US20120152492A1

US20120152492A1 - Gas thermal switch having a movable heat-exchange element

Info

Publication number: US20120152492A1
Application number: US12/306,326
Authority: US
Inventors: Lionel Duband; Dominique Lestelle
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2006-06-23
Filing date: 2007-06-22
Publication date: 2012-06-21
Also published as: FR2902868B1; WO2007147981A1; EP2032933A1; FR2902868A1

Abstract

The gas thermal switch (1) comprises:

- a first terminal (9) which is intended to be thermally connected to a first component:
- a second terminal (11) which is intended to be thermally connected to a second component;
- a chamber (33) for receiving a heat-exchange gas;
- a heat-exchange gas in the chamber (33);
- a least a first (13) and a second (15) heat-exchange elements which are arranged in the chamber (33) in order to be able to be thermally connected respectively to the first terminal (9) and second terminal (11).

At least one of the heat-exchange element (15) is movable relative to its respective terminal (11).

Description

The present invention relates to a gas thermal switch of the type comprising:

- a first terminal which is intended to be thermally connected to a first component,
- a second terminal which is intended to be thermally connected to a second component,
- a chamber for receiving a heat-exchange gas,
- at least a first and a second heat-exchange elements which are arranged in the chamber in order to be able to be thermally connected to the first terminal and second terminal, respectively, and to have facing heat-exchange surfaces which are separated by a space which is intended to be filled by the heat-exchange gas present in the chamber in order to exchange heat between the first and the second heat-exchange elements.

The invention is used, for example but not exclusively, to establish thermal connections between components which are placed at cryogenic temperatures, for example, for an application in the field of space.
It should be noted that cryogenic temperatures are temperatures of less than 120 K.
Generally, thermal switches allow a thermal connection between a first component and a second component to be established or disengaged as desired.
For an application in the field of space, the first component and the second component can be components of a coolant system, for example, an evaporator and a cold source.
Different types of thermal switch are known, including in particular mechanical switches and gas type switches.
Mechanical switches comprise two terminals which are each intended to be connected to one of the components between which the switch is arranged. The switch further comprises a movable element in order to establish mechanical contact, and therefore a thermal connection, between the two terminals. If it is desirable to reach high levels of thermal conductance, these switches are generally large and heavy and therefore unsuitable for use in the field of space.
Gas switches of the above-mentioned type are also known. The first heat-exchange element is a rod which is fixedly joined to the first terminal and the second heat-exchange element is a tube which is fixedly joined to the second terminal and which receives the rod, with a space being provided around it. This space is very thin, generally in the order of from one to a few times the mean free path of the gas used. For example, this thickness may be in the order of 100 μm when helium is used at a pressure of a few millibars at the operating temperature.
The chamber of the switch is delimited by the terminals, which are generally in the form of discs, and by an outer casing which is generally cylindrical and which surrounds the second heat-exchange element and whose ends are fixed to the two terminals. This outer casing forms a cross-member between the two terminals and it is therefore generally produced from a material which is a very poor conductor of heat, for example, from titanium, with a small thickness.
Furthermore, the switch also comprises a device for introducing and removing heat-exchange gas to/from the chamber. This device may be, for example, an adsorption pump.
When the gas is introduced into the chamber, the gas fills the space between the two heat-exchange elements and a heat exchange takes place between the two elements by means of conduction, owing to the particles of gas which will preferably collide with the elements and/or with each other.
When the thickness of the space is less than the mean free path of the particles of gas, the heat exchange between the two elements is proportional to the pressure of the gas, which allows this exchange to be modulated by controlling the pressure. In the majority of applications, however, the switch is used in an “all or nothing” manner, and, for this reason, the thickness of the space is selected to be slightly greater than the mean free path of the particles of gas, in order to maximise the heat exchange and render it independent of any fluctuations in the pressure of the exchange gas.
Furthermore, the heat exchange between the two elements will be substantially inversely proportional to the thickness of the space between the heat-exchange elements.
The switch thus allows a thermal connection to be provided between the two components to which the terminals thereof are connected. The switch can therefore be said to be in a closed configuration.
When the gas has been removed from the chamber, the only thermal connection existing between the two terminals is provided by the outer casing. Owing to its small thickness and the material selected, the thermal flux thus established is very small and acceptable. The switch can therefore be said to be in an open configuration.
Generally, one of the terminals of the switch is fixed directly to the corresponding component, for example, by means of screwing, and the other terminal is connected to the corresponding component by means of a flexible strand which allows mechanical decoupling to be provided.
In spite of this mechanical decoupling, the terminals of the switch may be subject to mechanical loads, in particular during a relative movement between the first component and the second component.
Owing to the small thickness of the space between the two heat-exchange elements, these loads may rapidly lead to the two elements coming into contact. Such contact brings about a thermal short-circuit regardless of the instruction given to the switch.
The mechanical stresses may therefore readily lead to operational degradation of the switch.
Stresses of greater amplitude, in particular in terms of flexion or shearing between the terminals, may bring about structural degradation of the casing or, if it has a sufficient degree of flexibility, structural degradation of the heat-exchange elements.
An object of the invention is to overcome this problem by providing a switch of the above-mentioned type whose operation is less sensitive to the mechanical forces which are capable of being applied to these terminals.
To this end, the invention relates to a switch of the above-mentioned type, characterised in that at least one of the heat-exchange elements can be moved relative to its respective terminal. This mobility is intended to be understood in its most general form, that is to say, in terms of translation and/or in terms of rotation.
According to specific embodiments, the invention may comprise one or more of the following features, taken in isolation or according to any technically possible combination:

