling system includes a lambda point refrigerator located at the cold location while the cold location may be located within an auxiliary coolant containing vessel. Alternatively, an item to be cooled could be connected directly to the closed path of the cooling system.
- the apparatus further comprises a second heat exchanger, located within the cryostat, and linking the supply and return lines such that coolant flowing in the supply line is cooled by coolant flowing in the return line, the second heat exchanger being upstream of the first heat exchanger with respect to coolant flow direction along the supply line.
- the use of the second heat exchanger enables additional precooling to be achieved thus further producing the power requirements on the refrigerator.
- further heat exchangers could be provided if required.
- a single stage refrigerator can be used but in the preferred examples, the refrigerator has an additional cooling stage, warmer than the one coolant stage, the additional cooling stage being located within the cryostat and being coupled to the supply line to cool the supply line at a location upstream of the first heat exchanger.
- the refrigerator has an additional cooling stage, warmer than the one cooling stage, the additional cooling stage being located within the cryostat and being coupled to a shield of the cryostat so as to cool the shield.
- the additional cooling stage cools both coolant in the supply line and the shield.
- this is preferably located upstream of the one cooling stage of the refrigerator with respect to the direction of flow of coolant along the supply line.
- the coolant which flows in the closed path of the cooling system comprises He although other coolants could be used depending upon the temperature required at the cold location.
- An alternative, for example is nitrogen.
- the refrigerator is typically an electrically powered mechanical refrigerator such as a pulse tube refrigerator since this has minimum vibration problems.
- a pulse tube refrigerator any cooler providing a low temperature cold stage and where coolant (such as 4 He) is consumed, could be used. Therefore, alternatives to pulse tube refrigerators include Stirling, Gifford-McMahon, Joule-Thomson refrigerators, dilution refrigerators and so on.
- the cooling apparatus can be utilized to cool a variety of objects but it is particularly suited to the cooling of electrical conductors to their superconducting condition as required, for example, in NMR, MRI and ICR where superconducting magnets are required.
- the magnets will define a bore, typically at room temperature and the surrounding vessels will be shaped to allow remote access to the bore.
- FIG. 1 is a schematic cross-section through a known cooling apparatus
- FIG. 2 illustrates the heat capacity of 4 He near the lambda point
- FIG. 3 is a view similar to FIG. 1 but of an example of the invention.
- a closed cooling system is provided defined by a supply line 26 extending from the pump 13 via a pump filter 13 a into the cryostat 20 to the lambda refrigerator 4 - 6 and a return line 28 extending from the lambda refrigerator back to the pump 13 .
- the supply line 26 opens into the second vessel 3 and is coupled to a second stage 16 of a two stage pulse tube refrigerator (PTR) 24 .
- PTR pulse tube refrigerator
- This second stage 16 recondenses helium vapour which boils in the vessel 3 and also condenses helium supplied along the supply line 26 . It absorbs typically a few 10 s to 100 s mW of power.
- the supply line Prior to reaching the second stage 16 of the PTR 24 , the supply line extends through a “first” heat exchanger 17 which links the supply line 26 with the return line 28 .
- This heat exchanger 17 allows the cold returning helium in the return line 28 to cool helium being supplied along the supply line 26 prior to reaching the first cooling stage 15 . This reduces the cooling power required at the first cooling stage 15 .
- This first heat exchanger 17 is particularly important because it keeps the cooling power requirement of the second stage 16 of the PTR 24 below about 1 W (the limit of current PTR technology at 4.2K).
- a “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 , 28 so as to further precool the helium in the supply line.
- a “third” heat exchanger 18 is provided coupled between the supply line 26 and the shield 8 .
- the shield 8 is connected to a first stage 15 of the PTR 24 which is used to cool the outer shield 8 to about 40K, requiring about 30 W for a typical large NMR magnet system. In view of these connections, the first stage 15 of the PTR 24 cools both the shield 8 and helium in the supply line 26 .
- the heat exchangers 17 and 19 utilise the enthalpy of the cold gas leaving the lambda point refrigerator that, in the prior-art system, was used to cool the shields 7 and 8 .
- the heat exchanger 18 adds a small heat load (a few watts) to the first stage 15 of the PTR 24 .
- the invention provides a zero boil-off (ZBO) system consuming no helium in normal operation and thus providing significant advantages of no disruptive and costly refilling being required.
