PULSE TUBE REFRIGERATOR

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2010-022473 filed on Feb. 3, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A certain aspect of this disclosure relates to a pulse tube refrigerator.

2. Description of the Related Art

Pulse tube refrigerators are commonly used to cool apparatuses, such as a magnetic resonance imaging (MRI) apparatus, that require a cryogenic environment.

In a pulse tube refrigerator, a refrigerant gas (e.g., helium gas), i.e., a working fluid, compressed by a compressor is repeatedly caused to flow into a regenerator tube and a pulse tube and to flow out of the regenerator tube and the pulse tube back into the compressor. As a result, “coldness” is generated at cold ends of the regenerator tube and the pulse tube. The cold ends are connected to a cooling stage and the cooling stage is brought into thermal contact with an object to draw heat from the object.

Take, for example, a pulse tube refrigerator used for an MRI cryostat. A cooling stage of the pulse tube refrigerator is disposed in a space communicating with a liquid helium tank that contains an MRI magnet so that the MRI magnet is cooled to a cryogenic temperature.

Here, to maintain the MRI magnet at the cryogenic temperature, liquid helium needs to be constantly supplied to the liquid helium tank to replace liquid helium vaporized by heat exchange. For this reason, a condenser is normally provided near the cooling stage (e.g., directly below the cooling stage) to condense the vaporized helium (helium gas) back into a liquid. Japanese Laid-Open Patent Publication No. 2006-214717, for example, discloses a pulse tube refrigerator where a condenser and a cooling stage are integrated.

SUMMARY OF THE INVENTION

There is provided a pulse tube refrigerator including a regenerator tube; a pulse tube; and a condenser condensing an atmospheric gas and disposed at cold ends of the regenerator tube and the pulse tube so as to also function as a cooling stage. The condenser includes a first surface and a second surface facing each other, a flow path having two openings on the first surface and connecting the cold end of the regenerator tube and the cold end of the pulse tube, and multiple holes extending from the second surface. When viewed from a direction parallel to the axis of the regenerator tube or the pulse tube, the holes formed in an area of the condenser defined by a circle having the center on a straight line connecting the centers of the openings of the flow path do not pass through the condenser up to the first surface. The circle is the smallest circle enclosing the openings of the flow path or a circumscribed circle circumscribing the openings of the flow path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a condenser having multiple holes;

FIG. 2 is a cut-away side view of a pulse tube refrigerator according to an embodiment of the present invention;

FIG. 3 is a cut-away side view of a condenser of a pulse tube refrigerator according to an embodiment of the present invention;

FIGS. 4A and 4B are a top view and a bottom view of a condenser of a pulse tube refrigerator according to another embodiment of the present invention;

FIG. 5 is a perspective view of a condenser of a pulse tube refrigerator according to another embodiment of the present invention;

FIG. 6 is a perspective view of a condenser of a pulse tube refrigerator according to another embodiment of the present invention; and

FIG. 7 is a cut-away side view of a condenser of a pulse tube refrigerator according to still another embodiment of the present invention.

DETAILED DESCRIPTION

The condensing efficiency (or the helium gas cooling efficiency) of the condenser improves as the area of thermal contact with the helium gas increases. Therefore, normally, multiple holes are formed in a condenser to increase its surface area.

FIG. 1 is a perspective view of a condenser 60 that has multiple holes and functions also as a cooling stage.

As illustrated in FIG. 1, the condenser 60 has an upper surface 2 and a lower surface 3. The upper surface 2 is connected with a cold end 42b of a regenerator tube 41 and a cold end 47b of a pulse tube 46. The cold end 42b of the regenerator tube 41 and the cold end 47b of the pulse tube 46 are connected to each other via a gas flow path 48 formed in the condenser 60. The condenser 60 also has multiple through holes 10 going through the condenser 60 from top to bottom. The through holes 10 are provided to increase the surface area of the condenser 60.

Although not illustrated in FIG. 1, the condenser 60 is housed in an insulating container. Also, a liquid helium tank (not shown) containing an MRI magnet is provided below the lower surface 3 of the condenser 60. Accordingly, a helium gas atmosphere is present in the insulating container.

