Cooling device and cooling preparation method

Abstract

To deactivate a cavity on a tank inner surface.SOLUTION: Pressure in a tank is changed from initial pressure (atmospheric pressure) to first pressure (negative pressure) so as to fill a liquid refrigerant from an external container into the tank (S18). After a fixed amount of the liquid refrigerant is filled into the tank (S20), the pressure in the tank is changed from the first pressure to second pressure (atmospheric pressure)(S22). Then, after elapse of a calming-down period, spontaneous bumping occurs in the liquid refrigerant in the tank (S26). The spontaneous bumping causes temporal rapid rise of the pressure in the tank, so as to liquefy air bubbles in the liquid refrigerant in the tank. This deactivates a cavity.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a cooling device and a cooling preparation method, and particularly to a cooling device having a tank for storing a liquid refrigerant.

Background Art

[0002] A cooling device is a device for cooling a cooled member. Cooling devices are used in various fields. For example, in the observation of a sample using a transmission electron microscope, a cooling device is used to maintain the temperature of the sample at an extremely low temperature or to maintain the temperature of a gas molecule trap at an extremely low temperature. The gas molecule trap is for preventing sample contamination or improving the vacuum degree. Generally, a cooling device includes a tank for storing a liquid refrigerant such as liquid nitrogen (see, for example, Patent Document 1).

[0003] When the liquid refrigerant in the tank absorbs heat from the outside, the liquid refrigerant boils. More specifically, the heat from the outside is transmitted to the liquid refrigerant through the inner surface of the tank. There are quite a few fine scratches, depressions, etc. on the inner surface of the tank. It is known that bubbles are likely to repeatedly generate in such scratches, depressions, etc. Such scratches, depressions, etc. are called cavities.

[0004] More specifically, due to the heat from the outside, the liquid refrigerant vaporizes in the cavity to generate minute bubbles. When the bubbles grow into large bubbles, the bubbles detach from the cavity and float upward. After the large bubbles detach from the cavity, minute bubbles remain in the cavity. The minute bubbles grow into large bubbles. Once bubbles generate in the cavity, the bubbles will repeatedly generate.

[0005] The repeated generation of bubbles in the tank vibrates the tank. In particular, the repeated generation of bubbles at the lower part of the tank greatly vibrates the tank. The vibration of the tank has an adverse effect on the device to which the tank is connected. For example, when the tank is connected to a transmission electron microscope, the vibration is transmitted from the tank to the transmission electron microscope. Such vibration becomes a major obstacle in sample observation. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Publication No. 5-45012 [Overview of the project] [Problems that the invention aims to solve]

[0007] The object of the present invention is to reduce vibrations in a tank storing liquid refrigerant. Alternatively, the object of the present invention is to deactivate cavities on the inner surface of a tank storing liquid refrigerant. [Means for solving the problem]

[0008] The cooling device according to the present invention includes a tank connected to a member to be cooled via a heat conduction member, and a pressure control device including a pump for drawing gas from the tank, wherein the pressure inside the tank is set to a first pressure during a first period in which liquid refrigerant is injected into the tank, and the pressure inside the tank is set to a second pressure higher than the first pressure during a second period following the first period, thereby causing spontaneous boiling of the liquid refrigerant inside the tank during the second period. Furthermore, the cooled member and the heat conductive member are provided outside the tank, and during the second period, the pressure inside the tank is increased to a second pressure, raising the boiling point of the liquid refrigerant inside the tank, and then, due to the heat flowing in from the outside through the tank, the temperature of the liquid refrigerant inside the tank reaches the boiling point, causing a portion of the liquid refrigerant inside the tank to become superheated, thereby causing the spontaneous bumping that deactivates the cavity inside the tank. It is characterized by the following:

[0009] Cooling according to the present invention preparation The method includes: a first step of lowering the pressure inside a tank connected to the cooled component via a heat conductor from an initial pressure to a first pressure, thereby lowering the boiling point of the liquid refrigerant injected into the tank; a second step of raising the pressure inside the tank from the first pressure to a second pressure after the first step, thereby raising the boiling point of the liquid refrigerant in the tank; and a third step of causing spontaneous boiling in the liquid refrigerant in the tank as the temperature of the liquid refrigerant in the tank rises after the second step, thereby liquefying the bubbles in the liquid refrigerant in the tank. Furthermore, the cooled member and the heat conductive member are provided outside the tank, and in the third step, the heat flowing in from the outside through the tank causes the temperature of the liquid refrigerant in the tank to reach the boiling point, and a portion of the liquid refrigerant in the tank becomes superheated, thereby causing the spontaneous bumping. It is characterized by the following: [Effects of the Invention]

[0010] According to the present invention, vibrations of a tank storing liquid refrigerant can be reduced. Alternatively, according to the present invention, cavities on the inner surface of a tank storing liquid refrigerant can be deactivated. [Brief explanation of the drawing]

[0011] [Figure 1] This figure shows an example of the configuration of a cooling device according to an embodiment. [Figure 2] This figure shows an example of the installation of a cooling device according to the embodiment. [Figure 3] This diagram shows the formation of bubbles in the cavity. [Figure 4] This figure shows an example of the operation of the cooling device according to the embodiment. [Figure 5] This figure shows the temperature and pressure changes during the cooling preparation process. [Figure 6] This figure shows a cooling device related to a comparative example. [Modes for carrying out the invention]

[0012] The embodiments will be described below with reference to the drawings.

[0013] (1) Outline of the Embodiment The cooling device according to this embodiment includes a tank and a pressure control device. The tank is connected to the component to be cooled via a heat conduction member. The pressure control device has a pump for drawing gas from the tank. The pressure control device raises the pressure inside the tank to a first pressure during a first period in which liquid refrigerant is injected into the tank. The pressure control device then raises the pressure inside the tank to a second pressure, which is higher than the first pressure, during a second period following the first period, thereby causing spontaneous boiling of the liquid refrigerant inside the tank during the second period.

[0014] According to the above configuration, in the first period, the boiling point of the liquid refrigerant in the tank, that is, the temperature of the liquid refrigerant (which is in a boiling state), is lowered. In the subsequent second period, the pressure in the tank is increased, which raises the boiling point of the liquid refrigerant in the tank. In other words, the boiling of the liquid refrigerant is temporarily suppressed. After this temporary calming process, the temperature of the liquid refrigerant in the tank reaches its boiling point again as the temperature of the liquid refrigerant rises due to the heat flowing in from the outside. However, because the heat inflow is relatively slow, the boiling of the liquid refrigerant does not occur immediately, and a portion of the liquid refrigerant becomes superheated. Subsequently, triggered by a small vibration or the like, a portion of the heated liquid refrigerant vaporizes all at once, that is, violent boiling occurs. This is the spontaneous bumping described above.

[0015] Spontaneous bumping causes a rapid and significant increase in pressure within the tank. This sudden pressure increase liquefies the bubbles in the liquid refrigerant. In other words, the bubbles in the cavity disappear, and the repeated generation of bubbles in the cavity stops. This effectively deactivates the cavity.

[0016] If a large amount of heat flows into the liquid refrigerant after a spontaneous bumping event occurs, the cavity will be reactivated. Therefore, it is desirable that a thermal equilibrium state be formed with the cooled component before the final spontaneous bumping event. Even if heat flows into the liquid refrigerant from the outside afterward, if the amount of heat is small, or if the heat flows to the top of the liquid refrigerant in the tank, the vibration due to boiling of the liquid refrigerant is usually small, and vibration-related problems basically do not occur. Note that the first and second pressures mentioned above are the steady-state pressures set by the pressure control equipment, respectively, and correspond to the baseline. Due to boiling (including bumping) of the liquid refrigerant, a pressure peak is superimposed on the baseline.

[0017] In an embodiment, the first pressure is lower than atmospheric pressure. The second pressure is atmospheric pressure. With this configuration, first, the pressure in the tank is made negative (vacuum), and then the pressure in the tank is returned to atmospheric pressure. By using atmospheric pressure, the configuration of the pressure adjustment equipment can be simplified, and the control of the pressure becomes easier. In the embodiment, at the beginning of the first period, the pressure in the tank is changed from the initial pressure (atmospheric pressure) to the first pressure.

