JPH11257769A - Cold storage refrigerating machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance refrigerating efficiency of a refrigerating machine comprising cold accumulators and expansion units arranged in multistage with a target temperature being set lower than 5K in the final cooling stage by specifying the volume of the final stage cold accumulator being occupied by refrigerant gas with respect to the expansion volume of a final stage expansion unit. SOLUTION: A two-stage expansion, two-stage cooling Gifford-MacMahon GM refrigerating machine having a design cooling temperature lower than 5K comprises a cold head 2 substantially disposed in a vacuum container 1, and a gas supply system 4 disposed in room temperature atmosphere. The cold head 2 comprises a displacer 12 contained in a stepped cylinder 11 reciprocally in the direction of gravity. The displacer 12 comprises first and second displacers 16, 18 of different diameter and cold accumulators 23, 25 of first and second stages are formed, respectively, in the displacers 16, 18. Volume of the second stage cold accumulator 25 is set 1.5-4 times as large as the expansion volume formed in a second expansion chamber 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a regenerative refrigerator, and more particularly, to a regenerative refrigerator in which a target temperature of a final cooling stage is set near a liquid helium temperature.

[0002]

2. Description of the Related Art As is well known, recently, a regenerative refrigerator having a small size and a sufficiently low reached temperature of 5 K or less has appeared. The appearance of a magnetic regenerator material having a large specific heat at an extremely low temperature greatly contributes to this.

[0003] Such a regenerative refrigerator, for example, a GM refrigerator employing a Gifford McMahon refrigeration cycle, has a compressor placed at room temperature, an expander placed at low temperature, and a compressor between the expander and the compressor. And a regenerator connecting the two. That is, the refrigerant gas compressed by the compressor is guided to the regenerator through the intake valve, where it is pre-cooled, and then generates cold corresponding to the expansion work in the expander, and is then again drawn through the regenerator. In order to cool the refrigerant gas, the temperature of the regenerator material rises while cooling the regenerator material, returns to room temperature, and is returned to the compressor via an exhaust valve. With this process as one cycle, cold is generated periodically.

In such a GM refrigerator, an expander and a regenerator are usually provided in a multi-stage configuration, for example, a two-stage configuration in order to lower the cooling temperature. Taking a two-stage GM refrigerator as an example, the one-stage cooling stage is often cooled to the liquid nitrogen temperature (77 K) level. For this reason, cold storage materials such as copper, stainless steel, and lead (net-like or spherical) are used in the single-stage regenerator.
Is used. On the other hand, two-stage cooling stage (final stage)
Is typically cooled to the 20K level. Therefore, lead (spherical material) is used as the cold storage material for the two-stage regenerator (final stage).
Is used. When cooling at the liquid helium temperature (4.2 K) level, a magnetic material having a higher specific heat at the above-mentioned temperature level than lead, such as Er ₃ Ni, is used as the cold storage material of the two-stage regenerator. In this case, helium gas is used as a refrigerant.

However, such a regenerative refrigerator, particularly one using helium gas as a refrigerant and setting the target temperature of the final cooling stage at less than 5K, has the following problems.

That is, the required refrigerant gas flow rate is greatly increased due to a decrease in the refrigeration temperature, and in order to operate at the same compression ratio, a compressor having a higher output than a compressor used in a chiller whose ultimate temperature is 20K level is used. Refrigeration efficiency is remarkably lower than that of a 20K level refrigerator.

[0007]

As described above, in a conventional regenerative refrigerator, in particular, in the case where helium gas is used as a refrigerant and the target temperature of the final cooling stage is set to less than 5K, the target temperature is high. There is a problem that the refrigerating efficiency is significantly lower than that of the refrigerator.

Accordingly, the present invention provides a regenerative refrigeration system which can improve refrigeration efficiency without complicating the structure even when helium gas is used as the refrigerant and the target temperature of the final cooling stage is set to less than 5K. The purpose is to provide a machine.

[0009]

In order to achieve the above object, the present invention relates to a regenerative refrigerator having multiple stages of regenerators and expanders. The inactive volume occupied by gas is 1.5 to 4 times the expansion volume of the expander located at the last stage.

[0010] The regenerator located at the last stage may be provided with a space which forms a part of an ineffective volume inside and at a portion located on the low temperature side, or may be provided inside and on the low temperature side and at a high temperature. A space that forms a part of the dead volume may be provided at a position intermediate the side.

The present invention is particularly effective for a regenerative refrigerator in which helium gas is used as the refrigerant gas and the target temperature of the final cooling stage is near the liquid helium temperature. By setting the ineffective volume occupied by the refrigerant gas in the volume of the regenerator located at the last stage in the above relationship, the use of a high-output compressor can be made unnecessary for the reason described later, and the refrigeration efficiency can be improved. Can be.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a regenerative refrigerator according to an embodiment of the present invention, in which a two-stage expansion type GM refrigerator in which the designed cooling temperature of a final stage, that is, a second stage cooling stage is less than 5K is shown. .

