WO2010139318A2

WO2010139318A2 - Displacer unit of a stirling cooling arrangement, and stirling cooling arrangement

Info

Publication number: WO2010139318A2
Application number: PCT/DK2010/000076
Authority: WO
Inventors: Stig Kildegaard Andersen; Klaus Reinwand; Snorri Jonsson
Original assignee: Danfoss Compressors Gmbh
Priority date: 2009-06-05
Filing date: 2010-06-01
Publication date: 2010-12-09
Also published as: WO2010139318A3; DE102009023968A1

Abstract

The invention concerns a displacer unit (3) of a Stirling cooling arrangement (1) with a housing (20), a displacer (10) that is arranged to reciprocate in a cylinder (11) between a cold side (12) and a hot side (13) of the displacer unit (3), and a regenerator (16) arranged between the cold side (12) and the hot side (13), a gas being permitted to flow through said regenerator (16) on its way from the cold side (12) to the hot side (13) and back. It is endeavoured to permit defrosting in a simple manner. For this purpose, at least one bypass conduit (23) is arranged in parallel to the regenerator (16), an opening or closing of said bypass conduit (23) being con- trolled by a control unit (22).

Description

Displacer unit of a Stirling cooling arrangement, and Stirling cooling arrangement

The invention concerns a displacer unit of a Stirling cooling arrangement with a housing, a displacer that is arranged to reciprocate in a cylinder between a cold side and a hot side of the displacer unit, and a regenerator arranged between the cold side and the hot side, a gas being permitted to flow through said regenerator on its way from the cold side to the hot side and back.

Further, the invention concerns a Stirling cooling arrangement with such a displacer unit and a pressure wave generator.

With a Stirling cooling arrangement, the pressure wave generator generates pressure waves in a gas that is supplied to the displacer unit. In the displacer unit, the displacer reciprocates in the cylinder, thus transporting gas from a hot side to a cold side and back. In this connection, the gas flows through the regenerator. On the way from the hot side to the cold side, the gas discharges heat to the regenerator, so that it reaches the cold side in a colder state. On the way back from the cold side to the hot side, the gas absorbs stored heat from the regenerator.

If the cold side is cooled to values below O⁰C, and a certain amount of humidity is available in the surrounding atmosphere, ice will be formed on the cold side of the housing or on a heat exchanger or a corresponding cooling member connected to the cold side. When ice has been formed, this ice prevents the heat exchange with the surrounding atmosphere, so that the efficiency of the Stirling cooling arrangement is deteriorated.

It is therefore known to defrost the cold side of the displacer unit from time to time, as this is also common with domestic refrigerators. One opportunity is to arrange an electrical heating on the cold side, said heating being supplied with current from time to time to activate it and make it defrost the cold side. Another opportunity is described in US 6,205,792 B1. Here, a phase relation between the movement of the displacer and the movement of the pistons in the pressure wave generator is changed, so that these elements will practically work in counterphase to their normal operation, thus reversing the Stirling cycle 5 process.

Both solutions require substantial efforts.

The invention is based on the task of permitting a defrosting of the cold side ofo the displacer unit in a simple manner.

With a displacer unit as mentioned in the introduction, this task is solved in that at least one bypass conduit is arranged in parallel to the regenerator, an opening or closing of the bypass conduit being controlled by a control unit. 5

With this embodiment, the Stirling cooling arrangement can be operated normally, when the bypass conduit has been closed by the control unit. In this case the same conditions occur than without the bypass conduit. When the cold side of the displacer unit has to be defrosted, the bypass conduit is opened by the o control unit to permit a flow. This causes that gas flowing from the hot side to the cold side no longer supplies the amount of heat contained in it to the regenerator but transports it to the cold side. This causes that the temperature on the cold side increases. Usually, already after a short while, the temperature has increased so much that a defrosting can take place. When the cold side of the5 displacer unit has been defrosted, the bypass conduit is closed again, so that the Stirling cooling arrangement can resume its task, namely to provide a certain cooling capacity. In this connection, changes in the operating conditions of the Stirling cooling arrangement do practically not occur.

o Preferably, the cold side of the regenerator is provided with a first heat exchanger and/or the hot side of the regenerator is provided with a second heat exchanger, and the bypass conduit is connected between the regenerator and at least one heat exchanger. Thus, only the regenerator and not the heat exchangers) is bypassed. This means that the heat exchangers can continue per- forming their task of absorbing heat from the environment into a chamber that is arranged on the cold side or to discharge heat to the environment from a gas in a chamber that is arranged on the hot side of the displacer unit.

