Helium liquefaction device and control method for helium liquefaction device

Abstract

To provide a helium liquefaction device and a control method thereof that enable a stable operating state even when pressure in a liquid helium storage tank fluctuates.SOLUTION: A helium liquefaction device 100 includes a compressor 10, a heat exchanger group 20, expansion turbines 30, 31, an ejector 40, a gas liquid separator 50 and a liquid helium storage tank 60. Helium gas is circulated by using a high pressure line L1 for supply to the gas liquid separator via the ejector and a low pressure line L2 for returning the helium gas derived from a gas phase portion of the gas liquid separator to the compressor 10. Heat exchange is performed in the heat exchanger group 20. Liquid helium is derived from a liquid phase portion of the gas liquid separator 50 to the liquid helium storage tank 60. The helium gas in the liquid helium storage tank is sucked as suction gas of the ejector 40. A pressure keeping valve is provided in a portion that is located on the low pressure line and in which a temperature of helium becomes higher than a critical temperature, and an opening of the pressure keeping valve is adjusted through cascade control to make pressure of the liquid helium storage tank 60 uniform.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a helium liquefaction apparatus and a control method thereof, and more particularly to a helium liquefaction apparatus and a control method thereof capable of obtaining a stable operating state even when the pressure in a liquid helium storage tank fluctuates.

Background Art

[0002] A helium liquefaction apparatus is an apparatus for liquefying helium gas, and generally includes a compressor, heat exchangers, expansion turbines, JT valves, a liquid helium storage tank, etc. Helium compressed to a high pressure by the compressor is cooled to about 6K by the heat exchanger and then partially liquefied by JT expansion (Joule-Thomson expansion) with a JT valve, and the liquefied liquid helium is stored in the liquid helium storage tank.

[0003] A configuration example of a helium liquefaction apparatus employing a typical ejector cycle as shown in Patent Document 1 is shown in FIG. 2. In Patent Document 1, a cooled body (4) is provided, but in FIG. 2, it is described as a liquid helium storage tank having the same function in terms of storing liquid helium.

[0004] The helium liquefaction apparatus 1 shown in FIG. 2 is composed of a compressor 10, a heat exchanger group 20 formed by combining a plurality of heat exchangers 20a to 20e, expansion turbines 30 and 31, an ejector 40, a gas-liquid separator 50, and a liquid helium storage tank 60. Helium is circulated by a high-pressure line L1 through which the helium gas pressurized by the compressor 10 is supplied to the gas-liquid separator 50 via the ejector, and a low-pressure line L2 for returning the helium gas derived from the gas phase part in the gas-liquid separator 50 to the compressor 10.

[0005] The helium gas pressurized by the compressor 10 passes through the high-pressure line L1, is precooled via the heat exchanger 20a with the cryogenic gas generated by doing work externally by the expansion turbines 30 and 31 described later, and is further introduced into the heat exchangers 20b to 20d and the JT heat exchanger 20e. This helium gas exchanges heat with the saturated helium gas at about 120 to 130 kPa (abs) introduced from the gas-liquid separator 50 to the low-pressure line L2 in the JT heat exchanger 20e to become a cryogenic gas at 6 to 8 K, and flows into the ejector 40 via the ejector inlet valve 70.

[0006] The cryogenic helium gas flowing into the ejector 40 flows into the gas-liquid separator 50 in a gas-liquid two-phase state due to Joule-Thomson expansion inside the ejector 40. The liquid helium inside the gas-liquid separator 50 is controlled by the liquid level indicating regulator LIC to open and close the liquid supply valve 71 communicating with the lower part of the gas-liquid separator 50 so that the liquid level height becomes constant. The liquid helium passing through the liquid supply valve 71 is stored in the liquid helium storage tank 60. The gaseous helium in the liquid helium storage tank 60 is sucked by the ejector 40 from the suction line L3 via the valve 72 and flows into the gas-liquid separator 50 to constitute a liquefaction cycle (ejector cycle).