- ZBO zero boil-off
- the cooling power of the outgoing helium flow does not need to be utilized for cooling any radiation shields (unlike in the prior art) since the first stage 15 achieves this cooling, this cooling power can be used to pre-cool the incoming helium flow.
- the circulation of outgoing and return or incoming flows is the same and so pre-cooling of the return flow from 45K down to about 5 K can be achieved using the heat exchanger 17 .
- the heating power required at the second stage 16 can be reduced to about 0.3-0.45 W.
- Such powers are readily available from commercially available pulse tube refrigerators.
- the system shown in the drawings can be used to cool a variety of items but particularly superconducting magnets which may be used in any conventional configuration such as MRI, NMR, and ICR.
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- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Mechanical Engineering (AREA)
- Thermal Sciences (AREA)
- General Engineering & Computer Science (AREA)
- Magnetic Resonance Imaging Apparatus (AREA)
- Containers, Films, And Cooling For Superconductive Devices (AREA)
Abstract
Cooling apparatus comprises a cooling system defining a closed path around which a coolant flows. The system includes a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump. The pump is located externally of the cryostat. A first heat exchanger is positioned within the cryostat and links the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line. A refrigerator is provided having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.
Description
- The invention relates to cooling apparatus, for example for use in cooling electrical conductors to a temperature at which they superconduct. The invention is particularly suited for cooling electromagnets to their superconducting condition for use in NMR (nuclear magnetic resonance) and ICR (ion cyclotron resonance) experiments.
- High field NMR magnets are often “sub-cooled” to a temperature a few Kelvin below the atmospheric boiling point of liquid 4He (4.2K) to improve the critical current capacity of the superconductor and allow a higher magnetic field to be generated. This is commonly achieved using a bath of liquid 4He in which the magnet is submerged. The magnet bath or vessel is commonly cooled to ˜2.2K, which is just above the superfluid transition temperature, or λ point, of 4He (Tλ=2.17K).
- 2.2K is the preferred operating temperature for two reasons. The specific heat capacity of 4He peaks at the λ point (
FIG. 2 ), so it is desirable to operate as near the lambda point as possible to improve the temperature stability of the system. However, it is generally considered undesirable to operate below the λ point. This is because a proportion of the liquid becomes superfluid, with zero viscosity, and it will flow, even against gravity, through the smallest cracks and orifices towards areas of the cryostat at higher temperature, thus causing a large heat leak and increasing boil-off (the so-called “superleak” phenomenon). - In early sub-cooled systems the magnet containing vessel which contained liquid He was simply pumped to a lower pressure, hence gradually evaporating the liquid bath and sub-cooling the magnet. With this simple design it is necessary to warm up the system, and hence de-energise the magnet, when the bath needs re-filling. To avoid this major cost and inconvenience, the lambda point refrigerator was invented by Roubeau and others (“The operation of superconducting magnets at temperatures below 4.2K”, Cryogenics, February 1972, p. 44-47, Biltcliffe, Hanley, McKinnon, Roubeau).
- More recently, as shown in
FIG. 1 , a “lambda point refrigerator” has been used. Referring toFIG. 1 a magnet 2 is submerged in liquid He in a firstcoolant containing vessel 1 at atmospheric pressure. A secondcoolant containing vessel 3, which is open to atmosphere, holds a reservoir of liquid He boiling at 4.2K; thisreservoir 3 may be refilled at any time. It is connected to thevessel 1 via aquench valve 14. Liquid He is conveyed from thesecond vessel 3 to aheat exchanger 5 in thefirst vessel 1 via an (optional)second heat exchanger 6 and anexpansion valve 4. Theheat exchanger 5 is typically a coiled loop tube immersed in the top of the liquid helium bath of thefirst vessel 1. The pressure in theloop 5, on the downstream side of thevalve 4, is reduced by pumping using anexternal pump 13, typically to 20-50 mbar. Helium liquid passing through thevalve 4 is partially vaporized and cooled by a few Kelvin due to the pressure drop across the valve. The reduced vapour pressure in the loop lowers the boiling temperature of the remaining liquid, which consequently evaporates, absorbing heat from the magnet bath and cooling it via heat exchange through 5. The vapour leaving theheat exchanger 5 is passed through the optionalsecond heat exchanger 6, which pre-cools the liquid entering the valve with the aim of reducing the fraction vaporized in the valve, and hence reducing the mass flow rate required for a given cooling power. - The cooling power of the lambda point refrigerator constituted by
components -
- where dm/dt is the total mass flow rate, H is enthalpy and λ is the fraction of liquid flashed to vapour in the valve.