The temperature of liquid helium in the liquid helium tank increases due to heat exchange with the MRI magnet. As a result, the liquid helium is vaporized and converted into a helium gas. When the helium gas contacts the condenser 60, the helium gas is cooled and condensed to a liquid again, and the liquid (liquid helium) returns to the liquid helium tank. Thus, the condenser 60 makes it possible to constantly supply liquid helium to the liquid helium tank to replace the vaporized liquid helium and thereby to maintain the MRI magnet at a cryogenic temperature (e.g., about 4K).

However, with the configuration of the condenser 60 as illustrated in FIG. 1, a part of the helium gas (the vaporized liquid helium) may easily flow from a lower space 75 via the through holes 10 of the condenser 60 to a space between the regenerator tube 41 and the pulse tube 46. This may increase the flow speed of the helium gas in the space between the regenerator tube 41 and the pulse tube 46 and thereby increase the convection heat loss. Also, strong convection may cause the temperatures of the regenerator tube 41 and the pulse tube 46 to vary and may reduce the cooling performance of the pulse tube refrigerator.

Embodiments of the present invention are described below with reference to the accompanying drawings.

FIG. 2 is a cut-away side view of an exemplary pulse tube refrigerator according to an embodiment of the present invention. In this example, the pulse tube refrigerator is implemented as a two-stage pulse tube refrigerator 100.

As illustrated in FIG. 2, the two-stage pulse tube refrigerator 100 includes a compressor 111, an upper housing unit 101, a flange 121, and a cold head 120 connected via the flange 121 to the upper housing unit 101.

The upper housing unit 101 includes a housing 105. The housing 105 houses a first stage reservoir 115A, a second stage reservoir 115B, a valve 112, a valve 113, and orifices 117. The valve 112 and the valve 113 are connected via piping 114 to the compressor 111.

The cold head 120 includes a first stage regenerator tube 131, a first stage pulse tube 136, a first cooling stage 130, a second stage regenerator tube 141, a second stage pulse tube 146, and a second cooling stage 160.

The first stage regenerator tube 131 includes a hollow cylinder 132 made of, for example, a stainless steel and a cold storage medium 133 filling the cylinder 132. The cold storage medium 133 is implemented, for example, by a wire mesh made of copper or a stainless steel. The first stage pulse tube 136 includes a hollow cylinder 137 made of, for example, a stainless steel. Hot ends 132a and 137a of the cylinders 132 and 137 are fixed to the flange 121 and cold ends 132b and 137b of the cylinders 132 and 137 are fixed to the first cooling stage 130. A heat exchanger 118a is provided at the hot end 137a of the first stage pulse tube 136 and a heat exchanger 118b is provided at the cold end 137b of the first stage pulse tube 136. A gas flow path 138 is formed in the first cooling stage 130 to connect the cold end 137b of the first stage pulse tube 136 and the cold end 132b of the first stage regenerator tube 131.

The second stage regenerator tube 141 includes a hollow cylinder 142 made of, for example, a stainless steel and a cold storage medium 143 filling the cylinder 142. The cold storage medium 143 is implemented, for example, by a wire mesh made of copper or a stainless steel. The second stage pulse tube 146 includes a hollow cylinder 147 made of, for example, a stainless steel. A hot end 142a of the second stage regenerator tube 141 is connected via the first cooling stage 130 to the cold end 132b of the cylinder 132 of the first stage regenerator tube 131, and a cold end 142b of the second stage regenerator tube 141 is connected to the second cooling stage 160. A hot end 147a of the second stage pulse tube 146 is fixed to the flange 121 and a cold end 147b of the second stage pulse tube 146 is fixed to the second cooling stage 160. A heat exchanger 119a is provided at the hot end 147a of the second stage pulse tube 146 and a heat exchanger 119b is provided at the cold end 147b of the second stage pulse tube 146. A gas flow path 148 is formed in the second cooling stage 160 to connect the cold end 147b of the second stage pulse tube 146 and the cold end 142b of the second stage regenerator tube 141.

