JP2005188405A - Heat insulation structure in piston type compressor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a piston type compressor improving heat insulation in a compression chamber. <P>SOLUTION: An aluminum made piston 25 is stored in a cylinder bore 111. The piston 25 partitions a compression chamber 112 in the cylinder bore 111. A synthetic resin made disc type heat insulating material 30 is adhered to a head end surface 251 of the piston 25 by an adhesive layer 43. The head end surface 251 of the piston 25 is fully covered with the heat insulating material 30. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is a piston type compression in which a piston is reciprocated in a cylinder bore based on rotation of a rotating shaft to suck refrigerant gas from a suction pressure region into a compression chamber and discharge refrigerant gas from the compression chamber into a discharge pressure region. It is related with the heat insulation structure in a machine.

The temperature of the refrigerant gas introduced into the compression chamber of a piston type compressor (see, for example, Patent Document 1) affects the performance of the compressor. The higher the temperature of the refrigerant gas introduced into the compression chamber, the lower the density of the refrigerant gas in the compression chamber, so the performance of the compressor decreases. Conversely, the lower the temperature of the refrigerant gas introduced into the compression chamber, the greater the density of the refrigerant gas in the compression chamber, so that the performance of the compressor is improved.
JP-T-2001-515174

  The temperature of the refrigerant gas in the compression chamber rises due to the compression of the refrigerant gas. Therefore, heat is transferred from the compressed refrigerant gas to the wall surface forming the compression chamber, and the temperature of the wall surface forming the compression chamber rises. The refrigerant gas newly sucked into the compression chamber after compressing and discharging the refrigerant gas receives heat from the wall surface forming the compression chamber and rises in temperature. Therefore, if the temperature rise of the wall surface forming the compression chamber is large, or if the thermal conductivity of the wall surface forming the compression chamber is large, the temperature rise of the refrigerant gas before compression in the compression chamber increases, and the compressor performance decreases. To do.

  An object of this invention is to improve the heat insulation in the compression chamber of a piston type compressor.

  To this end, the present invention provides a piston bore accommodated in a cylinder bore, a compression chamber is defined in the cylinder bore, and the piston is reciprocated in the cylinder bore based on the rotation of a rotating shaft, whereby a suction pressure region A piston-type compressor that sucks refrigerant gas from the compression chamber into the compression chamber and discharges refrigerant gas from the compression chamber to a discharge pressure region. In the invention of claim 1, the tip end surface of the piston is covered with a heat insulating material. did.

  The heat insulating material here means a material whose thermal conductivity is smaller than the thermal conductivity of the material of the piston. Moreover, the front end surface of the piston means the surface of the piston facing the compression chamber. The heat insulating material that covers the tip surface of the piston is not easily heated by the heat of the refrigerant gas in the compression chamber, and the temperature rise of the heat insulating material that covers the tip surface of the piston is suppressed. For this reason, the amount of heat received from the heat insulating material covering the front end surface of the piston by the refrigerant gas newly sucked into the compression chamber after compressing and discharging the refrigerant gas is increased, and the heat insulation in the compression chamber is enhanced. In addition, a heat insulating material may coat | cover only a part of tip surface of a piston, and may coat | cover all.

According to a second aspect of the present invention, in the first aspect, the tip end surface of the piston is covered with the heat insulating material having a flat chamber forming surface for forming the compression chamber.
Since the compression chamber forming end surface facing the tip end surface of the piston is generally a flat surface, it is possible to adopt a configuration in which the planar chamber forming surface of the heat insulating material and the compression chamber forming end surface are parallel to each other. The configuration in which the planar chamber forming surface of the heat insulating material and the forming end surface of the compression chamber are made parallel is advantageous in reducing the dead space in the compression chamber.

According to a third aspect of the present invention, in the second aspect, the chamber forming surface is circular, and the diameter of the chamber forming surface is greater than or equal to the diameter of the piston.
The front end surface of the piston can be entirely covered with a heat insulating material.

According to a fourth aspect of the present invention, in any one of the first to third aspects, the heat insulating material previously formed in a predetermined shape is fixed to the tip surface of the piston.
Such a heat insulating material is fixed to the piston by adhesion, press fitting, screwing or the like. When the heat insulating material formed in a predetermined shape is, for example, a flat plate shape, the thickness of the heat insulating material can be increased to enhance the heat insulating effect by the heat insulating material.