- The components described so far are located within a
cryostat 20 comprising a number of shields to be described below. - The cold vapour leaving the
second heat exchanger 6 passes up thecryostat 20 though anotherheat exchanger 10, absorbing heat from a gas cooledshield 7, which sits at a temperature at about 40K, and then through afinal heat exchanger 11, absorbing heat from thesecond shield 8. Theshields cryostat 20 reduce the radiation heat load on thehelium vessels 1 & 3, reducing total boil-off. Because theouter shield 8 sees the largest radiation load, it is common for it to have supplementary cooling from nitrogen boiling at atmospheric pressure (77K) in avessel 8 a thermally connected to theshield 8. The entire vessel assembly is enclosed in an evacuatedvessel 9 to reduce conduction and convection loss. Themagnet 2 and inner vessels are typically suspended using a web of fibreglass rods (not shown) to reduce conduction heat load. A bore tube (not shown) at room temperature and pressure passes through the assembly and through the magnet bore 22 to allow samples to be placed inside themagnet 2. - After passing through a
pump 13 located outside thecryostat 20 the helium gas is either vented to atmosphere and lost, or collected for later re-use (after re-liquefaction in a separate plant). - In the event of a magnet quench (a failure of superconducting state and release of stored magnetic energy as heat), the spring-closed pop-off
valve 14 allows the boiling helium in thefirst vessel 1 to escape to thesecond vessel 3, and hence to atmosphere, before a dangerous over-pressure condition develops. - Whilst the system described above solves the refilling problem, it consumes a large quantity of helium. The global supply of helium is limited and prices are expected to rise significantly in the next decade. It is therefore desirable to reduce the quantity of helium used in a sub-cooled cryostat.
- In accordance with the present invention, cooling apparatus comprises a cooling system defining a closed path around which a coolant flows, the system including a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump, the pump being located externally of the cryostat; a first heat exchanger positioned within the cryostat and linking the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line; and a refrigerator having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.
- We have developed a solution to the problem mentioned above by providing a cooling system defining a closed path around which the coolant, such as He, flows. This avoids the need to refill the cooling system and has the additional advantage that only small changes are required to be made to existing cooling systems in order to implement the invention. The solution involves utilizing a refrigerator having at least one cooling stage and assisting that cooling stage by including the first heat exchanger so as to precool the coolant before it reaches the first cooling stage. This reduces the power requirement of the first cooling stage to such an extent that conventional refrigerators such as pulse tube refrigerators, can be used. Typically, the cooling system includes a lambda point refrigerator located at the cold location while the cold location may be located within an auxiliary coolant containing vessel. Alternatively, an item to be cooled could be connected directly to the closed path of the cooling system.
- Although in some cases a single, first heat exchanger is sufficient, particularly when cooling to higher temperatures, preferably the apparatus further comprises a second heat exchanger, located within the cryostat, and linking the supply and return lines such that coolant flowing in the supply line is cooled by coolant flowing in the return line, the second heat exchanger being upstream of the first heat exchanger with respect to coolant flow direction along the supply line.
- The use of the second heat exchanger enables additional precooling to be achieved thus further producing the power requirements on the refrigerator. Of course, further heat exchangers could be provided if required.
- In some cases, a single stage refrigerator can be used but in the preferred examples, the refrigerator has an additional cooling stage, warmer than the one coolant stage, the additional cooling stage being located within the cryostat and being coupled to the supply line to cool the supply line at a location upstream of the first heat exchanger.
- In addition, or alternatively, the refrigerator has an additional cooling stage, warmer than the one cooling stage, the additional cooling stage being located within the cryostat and being coupled to a shield of the cryostat so as to cool the shield.
- In the most preferred embodiment, the additional cooling stage cools both coolant in the supply line and the shield.
- Where a second heat exchanger is provided, this is preferably located upstream of the one cooling stage of the refrigerator with respect to the direction of flow of coolant along the supply line.
