JP3416559B2 - Liquefied gas storage device, reliquefaction device, and method for reliquefaction of liquefied nitrogen - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquefied gas storage device and a re-liquefaction device, and more particularly to a re-liquefaction device for re-liquefying vapor vaporized from a liquefied state and returning it to a storage tank. The present invention relates to a liquefied gas storage device and a reliquefaction device having: 2. Description of the Related Art Liquefied gas is used in precision physical and chemical instruments such as NMR and electron microscopes to ensure a low-temperature environment. Here, the liquefied gas means a liquid state liquefied by cooling a gas in a gaseous state at normal temperature. In order to maintain a low temperature environment, it is necessary to replenish an amount of liquefied gas corresponding to the liquefied gas reduced by evaporation. In order to eliminate the need for liquefied gas replenishment,
A technique for reliquefying a vaporized liquefied gas is disclosed in Japanese Patent Laid-Open No. 8-32
No. 7171 and Japanese Patent Application Laid-Open No. 10-24647. In the reliquefaction apparatus disclosed in these publications, a liquefied gas storage tank and a low-temperature generation section of a cryogenic refrigerator are
Connect with a flexible heat insulating tube. When the evaporated liquefied gas reaches the low-temperature generation section through the heat insulating pipe, it is liquefied again here. The reliquefied liquefied gas returns to the liquefied gas storage tank through the heat insulating pipe. Since the heat insulating pipe has flexibility, it is possible to prevent the vibration generated by the cryogenic refrigerator from being transmitted to the liquefied gas storage tank. [0004] In the above-mentioned conventional reliquefaction apparatus, it is necessary to support the refrigerator with supporting means other than the liquefied gas storage tank. Also, because the heat insulation pipe becomes longer,
Heat easily penetrates, and the reliquefaction efficiency decreases. Another object of the present invention is to provide a reliquefaction method capable of suppressing the occurrence of a flooding phenomenon. According to one aspect of the present invention, a storage pipe for storing liquefied nitrogen is provided with a transport pipe communicating with a cooled reliquefaction chamber, whereby the storage tank is inserted. A method of reliquefying liquefied nitrogen that evaporates in the transport pipe, ascends in the transport pipe, liquefies the nitrogen gas that has reached the reliquefaction chamber, and returns to the storage tank through the transport pipe. Estimating the amount of evaporation v [cm ³ / h] of saturated liquefied nitrogen per unit time in terms of saturated vapor pressure under atmospheric pressure,
As the transport pipe, the cross-sectional area of the gas passage is S [cm
² ], a step of reliquefying the liquefied nitrogen using a tube satisfying S ≧ v / 432000 is provided. By using a transport pipe satisfying this condition, the occurrence of the flooding phenomenon can be suppressed. FIG. 1 is a sectional view of a liquefied gas storage device according to a first embodiment of the present invention. . In a vacuum vessel 1, a physicochemical apparatus 5 and a liquefied gas storage tank 2 operating in a low-temperature environment are arranged. The liquefied gas storage tank 2 surrounds the physics and chemistry equipment 5. In the liquefied gas storage tank 2,
Liquefied nitrogen 3 is filled. An opening 6 for carrying in / out the device is formed in the vacuum vessel 1. The opening 6 is a lid 7
It is airtightly closed. The liquefied gas storage tank 2 is provided with an inlet 10 for injecting a liquefied gas and an opening 11 for inserting a measuring instrument. In the opening 11, a liquid level gauge 12, a pressure gauge 1
3, and a pressure regulating valve 14 are attached. In addition,
In order to artificially control the amount of evaporation of the liquefied gas, an electric heater 4 is arranged in the liquefied gas storage tank 2. At the inlet 10, a reliquefaction device 20 is attached. The detailed configuration of the reliquefaction apparatus 20 will be described later with reference to FIG.
This will be described with reference to FIG. The reliquefaction apparatus 20 includes a pulse tube refrigerator 21. The gas compressor 22 periodically repeats supply and recovery of the refrigerant gas to the pulse tube refrigerator 21 via the connection pipe 23. At the hot end of the pulse tube of the pulse tube refrigerator 21, an orifice 24
Is connected to the reservoir tank 25 via the. This makes it possible to improve the refrigeration capacity. The connecting pipe 23 has flexibility. Therefore, it is possible to prevent the vibration generated in the gas compressor 22 from being transmitted to the pulse tube refrigerator 21. Since the pulse tube refrigerator 21 has no moving parts such as a piston, the vibration generated by the reliquefaction apparatus 20 itself is small. FIG. 2 is a sectional view of the reliquefaction apparatus according to the first embodiment. The low-temperature end of the pulse tube refrigerator 21
The vacuum container 35 is inserted into the inside of the vacuum container 35 through the opening.