  According to a fifth aspect of the present invention, in the fourth aspect of the present invention, in the fourth aspect of the present invention, a fitting recess is provided in the tip end surface of the piston, and the tip end surface of the piston is covered with the heat insulating material provided with a fitting projection fitted in the fitting recess. did.

  When the specific gravity of the heat insulating material that is driven to reciprocate integrally with the piston is smaller than the specific gravity of the material of the piston, the fitting protrusion of the heat insulating material fitted into the fitting concave portion has the total weight of the piston and the heat insulating material. It is lighter than the conventional metal piston alone.

  According to a sixth aspect of the present invention, in any one of the fourth and fifth aspects, an annular housing groove is formed in a peripheral portion of a tip surface of the piston, and a piston ring is housed in the housing groove. The piston ring was covered with the heat insulating material so that the piston ring could not be detached from the piston.

An endless piston ring can be easily attached to the piston. An endless piston ring has better sealing performance than an endless piston ring.
According to a seventh aspect of the invention, in any one of the first to sixth aspects, the heat insulating material is made of a synthetic resin.

Synthetic resin is suitable as a heat insulating material.
In an eighth aspect of the present invention, in any one of the first to seventh aspects, the refrigerant gas is carbon dioxide.

  The present invention is suitable for application to a piston type compressor using carbon dioxide as a refrigerant.

  The present invention has an excellent effect that heat insulation in the compression chamber can be enhanced.

A first embodiment in which the present invention is embodied in a variable displacement piston compressor will be described below with reference to FIGS.
As shown in FIG. 1, an aluminum front housing 12 is joined to the front end of an aluminum cylinder 11. An aluminum rear housing 13 as a cover housing is joined and fixed to the rear end of the cylinder 11 via a valve plate 14 and gasket type valve forming plates 15 and 16. As shown in FIG. 4, the valve forming plates 15, 16 are obtained by providing rubber layers 153, 154, 163, 164 on both surfaces of the metal plates 152, 162.

  As shown in FIG. 1, the cylinder 11, the front housing 12 and the rear housing 13 are coupled together by screws 53. The cylinder 11, the front housing 12, and the rear housing 13 constitute an entire housing of the variable displacement piston compressor 10.

  A rotary shaft 18 is rotatably supported via radial bearings 19 and 20 on the front housing 12 and the cylinder 11 forming the control pressure chamber 121. The rotating shaft 18 that protrudes outside from the control pressure chamber 121 obtains driving force from the vehicle engine 17 that is an external driving source via a pulley (not shown) and a belt (not shown).

  A rotary support 21 is fixed to the rotary shaft 18, and a swash plate 22 is supported so as to be slidable and tiltable in the axial direction of the rotary shaft 18. A connecting piece 23 is fixed to the swash plate 22, and a guide pin 24 is fixed to the connecting piece 23. A guide hole 211 is formed in the rotary support 21. The head of the guide pin 24 is slidably fitted into the guide hole 211. The swash plate 22 can be tilted in the axial direction of the rotary shaft 18 and can rotate integrally with the rotary shaft 18 by the linkage of the guide hole 211 and the guide pin 24. The tilt of the swash plate 22 is guided by the slide guide relationship between the guide hole 211 and the guide pin 24 and the slide support action of the rotary shaft 18.

  When the center portion of the swash plate 22 moves to the rotation support 21 side, the inclination angle of the swash plate 22 increases. The maximum inclination angle of the swash plate 22 is regulated by the contact between the rotary support 21 and the swash plate 22. The solid line position of the swash plate 22 in FIG. 1 indicates the maximum tilt angle state of the swash plate 22. When the central portion of the swash plate 22 moves to the cylinder 11 side, the inclination angle of the swash plate 22 decreases. The chain line position of the swash plate 22 in FIG. 1 indicates the minimum tilt angle state of the swash plate 22.

  As shown in FIG. 2, a plurality of cylinder bores 111 are provided through the cylinder 11. As shown in FIG. 1, an aluminum piston 25 (only one is shown in FIG. 2) is accommodated in the cylinder bore 111. The piston 25 includes a cylindrical head 252 and a neck 253. The head 252 is fitted into the cylinder bore 111, and the neck 253 is engaged with the swash plate 22 via the shoe 26. The rotational movement of the swash plate 22 is converted into the back-and-forth reciprocating movement of the piston 25 via the shoe 26, and the piston 25 is reciprocated within the cylinder bore 111. The piston 25 defines a compression chamber 112 in the cylinder bore 111.