- Typically, the coolant which flows in the closed path of the cooling system comprises He although other coolants could be used depending upon the temperature required at the cold location. An alternative, for example is nitrogen.
- The refrigerator is typically an electrically powered mechanical refrigerator such as a pulse tube refrigerator since this has minimum vibration problems. However, it will be appreciated that any cooler providing a low temperature cold stage and where coolant (such as 4He) is consumed, could be used. Therefore, alternatives to pulse tube refrigerators include Stirling, Gifford-McMahon, Joule-Thomson refrigerators, dilution refrigerators and so on.
- As explained above, the cooling apparatus can be utilized to cool a variety of objects but it is particularly suited to the cooling of electrical conductors to their superconducting condition as required, for example, in NMR, MRI and ICR where superconducting magnets are required. In these cases, the magnets will define a bore, typically at room temperature and the surrounding vessels will be shaped to allow remote access to the bore.
- An example of cooling apparatus according to the invention will now be described with reference to the accompanying drawings, in which:—
-
FIG. 1 is a schematic cross-section through a known cooling apparatus; -
FIG. 2 illustrates the heat capacity of 4He near the lambda point; and, -
FIG. 3 is a view similar toFIG. 1 but of an example of the invention. - In the following description, those components of the apparatus shown in
FIG. 3 which are the same as those shown inFIG. 1 have been given the same reference numerals and will not be described again in detail. - In the
FIG. 3 example, a closed cooling system is provided defined by asupply line 26 extending from thepump 13 via apump filter 13 a into thecryostat 20 to the lambda refrigerator 4-6 and areturn line 28 extending from the lambda refrigerator back to thepump 13. Thesupply line 26 opens into thesecond vessel 3 and is coupled to asecond stage 16 of a two stage pulse tube refrigerator (PTR) 24. Thissecond stage 16 recondenses helium vapour which boils in thevessel 3 and also condenses helium supplied along thesupply line 26. It absorbs typically a few 10 s to 100 s mW of power. - Prior to reaching the
second stage 16 of thePTR 24, the supply line extends through a “first”heat exchanger 17 which links thesupply line 26 with thereturn line 28. Thisheat exchanger 17 allows the cold returning helium in thereturn line 28 to cool helium being supplied along thesupply line 26 prior to reaching thefirst cooling stage 15. This reduces the cooling power required at thefirst cooling stage 15. Thisfirst heat exchanger 17 is particularly important because it keeps the cooling power requirement of thesecond stage 16 of thePTR 24 below about 1 W (the limit of current PTR technology at 4.2K). - A “second”
heat exchanger 19 is provided upstream of theheat exchanger 17 with respect to the supply line. Theheat exchanger 19 allows further heat exchange between the supply and returnlines - A “third”
heat exchanger 18 is provided coupled between thesupply line 26 and theshield 8. Theshield 8 is connected to afirst stage 15 of thePTR 24 which is used to cool theouter shield 8 to about 40K, requiring about 30 W for a typical large NMR magnet system. In view of these connections, thefirst stage 15 of thePTR 24 cools both theshield 8 and helium in thesupply line 26. - The
heat exchangers shields heat exchanger 18 adds a small heat load (a few watts) to thefirst stage 15 of thePTR 24. - Current PTR technology is limited to a cooling power in the
second stage 16 of about 1 W at 4.2K. Without theheat exchanger 17 it would not be possible to re-condense all the helium boil-off from the lambda fridge in a typical state-of-the-art high-field NMR magnet cryostat. By exchanging the enthalpy of the warm gas with the fridge exhaust in exchanger 17 (and preferably also 19) the problem is solved. - It will be seen therefore that the invention provides a zero boil-off (ZBO) system consuming no helium in normal operation and thus providing significant advantages of no disruptive and costly refilling being required.
- 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, then in the absence of theheat exchanger 17, the cooling power required by thesecond stage 16 must be:
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 Watt -
- Where L=83 J/Mol—helium latent heat,
- n′=flow rate of helium,
- HT=Helium enthalpy
- Where L=83 J/Mol—helium latent heat,
- However, since the cooling power of the outgoing helium flow does not need to be utilized for cooling any radiation shields (unlike in the prior art) since the
first stage 15 achieves this cooling, this cooling power can be used to pre-cool the incoming helium flow. The circulation of outgoing and return or incoming flows is the same and so pre-cooling of the return flow from 45K down to about 5 K can be achieved using theheat exchanger 17. As a result, the heating power required at thesecond stage 16 can be reduced to about 0.3-0.45 W. Such powers are readily available from commercially available pulse tube refrigerators. - As explained earlier, the system shown in the drawings can be used to cool a variety of items but particularly superconducting magnets which may be used in any conventional configuration such as MRI, NMR, and ICR.