The high-temperature end of the pulse tube refrigerator 21 is arranged outside the vacuum vessel 35. The lid 36 closes the opening of the vacuum container 35, and the airtightness in the vacuum container 35 is maintained. The vacuum vessel 35 includes a support 33 and a fastener 31.
Are supported by the vacuum vessel 1. A support surface 16 having physical supporting force is defined around the inlet 10 of the vacuum vessel 1. The fastener 31 defines a supported surface 30 having a shape that matches the support surface 16. Supported surface 3
By aligning 0 with the support surface 16, the fastener 31 is stably supported by the vacuum vessel 1. The support surface 16 and the supported surface 30 have, for example, rotationally symmetric shapes with respect to a common axis. With such a rotationally symmetric shape, the relative positions of the two can be easily fixed stably. A flexible film 32 is inserted between the support surface 16 and the supported surface 30. By inserting the membrane 32, the fastening of the fastener 31 can be stabilized and the transmission of vibration can be suppressed. The film 32 is formed of, for example, rubber or a foam material. A cold head 38 made of copper is attached to the low temperature end of the pulse tube refrigerator 21. A reliquefaction chamber 40 is defined by the cold head 38 and the copper reliquefaction chamber container 39. Fins or grooves are formed in portions of the cold head 38 that define the inner surface of the reliquefaction chamber 40. Thereby, heat transfer efficiency is enhanced. The inner pipe 42 is connected to the reliquefaction chamber container 39 via the vacuum bellows 43, and the space inside the inner pipe 42 communicates with the reliquefaction chamber 40. Outer tube 45 is vacuum bellows 4
6 is connected to the vacuum container 35. The inner tube 42 and the outer tube 45 are both formed of stainless steel. Outer tube 45
Is arranged so as to wrap a part of the inner tube 42, and the vacuum bellows 46 is arranged so as to wrap the vacuum bellows 43. A gap 47 is defined between the outer peripheral surface of the inner tube 42 and the inner peripheral surface of the outer tube 45. The gap 47 communicates with a space in the vacuum vessel 35 via a space between the vacuum bellows 46 and the vacuum bellows 43. The tip of the outer tube 45 is welded to the outer peripheral surface of the inner tube 42. Thereby, the airtightness of the space in the vacuum container 35 and the gap 47 is maintained. The vacuum bellows 43 and 46 prevent the vibration of the pulse tube refrigerator 21 from being transmitted to the inner tube 42 and the outer tube 45. The inlet 10 defines an elongated cylindrical space that connects the space inside the liquefied gas storage tank 2 and the outside space. A part of the outer tube 45 and a part of the inner tube 42 are inserted into the inlet 10 of the vacuum vessel 1. An O-ring 50 is loaded between the tip of the opening of the inlet 10 and a step formed on the outer peripheral surface of the outer tube 45, and the airtightness of the space in the liquefied gas storage tank 2 is maintained. Further, the cylindrical leakage prevention rubber 51 covers the outer peripheral surface near the distal end of the inlet 10 to the outer peripheral surface of the outer tube 45 located outside the inlet 10. By tightening the rubber 51 for preventing leakage with the rubber band 52, the rubber 51 for preventing leakage comes into close contact with the outer peripheral surface of the tip of the inlet 10 and the outer peripheral surface of the outer tube 45. By operating the pulse tube refrigerator 21, the cold head 38 is cooled down to an absolute temperature of about 74K. The evaporated nitrogen gas is transported through the inner pipe 42 to the reliquefaction chamber 40. The nitrogen gas in the reliquefaction chamber 40 is again liquefied and returned to the liquefied gas storage tank 2 via the inner pipe 42. In the reliquefaction apparatus according to the first embodiment, the portion of the inner pipe 42 outside the inlet 10 and the vacuum bellows 4
The circumference of 3 is kept in a vacuum. Further, a evacuated gap 47 is inserted to the inside of the inlet 10. For this reason, heat intrusion from the outside into the inner tube 42 can be reduced. In the first embodiment, the reliquefaction apparatus 20