- the switch also comprises a device for introducing the heat-exchange gas into the chamber and removing the heat-exchange gas from the chamber;
- the movable heat-exchange element can be moved between at least one position for greater heat transfer with the terminal and the element, and a position for lesser heat transfer between the terminal and the element, and the switch comprises a device for moving the heat-exchange element;
- in its position of greater heat-transfer, the movable heat-exchange element is in contact with its respective terminal;
- the movable heat-exchange element and the respective terminal thereof have substantially complementary surfaces to position the heat-exchange element relative to the respective terminal thereof;
- one of the heat-exchange elements is a male element and the other heat-exchange element is a female element for receiving the male element;
- the movable heat-exchange element is the female element;
- the movable female heat-exchange element is a tube, one end of which is closed by means of a base;
- the tube is provided with passages for circulation of the heat-exchange gas in the region of the base and/or at the base;
- the male heat-exchange element and the female heat-exchange element have substantially complementary shapes;
- the male heat-exchange element and the female heat-exchange element extend along an axis, and the movable heat-exchange element can be moved in translation along the axis;
- the substantially complementary surfaces converge towards the axis;
- the surfaces are spherical caps;
- the movement device comprises a ferromagnetic core which is fixedly joined to the movable heat-exchange element and means for creating a magnetic field in the chamber; and
- the movement device is capable of moving the tube in a controlled manner to at least one intermediate position between the position of greater heat transfer and the position of lesser heat transfer.

The invention will be better understood from a reading of the following description, given purely by way of example, and with reference to the single appended FIGURE which is a schematic perspective view of a gas switch according to the invention, one half of the switch being broken-away by a section taken along a longitudinal centre plane.

The FIGURE illustrates a gas thermal switch 1 which is intended to operate in a temperature range of between 0 and 10K and which uses helium as a heat-exchange gas. This switch 1 is intended, for example, for an application in space, for example, to thermally connect a first component and a second component which may be components of a coolant system.
The switch 1 principally comprises a body 3, a device 5 for moving a heat-exchange element inside the body 3, a device 7 for introducing and removing the heat-exchange gas and a system 8 for controlling the devices 5 and 7 in order to carry out the operation described below.
In the example illustrated, the body 3 has a generally cylindrical shape about a longitudinal axis L. The body 3 is substantially rotationally symmetrical about the axis L but, as mentioned below, other forms may be envisaged.
The body 3 principally comprises:

- a first terminal 9, or electrode, which is intended to be connected to the first component,
- a second terminal 11 which is intended to be connected to the second component,
- a first heat-exchange element 13,
- a second heat-exchange element 15 and
- an outer casing 17.