In the pulse tube refrigerator 100, a high-pressure refrigerant gas is supplied from the compressor 111 via the valve 112 and the piping 114 to the first stage regenerator tube 131, and a low-pressure refrigerant gas is discharged from the first stage regenerator tube 131 via the piping 114 and the valve 113 to the compressor 111. The hot end 137a of the first stage pulse tube 136 is connected via the orifice 117 and piping 116 to the first stage reservoir 115A. The hot end 147a of the second stage pulse tube 146 is connected via the orifice 117 and piping 116 to the second stage reservoir 115B. The orifices 117 adjust the phase difference between a pressure change and a volume change of the refrigerant gas that occur periodically in the first stage pulse tube 136 and the second stage pulse tube 146.

The cold head 120 of the pulse tube refrigerator 100 also includes a first insulating container 150 enclosing a space between the flange 121 and the first cooling stage 130 and filled with a helium gas.

The cold head 120 of the pulse tube refrigerator 100 further includes a second insulating container 152 enclosing a space (hereafter called an upper space 165) between the first cooling stage 130 and the second cooling stage 160. The second insulating container 152 also encloses a space (hereafter called a lower space 175) below the second cooling stage 160. A liquid helium tank 153 is provided in the second insulating container 152. The liquid helium tank 153 contains liquid helium 154 and an MRI magnet 155. The liquid helium tank 153 is disposed in the second insulating container 152 so as to face the second cooling stage 160 via the lower space 175.

The second cooling stage 160 also functions as a condenser and therefore may be called a condenser 160 in the descriptions below.

Next, operations of the pulse tube refrigerator 100 are described. When the valve 112 is opened and the valve 113 is closed, a high-pressure refrigerant gas flows from the compressor 111 to the first stage regenerator tube 131. The refrigerant gas flowing into the first stage regenerator tube 131 is cooled by the cold storage medium 133, passes through the cold end 132b of the first stage regenerator tube 131 and the gas flow path 138, and flows into the first stage pulse tube 136. The high-pressure refrigerant gas flowing into the first stage pulse tube 136 compresses a low-pressure refrigerant gas that is originally in the first stage pulse tube 136. As a result, the pressure of the refrigerant gas in the first stage pulse tube 136 becomes greater than the pressure in the first stage reservoir 115A, and the refrigerant gas flows via the orifice 117 and the piping 116 into the first stage reservoir 115A.

A part of the high-pressure refrigerant gas cooled at the first stage regenerator tube 131 also flows into the second stage regenerator tube 141. The refrigerant gas is further cooled by the cold storage medium 143, passes through the cold end 142b of the second stage regenerator tube 141 and the gas flow path 148, and flows into the second stage pulse tube 146. The high-pressure refrigerant gas flowing into the second stage pulse tube 146 compresses a low-pressure refrigerant gas that is originally in the second stage pulse tube 146. As a result, the pressure of the refrigerant gas in the second stage pulse tube 146 becomes greater than the pressure in the second stage reservoir 115B, and the refrigerant gas flows via the orifice 117 and the piping 116 into the second stage reservoir 115B.

When the valve 112 is closed and the valve 113 is opened, the refrigerant gas in the first stage pulse tube 136 and the refrigerant gas in the second stage pulse tube 146, respectively, pass through the first stage regenerator tube 131 and the second stage regenerator tube 141 and thereby cool the cold storage medium 133 and the cold storage medium 143. The refrigerant gas that has passed through the second stage regenerator tube 141 then passes through the first stage regenerator tube 131, the hot end 132a of the first stage regenerator tube 131, and the valve 113, and returns to the compressor 111. Since the first stage pulse tube 136 and the second stage pulse tube 146 are connected, respectively, via the orifices 117 to the first stage reservoir 115A and the second stage reservoir 115B, a certain phase difference occurs between the phase of the pressure change and the phase of the volume change of the refrigerant gas. The phase difference causes the refrigerant gas to expand and thereby to generate “coldness” at the cold end 137b of the first stage pulse tube 136 and the cold end 147b of the second stage pulse tube 146. The pulse tube refrigerator 100 repeats the above process to cool an object.