Preferably, the control unit interacts with a closing element at both ends of the bypass conduit. Thus, the bypass conduit can be closed or opened at both its ends. This causes that during normal operation of the Stirling cooling arrangement no additional gas volume is available that could change the operational behaviour. It is, however, not required for the closing elements to be arranged directly at the ends of the bypass conduit, to avoid the existence any volume in the bypass conduit, when the Stirling cooling arrangement operates normally. A small additional gas volume is not harmful.

Preferably, the bypass conduit is arranged outside the housing. This is a rela- tively simple way of providing the bypass conduit without substantial changes to the displacer unit.

In an alternative embodiment, the bypass conduit is arranged at least partly in a wall of the housing. Often, the housing of the displacer unit must anyway have a certain wall thickness in order to provide a certain pressure resistance. In the thicker areas of the wall, the bypass conduit can then be arranged without problems. In this connection, the bypass conduit must not necessarily extend inside the housing over its whole length; possibly only the thicker areas of the housing wall can be used for accommodating the bypass conduit.

In an alternative embodiment, the bypass conduit can also extend through the regenerator. This will cause a small loss of flow cross-section in the regenerator, as this flow cross-section will be occupied by the bypass conduit. It, however, makes the guiding of the bypass conduit relatively simple.

Finally, the bypass conduit can also be located in the cylinder. The cylinder must have a relatively large wall thickness to ensure the required pressure resistance. This wall thickness can be utilised for accommodating a thin bypass conduit. In all cases, it is merely required that the bypass conduit has a lower flow resistance than the regenerator, so that after opening or releasing the bypass conduit the main share of the gas will flow through the bypass conduit and only a minor share will flow through the regenerator.

Preferably, the control unit interacts with an actuator unit arranged in the housing, said actuator unit being provided for touch-free actuation from the outside of the housing. As mentioned above, the housing must have a certain pressure resistance. When the actuator unit can be activated from the outside without requiring insertion of an actuator element through the housing, problems with the pressure tightness of the housing can be avoided.

It is preferred that the control unit controls an electromagnet interacting with the actuator unit. An electromagnet generates a magnetic field. This magnetic field can then activate the actuator unit, for example to lift a valve element from a valve seat. When the electromagnet is switched off, there are no longer any forces keeping the valve element away from the valve seat. A return spring can then draw the valve element back towards the valve seat and close the bypass conduit.

It is preferred that the actuator unit comprises a permanent magnet. If then the electromagnet is switched on, particularly favourable force conditions occur, which cause a reliable actuation of the actuator unit.

The invention also concerns a Stirling cooling arrangement with a displacer as described above, and a pressure wave generator, whose pressure amplitude can be reduced to a defrosting level.

When the bypass conduit has been opened, the gas flowing from the hot side to the cold side and vice versa no longer passes only through the regenerator. On the contrary, the largest share flows through the bypass conduit. The bypass conduit has a substantially lower flow resistance than the regenerator. Accordingly, also a substantially smaller pressure difference is required to bring gas from the hot side to the cold side and back. Accordingly, only a very small pressure wave is required to reciprocate the displacer that can also be called dis- placer piston in such a manner that it transports a sufficient amount of gas from the hot side to the cold side to supply the desired amount of heat to the cold side.

It is preferred that the control unit is connected to a drive of the pressure wave generator. Thus, when the control unit opens the bypass conduit, it simultaneously reduces the amplitude of the pressure wave generator to prevent damage to the Stirling cooling arrangement. In particular, the amplitude reduction prevents damage to the displacer unit that could be caused by excessive movement (overdriving) of the displacer at open bypass.

In the following, the invention is described on the basis of preferred embodi- ments in connection with the drawings, showing:

Fig. 1 a schematic view of a Stirling cooling arrangement,

Fig. 2 a modified embodiment of a displacer unit,

Fig. 3 a third embodiment of a displacer unit, and

Fig. 4 a fourth embodiment of a displacer unit.

Fig. 1 is a schematic view of a Stirling cooling arrangement 1 with a pressure wave generator 2 and a displacer unit 3 connected to one another via a pipe 4.