[0007] Also, a part of the helium gas precooled via the heat exchanger 20a in the high-pressure line L1 branches into the branch line L4 and is introduced into the first expansion turbine 30, the heat exchanger 20c, and the second expansion turbine 31. In the first expansion turbine 30 and the second expansion turbine 31, the helium gas undergoes isentropic expansion to generate the cold required for the process, and the temperature of the helium gas itself decreases. This helium gas is led out from the gas-phase part of the gas-liquid separator 50 to the low-pressure line L2, merges with the helium gas that has passed through the heat exchanger (20e), and then exchanges heat with the helium gas pressurized by the compressor 10 in the heat exchangers 20a to 20d. The helium gas that has reached room temperature by heat exchange is introduced into the compressor 10.

[0008] The liquid helium in the liquid helium storage tank 60 is sucked by a pump or the like (not shown) and used as product liquid helium.

[0009] As a control method for this helium liquefaction apparatus 1, there is known a method of adjusting the opening degree of the ejector inlet valve 70 with a temperature indicating controller TIC so that the temperature at the outlet of the second expansion turbine 31 becomes a constant temperature, and adjusting the throughput of the second expansion turbine 31.

[0010] Further, in this helium liquefaction apparatus 1, in the system shown in FIG. 2, the suction pressure of the compressor 10 is operated at about atmospheric pressure 101.3 kPa (abs), and the operating pressure of the gas-liquid separator is often higher than that, about 120 to 130 kPa (abs).

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] Here, in the helium liquefaction apparatus 1 of FIG. 2, the pressure in the liquid helium storage tank 60 fluctuated due to influences such as heat intrusion from the outside, supply of liquid helium through the liquid feed valve 71 from the gas-liquid separator 50, and utilization of the product liquid helium led out from the liquid helium storage tank 60. Also, it was necessary to change the liquefaction capacity of the apparatus according to the required amount of liquid helium, and accordingly, the pressure in the liquid helium storage tank 60 also fluctuated.

[0013] When the pressure inside the liquid helium storage tank 60 rises above a predetermined level, it becomes inevitable to release the cryogenic helium gas, which has been a major hindrance to the effective and efficient use of cryogenic helium. Therefore, there is a method of preventing the release by recovering the cryogenic helium gas from the liquid helium storage tank 60 into the gas-liquid separator 50 by suction with the ejector 40. Also, by suction with the ejector 40, even when the pressure of the liquid helium storage tank 60 is low, the helium gas inside the liquid helium storage tank 60 can be introduced into the ejector 40. In this case, the amount of cryogenic helium gas sucked by the ejector 40 is governed by the flow rate flowing through the ejector 40 (ejector throughput). Basically, as the ejector throughput increases, the flow rate of cryogenic helium gas sucked by the ejector (ejector suction amount) tends to increase.

[0014] However, in order to automatically control the inlet flow rate of the ejector 40 according to the amount of cold generated by the expansion turbines 30, 31 to control the liquefaction amount, the opening and closing of the ejector inlet valve 70 is adjusted. Therefore, the flow rate flowing through the ejector itself (ejector throughput) always changes regardless of the amount of helium gas that needs to be sucked from the liquid helium storage tank 60. Furthermore, the relationship between the ejector throughput and the ejector suction amount varies depending on the ejector and is not necessarily a proportional relationship as shown by an example in FIG. 3. Therefore, if the flow rate of the sucked helium gas is not appropriate, the pressure inside the liquid helium storage tank 60 will tend to rise or fall with respect to the target value, resulting in a problem that a stable operating state cannot be obtained.

[0015] That is, conventionally, it has not been possible to control the pressure of the gas-liquid separator 50 and the amount of helium gas sucked by the ejector 40, and it has been left to chance. In other words, it has been difficult to suck an appropriate amount of helium gas with the ejector according to the ejector throughput and control the increase in the pressure inside the liquid helium storage tank 60.

[0016] In addition, it is necessary to change the liquefaction capacity of the helium liquefaction device 1 according to the required amount of liquid helium. For example, Table 1 shows an example of an operating state where the liquefaction amount of helium is 180 L / h to 220 L / h. When changing from an operation with a liquefaction amount of 200 L / h to a reduced operation of 180 L / h, in order to maintain the pressure P0 of the liquid helium storage tank 60 at 102 kPa, it is necessary to increase the pressure of the gas-liquid separator 50 to 126.8 kPa. Conversely, if the pressure of the gas-liquid separator 50 is kept at 120 kPa, the pressure of the liquid helium storage tank 60 will drop. On the other hand, in order to maintain the pressure of the liquid helium storage tank 60 at 102 kPa during an increased operation to 220 L / h, it is necessary to lower the pressure of the gas-liquid separator 50 to 114.2 kPa. Conversely, if the pressure of the gas-liquid separator 50 is kept at 120 kPa, the pressure of the liquid helium storage tank 60 will rise.