The GM refrigerator has a refrigerator cold head 2 and a room temperature atmosphere 3
And a gas supply system 4 disposed therein. In the cold head 3 of the refrigerator, a displacer 12 formed of a heat insulating material is housed in a closed cylinder 11 so as to be reciprocally movable, for example, parallel to the direction of gravity. Cylinder 11
Is a large-diameter first cylinder 13 and the first cylinder 13
And a small-diameter second cylinder 14 coaxially connected to the second cylinder 14. The first cylinder 13 and the second cylinder 1
4 is usually formed of a thin stainless steel plate or the like.

In the first cylinder 13, a first displacer 16 forming an expansion chamber 15 of the first stage expander between the first cylinder 13 and a head wall of the first cylinder 13 is provided between the first cylinder 13 and the inner peripheral surface of the first cylinder 13. A predetermined gap is provided between them, and they are arranged so as to be able to reciprocate freely. Similarly, in the second cylinder 14, the second cylinder
A second displacer 18 that forms an expansion chamber 17 of a second-stage (final-stage) expander between the head wall of the cylinder 14 and the cylinder wall is reciprocally movable.

The first displacer 16 and the second displacer 18 are both formed of a heat insulating resin such as bakelite, for example, and are connected in the axial direction by a connecting portion 19 having substantially the same diameter as the second displacer 18. Also, the first
Annular grooves are formed on the outer peripheral surfaces of the displacer 16 and the second displacer 18, respectively. Seal rings 20, 21 for sealing the first and second cylinders 13 and 14 without lubrication are formed in the respective annular grooves.
Is installed.

A fluid passage 22 for forming a first stage regenerator is formed in the first displacer 16 in the axial direction. The fluid passage 22 has a mesh structure cold accumulator made of copper, for example. The material 23 is accommodated. Similarly, a fluid passage 24 for forming a second stage (final stage) regenerator is formed inside the second displacer 18, and a magnetic phase such as Er ₃ Ni is formed in the fluid passage 24. A regenerator material 25 made of granular magnetic regenerator material utilizing abnormal magnetic specific heat or the like accompanying the transition is accommodated.

Here, in the regenerative refrigerator according to this embodiment,
As equivalently shown in FIG. 2, the so-called refrigerant gas other than the volume occupied by the solid members such as the cold storage material 25, the partition plate and the rectifying plate (not shown) in the volume of the second stage regenerator, that is, the volume of the fluid passage 24. Is set to 1.5 to 4 times the expansion volume formed by the expansion chamber 17. In this example, a space 26 which forms a part of the above-described ineffective volume is provided in a portion located on the low temperature side in the fluid passage 24.

The upper end of the first displacer 16 in FIG.
The motor 2 is connected via the connecting rod 27 and the crankshaft 28.
9 rotation shafts. Therefore, the motor 29
Rotates, the displacer 12 is synchronized with this rotation.
Reciprocate up and down as shown by the solid arrow 30 in the figure. Due to this reciprocation, the volumes of the first-stage expansion chamber 15 and the second-stage expansion chamber 17 change. As can be seen from this, in this example, the first stage cooling stage 31 is constituted by the head wall of the first cylinder 13, and the second stage (final stage) cooling stage 32 is constituted by the head wall of the second cylinder 14. I have.

The space above the first displacer 16 in FIG. 1 is connected to the gas supply system 4 via an intake bubble 33 and an exhaust valve 34. Here, the intake valve 33 and the exhaust valve 34 are controlled to open and close in interrelation with the reciprocating motion of the displacer 12 under the intervention of a cam or the like.

The gas supply system 4 includes a compressor 35 which compresses helium gas as a refrigerant sucked through an exhaust valve 34 to a predetermined pressure and supplies the compressed helium gas to an intake valve 33.

In FIG. 1, reference numeral 40 denotes a first cooling stage 3;
Reference numeral 1 denotes a heat shield plate thermally connected, and 41 denotes a second heat shield plate.
An object to be cooled such as a superconducting coil thermally connected to the stage cooling stage 32 is shown, and reference numeral 42 is a heat exchanger for cooling the helium gas compressed by the compressor 35 with water or the like.

When the motor 29 starts rotating, the displacer 12 reciprocates between a bottom dead center (the top point in FIG. 1) and a top dead center (the bottom point in FIG. 1). In this example, it reciprocates 0.3 to 2.0 times per second, for example, once per second. Therefore, the volume of the second-stage expansion chamber 17 changes from substantially zero to the maximum value during one cycle.