  As shown in FIGS. 4 and 5, a piston ring 46 made of synthetic resin is accommodated in the head 252 of the piston 25. The piston ring 46 is a cut ended piston ring. The piston ring 46 prevents the refrigerant gas in the compression chamber 112 from leaking to the control pressure chamber 121 via the space between the peripheral surface of the piston 25 and the peripheral surface of the cylinder bore 111.

  A disc-shaped heat insulating material 30 made of synthetic resin is bonded to the front end surface 251 of the piston 25 by an adhesive layer 43. The front end surface 251 of the piston 25 is entirely covered with the heat insulating material 30. The disc-shaped heat insulating material 30 includes a flat chamber forming surface 301 for forming the compression chamber 112. The chamber forming surface 301 is parallel to the valve plate 14 and the valve forming plate 15. That is, the chamber forming surface 301 is parallel to the surface 155 (the surface of the rubber layer 153) of the valve forming plate 15 that is the forming end surface of the compression chamber 112. The diameter of the chamber forming surface 301 is the same as the diameter of the piston 25 (that is, the diameter of the head 252).

  As shown in FIGS. 1 and 3, in the rear housing 13, a suction chamber 27 that is a suction pressure region and a discharge chamber 28 that is a discharge pressure region are defined by an annular partition wall 29. The suction chamber 27 is on the outer peripheral side of the rear housing 13 and surrounds the discharge chamber 28 around the axis 181 of the rotation shaft 18. As shown in FIG. 1, valve forming plates 15 and 16 and a retainer 31 are coupled to the valve plate 14 by tightening screws 32 in the discharge chamber 28.

  As shown in FIGS. 1 and 4, a suction port 141 is formed in the valve plate 14 and the valve forming plate 16, and a discharge port 142 is formed in the valve plate 14 and the valve forming plate 15. A suction valve 151 is formed on the valve forming plate 15, and a discharge valve 161 is formed on the valve forming plate 16. The gaseous refrigerant in the suction chamber 27 is sucked into the compression chamber 112 by pushing the suction valve 151 away from the suction port 141 by the backward movement of the piston 25 (movement from the right side to the left side in FIG. 1).

  The suction valve 151 is in contact with the bottom of the position restricting recess 113 and the opening degree is restricted. The gaseous refrigerant sucked into the compression chamber 112 is discharged into the discharge chamber 28 by pushing the discharge valve 161 away from the discharge port 142 by the forward movement of the piston 25 (movement from the left side to the right side in FIG. 1). The discharge valve 161 is in contact with the retainer 31 and the opening degree is regulated.

  A suction passage 33 and a discharge passage 34 are formed in the rear housing 13. The suction passage 33 for introducing the gaseous refrigerant into the suction chamber 27 and the discharge passage 34 for discharging the gaseous refrigerant from the discharge chamber 28 are connected by an external refrigerant circuit 35. On the external refrigerant circuit 35, a heat exchanger 36 for removing heat from the refrigerant, a fixed throttle 37, a heat exchanger 38 for transferring ambient heat to the refrigerant, and an accumulator 39 are interposed. The accumulator 39 is for sending only the gaseous refrigerant to the compressor. The refrigerant in the discharge chamber 28 flows into the suction chamber 27 via the discharge passage 34, the heat exchanger 36, the fixed throttle 37, the heat exchanger 38, the accumulator 39, and the suction passage 33.

  The discharge chamber 28 and the control pressure chamber 121 are connected by a supply passage 40. The control pressure chamber 121 and the suction chamber 27 are connected by a discharge passage 41. The refrigerant in the control pressure chamber 121 flows out to the suction chamber 27 through the discharge passage 41.