The pressure wave generator 2 comprises two pistons 5, 6 that are located in a cylinder block 7. The two pistons 5, 6 are movable by drives 8, 9 in opposite di- rections, that is, they move either towards each other or away from each other. Gas that is available between the two pistons 5, 6 in the cylinder block 7 is displaced through the conduit 4 to the displacer unit 3 or sucked back from there. This causes pressure waves in the displacer unit 3. Also the displacer unit 3 is only shown schematically. It comprises a displacer 10, that is, a piston that reciprocates in a cylinder 11. The displacer 10 is arranged between a cold side 12 and a hot side 13. At the cold side 12, an expansion chamber 14 is located, and at the hot side 13, a compression chamber 15 is located. Between the compression chamber 15 and the expansion chamber 14 is arranged a regenerator 16, heat exchangers 17, 18 being arranged at either ends of the regenerator 16. The displacer 10 is connected to a return spring 19.

The functioning mode of such a Stirling cooling arrangement is known per se. Due to the pressure waves generated by the pressure wave generator 2; the displacer 10 is periodically reciprocated in the cylinder 11. Thus, it displaces gas from the compression chamber 15 into the expansion chamber 14. The gas flows through the regenerator 16 and discharges heat to the regenerator 16. During a return movement of the displacer 10, the displacer 10 displaces gas from the expansion chamber 14 and returns it to the compression chamber 15. The gas flows through the regenerator 16 again, where it absorbs heat. The heat exchanger 17 serves the purpose of absorbing heat from the environment, so that the cold side 12 is cooled. The heat exchanger 18 serves the purpose of discharging heat to the environment, so that the hot side 13 is heated.

When the cold side 12 is cooled below O⁰C, and the environment of the cold side 12 contains humidity, first white frost and eventually ice will be formed at the outside of the housing 20 of the displacer unit 3. The ice layer impedes heat exchange with the environment. This also applies if the environment is not in direct contact with the outside of the housing 20, but only through heat conducting means, such as a cooling member or the like.

A defrosting sensor 21 that is connected to a control unit 22 can determine, whether a defrosting process is required or not. When a defrosting process is required, the control unit triggers this defrosting process.

The displacer unit 3 comprises a bypass conduit 23 that is arranged in parallel with the regenerator 16 and connects the cold side 12 and the hot side 13, or rather, the expansion chamber 14 and the compression chamber 15, to each other. However, the bypass conduit 23 is closed by a closing unit 24. However, the control unit 22 can open the closing unit 24, when a defrosting process is required. In this case, a flow through the bypass conduit 23 is possible. When the defrosting process has been finished, the control unit 22 closes the closing unit 24 again, so that the bypass conduit 23 is closed and a further gas flow through the bypass conduit 23 is prevented.

Simultaneously with the opening of the bypass conduit 23 the control unit 22 reduces the driving performance of the drives 8, 9, so that the pressure wave generator 2 reduces a pressure amplitude to a defrosting level.

The opened bypass conduit 23 has a substantially lower flow resistance than the regenerator 16. Firstly, this causes that with the open bypass conduit 23 the largest share of the gas flowing between the compression chamber 15 and the expansion chamber 14 passes through the bypass conduit 23 and not through the regenerator. Secondly, it causes that the power required to move the dis- placer 10 has been substantially reduced, as the displacer 10 no longer has to supply the gas through the regenerator 16. Accordingly, a small pressure ampli- tude from the pressure wave generator 2 will be sufficient to move the displacer during the defrosting process.

The gas that flows from the compression chamber 15 to the expansion chamber 14, however bypassing the regenerator 16 and flowing through the bypass con- duit 23, no longer discharges its heat to the regenerator 16, but transports this heat into the expansion chamber 14, that is, to the cold side 12. This heat transport then causes that the temperature at the cold side 12 increases until it has reached a value of more than O⁰C, so that an ice layer at the cold side 12 can melt.

As soon as the defrosting process has been finished, which can, for example, be determined by the defrosting sensor 21 , the bypass conduit 23 is closed again and the displacer unit 3 can revert to its normal operation. Instead of a defrosting sensor 21 also other means can be used to avoid or remove ice. For example, the defrosting can be performed periodically at predetermined time intervals.

For the embodiment of the bypass conduit 23, there are various opportunities, which will be explained by means of the Figs. 2 to 4. In these Figures, elements equal to or functionally equal to elements of Fig. 1 are provided with the same reference numbers.

Fig. 2 shows a displacer unit 3, in which the bypass conduit 23 that, as shown in the embodiment of Fig. 1 , extends between the regenerator 16 and the two heat exchangers 17, 18 inside of the housing 20 is arranged outside the housing 20 in parallel to the regenerator 16. At either end, the bypass conduit 23 has a valve 25, 26, both valves 25, 26 being activated by the control unit 22 in a man- ner not shown in detail. During the defrosting process the two valves 25, 26 are opened, and the gas can flow through the bypass conduit 23 at a lower differential pressure. Further, they ensure that the bypass conduit 23 does not form an additional free volume that has to be considered by the control of the displacer 10.