Are supported by a support surface 16 provided on the vacuum vessel 1. Since a large-scale support mechanism is not required, the size of the liquefied gas storage device can be reduced. The inner pipe 42 and the outer pipe 45 of the reliquefaction apparatus 20 according to the first embodiment are preferably arranged so as to be substantially vertical. With the vertical arrangement, the reliquefied liquefied gas can be efficiently returned into the liquefied gas storage tank. If the angle between the axial direction of the inner tube 42 and the vertical direction is 20 ° or less, the same effect as when the axial direction of the inner tube 42 is vertical will be obtained. In the first embodiment, the vaporized nitrogen gas rises in the inner pipe 42, and the reliquefied liquefied nitrogen falls in the inner pipe 42. When the flow rate of the ascending flow of the nitrogen gas is increased, the liquefied nitrogen which is going to descend in the inner pipe 42 is wound up and cannot reach the liquefied gas storage tank. This phenomenon is called a flooding phenomenon. When the flooding phenomenon occurs, the reliquefaction device does not function properly. If the inner diameter of the inner tube 42 is too small, a flooding phenomenon is likely to occur. Next, a preferred range of the thickness of the inner tube 42 will be described. FIG. 3 shows the flow rate of nitrogen gas and the maximum reliquefaction rate of liquefied nitrogen in the inner tube 42 when the inner diameter of the inner tube 42 is changed. The horizontal axis represents the inner diameter of the inner tube in mm, the right vertical axis represents the nitrogen gas flow rate in m / s, and the left vertical axis represents the maximum reliquefaction rate in cm ³ / hour. In the figure, open squares indicate the flow rate of saturated nitrogen vapor under atmospheric pressure, and solid circles indicate the maximum reliquefaction rate. Hereinafter, a method of measuring the data shown in FIG. 3 will be described. First, an inner tube having an inner diameter of 12 mm was used. Open the interior of the liquefied gas storage tank 2 to the atmosphere,
The specific evaporation amount Q1 from the liquefied gas storage tank 2 is measured by the liquid level meter 12. The intrinsic evaporation amount Q1 is the amount of the liquefied gas that constantly evaporates from the liquefied gas storage tank 2 due to heat intrusion from the external atmosphere and heat generation from the physical and chemical equipment 5. The operation of the refrigerator 21 is started. Reliquefaction room 4
When the temperature in 0 becomes 77K or less, reliquefaction of the evaporated nitrogen gas starts. Therefore, the temperature in the reliquefaction chamber 40 is 7
It becomes constant at a temperature slightly lower than 7K. The liquefied gas storage tank 2 is sealed, and the internal pressure is measured with a pressure gauge 13. As the reliquefaction proceeds, the pressure in the liquefied gas storage tank 2 decreases. The power supplied to the electric heater 4 is adjusted so that this pressure becomes substantially equal to the atmospheric pressure. The power supplied to the electric heater 4 when the pressure in the liquefied gas storage tank 2 is stable and substantially equal to the atmospheric pressure is recorded. The latent heat of vaporization of liquefied nitrogen under atmospheric pressure is 19
8.64 J / g. From the electric power supplied to the electric heater 4 and the latent heat of evaporation, an amount Q2 of liquefied nitrogen forcibly evaporated by the electric heater 4 is obtained. For example, if the supply power is 10
In the case of W, the forced evaporation amount Q2 is 181.23 g / h
It is. The density of saturated liquefied nitrogen under atmospheric pressure is 0.8086
Since the unit used as a reference for the forced evaporation amount Q2 is converted from mass to volume, the forced evaporation amount Q2 is 2 g / cm ^3.
24.13 cm ³ / h. The maximum reliquefaction rate shown in FIG. 3 is Q1 + Q2
Given by The same measurement is performed by setting the inner diameter of the inner tube 42 to 9 mm, 7 mm, and 5 mm. The maximum reliquefaction rate in these cases was about 300 cm ³ / h as shown in FIG. When the inner diameter of the inner tube 42 is 5 mm or more, the reliquefaction rate does not increase even if the inner diameter is made larger. This is probably because the speed is controlled by the refrigerating capacity of the refrigerator 21. When the refrigerating capacity of the refrigerator 21 is higher, if the inner diameter of the inner pipe 42 is set to 5 mm or more, the maximum reliquefaction rate becomes 3
It is considered to be larger than 00 cm ³ / h. When the flooding phenomenon occurs, the temperature of the reliquefaction chamber 40 gradually decreases, and the pressure in the liquefied gas storage tank 2 increases. This rise in pressure is measured with a pressure gauge 13
, The presence or absence of the occurrence of the flooding phenomenon can be determined. When the inner diameter of the inner tube 42 was 5 mm or more, no flooding phenomenon occurred. Inner tube 4
When the inner diameter of No. 2 was 3.8 mm, the occurrence of the flooding phenomenon was observed. The maximum reliquefaction rate in this case is not limited by the capacity of the refrigerator 21 but is limited by the inner pipe 42.
