EP0212093B1 - Method of cooling an object by means of superfluid helium (he ii) and device for carrying out the method - Google Patents

Description

6. The present invention relates to a method for cooling an object with the aid of superfluid helium (He 11) and a device for carrying out the method according to the first part of claims 1 and 6 respectively.

The abnormally good thermal conductivity of the He II in the temperature range between 1.7 K and 2.1 K and the superfluidity are excellent properties for cooling superconducting magnet windings. So far, such 1.8 K-cooled coils could only be realized on the principle of bath-cooled windings. This conventional technique is known to have the following disadvantages: the need for a tight coil housing, poor mechanical fixation of the conductor and poor high-voltage strength. These disadvantages can be overcome by changing from such a bath cooling with external cooling of the conductor to an internal cooling with forced helium flow. In the absence of suitable pumps, this concept could not previously be realized with cooling with superfluid helium (He II).

Fountain pumps with a super filter for stimulating convection of He II in ³ He- ⁴ He cooling machines are known, as described in the Dutch patent application ^p. 7 902 438.

In all known applications, the pumping effect is achieved by additional electrical heating at the warm end of the super filter. Since the thermomechanical efficiency of such pumps is inherently very low (less than 10% of the heat supplied can be converted into work), this process leads to an uneconomically high load on the refrigeration system, especially when the helium rate is to be circulated.

The invention has for its object the cooling of objects, such as superconducting large magnets, with liquid helium, to make them more effective and economical.

According to the first part of claims 1 and 6, the above object is achieved according to the invention by their characterizing features. The remaining patent claims represent advantageous developments and forms of training of both the method according to the invention and the device according to the invention for carrying it out as well as a particular field of application of the invention.

The particular advantage of the invention is that the heat loss of the object to be cooled is used to generate the forced flow in its own cooling circuit, the heat loss being coupled into a fountain pump with a super filter and heat being supplied to the warm end of the super filter that no additional Drive power is required and the throughput corresponding to the respective load is set automatically. In a particularly advantageous manner, these pumps have no mechanically moving parts.

The invention creates the possibility of internally cooled conductor concepts at extremely low temperatures below the 1-line of liquid helium, i.e. with superfluid helium (He 11), whereby the undisputed advantages of cooling by forced flow are combined with the advantages of the extremely good cooling properties of He II in the construction of large superconductor windings.

The opposite of the z.Z. Usually conventional cooling at temperatures around 4 K necessarily increased expenditure for providing the cooling capacity is often compensated for by the use of cheaper materials (NbTi instead of NbaSn) or by the higher magnetic fields and current densities that can be achieved at 1.8 K.

• Figur 1 stellt das Schema des erfindungsgemäßen Kühlkreislaufes dar;
• Figur 1a zeigt eine erweiterte Version des Kühlkreislaufes nach Figur 1;
• Figur 2 zeigt die Austrittstemperatur T₂ in Abhängigkeit von der normierten Belastung bei unterschiedlichen Systemdrücken po. Die Eintrittstemperatur ist T_i = 1,8 K;
• Figur 2a zeigt den Heliumdurchsatz in Kühlkanälen mit dem Durchmesser D = 4 mm bei unterschiedlichen Längen in Abhängigkeit von der thermischen Belastung Q;
• Figur 2b zeigt den Fountaindruck Ap in Abhängigkeit von der Austrittstemperatur T₂;
• Figur 3 zeigt ein Ausführungsbeispiel einer typischen Fountainpumpe;
• Figur 4 stellt ein Kühlschema mit parallel geschalteten Kühlkanälen dar;
• Figur 5 stellt ein Kühlschema für den Fall dar, daß parallele Kühlkanäle 11, 11 a ungleich belastet sind;
• Figur 6 zeigt eine weitere Konzeption eines Kühlsystems, wobei außer der Verlustwärme des zu kühlenden Objektes noch andere Wärmequellen zur Verstärkung der Zwangsströmung genutzt werden.

The invention is described below with reference to FIGS. 1 to 6 using a few examples.

• Figure 1 shows the scheme of the cooling circuit according to the invention;
• Figure 1a shows an expanded version of the cooling circuit of Figure 1;
• Figure 2 shows the outlet temperature T ₂ as a function of the normalized load at different system pressures po. The inlet temperature is T _i = 1.8 K;
• Figure 2a shows the helium throughput in cooling channels with the diameter D = 4 mm at different lengths depending on the thermal load Q;
• FIG. 2b shows the form impression Ap as a function of the outlet temperature T ₂ ;
• Figure 3 shows an embodiment of a typical fountain pump;
• FIG. 4 shows a cooling scheme with cooling channels connected in parallel;
• Figure 5 shows a cooling diagram for the case that parallel cooling channels 11, 11 a are loaded unequally;
• FIG. 6 shows a further conception of a cooling system, wherein besides the heat loss of the object to be cooled, other heat sources are used to reinforce the forced flow.