The terminals 9 and 11 are generally of disc-like form and are each arranged at a longitudinal end of the body 3. The terminals 9 and 11 are extended longitudinally by lugs 19 and 21, respectively, for mechanical and thermal connection to the first and second component, for example, by means of screwing directly to these components or by means, for example, of flexible strands of copper wires. To this end, the lugs 19 and 21 may be provided with orifices 23.
In this manner, this mechanical connection of the lugs 19 and 21 to the first component and the second component allows a thermal connection to be provided between the terminals 9 and 11 and these two components.
A channel 25 which is connected to the introduction and removal device 7 extends through the first terminal 9 (at the left-hand side in the Figure). Another channel (not illustrated) also extends through the terminal 9 and is intended to allow a first filling operation with heat-exchange gas, as will be described below.
The casing 17 is constituted, in the example illustrated, by a narrow tube having a circular base which is produced from a material with a low level of thermal conductivity and which may be non-magnetic in accordance with the type of movement device 5 used.
In the example described, this tube is produced from titanium which is a non-magnetic material.
The casing 17 is fixed, with the two longitudinal ends thereof, to the terminals 9 and 11, for example, by means of fitting to protrusions 29 and 31 of the terminals 9 and 11 and generally soldering thereto, although adhesive-bonding may also be used. In this manner, the outer casing 17 forms a cross-member and it delimits internally, with the terminals 9 and 11, a sealed chamber 33 which is intended to contain the heat-exchange gas.
The first heat-exchange element 13 is a cylindrical rod having a circular base which is centred on the axis L. The rod 13 extends the first terminal 9 towards the second terminal 11 and extends over the main part of the length of the chamber 33.
The rod 13 is integral with the first terminal 9 and in particular with the protrusion 29, and is therefore constructed in one piece therewith. In this example, the rod 13 is therefore fixedly joined to the first terminal 9.
The second heat-exchange element 15 is a tube having a circular cross-section which is also centred on the axis L. In the example illustrated, it is a blind tube, one end of which, in this instance, the one facing the second terminal 11, is closed by a base 35. The inner diameter of the heat-exchange tube 15 is slightly greater than the outer diameter of the heat-exchange rod 13 and the rod 13 is arranged inside the tube 15.
In this manner, the rod 13 and the tube 15 together delimit a space 37 which has a small thickness e and which is slightly greater than the mean free path of the heat-exchange gas. This space 37 is intended to be filled by the heat-exchange gas, as will be seen below.
Since the tube 15 covers the rod 13 over a significant portion of the length of the chamber 33, the rod 13 and the tube 15 have facing surfaces 39 and 41 with large surface-areas so that, when the heat-exchange gas is present in the space 37, significant heat-exchange is provided by conduction between the rod 13 and the tube 15.
Passages 43 for circulation of the heat-exchange gas are provided in the tube 15 in the region of the base 35. In some variants, passages are also provided in the base 35.
These passages 43 allow the filling of the space 37 with heat-exchange gas to be accelerated, as will be described below.
The surface 45 of the base 35 positioned opposite the second terminal 11 is preferably in the form of a spherical cap centred on the axis L. The surface 47 of the terminal 11 positioned opposite has a complementary shape. These two surfaces 45 and 47 have the same radius of curvature. These two surfaces 45 and 47 therefore converge towards the axis L, in the direction from the first terminal 9 towards the second terminal 11.
The heat-exchange tube 15 can be moved in the chamber 33, relative to the second terminal 11, between a first position of lesser heat transfer with the second terminal 11 and a position of greater heat transfer with the second terminal 11. These two positions are extreme positions, that is to say, at the end of the path.
The first position of lesser heat transfer is illustrated in the Figure. In this position, the base 35 is longitudinally spaced from the second terminal 11 and is supported longitudinally, in the direction towards the first terminal 9, against the end of the rod 13 remote from the first terminal 9.
In the second position of greater heat transfer (not illustrated), the base 35 is longitudinally spaced from the rod 13 and is supported longitudinally against the terminal 11.
The surfaces 45 and 47 are then fitted one inside the other thus centering the tube 15 with respect to the terminal 11.
The movement of the heat-exchange tube 15 is provided owing to the device 5 whose structure and operation will now be described.
The device 5 comprises a ferromagnetic core 49 which is, for example, a ring of soft iron, and means 51 for creating a magnetic field in the chamber 33, in this instance with field lines which are directed longitudinally. The core 49 is fixed to the end of the tube 15 opposite the base 35. More precisely, the core 49 extends the tube 15 towards the first terminal 9 and surrounds the rod 13.
The means 51 for creating a magnetic field comprise a first coil 53 and a second coil 55 which are arranged longitudinally beside each other. The first coil 53 is adjacent to the first terminal 9 and the second coil 55 is adjacent to the first coil 53. The means 51 for creating a magnetic field are mounted on the body 3 at the outer side of the casing 17 and are, for example, fixed to the terminal 9 by means of screwing.
The means 51 for creating a magnetic field extend practically over only the left half (in the FIGURE) of the body 3.
In the intermediate region 57 between the coils 53 and 55, the means 51 comprise a partition wall 59.