During the process, a part of the liquid helium 154 in the liquid helium tank 153 is vaporized due to heat exchange with the MRI magnet 155. Accordingly, a helium gas (atmospheric gas) is present in the lower space 175 and the upper space 165 communicating with the lower space 175. When the helium gas contacts the second cooling stage 160, i.e., the condenser 160, the helium gas is cooled and condensed to a liquid and the liquid (liquid helium) returns to the liquid helium tank 153. This recycling mechanism makes it possible to constantly supply liquid helium to the liquid helium tank 153 to replace the vaporized liquid helium and thereby makes it possible to maintain the MRI magnet 155 at a cryogenic temperature.

The condensing efficiency (or the helium gas cooling efficiency) of a condenser improves as the area of thermal contact with the helium gas increases. Therefore, normally, multiple holes are formed in a condenser to increase its surface area.

However, with the configuration of the condenser 60 illustrated in FIG. 1, a part of the helium gas (the vaporized liquid helium) may easily flow from the lower space 75 via the through holes 10 of the condenser 60 to the space between the regenerator tube and the pulse tube 46. This may increase the flow speed of the helium gas in the space between the regenerator tube 41 and the pulse tube 46 and thereby increase the convection heat loss. Also, strong convection may cause the temperatures of the regenerator tube 41 and the pulse tube 46 to vary and may reduce the cooling performance of the pulse tube refrigerator.

For this reason, in this embodiment, the condenser 160 is configured such that the helium gas does not easily flow from the lower space 175 via holes of the condenser 160 to the space between the second stage regenerator tube 141 and the second stage pulse tube 146. This configuration makes it possible to effectively prevent convection in the space between the second stage regenerator tube 141 and the second stage pulse tube 146.

FIG. 3 is a cut-away side view of the condenser 160 (the second cooling stage 160) of the pulse tube refrigerator 100 according to an embodiment of the present invention. In FIG. 3, the second stage regenerator tube 141, the cold end 142b, the second stage pulse tube 146, the cold end 147b, and the gas flow path 148 are omitted for brevity.

As illustrated in FIG. 3, multiple holes 110 are formed in the condenser 160 to increase the surface area. The holes 110 have openings on a lower surface 103 of the condenser 160 and extend toward an upper surface 102 of the condenser 160. However, the holes 110 are “non-through” holes that do not pass through the condenser 160 up to the upper surface 102.

With this configuration, unlike the condenser 60 of FIG. 1, the helium gas does not directly flow from the lower space 175 via the holes 110 to the upper space 165. Thus, this configuration makes it possible to effectively prevent convection in the space between the second stage regenerator tube 141 and the second stage pulse tube 146 and thereby makes it possible to prevent reduction in the cooling performance of the pulse tube refrigerator 100. With the configuration of FIG. 3, the helium gas flows between the lower space 175 and the upper space 165 only through a gap between the outer surface of the condenser 160 (the second cooling stage 160) and the inner surface of the second insulating container 152.

In the example shown in FIG. 3, the upper surface 102 is substantially horizontal (i.e., orthogonal to the vertical). Alternatively, the upper surface 102 may be tilted at an angle with respect to the horizontal or may be shaped like a circular cone or a circular truncated cone. This configuration makes it easier for the condensed helium gas (i.e., liquid helium) on the upper surface 102 of the condenser 160 to fall into the liquid helium tank 153.

In FIG. 3, all of the holes 110 of the condenser 160 are non-through holes. Alternatively, some of the holes 110 may be formed as through holes.

FIGS. 4A and 4B are a top view and a bottom view of a condenser 160-2 according to another embodiment of the present invention. FIG. 4A illustrates an upper surface 102 of the condenser 160-2 and FIG. 4B illustrates a lower surface 103 of the condenser 160-2. In FIG. 4A, the outline of the cold end 142b of the second stage regenerator tube 141 and the outline of the cold end 147b of the second stage pulse tube 146 are indicated by dotted lines. Also, in FIGS. 4A and 4B, the gas flow path 148 is indicated by dotted lines.