Figure 1 shows the diagram of the cooling circuit according to the invention. For better understanding, the figure also contains a cooling system according to the prior art, with which the 1.8 K operating temperature is achieved. Liquid helium, which boils in a storage container 1 under a pressure of, for example, 1 bar, is guided through a tube 2 designed as a heat exchanger to the expansion valve 3. By relaxing to a pressure of about 15 mbar, an operating temperature of about 1.8 K is reached in a recooling bath 4. The steam is drawn off via line 5 and returned to the condenser. The recooling bath 4 is in good thermal contact with the supply bath 7 via the wall 6 designed as a heat exchanger, which takes on the same pressure as in the storage container 1 via a pressure equalization line 1a. 8 K and the pressure of, for example, 1 bar, as in the storage container 1. The pressure compensation line 1a is to be designed as a so-called thermal barrier, in order to prevent the heat reduce the flow from the storage container 1 to the supply bath 7 to a permissible level.

The superfluid helium (He 11) from the supply bath 7 is brought to the temperature of the recooling bath 4 by means of a thermomechanical pump (fountain pump), which consists of a fine porous filter 8 (super filter) with a downstream heating bath 9, after recooling in a first heat exchanger 10, into a cooling channel 11 of the object 16 to be cooled, for example a superconducting winding. In this cooling section 11, the He II absorbs the heat to be dissipated from there. The emerging, heated He then flows through a second heat exchanger 12, wherein it gives off part of the heat absorbed to the heating bath 9.

Due to the thermomechanical effect, a specific effect that occurs in the He II, superfluid He flows largely dissipation-free from the supply bath 7 into the heating bath 9 when the heating bath 9 has a higher temperature than in the supply bath 7. This is achieved by coupling the heat absorbed in the cooling section 11 into the heating bath 9 of the fountain pump.

The super filter 8 acts as an entropy filter. Figuratively speaking, the heat is stripped off the He II when it flows through this filter. The result of this is that when there is a flow in the supply bath 7, heat is generated which is released by the heat exchanger 6 to the recooling bath 4. On the other hand, a cooling effect occurs at the outlet of the super filter 8. Part of the heat supplied via the heat exchanger 12 to the heating bath 9 is hereby removed. The helium emerging from the second heat exchanger 12 is then recooled to the initial temperature in a downstream third heat exchanger 13 and returned to the supply bath 7.

FIG. 1a shows an expanded version of the device according to FIG. 1, the heat exchangers 10 and 13 being preceded by a fourth heat exchanger 14 and a fifth heat exchanger 15 which are precooled within the exhaust gas line 5 in order to reduce the heat load on the recooling bath 4.

The calculated cooling characteristic of the cooling system according to the invention is shown in FIG. Here, the fluid temperature T ₂ on leaving the cooling duct 11 heated with the output Q is plotted with the length L, the flow cross section F and the hydraulic diameter D over the "standardized" heating output. The calculation was carried out for two different system pressures (po = 1.0 and 7.5 bar).

FIG. 2a shows the helium throughput which is established in channels with a diameter of D = 4 mm when the heat flow Q is removed therefrom. This illustrates that with the cooling system according to the invention considerable heat loads can also be dissipated from long ducts. The cooling also does not stop when the outlet temperature T ₂ leaves the He 11 range.

As shown in FIG. 2b, the delivery pressure (bottom impression ep _F ) increases even up to an outlet temperature of T ₂ max = 3.5 K. It is therefore to be expected that in such a cooling system there will be a continuous transition from He II to forced He I cooling. This can also be seen as an advantage over He II bath cooling.

FIG. 3 shows an exemplary embodiment of a typical fountain pump for a maximum delivery rate of about 10 g / sec with a small pressure drop and for about 0.3 bar maximum delivery pressure with a low throughput. With such a unit, for example from a cooling channel of 5 mm in diameter and 100 mm in length, an output of approximately 3 watts can be dissipated if the inlet temperature T _{1 is} 1.8 K and the outlet temperature T _{2 is} 2.16 K. The super filter 8 consists here, for example, of A1 ₂ 0 ₃ powder with an average particle size of 1.5 μm, which is pressed with a fill factor of about 50% into a tube of about 100 mm in length and 35 mm in diameter. Of course, other materials with similar porosity can also be used. The cross section and length of the filter units must be adapted to the special requirements with regard to mass throughput and delivery pressure.