It should be noted that the core 49 remains radially opposite, or in any case close, to the wall 59, and therefore the intermediate region 57 between the two coils 53 and 55, during the movement of the tube 15.
When the tube 15 is in a central position between the two above-mentioned extreme positions thereof, the core 49 of soft magnetic material is substantially centred on the partition wall 59.
When, for example, voltage is applied to only the first coil 53, the core 49 is positioned in the direction towards the first coil 53 in order to maximise the magnetic flux which passes through the core 49, that is to say, to move the tube 15 to its first position. That is to say, as in variable reluctance motors, the core 49 will move so that a maximum induction flux can pass through it and thus reduce the reluctance.
In contrast, when only the second coil 55 is supplied with electrical power, a force is produced which tends to move the tube 15 into its second position.
By supplying one or other of the coils 53 and 55 with electrical power, it is therefore possible to move the tube 15 between the first and second position thereof, and even to control the movement of the tube 15 in a plurality of intermediate positions between the two extreme positions thereof. To this end, the switch 1 may comprise a sensor 60 for detecting the position of the tube 15 connected to the control system 8. It should be noted that the polarisation of the coils 53 and 55 has no influence on the movement of the tube 15 since the core is of soft magnetic material.
For an application in the field of space, the force required to move the tube 15 is extremely small since only the friction forces are to be overcome.
For an application on the ground, the force required to move the tube 15 may be very small, for example, in the order of 0.4 N and each coil may be a coil of 1500 ampere turns. Again in applications on the ground, the force required for the movement may also be reduced in arrangements where the switch 1 is placed in a horizontal position. Also in this instance, in the same manner as in applications in space, the coil 53 or the coil 55 may be supplied with electrical power only intermittently simply to move the tube 15 into the desired position, the electrical power supply being stopped after the tube 15 has reached the required position.
In order to reduce the occurrences of heating owing to the Joule effect, the coils 53 and 55 may be produced from superconductive wire.
The device 7 for introducing and removing the heat-exchange gas 7 may be, for example, a miniature adsorption pump. Such a pump conventionally comprises an adsorbant block 61 and a heating resistor 63.
When the block 61 is not heated by the resistor 63, it adsorbs the molecules of the heat-exchange gas which has been introduced beforehand into the chamber 33 via the first filling channel (not illustrated). The device 7 then removes the gas from the chamber 33.
In order to again introduce the gas into the chamber 33 in a controlled manner, the block 61 which desorbs the molecules which have been adsorbed previously is heated via the resistor 63.
The switch 1 has an open configuration and a closed configuration.
In the open configuration, the heat-exchange gas has been removed from the chamber 33 by the device 7 and the tube 15 is in the first position thereof. The only heat exchange between the terminals 9 and 11 is provided by thermal conduction via the outer casing 17. However, the casing has been sized and the material thereof selected in such a manner that the corresponding thermal flux is very weak and acceptable. The terminals 9 and 11 therefore establish practically no thermal connection between the first and the second components.
In the closed configuration, the heat-exchange gas has been introduced into the chamber 33 via the device 7 and fills in particular the space 37 between the tube 15 and the rod 13. The molecules of gas thus collide with the surfaces 39 and 41 of the tube 15 and the rod 13, thereby providing significant heat exchange by means of conduction between the rod 13 and the tube 15.
Furthermore, the tube 15 is in the second position thereof. The surface 45 of the tube 15 is therefore in contact with the surface 47 of the terminal 11 and thus provides heat transfer between the terminal 11 and the tube 15. In this manner, a significant thermal flux can be transmitted from the terminal 9 to the terminal 11 and therefore between the two components to which the two terminals 9 and 11 are connected.
In the open configuration, since the tube 15 is not fixedly joined to the terminal 11 and is therefore decoupled or free relative thereto, the risks of inadvertent contact of the tube 15 and the rod 13 when force is applied to the terminals 9 and 11 are very significantly reduced. The risks of operational degradation of the switch 1 are therefore also reduced.
Consequently, it is further possible to reduce the thickness e of the space 37 in order to reach a few μm or some tens of μm. The thermal output levels of the switch 1 can thus be increased compared with those of conventional gas switches.
The thermal conductance is also increased compared with that of conventional mechanical switches, whilst retaining a reduced mass and spatial requirement and reliable operation.
Furthermore, it should be noted that the time for moving between the closed configuration and the open configuration may be reduced compared with the conventional operation of a gas thermal switch. In order to reduce this movement time, it is possible, in order to move into the open configuration, to begin by moving the tube 15 to the first position thereof, which will cause the heat transfer between the tube 15 and the terminal 11 and therefore between the first and the second component to fall very rapidly.
The passages 43 also allow the heat-exchange gas to rapidly fill and leave the space 37 and therefore allow the time to be reduced for moving from the open configuration to the closed configuration, and vice-versa.