  An electromagnetic capacity control valve 42 is interposed on the supply passage 40. The capacity control valve 42 is in a valve-closed state where refrigerant cannot flow in the demagnetized state, and refrigerant supply from the discharge chamber 28 to the control pressure chamber 121 via the supply passage 40 is not performed. Since the refrigerant in the control pressure chamber 121 flows out to the suction chamber 27 through the discharge passage 41, the pressure in the control pressure chamber 121 decreases. Accordingly, the inclination angle of the swash plate 22 increases and the discharge capacity increases. The capacity control valve 42 is in an open state in which the refrigerant can flow by excitation, and the refrigerant is supplied from the discharge chamber 28 to the control pressure chamber 121 via the supply passage 40. Therefore, the pressure in the control pressure chamber 121 increases, the inclination angle of the swash plate 22 decreases, and the discharge capacity decreases.

In the present embodiment, carbon dioxide is used as the refrigerant.
In the first embodiment, the following effects can be obtained.
(1-1) As the piston 25 moves backward, the refrigerant gas in the suction chamber 27 is sucked into the compression chamber 112 via the suction port 141, and as the piston 25 moves forward, The refrigerant gas is compressed and discharged to the discharge chamber 28 via the discharge port 142. When the refrigerant gas in the compression chamber 112 is compressed, the temperature rises. The heat insulating material 30 that covers the front end surface 251 of the piston 25 is not easily heated by the heat of the refrigerant gas in the compression chamber 112, and the temperature rise of the heat insulating material 30 that covers the front end surface 251 of the piston 25 is suppressed. Therefore, the amount of heat received by the refrigerant gas newly sucked into the compression chamber 112 after compressing and discharging the refrigerant gas from the heat insulating material 30 covering the front end surface 251 of the piston 25 is small. That is, the refrigerant gas in the compression chamber 112 before being compressed is suppressed from rising in temperature due to the presence of the heat insulating material 30. The presence of the heat insulating material 30 increases the heat insulating property in the compression chamber 112 and contributes to the improvement of the performance of the variable displacement piston compressor 10.

  (1-2) The piston 25 shown in FIG. 1 is at the top dead center position, and between the chamber forming surface 301 of the heat insulating material 30 and the surface 155 of the valve forming plate 15 when the piston 25 is at the top dead center position. Has a dead space. This dead space should be small to increase volumetric efficiency and improve compressor performance. The configuration in which the planar chamber forming surface 301 of the heat insulating material 30 and the surface 155 that forms the forming end surface of the compression chamber 112 are parallel to each other so that the interval between the chamber forming surface 301 and the surface 155 is as small as possible. This is advantageous in reducing the dead space in the chamber 112.

(1-3) In the heat insulating material 30 formed in a predetermined shape called a disk shape in advance, the heat insulating effect of the heat insulating material 30 can be enhanced by increasing the plate thickness.
(1-4) About the heat insulating material 30 which coat | covers the front end surface 251 of the piston 25, it is not necessary to worry about the problem of abrasion by sliding contact with another member. Therefore, it is not necessary to stick to a material excellent in wear resistance as the material of the heat insulating material 30, and the degree of freedom in selecting the material of the heat insulating material 30 is high.

  (1-5) The heat insulating material 30 is made of a synthetic resin having a low thermal conductivity. The heat insulating material 30 reduces heat transfer from the aluminum piston 25 having a high thermal conductivity to the refrigerant gas in the compression chamber 112. The heat insulating material 30 made of synthetic resin contributes to improving the performance of the compressor.

The greater the specific heat of the heat insulating material 30, the harder the heat insulating material 30 is heated. A synthetic resin having a large specific heat is suitable as a material for the heat insulating material 30 from the viewpoint of specific heat.
(1-6) When the variable displacement piston compressor 10 cannot be used, the heat insulating material 30 can be reused by removing the heat insulating material 30 from the piston 25.

  (1-7) Carbon dioxide used as a refrigerant at a higher pressure than chlorofluorocarbon gas requires a small gas flow rate. The smaller the gas flow rate, the more important it is to prevent the refrigerant gas from being heated in the compression chamber 112. A variable displacement piston compressor 10 that uses carbon dioxide as a refrigerant is suitable as an application target of the present invention.

In the present invention, the embodiments of FIGS. 6 to 15 are also possible. In each of these embodiments, the same reference numerals are used for the same components as those in the first embodiment.
In the second embodiment of FIG. 6, a fitting recess 254 is formed in the tip surface 251 of the piston 25, and a heat insulating material 44 made of synthetic resin is press-fitted into the fitting recess 254. The surface in the fitting recess 254 is a part of the front end surface of the piston 25 facing the compression chamber 112. The surface in the fitting recess 254 is covered with a heat insulating material 44.