While the compressor 35 is being driven,
When the displacer 12 reaches the top dead center, the intake valve 33
Is opened, for example, helium gas of 2.5 MPa flows into the cold head 2 of the refrigerator. Then, as the displacer 12 moves to the bottom dead center side, the intake valve 33 closes. At this time, the exhaust valve 34 is kept closed.

Since the displacer 12 continues to move toward the bottom dead center, the helium gas of 2.5 MPa
The fluid flows to the first stage expansion chamber 15 through a fluid passage 22 formed in the displacer 16, and to the second stage expansion chamber 17 through a fluid passage 24 formed in the second displacer 18. That is, the helium gas of 2.5 MPa contacts the first-stage expansion chamber 15 and the second
It flows to the step expansion chamber 17.

Here, when the displacer 12 reaches the bottom dead center, the exhaust valve 34 is opened. When the exhaust valve 34 is opened in this manner, the pressure in the first-stage expansion chamber 15 and the pressure in the second-stage expansion chamber 17 rapidly decrease to, for example, 0.5 MPa. The helium gas adiabatically expands due to this pressure drop to generate cold. The first cooling stage 31 and the second cooling stage 32 are gradually cooled by this cold.

When the displacer 12 moves to the top dead center again, the volumes in the first-stage expansion chamber 15 and the second-stage expansion chamber 17 decrease accordingly. 24, and cools the cold storage materials 23 and 25 during this passage. The helium gas whose temperature has risen is discharged to the compressor 35 via the exhaust valve 34. The above-described cycle is repeated until the first cooling stage 31 is cooled to about 30K and the second cooling stage 32 is cooled to less than the target temperature of 5K.

As described above, in the regenerative refrigerator according to this embodiment, the ineffective volume occupied by the refrigerant gas in the volume of the regenerator located at the last stage is 1.times. The expansion volume of the expander located at the final stage. Since it is set to be 5 to 4 times,
While maintaining the refrigerating performance of the second cooling stage 32, the flow rate of the refrigerant gas passing through the inlet (high-temperature end) of the regenerator located at the final stage can be minimized, and the compressor 3
The reason for this will be described below.

First, helium gas as a refrigerant is 10K
At the above temperature, it can be treated as an ideal gas. In this temperature range, the existence of the ineffective volume in the regenerator does not participate in the refrigeration cycle, and the helium gas present in this ineffective volume only repeats expansion and compression in a finite closed space. This leads to an increase in the flow rate and a decrease in the refrigeration efficiency. Therefore, in the above-mentioned temperature range, it is advantageous to reduce the ineffective volume as much as possible in order to improve the refrigeration efficiency as long as the heat exchange between the cold storage material and the gas can be performed without any problem.

On the other hand, in a liquid helium temperature region, helium gas as a refrigerant cannot be treated as an ideal gas. Helium gas in this temperature range behaves incompressible like a liquid, and the density difference due to the difference in pressure is reduced. In addition, the specific heat is remarkably increased, and has the same level as that of the magnetic material used as the cold storage material. For this reason, in the intake and exhaust cycles, even a regenerator having a regenerative efficiency of 100% generates a large temperature oscillation in the regenerator. For example,
When the exhaust is completed, the low-pressure 4K gas in the expander cools most of the regenerator to near 4K and lowers the average temperature of the regenerator. Conversely, when the intake is completed, the high-pressure gas that flows in warms the regenerator and raises the average temperature. Due to such a property, the last-stage regenerator having a finite ineffective volume works effectively conversely, suppressing an increase in the gas flow rate at the high-temperature end of the regenerator and improving the refrigeration efficiency. FIG. 3 shows the calculation results in consideration of the physical properties of helium gas.

The horizontal axis in FIG. 3 is a value obtained by dividing the ineffective volume of the final stage (second stage) regenerator by the expansion volume of the final stage (second stage), and the vertical axis is the gas flow rate at the hot end of the final stage regenerator. Show. The broken line in the figure indicates the gas flow rate at the high temperature end of the regenerator when treated as an ideal gas,
The solid line indicates the gas flow rate at the high temperature end of the regenerator when treated as a real gas.

When the dead volume of the last-stage regenerator is zero or small enough to be regarded as zero, the flow rate of the helium gas flowing through the last-stage regenerator is equal to the flow rate of the high-pressure helium gas stored in the maximum expansion volume per cycle. In the case of a regenerator which is equal to the amount M2 but has a finite ineffective volume, it does not become M2 due to the effect of the ineffective volume. In the case of an ideal gas, an increase in the ineffective volume simply causes an increase in the flow rate.

On the other hand, in the case of real gas, the gas flow rate becomes minimum at a certain invalid volume. This is because at the time of evacuation, helium gas remains at a high density of 4K in the dead volume, and the helium gas exhausted from the high-temperature end of the final stage regenerator is only a part of the gas originally in the regenerator. It is. Conversely, when the intake is completed, the interior of the regenerator is warmed, the gas in the ineffective volume is in a low density state, and the amount of gas that must be inhaled from the high temperature end of the regenerator is small. FIG. 3 shows calculation results for a refrigerator having a certain predetermined expansion volume, but the same can be said for a 4K level refrigerator of an arbitrary size.