[0017]

Table 1

[0018] Here, regarding the flow rate m of the flash gas that vaporizes when liquid helium is sent from the gas-liquid separator 50 to the liquid helium storage tank 60 f for the liquefaction amount m L and the flow rate m of the flash gas f the ratio to the sum of is defined as the flash loss.

[0019] Next, Table 2 shows the liquid flash loss when liquid is sent from the gas-liquid separator 50 to the liquid helium storage tank 60. Table 2 shows an example when the pressure of the gas-liquid separator 50 is changed while the pressure of the liquid helium storage tank 60 is kept constant. The flash loss varies in the range of 2.2 to 10.4% when the pressure of the gas-liquid separator 50 is changed from 110 to 140 kPa (abs). That is, the higher the pressure of the gas-liquid separator 50 (the greater the difference from the pressure of the liquid helium storage tank 60), the greater the ratio of the vaporized liquid helium supplied to the liquid helium storage tank 60. In other words, when the liquid helium storage tank 60 is operated at a constant pressure, if the pressure of the gas-liquid separator 50 is not left to its natural state but is appropriately controlled, the amount of helium gas itself supplied from the gas-liquid separator 50 to the liquid helium storage tank 60 can be reduced.

[0020]

Table 2

[0021] Also, when the liquid helium in the gas-liquid separator 50 is introduced into the liquid helium storage tank 60, the helium gas present in the liquid helium storage tank 60 corresponding to the volume of the supplied liquid will be introduced into the suction line L3, and this gas is referred to as rising gas.

[0022] In addition, the amount of liquid helium introduced from the gas-liquid separator 50 to the liquid helium storage tank 60 is determined by the amount of product liquid helium that needs to be sampled from the liquid helium storage tank 60, and the helium gas corresponding to the volume of the liquid helium will be pushed out from the liquid helium storage tank 60. That is, since the amount of rising gas is uniquely determined by the amount of product liquid helium that needs to be sampled, the flow rate of the rising gas m r also always fluctuates, which also contributes to the pressure fluctuation of the liquid helium storage tank 60 in this regard.

[0023] When organizing the technical problems to be solved by the present invention, it is as follows. (1) For the sake of the stability of the process of the helium liquefaction device 1, it is desirable to operate the pressure P1 of the gas-liquid separator 50 and the pressure P0 of the liquid helium storage tank 60 in a stable state. However, according to the demand for the product liquid helium, the operating state 1 of the helium liquefaction device (especially the circulation flow rate of helium) has to be varied. (2) Accordingly, the ejector throughput changes, and as a result, the suction amount of the ejector from the suction line L3 changes. Moreover, it is difficult to uniquely determine the suction amount from the ejector throughput. (3) Due to the nature of the process, there is a pressure difference between the gas-liquid separator 50 and the liquid helium storage tank 60, and a part of the helium introduced from the gas-liquid separator 50 to the liquid helium storage tank 60 vaporizes as flash gas (flash loss). Also, due to heat loss or the like, a part of the liquid helium in the liquid helium storage tank 60 vaporizes to generate helium gas. Therefore, the amount of suction helium gas from the liquid helium storage tank 60, which is necessary for the stable operation of the liquid helium storage tank 60, also constantly changes. (4) Also, it is necessary to change the liquefaction capacity of the device according to the required amount of product liquid helium, and thereby, the flow rate m f of the flash gas and the amount m r of the rising gas, etc. change, and thereby the pressure in the liquid helium storage tank 60 fluctuates.

[0024] Therefore, an object of the present invention is to solve the above problems of the prior art and provide a helium liquefaction device and its control method capable of obtaining a stable operating state even when the pressure in the liquid helium storage tank fluctuates.