In the third embodiment of FIG. 7, the heat insulating material 30 is fixed to the front end surface 251 of the piston 25 by a screw 45.
In the fourth embodiment of FIG. 8, an annular housing groove 255 is formed in the peripheral edge portion of the tip surface 251 of the piston 25, and a synthetic resin piston ring 46 </ b> A is housed in the housing groove 255. The piston ring 46A is an endless piston ring without a break. The heat insulating material 30 fixed to the front end surface 251 of the piston 25 with the screw 45 holds the piston ring 46 </ b> A in the housing groove 255 so as not to be detached from the housing groove 255. The piston ring 46 </ b> A prevents the refrigerant gas in the compression chamber 112 from leaking to the control pressure chamber 121 via the space between the peripheral surface of the piston 25 and the peripheral surface of the cylinder bore 111.

  In the fourth embodiment, the endless piston ring 46A can be easily attached to the piston 25. The endless piston ring 46A is superior in sealing performance as compared to the endless piston ring.

  In the fifth embodiment of FIG. 9, the fitting cylinder portion 471 is formed in the heat insulating material 47, and the heat insulating material 47 is fitted to the tip end portion of the piston 25 so that the fitting cylinder portion 471 enters the receiving groove 255. Has been. The fitting tube portion 471 of the heat insulating material 47 holds the piston ring 46 </ b> A in the housing groove 255 so as not to be detached from the housing groove 255. In the fifth embodiment, the same effect as in the fourth embodiment can be obtained.

  In the sixth embodiment shown in FIG. 10, a fitting recess 256 is formed in the distal end surface 251 of the piston 25, and a fitting projection 481 is formed in the heat insulating material 48. The fitting protrusion 481 is fitted into the fitting recess 256. The fitting protrusion 481 is fixed in the fitting recess 256 by adhesion or press fitting. The surface in the insertion recess 256 is a part of the tip surface of the piston 25 facing the compression chamber 112. The inner surface of the insertion recess 256 is covered with the insertion protrusion 481 of the heat insulating material 48, and the tip surface 251 is covered with the flange-shaped portion 482 of the heat insulating material 48.

  The fitting protrusion 481 fitted in the fitting recess 256 makes the total weight of the piston 25 and the heat insulating material 48 lighter than the weight of the conventional metal piston alone. Such weight reduction improves the accuracy of capacity control in the variable displacement piston compressor 10 that controls the tilt angle of the swash plate 22 by controlling the pressure in the control pressure chamber 121 that accommodates the swash plate 22 in a variable tilt angle. It is effective in doing.

  In the seventh embodiment of FIG. 11, the length of the fitting protrusion 481 is made larger than the depth of the fitting recess 256. Between the front end surface 251 of the piston 25 and the flange-shaped portion 482 is an annular housing portion 49, and the piston ring 46 </ b> A is housed in the housing portion 49. The accommodating part 49 functions as an accommodating groove.

In the seventh embodiment, the same effects as in the sixth embodiment and the fourth embodiment can be obtained.
In the eighth embodiment shown in FIGS. 12A and 12B, the fitting tube portion 501 is integrally formed with the heat insulating material 50, and the ring portion 502 is connected to the fitting tube portion 501 in the fitting tube portion 501. It is integrally formed so as to surround it. The heat insulating material 50 is fitted to the distal end portion of the piston 25 so that the fitting cylinder portion 501 and the ring portion 502 enter the accommodation groove 255.

  When the piston 25 moves forward to compress the refrigerant gas in the compression chamber 112, the ring portion 502 is pressed against the circumferential surface of the cylinder bore 111. That is, the ring portion 502 plays the same role as the piston ring 46A.

  In the ninth embodiment shown in FIGS. 13A and 13B, the heat insulating material 51 including the ring portion 511 is fixed to the tip surface 251 of the piston 25. A plurality of slits 512 are formed in the ring portion 511 by cutting. The slit 512 extends in a direction oblique to the circumference of the ring portion 511. When the piston 25 moves forward and compresses the refrigerant gas in the compression chamber 112, the ring portion 511 is pressed against the circumferential surface of the cylinder bore 111 so that the thickness of the slit 512 is reduced. That is, the ring portion 511 plays the same role as the piston ring 46A.