[0018] Note that, on the premise of heat inflow from the outside, in order to cause spontaneous boiling in the liquid refrigerant, an operation of raising the boiling point of the liquid refrigerant from the first temperature to the second temperature is important. Therefore, the first temperature may be set to an arbitrary temperature, and the second temperature may be determined based on it.

[0019] The cooling device according to the embodiment includes a sensor that detects the liquid level height of the liquid refrigerant in the tank. The pressure control equipment determines the timing to change the pressure in the tank to the second pressure based on the output signal of the sensor. A part of the liquid refrigerant injected into the tank is vaporized by its boiling. It is difficult to specify the liquid level (liquid volume) of the liquid refrigerant in the tank from the supply amount of the liquid refrigerant. According to the above configuration, it is possible to control the pressure in the tank based on the actual liquid level height of the liquid refrigerant.

[0020] The cooling device according to the embodiment has an external container into which the liquid refrigerant is injected and a supply pipe for sending the liquid refrigerant in the external container to the tank. The first pressure is lower than atmospheric pressure. In the first period, the liquid refrigerant in the external container is taken into the tank through the supply pipe. According to this configuration, due to the negative pressure in the tank, the liquid refrigerant is naturally taken into the tank. Therefore, the injection operation of the liquid refrigerant becomes easy.

[0021] In an embodiment, the external container is installed at a position lower than the tank. According to this configuration, the filling operation of the liquid refrigerant into the external container becomes easy. By avoiding work at heights, the safety of the operator can be enhanced.

[0022] In this embodiment, a porous member is provided at the suction port of the supply piping. This configuration offers several advantages. First, the porous member can function as a filter, preventing foreign matter such as ice from entering the tank. Second, when the porous member is exposed, if gas generated in the tank flows out through the porous member, the surface of the porous member becomes covered with gas. This prevents air from entering the supply piping. Third, because the porous member increases the flow resistance, a large pressure increase can be generated in the tank when spontaneous boiling occurs, without the need to install an on-off valve in the middle of the supply piping and operate it to close.

[0023] In one embodiment, the pressure control system includes a suction pipe provided between the pump and the tank, and a detector that detects temperature at a detection position on the suction pipe. Based on the temperature detected by the detector, overflow of liquid refrigerant from the tank is determined. This configuration allows for the determination of liquid refrigerant overflow with a simple setup. In an embodiment described later, the control unit functions as an overflow detector.

[0024] In this embodiment, the pressure control system includes a relief valve that operates when the pressure inside the tank reaches a predetermined pressure. The relief valve operates when the pressure inside the tank exceeds a predetermined value (upper limit) beyond the pressure required to deactivate the cavity. This configuration enhances safety.

[0025] The cooling preparation method according to this embodiment comprises a first step, a second step, and a third step. In the first step, the pressure inside a tank connected to the member to be cooled via a heat conductor is reduced from the initial pressure to a first pressure. This lowers the boiling point of the liquid refrigerant injected into the tank. In the second step, after the first step, the pressure inside the tank is increased from the first pressure to a second pressure. This raises the boiling point of the liquid refrigerant in the tank. This temporarily suppresses boiling of the liquid refrigerant in the tank. In the third step, after the second step, spontaneous boiling occurs in the liquid refrigerant in the tank as the temperature of the liquid refrigerant in the tank rises. This causes the bubbles in the liquid refrigerant in the tank to liquefy. That is, the cavities on the inner surface of the tank are deactivated. By suppressing the repeated generation of bubbles in the liquid refrigerant in the tank, vibrations occurring in the tank during the cooling state of the member to be cooled can be reduced.

[0026] (2) Details of the embodiment Figure 1 schematically shows an example of the configuration of a cooling device according to an embodiment. The cooling device according to the embodiment is, for example, a device for cooling a component to be cooled included in an electron microscope. The cooling device according to the embodiment may be used for other purposes. In this embodiment, the liquid refrigerant is specifically liquid nitrogen. Other liquid refrigerants may be used.