Fig. 3 shows an embodiment, in which the housing 20 comprises several sections 27, 28 that are relatively massive. These sections are particularly located in areas, which are, for example, acted upon by large clamping forces of the heat exchangers. Thus, for example, they surround the expansion chamber 14 and partly the compression chamber 15, at least in the area of the heat exchanger 18 arranged there.

Also here, the bypass conduit 23 has two valves 25, 26 located at the ends of the bypass conduit 23. Sections 29, 30 of the bypass conduit 23, however, are guided through the massive sections 27, 28 of the housing 20.

Fig. 4 shows a further embodiment of a displacer unit 3. Fig. 4a shows the displacer unit in a section and Fig. 4b shows an enlarged view of a unit for closing or opening the bypass conduit 23. In this case, the bypass conduit 23 is arranged in the wall of the cylinder 10 in parallel to the regenerator 16. A radial opening 31 provides a connection to the expansion chamber 14. A radial opening 32 provides a connection to the com- 5 pression chamber 15. The wall 33 of the cylinder 10 is sufficiently pressure resistant.

A valve 25 has a valve element 34 and a valve seat 35. The valve element 34 is connected to a closing spring 37 via a rod 36. At the end facing away from theo closing spring 37, the rod 36 is connected to a permanent magnet 38. Outside the housing 20 is arranged an electromagnet 39 that is connected to the control unit 22. Thus, the control unit 22 is able to provide the electromagnet 39 with current, so that it generates a magnetic field that attracts the permanent magnet 38. When the permanent magnet 38 is attracted by the electromagnet 39, it lifts5 the valve element 34 off from the valve seat 35, and the bypass channel 23 is opened. When the electromagnet 39 is switched off, the return spring 37 pulls the valve element 34 back onto the valve seat 35 and the bypass conduit 23 is closed. This actuation is touch-free, that is, the permanent magnet 38 with the rod 36 and the closing spring 37 form an actuator unit that can be activated in a o touch-free manner from the outside of the housing 20.

Claims

Patent Claims

1. Displacer unit of a Stirling cooling arrangement with a housing, a dis- 5 placer that is arranged to reciprocate in a cylinder between a cold side and a hot side of the displacer unit, and a regenerator arranged between the cold side and the hot side, a gas being permitted to flow through said regenerator on its way from the cold side to the hot side and back, characterised in that at least one bypass conduit (23) is arranged in parallelo to the regenerator (16), an opening or closing of said bypass conduit (23) being controlled by a control unit (22).

2. Displacer unit according to claim 1 , characterised in that the cold side (12) of the regenerator (16) is provided with a first heat exchanger (17)5 and/or the hot side (13) of the regenerator (16) is provided with a second heat exchanger (18), and the bypass conduit (23) is connected between the regenerator (16) and at least one heat exchanger (17, 18).

3. Displacer unit according to claim 1 or 2, characterised in that the control o unit (22) interacts with a closing element (25, 26) at both ends of the bypass conduit (23).

4. Displacer unit according to one of the claims 1 to 3, characterised in that the bypass conduit (23) is arranged outside the housing (20). 5

5. Displacer unit according to one of the claims 1 to 4, characterised in that the bypass conduit (23) is arranged at least partly in a wall (27, 28) of the housing (20).

0 6. Displacer unit according to one of the claims 1 to 3, characterised in that the bypass conduit (23) extends through the regenerator (16).

7. Displacer unit according to one of the claims 1 to 3, characterised in that the bypass conduit (23) is located in a wall (33) of the cylinder (10).

8. Displacer unit according to one of the claims 1 to 7, characterised in that the control unit (22) interacts with an actuator unit (36-38) arranged in the housing (20), said actuator unit (36-38) being provided for touch-free ac-

5 tuation from the outside of the housing (20).

9. Displacer unit according to claim 8, characterised in that the control unit (22) controls an electromagnet (39) interacting with the actuator unit (36- 38). 0

10. Displacer unit according to claim 9, characterised in that the actuator unit (36-38) comprises a permanent magnet (38).

11. Stirling cooling arrangement with a displacer (3) according to one of the5 claims 1 to 10, and a pressure wave generator (2), whose pressure amplitude can be reduced to a defrosting level.

12. Stirling cooling arrangement according to claim 11 , characterised in that o the control unit (22) is connected to a drive (8, 9) of the pressure wave generator (2).