Is limited by the inside diameter of Next, a method of measuring the maximum reliquefaction rate when the inner diameter of the inner tube 42 is 3.8 mm, 2.7 mm, and 2 mm will be described. First, the refrigeration capacity is gradually reduced by changing the operating pressure of the refrigerator 21. When the refrigeration capacity is excessively reduced, the pressure in the liquefied gas storage tank 2 gradually increases. If the refrigerating capacity is high and the amount of heat generated by the electric heater 4 exceeds a certain value, a flooding phenomenon occurs. While finely adjusting the power supplied to the electric heater 4 and the refrigerating capacity of the refrigerator 21, the refrigerating capacity and the power supplied to the electric heater 4 immediately before the occurrence of the flooding phenomenon are found. From the electric power supplied to the electric heater 4, a forced evaporation amount Q2 is obtained. Also in this case, the evaporation amount Q1 + Q2 corresponds to the maximum reliquefaction rate. Next, how to determine the flow rate of the nitrogen gas in the inner pipe 42 will be described. At atmospheric pressure, the saturated liquid density of nitrogen is 0.80861 g / cm ³ and the saturated vapor density is 0.004612 g / cm ³ . The nitrogen gas reliquefied in the reliquefaction chamber 40 is all replenished by the nitrogen gas rising in the inner pipe 42. Therefore, the mass flow rate of the nitrogen gas rising in the inner pipe 42 is equal to the mass flow rate of the liquefied nitrogen descending in the inner pipe 42. This mass flow rate is determined by multiplying the reliquefaction rate by the saturated liquid density. By dividing the obtained mass flow rate by the saturated vapor density, the volume flow rate of the nitrogen gas is obtained. By dividing the volume flow rate by the cross-sectional area of the flow path of the inner pipe 42, the flow rate of the nitrogen gas is determined. The unit of the flow velocity obtained by the calculation so far is cm / h. A value obtained by converting the unit of the flow rate to m / s corresponds to the flow rate shown on the right vertical axis of FIG. As shown in FIG. 3, when the maximum reliquefaction rate is limited by the capacity of the refrigerator 21, the inner pipe 42
As the inner diameter of the gas becomes larger, the flow rate of the nitrogen gas decreases. When the maximum reliquefaction rate is controlled by the inner diameter of the inner pipe 42, the flow rate of the nitrogen gas is about 1.2 m / s. This is regardless of the size of the inner diameter of the inner tube 42.
If the flow rate of the nitrogen gas exceeds 1.2 m / s, it means that the flooding phenomenon occurs. From this, it is understood that the inner diameter of the inner pipe 42 may be determined by the following method. First, the evaporation amount v [cm ³ / h] of the liquefied gas in the liquefied gas storage tank per unit time in terms of saturated vapor under atmospheric pressure is predicted. The cross-sectional area of the passage of the inner pipe 42 is determined so that the flow rate when the evaporated gas rises inside the inner pipe 42 is 1.2 m / s or less. That is, assuming that the sectional area of the passage is S [cm ² ], v / S ≦ 1.2 [m / s] × 100 [cm / m] ×
It is sufficient to satisfy 3600 [s / h] S ≧ v / 432000. By designing in this way, it is possible to reliquefy the nitrogen gas corresponding to the predicted evaporation amount without causing the flooding phenomenon. Note that the predicted evaporation amount v may be a value obtained by multiplying the actually predicted evaporation amount by a predetermined safety coefficient, for example, 1.2. Next, a second embodiment of the present invention will be described with reference to FIG. Here, only differences from the reliquefaction apparatus according to the first embodiment shown in FIG. 2 will be described. In the first embodiment, the reliquefaction chamber 40 is defined by the cold head 38 and the reliquefaction chamber container 39. However, in the second embodiment, the reliquefaction chamber 40 is defined only by the reliquefaction chamber container 39a. Is defined. Reliquefaction chamber container 39a
Are thermally coupled to the cold head 38. With such a configuration, the refrigerator 21 can be easily removed from the reliquefaction chamber 40. In the reliquefaction chamber 40, the first inner pipe 42a and the second