On the other hand, it is possible not to use any passages 43 so that the tube 15 and the rod 13 form a gas damping device which allows impacts to be absorbed, in particular in the case of application in the field of space.
Furthermore, in the example described above, using the device 5, it is possible to control the movement of the tube 15 to a plurality of intermediate positions between the two extreme positions thereof.
The switch 1 thus provides a plurality of intermediate configurations between the open configuration and the closed configuration, these configurations corresponding to increasing levels of thermal conductance. It should be noted that controlling the switch 1 to reach these intermediate configurations is much more straightforward owing to the fact that it can be carried out by moving the tube 15 rather than controlling the pressure of the exchange gas in the chamber 33.
Since the two heat-exchange elements 13 and 15 are arranged in the chamber 33, it is possible to obtain facing heat-exchange surfaces with large surface-areas, which allows significant heat exchange to be achieved whilst preserving small dimensions for a thermal switch.
In the example described above, the movement device 5 does not use a permanent magnet and is therefore less sensitive with respect to the high temperatures which may be reached, in particular during the soldering of the outer casing 17 to the terminals 9 and 11.
Generally, however, other movement devices 5 may be envisaged, for example, devices comprising permanent magnets in place of the core 49.
In this manner, according to one variant, the core 49 may be of hard magnetic material, which is magnetised in the same direction as the magnetic field produced by the means 51, the casing 17 still being of non-magnetic material.
However, in the proximity of the hard core 49, during the assembly of the thermal switch, this requires a welding operation to be carried out which raises the temperature beyond a value which allows the magnetisation to be maintained.
This may be carried out, for example, by adhesively-bonding the casing 17 to the terminals 9 and 11 at the two ends thereof.
The coils 53 and 55 may advantageously be replaced with the same coil, for example, located at a central position between the positions of the coils 53 and 55 of the preceding embodiment. The configuration of the switch 1 is determined by the direction of the current which passes through the single coil which replaces the coils 53 and 55.
In the example described above, the complementary surfaces 45 and 47 allow the tube 15 to be centred relative to the terminal 11 and therefore the axis L.
In other variants, which may optionally be combined with each other and also with other variants described, it is possible, in order to prevent the phenomenon of adhesive-bonding or adhesion of the surfaces 45 and 47 which could occur after they have been brought into contact, to use surfaces which, whilst remaining substantially complementary, are not completely complementary. In this manner, it is possible to use different radii of curvature for the surfaces 45 and 47.
Again in order to prevent the phenomenon of adhesive-bonding, it is possible to process the surfaces 45 and/or 47 so that they have a degree of roughness, and/or to provide cross-members in order to prevent, in the second position of the tube 15, the facing surfaces 45 and 47 from being in contact over the main part of their surface-areas.
In the same manner, in the second position of the tube 15, a space may be provided between the surfaces 45 and 47, for example, having a thickness which is equal to e and which is more generally less than the mean free path of the heat-exchange gas selected, that is to say, a few μm or some tens of μm. In this instance, in the second position of the tube 15, the heat transfer between the tube 15 and the terminal 11 is provided by means of conduction between the surfaces 45 and 47.
In order to centre the tube 15 and more generally position it relative to the terminal 11, other forms for the surfaces 45 and 47 may be envisaged, for example, converging and in particular conical surfaces. It should be noted generally that this positioning does not necessarily involve centering.
Even more generally, one of the heat-exchange elements 13 and 15 will be a male element and the other heat-exchange element a female element which receives the male element. Their shapes may be varied. For example, they may be cylinders having bases which have shapes other than circular, for example, triangular, square . . . . It should also be noted that the female element is not necessarily blind as described above, but instead may be open at both ends thereof.
Even more generally, the elements 13 and 15 are not necessarily male and female elements. They may, for example, be combs whose teeth are intended to engage with each other.
It is also possible for the tube 15 to be able to be moved relative to the terminal 11 in order to provide the mechanical decoupling, but without this mobility constituting positions corresponding to substantially different heat transfers between the terminal 11 and the tube 15. That is to say, the thermal conductivity of the switch 1 remains substantially constant relative to the terminal 11, regardless of the position of the tube 15.
In the same manner, the movable element is not necessarily the element 15 described above, but instead may be the element 13 which is intended to be thermally connected to the first terminal 9. In the same manner, it is possible to envisage that the two elements 13 and 15 can be moved relative to the terminals 9 and 11.
In some variants which are, for example, intended to operate at temperatures close to ambient temperatures, the switch 1 may not comprise a device 7 for introducing and removing heat-exchange gas.
The heat-exchange gas therefore permanently remains in the chamber 33 even when the switch is in an open configuration.
The gas thermal switch 1 described above and the different variants thereof may be used in a number of applications and are by no means limited to cryogenic temperatures and the field of space.