In order to adapt such a device to the respective conditions with regard to the heat to be dissipated as well as the cross section and length of the cooling channels, several cooling channels 11 or several pump units can be combined with one another in a suitable manner.

With such combinations, the fact must be taken into account that the delivery pressure achievable with such fountain pumps is limited to relatively low values of less than about 0.5 bar due to physical effects. For a given filter material, however, the throughput that can be achieved depends only on the heat supplied and on the filter cross-section. As a result, it is not possible to operate cooling ducts of any length, but there is no physical limit to dividing them into several parallel ducts.

FIG. 4 shows a cooling diagram with cooling ducts 11 connected in parallel, as is possible with a large heat load or narrow cooling duct cross sections. This cooling system differs from that shown in FIG. 1 only in that in the object to be cooled (for example a superconducting winding) the He II current is split into several partial flows. The cross section of the cooling channels 11 of the super filter 8 and the heat exchangers 10, 12, 13, 14 and 15 must be adapted to the increased throughput. Such a system appears expedient if all parallel branches have the same flow resistances and the same thermal loads.

Figure 5 shows a cooling device in the event that parallel cooling channels 11 and 11 a are unequally loaded and have unequal flow resistances. Each of the cooling channels 11 and 11 a has its own pump, which ensures that a throughput corresponding to the respective load is set in each cooling channel 11 and 11 a.

The heated He discharged from the center of the winding 16 (or also from any intermediate point is first passed through the second heat exchanger 12 and thus stimulates a first mass flow 17 which, after recooling in the heat exchangers 15 and 10, the cooling channel 11 of the winding 16 After exiting the heat exchanger 12, the He is passed into a sixth heat exchanger 12a of a second fountain pump because of the already partially lowered temperature of the refrigerant when it enters this second pump, only a comparatively lower second mass flow 17a can be excited there than in that After recooling in the heat exchangers 15a and 10a, this helium flow is led through the second cooling channel 11a of the winding 16. A self-excited cooling system is thus obtained, with which different coolant flows 17 and 17a are generated in the two winding parts same principle n more than two parallel cooling circuits can also be set up.

Such cooling circuits with graded cooling capacities can be of particular interest for windings with inhomogeneous thermal loads. Such a case is e.g. with a toroidal field coil of a TOKAMAK fusion reactor. Here, in the winding layers that are closest to the plasma, due to the absorption of neutrons, a considerably higher load occurs than further out. In relation to the cooling system shown in FIG. 5, the larger mass flow 17 would be led through the inner windings in this case.

• a) ein erster Wärmestrom 18, durch die als thermische Barriere auszubildende Druckausgleichsverbindung 1a zwischen dem He I und dem He II-System, und
• b) ein zweiter Wärmestrom 19, der mit dem vom He I-Vorratsbehälter 1 über einen achten Wärmeaustauscher 2a zum Rückkühlbad 4 einströmenden He verknüpft ist.

FIG. 6 shows a further design of a cooling system, the circulation of the He II being stimulated not only by the heat fed back from the cooling object to the fountain pump, but also additionally by other heat flows which, at other points in the overall cooling system, between the temperature level of the He I storage container 1 and the He ll warm-up bath 9 flow. In the examples discussed here, there are two parts:

• a) a first heat flow 18, through the pressure compensation connection 1 a to be designed as a thermal barrier, between the He I and the He II system, and
• b) a second heat flow 19 which is linked to the He flowing in from the He I storage container 1 via an eighth heat exchanger 2a to the cooling bath 4.

Both heat flows 18 and 19 burden the warm-up bath 9 and thus make a contribution to increased convection. This measure reduces the thermal load on the cooling bath 4. If a pressure line 21, which is thermally coupled to the supply bath 1 via a heat exchanger 20, is hydraulically decoupled from the supply bath 1 instead of the pressure compensation line 1a, any pressure can be applied to the He ll system 4 via this pressure line 21.