As illustrated in FIGS. 4A and 45, the condenser 160-2 has first holes 110a and second holes 110b. The first holes 110a are non-through holes that do not have openings on the upper surface 102 of the condenser 160-2. The second holes 110b are through holes that pass through the condenser 160-2 from the lower surface 103 to the upper surface 102.

The first holes 110a may be formed in any positions as long as the first holes 110 do not interfere with the gas flow path 148. Meanwhile, the second holes 110b are formed outside of an area S defined by a (imaginary) curved line R.

As illustrated in FIG. 4A, the gas flow path 148 has openings 148A and 148B on the upper surface 102. When the condenser 160-2 is viewed from above or below (i.e. from a direction parallel to the axis of the second stage regenerator tube 141 or the second stage pulse tube 146), the curved line R is a circumscribed circle having a center O on a straight line L connecting a center O₁of the opening 148A and a center O₂of the opening 148B and circumscribing the openings 148A and 148B of the gas flow path 148 (i.e., the openings 148A and 148B of the gas flow path 148 are inscribed in the circle R). When the openings 148A and 148B of the gas flow path 148 have a shape other than a circle, the curved line R may be a smallest circle enclosing the openings 148A and 148B.

Also with the condenser 160-2 configured as described above, the helium gas does not directly flow from the lower space 175 to the space between the second stage regenerator tube 141 and the second stage pulse tube 146 through the condenser 160-2. Thus, the condenser 160-2 having the holes 110a and 110b also provides advantageous effects as described above.

The holes 110a and 110b may be formed in any positions other than those illustrated in FIGS. 4A and 4B as long as the through holes 110b are formed outside of the area S. Also the shape of the holes 110a and 110b is not limited to a circle. Further, the numbers of the holes 110a and 110b are not limited to specific numbers.

However, the effect of preventing convection in the space between the second stage regenerator tube 141 and the second stage pulse tube 146 increases as the number of the through holes 110b decreases.

FIG. 5 illustrates a condenser 160-3 according to another embodiment of the present invention. In FIG. 5, the gas flow path 148 is omitted for brevity. The condenser 160-3 has holes 110c. The holes 110c are shaped like an inverted L or an elbow and have openings on a lower surface 103 and a side surface 104 (connecting the lower surface 103 and an upper surface 102) of the condenser 160-3. A part of the hole 110c extending horizontally and a part of the hole 110c extending vertically are not necessarily at a 90-degree angle with each other. Also, the hole 110c may have a shape other than an inverted L or an elbow and may extend substantially linearly from the side surface 104 to the lower surface 103.

Although only the holes 110c shaped like an inverted L or an elbow are illustrated in FIG. 5, the condenser 160-3 may also have non-through holes having openings on the lower surface 103. Further, the condenser 160-3 may have through holes having openings on the upper surface 102 and the lower surface 103 outside of an area corresponding to the area S illustrated in FIGS. 4A and 4B.

Also with the condenser 160-3 configured as described above, the helium gas does not directly flow from the space 175 to the space between the second stage regenerator tube 141 and the second stage pulse tube 146 through the condenser 160-3. Thus, the condenser 160-3 having the holes 110c also provides advantageous effects as described above.

FIG. 6 illustrates a condenser 160-4 according to another embodiment of the present invention. In FIG. 6, the gas flow path 148 is omitted for brevity. The condenser 160-4 has an upper surface 102, a lower surface 103, and a side surface 104 having an indented part 190 along the circumference of the side surface 104. In other words, the condenser 160-4 includes the indented part 190, a first part 210 above the indented part 190, and a second part 220 below the indented part 190. The first part 210 has a horizontal surface 215 that is parallel to the upper surface 102 and the lower surface 103. The second part 220 has a horizontal surface 225 that is parallel to the upper surface 102 and the lower surface 103. The second part 220 also has multiple through holes 110d passing through the second part 220 from the lower surface 103 to the horizontal surface 225.