Means for Solving the Problems

[0025] To achieve the above object, the helium liquefaction apparatus of the present invention includes a compressor for compressing and circulating helium gas, a heat exchanger group combining a plurality of heat exchangers, an expansion turbine, an ejector, a gas-liquid separator, and a liquid helium storage tank. A high-pressure line through which the helium gas pressurized by the compressor is supplied to the gas-liquid separator via the ejector, and a low-pressure line for returning the helium gas derived from the gas-phase portion of the gas-liquid separator to the compressor circulate the helium gas. In the heat exchanger group, the helium gas in the high-pressure line and the low-pressure line is heat-exchanged. Liquid helium is derived from the liquid-phase portion of the gas-liquid separator to the liquid helium storage tank. The helium gas in the liquid helium storage tank is sucked as the suction gas of the ejector. A part of the helium gas in the high-pressure line is introduced into the expansion turbine, and after generating the cold required for the apparatus, it is introduced into the low-pressure line. In the helium liquefaction apparatus, a pressure-maintaining valve is provided on the low-pressure line at a portion where the temperature of helium is higher than the critical temperature, and the opening degree of the pressure-maintaining valve is adjusted based on the pressure of the gas-liquid separator.

[0026] Further, the control method of the helium liquefaction apparatus of the present invention is such that a primary pressure indicating regulator measures the pressure of the liquid helium storage tank, outputs a pressure value of the gas-liquid separator such that the liquid helium storage tank reaches a set pressure, and uses it as a set target value of a secondary pressure indicating regulator. The secondary pressure indicating regulator adjusts the opening degree of the pressure-maintaining valve such that the pressure of the gas-liquid separator reaches the set target value.

Advantages of the Invention

[0027] According to the helium liquefaction apparatus and its control method of the present invention, a pressure-maintaining valve is provided at a portion on the low-pressure line where the temperature of helium is higher than the critical temperature, and the opening degree is adjusted based on the pressure of the gas-liquid separator. By performing cascade control, it is possible to stabilize the pressure of the liquid helium storage tank and prevent the release of low-temperature helium gas.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0029] FIG. 1 shows an exemplary form of the helium liquefaction apparatus 100 of the present invention. The helium liquefaction apparatus 100 shown in FIG. 1, similar to the conventional apparatus shown in FIG. 2, is composed of a compressor 10, a heat exchanger group 20 formed by combining a plurality of heat exchangers 20a to 20e, expansion turbines 30 and 31, an ejector 40, a gas-liquid separator 50, a liquid helium storage tank 60, and an ejector inlet valve 70. Helium is circulated by a high-pressure line L1 through which the helium gas pressurized by the compressor 10 is supplied to the gas-liquid separator 50 via the ejector, and a low-pressure line L2 that returns the helium gas derived from the gas phase portion in the gas-liquid separator 50 to the compressor 10. Regarding the same configuration as in FIG. 2, the same reference numerals are given and the description thereof is omitted.

[0030] The difference between the helium liquefaction apparatus 100 and the conventional apparatus shown in FIG. 2 is that a pressure maintaining valve 80 is provided in the low-pressure line L2 in order to control the pressure of the liquid helium storage tank 60. As shown in FIG. 1, the pressure maintaining valve 80 is preferably provided between the cold flow temperature end position of the JT heat exchanger 20e, the low-pressure line L2, and the confluence point of the branch line L4. This is because the helium gas immediately after being withdrawn from the gas-liquid separator 50 is in a dew point (saturation temperature at that pressure) state, and there is a possibility that a part of the helium gas may liquefy and become a two-phase flow state due to the temperature drop caused by the pressure drop of the pressure maintaining valve, so it needs to be provided at a position above the critical temperature (5.2K) of helium. Also, the temperature of the helium gas processed by the pressure maintaining valve 80 can be made approximately the same as the outlet temperature of the second expansion turbine 31.