Reference symbol list:

• ¹ storage container
• 2 pipe
• 2a eighth heat exchanger
• 3 relief valve
• 4 recooling bath
• 5 exhaust pipe
• 6 wall
• 7 supply bath
• 8 super filters
• 9 warm-up bath
• 10 first heat exchanger
• 11 Cooling channel / cooling section
• 12 second heat exchanger
• 12a sixth heat exchanger
• 13 third heat exchanger
• 14 fourth heat exchanger
• 15 fifth heat exchanger
• 16 object too cool
• 17 first mass flow
• 17a second mass flow
• 18 first heat flow
• 19 second heat flow
• 20 seventh heat exchanger
• 21 pressure line

Claims (11)

1. Method of cooling an object by means of super- fluid helium (He II), wherein a fountain pump with a superfilter (8) is used to produce a forced flow of the He II, and heat is supplied to the hot end of the superfilter (8), characterised in that the forced flow is activated as a result of the heat, which is removed from the object (16) to be cooled, being incorporated into a heating-up bath (9), which is provided at the hot end of the superfilter (8).

2. Method according to claim 1, characterised in that

a) heat is removed from the helium, which has been heated-up from the inlet temperature T₁ to the outlet temperature T₂ in the object (16) to be cooled, in the heating-up bath (9) of the fountain pump, whereupon the helium is re-cooled further in a re-cooling bath (4) and supplied to the supply bath (7) of the fountain pump, and

b) the pressurised helium, which has thus been heated in the heating-up bath (9) of the fountain pump, is removed and, after it has been re-cooled to the inlet temperature T₁ in the re-cooling bath (4), it is supplied to the object (16) to be cooled.

3. Method according to claims 1 and 2, characterised in that the He II, which has been removed from the heating-up bath (9) of the fountain pump, is split- up into a plurality of partial streams of He II (17, 17a), which are to be supplied to the object (16) at various locations.

4. Method according to claim 3, characterised in that

a) when there are two or more partial streams of He II (17, 17a) with different thermal loadings, each partial stream (17, 17a) is removed from its own particular fountain pump, which is adapted to the actual thermal loading, and is supplied to the object (16) to be cooled, and

b) after the heat has been absorbed in the object (16) to be cooled, the partial streams of He (17, 17a) are combined to form a total stream of He (17, 17a), which supplies the heating-up baths (9) of the fountain pumps in succession, according to the thermal loading of the associated partial streams in order to activate the forced flow, the fountain pump of the partial stream with the greater thermal loading taking precedence.

5. Method according to claims 1 to 4, characterised in that, in order to intensify the forced flow, heat which has been removed from the heating-up bath (9) of the fountain pump, in addition to that removed from the object (16) to be cooled, is incorporated with heat from the He I storage container (1).

6. Device for carrying out the method according to claim 1, having a fountain pump, which comprises a superfilter (8), a supply bath (7) which is provided at the cold end of the superfilter (8) and supplies the superfilter (8) with He II, and a heating-up bath (9) which is provided at the hot end of the superfilter (8), said pump having a re-cooling bath (4) for cooling the supply bath (7), and having a closed helium circuit in which the following components are disposed in series:

- a first heat exchanger (10), which is disposed in the te-cooling bath (4) and is connected, at its inlet end, to the heating-up bath (9) and, at its outlet end, to a cooling channel (11) of the object to be cooled;

- a second heat exchanger, which is disposed in the heating-up bath (9) and is connected to the outlet of the cooling channel (11);

- a third heat exchanger (13), which is disposed in the re-cooling bath (4) and is connected, at its inlet end, to the second heat exhanger (12), the outlet of said third heat exchanger extending into the supply bath (7).

7. Device according to claim 6, characterised in that a fourth heat exchanger (14) is introduced between the second heat exchanger (12) and the third heat exchanger (13) and exchanges heat with the waste gas pipe (5) of the re-cooling bath (4).

8. Device according to claims 6 and 7, characterised in that a fifth heat exchanger (15) is introduced betwen the heating-up bath (9) and the first heat exchanger (10) and exchanges heat with the waste gas pipe (5).

9. Device according to claims 6 to 8, characterised in that a storage container (1) is thermally coupled with the heating-up bath (9) of the fountain pump via a sixth heat exchanger (2a), the inlet and outlet of which are each connected to a tube (2b and 2c respectively), which exchanges heat with the waste gas pipe (5), the end of said tube being provided with a relief valve (3) and extending into the re-cooling bath (4).

10. Device according to claims 6 to 9, characterised in that a pressure-equalising connection (1a), which is normally situated between a storage container (1) and the supply bath (7), extends directly into the heating-up bath (9).

11. Device according to claim 10, characterised in that, instead of using the pressure-equalising connection (1 a), a pressure line (21) extends into the heating-up bath (9) via a seventh heat exchanger (20) in the storage container (1).

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