Although only the holes 110d are illustrated in FIG. 6, the second part 220 may also have non-through holes having openings on the lower surface 103. Also, the first part 210 may have multiple non-through holes having openings on the horizontal surface 215. Further, the first part 210 may have through holes having openings on the upper surface 102 and the horizontal surface 215 outside of an area corresponding to the area S illustrated in FIGS. 4A and 4B.

Also with the condenser 160-4 configured as described above, the helium gas does not directly flow from the lower space 175 to the space between the second stage regenerator tube 141 and the second stage pulse tube 146 through the condenser 160-4. Thus, the condenser 160-4 of this embodiment also provides advantageous effects as described above.

FIG. 7 illustrates a condenser 160-5 according to still another embodiment of the present invention. In FIG. 7, the gas flow path 148 is omitted for brevity. The condenser 160-5 has a configuration similar to that of the condenser 160-4 except that through holes 110e shaped like an inverted Z or an elbow are formed through the second part 220 and the indented part 190 of the condenser 160-5. The through holes 110e have openings on a side 230 of the indented part 190. In this case, the through holes 110d formed in the second part 220 may be omitted. Meanwhile, the second part 220 may also have multiple non-through holes having openings on the lower surface 103. Further, the first part 210 may have multiple non-through holes having openings on the horizontal surface 215.

In the above embodiments, a two-stage pulse tube refrigerator is used as an example. However, the above embodiments may also be applied to a pulse tube refrigerator having a single-stage or three or more stages. When a pulse tube refrigerator includes multiple cooling stages, the condenser may be integrated with one of the cooling stages that provides the lowest temperature in the pulse tube refrigerator.

Also in the above embodiments, a helium gas is used as an atmospheric gas in the first insulating container 150 and the second insulating container 152. However, any other appropriate gas may be used as an atmospheric gas in the first insulating container 150 and the second insulating container 152. For example, for a single-stage pulse tube refrigerator where the temperature of the cooling stage is about 40 K to 50 K, a nitrogen gas may be used as the atmospheric gas and a liquid nitrogen tank may be provided instead of a liquid helium tank.

Although a condenser and a cooling stage are integrated in the above embodiments, a condenser and a cooling stage may be provided separately. For example, in FIG. 2, a condenser may be provided below a cooling stage such that the condenser is in contact with the lower surface of the cooling stage.

Experiments were performed using a two-stage pulse tube refrigerator configured as illustrated in FIG. 2 and the temperatures of the first cooling stage and the second cooling stage were measured.

In a first experiment, a condenser with multiple through holes extending vertically from the lower surface to the upper surface as illustrated in FIG. 1 was used. In a second experiment, a condenser with multiple non-through holes extending vertically and having openings only on the lower surface as illustrated in FIG. 3 is used. Both in the first and second experiments, the number of the holes was 30 and the diameter of the holes was about 4 mm. Also, the positions of the holes were the same in the first and second experiments. The holes were arranged at substantially regular intervals so as not to interfere with the gas flow path connecting the pulse tube and the regenerator tube. A helium gas was used as the atmospheric gas in the first and second insulating containers 150 and 152.

The results of the first and second experiments are shown in table 1 below.

TABLE 1

Temperature of first
Temperature of

Experiments
cooling stage
second cooling stage

First
45.9 K
4.35 K

experiment

Second
45.5 K
4.31 K

experiment

In the first and second experiments, the heat load of the first cooling stage was 30 W and the heat load of the second cooling stage was 1.0 W. As shown in table 1, in the first experiment, the temperature of the first cooling stage was 45.9 K and the temperature of the second cooling stage was 4.35 K. Meanwhile, in the second experiment, the temperature of the first cooling stage was 45.5 K and the temperature of the second cooling stage was 4.31 K.

As the results show, a condenser according to an embodiment of the present invention makes it possible to effectively reduce the temperature of the second cooling stage.

The embodiments of the present invention may be applied to a regenerator refrigerator such as a pulse tube refrigerator including a condenser for condensing an atmospheric gas.

The embodiments of the present invention provide a pulse tube refrigerator including a condenser that makes it possible to effectively prevent convection in a space between a regenerator tube and a pulse tube.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

PULSE TUBE REFRIGERATOR

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