Claims

1. Gas thermal switch of the type comprising:

a first terminal which is intended to be thermally connected to a first component,

a second terminal which is intended to be thermally connected to a second component,

a chamber for receiving a heat-exchange gas,

a heat-exchange gas in the chamber,

at least a first heat-exchange element and a second heat-exchange element which are arranged in the chamber in order to be able to be thermally connected to the first terminal and second terminal, respectively, and to have facing heat-exchange surfaces which are separated by a space which is intended to be filled by the heat-exchange gas present in the chamber in order to exchange heat between the first heat-exchange element and the second heat-exchange element,

characterized in that at least one of the heat-exchange elements can be moved relative to its respective terminal.

2. Switch according to claim 1, characterized in that it also comprises a device for introducing the heat-exchange gas into the chamber and removing the heat-exchange gas from the chamber.

3. Switch according to claim 1, characterized in that the movable heat-exchange element can be moved between at least one position for greater heat transfer with the terminal and the element, and a position for lesser heat transfer between the terminal and the element, and in that the switch comprises a device for moving the heat-exchange element.

4. Switch according to claim 3, characterized in that, in its position of greater heat-transfer, the movable heat-exchange element is in contact with its respective terminal.

5. Switch according to claim 4, characterized in that the movable heat-exchange element and the respective terminal thereof have substantially complementary surfaces to position the heat-exchange element relative to the respective terminal thereof.

6. Switch according to claim 1, characterized in that one of the heat-exchange elements is a male element and the other heat-exchange element is a female element for receiving the male element.

7. Switch according to claim 6, characterized in that the movable heat-exchange element is the female element.

8. Switch according to claim 7, characterized in that the movable female heat-exchange element is a tube, one end of which is closed by means of a base.

9. Switch according to claim 8, characterized in that the tube is provided with passages for circulation of the heat-exchange gas in the region of the base and/or in the base.

10. Switch according to claim 6, characterized in that the male heat-exchange element and the female heat-exchange element have substantially complementary shapes.

11. Switch according to claim 6, characterized in that the male heat-exchange element and the female heat-exchange element extend along an axis, and in that the movable heat-exchange element can be moved in translation along the axis.

12. Switch according to claim 4, characterized in that the substantially complementary surfaces converge towards the axis.

13. Switch according to claim 12, characterized in that the surfaces are spherical caps.

14. Switch according to claim 3, characterized in that the movement device comprises a ferromagnetic core which is fixedly joined to the movable heat-exchange element and means for creating a magnetic field in the chamber.

15. Switch according to claim 3, characterized in that the movement device is capable of moving the tube in a controlled manner to at least one intermediate position between the position of greater heat transfer and the position of lesser heat transfer.