Claims (18)
1. Cooling apparatus comprising a cooling system defining a closed path around which a coolant flows, the system including a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump, the pump being located externally of the cryostat; a first heat exchanger positioned within the cryostat and linking the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line; and a refrigerator having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.
2. Apparatus according to claim 1 , wherein the cooling system includes a coolant containing vessel forming part of the supply line, the cooling stage of the refrigerator being located in the coolant containing vessel.
3. Apparatus according to claim 2 , wherein the coolant containing vessel is also connected to an auxiliary coolant containing vessel at the cold location whereby a portion of the coolant can flow from the coolant containing vessel to the auxiliary coolant containing vessel.
4. Apparatus according to claim 1 , wherein the cooling system includes a helium lambda-point refrigerator positioned at the cold location.
5. Apparatus according to claim 1 , further comprising a second heat exchanger, located within the cryostat, and linking the supply and return lines such that coolant flowing in the supply line is cooled by coolant flowing in the return line, the second heat exchanger being upstream of the first heat exchanger with respect to coolant flow direction along the supply line.
6. Apparatus according to claim 1 , wherein the refrigerator has an additional cooling stage, warmer than the one coolant stage, the additional cooling stage being located within the cryostat and being coupled to the supply line to cool the supply line at a location upstream of the first heat exchanger.
7. Apparatus according to claim 1 , wherein the refrigerator has an additional cooling stage, warmer than the one cooling stage, the additional cooling stage being located within the cryostat and being coupled to a shield of the cryostat so as to cool the shield.
8. Apparatus according to claim 6 , wherein the shield is coupled via a third heat exchanger to the supply line so that the additional cooling stage of the refrigerator cools both the shield and coolant in the supply line.
9. Apparatus according to claim 8 , wherein the second heat exchanger is located outside the shield.
10. Apparatus according to claim 7 , wherein the shield is also cooled by a second coolant contained within a second, coolant containing vessel attached to the shield.
11. Apparatus according to claim 1 , wherein the coolant in the cooling system comprises helium.
12. Apparatus according to claim 1 , wherein the refrigerator comprises a pulse tube cryocooler.
13. Apparatus according to claim 1 , wherein the refrigerator comprises a Stirling, Gifford-McMahon, Joule-Thomson or dilution refrigerator.
14. Apparatus according to claim 1 , wherein the one cooling stage of the refrigerator provides a temperature of about 4K.
15. Apparatus according to claim 6 , wherein the additional cooling stage of the refrigerator provides a temperature in the range 40-50K.
16. Apparatus according to claim 1 , further comprising a superconducting magnet located at the cold location.
17. Apparatus according to claim 16 , wherein the superconducting magnet defines a room temperature bore adapted to receive a sample.
18. NMR or ICR apparatus including cooling apparatus according to claim 17; and a probe for insertion into the bore, the probe having means to support a sample.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GBGB0408312.7A GB0408312D0 (en) | 2004-04-14 | 2004-04-14 | Cooling apparatus |
GB0408312.7 | 2004-04-14 |
Publications (1)
Publication Number | Publication Date |
---|---|
US20050229609A1 true US20050229609A1 (en) | 2005-10-20 |
Family
ID=32320814
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US10/960,281 Abandoned US20050229609A1 (en) | 2004-04-14 | 2004-10-08 | Cooling apparatus |
Country Status (4)
Country | Link |
---|---|
US (1) | US20050229609A1 (en) |
EP (1) | EP1586833A3 (en) |
JP (1) | JP2005351613A (en) |
GB (1) | GB0408312D0 (en) |
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Also Published As
Publication number | Publication date |
---|---|
JP2005351613A (en) | 2005-12-22 |
EP1586833A3 (en) | 2006-10-11 |
EP1586833A2 (en) | 2005-10-19 |
GB0408312D0 (en) | 2004-05-19 |
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