[0031] Next, a control method of the helium liquefaction apparatus 100 using the pressure maintaining valve 80 will be described in detail. The primary pressure indicating regulator PIC that senses the pressure in the liquid helium storage tank 60 outputs the pressure value (MV) of the helium gas from the gas-liquid separator 50 such that the liquid helium storage tank 60 reaches the set pressure, and this is used as the set target value (SV) of the secondary pressure indicating regulator PIC. The secondary pressure indicating regulator PIC controls the opening degree output of the pressure maintaining valve 80 such that the pressure of the helium gas in the low-pressure line L2 withdrawn from the gas-liquid separator 50 reaches the set target value (SV). In this way, by setting the set target value for controlling the pressure of the gas-liquid separator 50 to cascade control of the pressure of the liquid helium storage tank 60, it is possible to stabilize the pressure of the liquid helium storage tank 60 and prevent the release of low-temperature helium gas. That is, by indirectly adjusting the pressure of the gas-liquid separator 50, stable operation of the entire process of the helium liquefaction apparatus 100 can be achieved.

[0032] Here, the flow rate m of the flash gas f and the flow rate m of the rising gas r The sum of which corresponds to the suction gas flow rate m of the ejector 40 of the helium gas e Let it be. That is, if the pressure P0 of the liquid helium storage tank 60 is rising, then m e <m f +m r If the pressure P0 of the liquid helium storage tank 60 is falling, then m e >m f +m r It can be judged. In the helium liquefaction apparatus 100, in order to stabilize the pressure of the liquid helium storage tank 60, m e =m f +m r It is necessary to set it like this (however, m r >>m f ).

[0033] However, it is difficult to control m e , m f , m r individually to a predetermined flow rate. On the other hand, due to the characteristics of the helium liquefaction apparatus 100, if the pressure P1 of the gas-liquid separator 50 is set high, then m e tends to decrease, mf shows an increasing trend. If the pressure P1 of the gas-liquid separator 50 is set low, m e shows an increasing trend, m f shows a decreasing trend. Therefore, paying attention to the pressure P1 of the gas-liquid separator 50, even if it is constantly changing during operation, if the pressure P1 of the gas-liquid separator 50 is appropriately controlled, it is noted that the pressure of the liquid helium storage tank 60 can also be stabilized, and cascade control is adopted.

[0034] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included. Note that cascade control may be used to sense the pressure of the gas-liquid separator 50 and determine the set target value for the pressure control of the liquid helium storage tank.

Description of Reference Numerals

[0035] 1... Helium liquefaction device, 10... Compressor, 20... Heat exchanger group, 20a - d... Heat exchangers, 20e... JT heat exchanger, 30... First expansion turbine, 31... Second expansion turbine, 40... Ejector, 50... Gas-liquid separator, 60... Liquid helium storage tank, 71... Liquid supply valve, 72... Valve, 100... Helium liquefaction device, 80... Pressure maintaining valve

Claims

1. A helium liquefier comprising a compressor for compressing and circulating helium gas, a heat exchanger group combining a plurality of heat exchangers, an expansion turbine, an ejector, a gas-liquid separator, and a liquid helium storage tank, wherein helium gas is circulated by a high-pressure line through which the helium gas pressurized by the compressor is supplied to the gas-liquid separator via the ejector and a low-pressure line through which the helium gas derived from the gas-phase portion of the gas-liquid separator is returned to the compressor, wherein in the heat exchanger group, the helium gas in the high-pressure line and the low-pressure line is heat-exchanged, wherein liquid helium is derived from the liquid-phase portion of the gas-liquid separator to the liquid helium storage tank, wherein the helium gas in the liquid helium storage tank is sucked as the suction gas of the ejector, wherein a part of the helium gas in the high-pressure line is introduced into the expansion turbine, and after generating the required cold for the apparatus, is introduced into the low-pressure line in a helium liquefier, a pressure maintaining valve is provided on the low-pressure line at a portion where the temperature of helium is higher than the critical temperature, and the opening degree of the pressure maintaining valve is adjusted based on the pressure of the gas-liquid separator. A helium liquefier characterized by this.

2. A control method for the helium liquefier according to Claim 1, wherein a primary pressure indicating regulator measures the pressure of the liquid helium storage tank and outputs a pressure value of the gas-liquid separator such that the liquid helium storage tank reaches a set pressure, which is used as a set target value of a secondary pressure indicating regulator, and the secondary pressure indicating regulator adjusts the opening degree of the pressure maintaining valve such that the pressure of the gas-liquid separator reaches the set target value. A control method for a helium liquefier characterized by this.

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