  In the tenth embodiment of FIG. 14, a synthetic resin coating layer 52 covers the tip surface 251 of the piston 25. The coating layer 52 is a heat insulating material that covers the tip surface 251.

  In the eleventh embodiment of FIG. 15, the peripheral surface of the piston 25 is covered with a coating layer 54 made of polytetrafluoroethylene having excellent wear resistance, and the tip surface 251 is a synthetic resin different from polytetrafluoroethylene. The coating layer 52 is covered.

In the present invention, the following embodiments are also possible.
(1) Rubber or ceramic may be used as the material of the heat insulating material that covers the tip surface of the piston 25.

(2) The present invention may be applied to a piston type compressor in which a discharge chamber is provided on the outer peripheral side of the rear housing 13 and the suction chamber is surrounded by the discharge chamber around the axis 181 of the rotating shaft 18.
(3) The present invention may be applied to a fixed displacement type piston compressor.

  (4) The present invention may be applied to a compressor using a refrigerant other than carbon dioxide.

The side sectional view of the whole compressor which shows a 1st embodiment. AA sectional view taken on the line AA of FIG. FIG. 3 is a sectional view taken along line BB in FIG. 1. The principal part expanded side sectional view. The side view of a piston. The principal part side sectional view showing a 2nd embodiment. The principal part sectional side view which shows 3rd Embodiment. The principal part sectional side view which shows 4th Embodiment. The principal part sectional side view which shows 5th Embodiment. The principal part sectional side view which shows 6th Embodiment. The principal part sectional side view which shows 7th Embodiment. The 8th Embodiment is shown, (a) is principal part sectional drawing. (B) is CC sectional view taken on the line of (a). A 9th embodiment is shown and (a) is an important section side sectional view. (B) is the DD sectional view taken on the line of (a). The principal part side sectional view showing a 10th embodiment. The principal part sectional side view which shows 11th Embodiment.

Explanation of symbols

  10: Variable displacement piston compressor. 11 ... Cylinder. 111 ... Cylinder bore. 112: Compression chamber. 155 ... A surface to be a forming end surface of the compression chamber. 18 ... Rotating shaft. 181 ... axis. 25 ... Piston. 251 ... The tip surface. 255: Housing groove. 256 ... insertion recess. 27: A suction chamber as a suction pressure region. 28: A discharge chamber as a discharge pressure region. 30, 44, 47, 48, 50, 51 ... heat insulating material. 301: chamber forming surface. 46A ... Piston ring. 481 ... Insertion protrusion.

Claims (8)

A piston is accommodated in a cylinder bore formed in the cylinder, a compression chamber is defined in the cylinder bore, and the piston is reciprocated in the cylinder bore based on rotation of a rotating shaft, so that a refrigerant is transferred from the suction pressure region to the compression chamber. In a piston compressor that sucks in gas and discharges refrigerant gas from the compression chamber to a discharge pressure region,
The heat insulation structure in the piston type compressor which coat | covered the front end surface of the said piston with the heat insulating material.   The said heat insulating material is a heat insulation structure in the piston type compressor of Claim 1 provided with the planar shape chamber formation surface for forming the said compression chamber.   The heat insulation structure in the piston type compressor according to claim 2, wherein the chamber forming surface is circular and the diameter of the chamber forming surface is equal to or larger than the diameter of the piston.   The heat insulation structure in the piston type compressor according to any one of claims 1 to 3, wherein the heat insulating material formed in advance in a predetermined shape is fixed to a front end surface of the piston.   The heat insulation structure in the piston type compressor according to claim 4, wherein the piston includes a fitting recess recessed in the tip surface, and the heat insulating material includes a fitting protrusion fitted into the fitting recess. .   An annular housing groove is formed in the peripheral edge of the piston front end surface, the piston ring is received in the housing groove, and the piston ring is covered with the heat insulating material so that the piston ring cannot be detached from the piston. The heat insulation structure in the piston type compressor according to any one of claims 4 and 5.   The heat insulating structure for a compressor according to any one of claims 1 to 6, wherein the heat insulating material is made of a synthetic resin.   The heat insulation structure for a compressor according to any one of claims 1 to 7, wherein the refrigerant gas is carbon dioxide.

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