FIG. 4 is a calculation result showing that a certain finite ineffective volume improves the refrigeration efficiency. The horizontal axis represents the value obtained by dividing the invalid volume by the expanded volume, as in FIG. 3, and the vertical axis represents the refrigeration efficiency obtained by dividing the refrigeration capacity by the compression work. FIG. 4 as in FIG.
The broken line in the middle indicates the gas flow rate at the high temperature end of the final stage regenerator when treated as an ideal gas, and the solid line indicates the gas flow rate at the high temperature end of the final stage regenerator when treated as an actual gas. As can be seen from this figure, the refrigeration efficiency peaks at a certain value of the ratio of the ineffective volume to the expanded volume, and a high refrigeration efficiency of 0.1 or more is obtained particularly between 1.5 and 4.

FIG. 5 shows the results of actual testing of the refrigerator. In the test, the operating conditions such as the intake and exhaust pressures of the refrigerator were fixed, and only the invalid volume was changed. As can be seen from this figure, the gas flow rate increases even if the invalid volume is too small, and the gas flow rate increases if the invalid volume is too large. The condition with the highest refrigeration efficiency is within the range of the previous calculation result.

Therefore, in order to obtain a refrigerating efficiency of 0.1 or more, an ineffective space having a volume falling within a range of 1.5 to 4 times the expansion volume of the final stage is positively provided in the regenerator of the final stage. It is effective to provide. As a result, the gas flow rate can be reduced, the required compression work can be reduced, and the refrigeration efficiency can be improved.

In the above-described example, the space 26 which forms a part of the above-mentioned ineffective volume is provided in a portion located on the low temperature side in the fluid passage 24 constituting the final stage regenerator.
As shown in the equivalent diagram of FIG. 6, a space 26 which forms a part of the dead volume may be provided in the fluid passage 24 constituting the final stage regenerator at an intermediate position between the low temperature side and the high temperature side.

In the above-described example, the displacer forming the expansion space also serves as a cold storage material container. However, the present invention can be applied to a case where the displacer and the cold storage material container are separated from each other. It is not limited to refrigerators.

[0038]

As described above, according to the present invention, it is possible to reduce the required refrigerant flow rate particularly in the case where helium gas is used as the refrigerant and the target temperature of the final cooling stage is set to less than 5K. As a result, the refrigeration efficiency can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a regenerative refrigerator according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining a configuration of a last-stage regenerator and a last-stage expander of the refrigerator.

FIG. 3 is a diagram for explaining a result obtained by calculating a relationship between a size of an invalid volume set in a last-stage regenerator and a gas flow rate;

FIG. 4 is a diagram for explaining a result obtained by calculating a relationship between the size of an invalid volume set in the last-stage regenerator and refrigeration efficiency.

FIG. 5 is a diagram for explaining an actual measurement result of a relationship between the size of an invalid volume set in the last-stage regenerator and a gas flow rate.

FIG. 6 is a diagram showing another example in which a space that becomes a part of the dead volume is set in the last-stage regenerator;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Vacuum container 2 ... Cold machine cold head 3 ... Room temperature atmosphere 4 ... Gas supply system 11 ... Cylinder 12 ... Displacer 13 ... 1st cylinder 14 ... 2nd cylinder 15 ... 1st stage expansion chamber 16 ... 1st displacer 17 ... 1st Two-stage expansion chamber 18 Second displacer 20, 21 Seal ring 23, 25 Cold storage material 26 Space 31 First-stage cooling stage 32 Second-stage cooling stage 33 Intake valve 34 Exhaust valve 35 Compressor

 ──────────────────────────────────────────────────の Continued on the front page (72) Takashi Sasaki Inventor 2-4 Suehirocho, Tsurumi-ku, Yokohama-shi, Kanagawa Pref.

Claims (4)

[Claims] In a regenerative refrigerator having multiple stages of regenerators and expanders, the ineffective volume occupied by refrigerant gas in the volume of the regenerator located at the last stage is the expansion of the expander located at the last stage. A regenerative refrigerator having a capacity of 1.5 to 4 times the volume. 2. The regenerator located at the last stage has:
The regenerative refrigerator according to claim 1, further comprising a space that forms a part of the ineffective volume in a portion located on a low temperature side. 3. The regenerator located at the last stage has:
2. The regenerative refrigerator according to claim 1, further comprising a space that forms a part of the ineffective volume at an intermediate position between the low temperature side and the high temperature side. 3. 4. A helium gas is used as said refrigerant gas,
The regenerative refrigerator according to claim 1, wherein the target temperature of the last cooling stage is around the liquid helium temperature.