Is connected to the inner tube 42b. In the case of the first embodiment, the inner pipe 42 and the reliquefaction chamber 40 are connected via the vacuum bellows, but in the second embodiment, both are directly connected without the use of the vacuum bellows. First inner tube 42
a and a part of the second inner tube 42b are both wrapped by the outer tube 45. In FIG. 4, the first inner pipe 42a, the second inner pipe 42b, and a part of the outer pipe 45 are inclined because:
This is because a case where a sufficient space for installing the reliquefaction apparatus 20 is not secured near the injection port 10 of the vacuum vessel 1 shown in FIG. Note that, in order to efficiently return the reliquefied liquefied gas into the liquefied gas storage tank 2, the angle between the axial direction of the inclined portion and the vertical direction is preferably 20 ° or less. The second inner tube 42b is inserted to a position deeper than the first inner tube 42a. When the liquefied gas is stored in the liquefied gas storage tank 2, the tip of the second inner pipe 42 b reaches below the liquid level of the liquefied gas and the tip of the first inner pipe 42 a does not reach the liquid level. Is done. Therefore, the evaporated liquefied gas is transported into the reliquefaction chamber 40 through the first inner pipe 42a. The tip of the first inner pipe 42 a on the side of the reliquefaction chamber 40 slightly protrudes from the bottom surface of the reliquefaction chamber 40. For this reason, the liquefied gas reliquefied in the reliquefaction chamber 40 is the first gas.
Does not flow into the inner pipe 42a, but flows into the second inner pipe 42b. As described above, the first inner pipe 42a for transporting the vapor of the liquefied gas is separated from the second inner pipe 42b for transporting the liquefied gas. For this reason, the occurrence of the flooding phenomenon can be prevented. The present invention has been described in connection with the preferred embodiments.
The present invention is not limited to these. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made. As described above, according to the present invention,
A reliquefaction apparatus that is small and convenient to handle is provided. In addition, a design guide for preventing the flooding phenomenon is provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of a liquefied gas storage device according to a first embodiment of the present invention. FIG. 2 is a sectional view of the reliquefaction apparatus according to the first embodiment. FIG. 3 is a graph showing the maximum reliquefaction rate and the flow rate of nitrogen gas as a function of the inner diameter of the inner tube when reliquefying liquefied nitrogen using the reliquefaction apparatus according to the first embodiment. FIG. 4 is a sectional view of a reliquefaction apparatus according to a second embodiment. [Description of Signs] 1 vacuum vessel 2 liquefied gas storage tank 3 liquefied nitrogen 4 electric heater 5 physicochemical equipment 6 opening 7 lid 10 inlet 11 opening 12 liquid level gauge 13 pressure gauge 14 pressure regulating valve 16 support surface 20 reliquefaction Apparatus 21 Pulse tube refrigerator 22 Gas compressor 23 Connecting pipe 24 Orifice 25 Reservoir tank 30 Supported surface 31 Fastener 32 Membrane 33 Support post 35 Vacuum container 36 Cover 38 Cold head 39 Reliquefaction chamber forming member 40 Reliquefaction chamber 42 Inner pipe 43, 46 Vacuum bellows 45 Outer tube 47 Gap 50 O-ring 51 Leak prevention rubber 52 Rubber band

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-73264 (JP, A) JP-A-61-38363 (JP, A) JP-A-11-63766 (JP, A) 109795 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) F25B 9/00

Claims (1)

(57) [Claim 1] By inserting a transport pipe communicating with a cooled reliquefaction chamber into a storage tank for storing liquefied nitrogen,
A method of reliquefying liquefied nitrogen that evaporates in the storage tank, rises in the transport pipe, liquefies the nitrogen gas that has reached the reliquefaction chamber, and returns to the storage tank through the transport pipe. A step of predicting an evaporation amount v [cm ³ / h] of a liquefied nitrogen in a storage tank in terms of a saturated vapor pressure under an atmospheric pressure per unit time; cm
² ], a step of re-liquefying the liquefied nitrogen using a tube satisfying S ≧ v / 432000.