[0027] In Figure 1, the cooling device 10 includes a tank 12, an external container 14, pressure control equipment 16, supply piping 20, and suction piping 22. The tank 12 is a container for storing liquid nitrogen. The side walls of the tank 12 are made of, for example, stainless steel, and the bottom wall of the tank 12 is made of, for example, copper. The tank 12 is equipped with a vacuum insulation structure 50.

[0028] The bottom surface 12A of the tank 12 is connected to a heat conduction member 18. The heat conduction member 18 is connected to a member to be cooled (not shown). The heat conduction member 18 is made of, for example, a copper plate, a copper wire mesh, etc.

[0029] A temperature sensor 44 is provided on the outer surface of the lower part of the tank 12. A temperature sensor 46 is provided on the outer surface of the upper part of the tank 12. Using the height of the inner bottom surface of the tank 12 as a reference, the temperature sensor 44 is installed at a first height and the temperature sensor 46 is installed at a second height. The second height is a reference height for determining the timing to stop the supply of liquid nitrogen. The temperature sensor 44 is provided to detect the presence or absence of liquid nitrogen in the tank 12. The control unit 34, which will be described later, determines, based on the output signal of the temperature sensor 46, that the liquid level of the liquid nitrogen has reached or exceeded the second height.

[0030] The external container 14 is a dewar for storing liquid nitrogen. The external container 14 is equipped with a vacuum insulation structure or an insulating member 60. The external container 14 has an opening 14A through which liquid nitrogen is injected into the external container 14 (see reference numeral 52). A temperature sensor 56 is provided on the inner surface of the lower part of the external container 14, and a temperature sensor 58 is provided on the inner surface of the upper part of the external container 14. Specifically, with respect to the height of the inner bottom surface of the external container 14, the temperature sensor 56 is provided at a third height, and the temperature sensor 58 is provided at a fourth height. The temperature sensors 56 and 58 may also be provided on the outer surface of the external container 14.

[0031] The temperature sensor 56 is provided to determine whether a certain amount or more of liquid nitrogen is present in the outer container 14, and more specifically, to confirm that the liquid nitrogen level is higher than the upper surface of the porous member 54, which will be described later. The temperature sensor 58 is provided to prevent nitrogen gas from overflowing from the outer container 14.

[0032] Although an insulating material is provided on the outside of the supply pipe 20, its illustration is omitted. The supply pipe 20 is composed of a first section 20A extending vertically, a second section 20B extending horizontally, and a third section 20C extending vertically.

[0033] The lower end opening (suction port) of the first section 20A is covered by a porous member 54. The porous member 54 has numerous micropores, each micropore allowing the passage of liquids and gases. The porous member 54 is made of, for example, sintered metal. The porous member 54 may also be made of ceramic or the like.

[0034] The porous member 54 functions as a filter. In other words, the passage of foreign matter such as ice is prevented by the porous member 54. Furthermore, the porous member 54, in conjunction with the small inner diameter of the supply pipe 20, contributes to an increase in flow resistance. After liquid nitrogen is injected into the tank 12, nitrogen gas from the tank flows out from the surface of the porous member 54 to the outside. When the amount of liquid nitrogen in the outer container 14 decreases, the porous member 54 is exposed, but since the porous member 54 is surrounded by nitrogen gas, the entry of air into the supply pipe 20 is prevented. At the same time, condensation and ice formation on the surface of the porous member 54 are prevented.

[0035] As shown in the figure, the outer container 14 is installed at a lower position than the tank 12; specifically, the bottom wall of the outer container 14 is installed at a lower position than the bottom wall of the tank 12. It is also possible to pour liquid nitrogen into the tank 12 from a position higher than the tank 12, but in that case, it would require workers to work at height. According to the configuration of this embodiment, workers only need to work at a low position, thus reducing the burden on workers and increasing worker safety.

[0036] The lower end 40 of the third section 20C of the supply piping 20 enters the tank 12. This lower end 40 functions as a discharge nozzle.

[0037] The suction pipe 22 has a first portion 22A extending vertically, a second portion 22B extending horizontally, a U-shaped portion 24C belonging to the detection unit 24, etc. Insulation material is also provided on the outside of the suction pipe 22 as needed. The U-shaped portion 24C consists of a portion extending downward, a curved portion, and a portion extending upward. A number of devices, which will be described in detail below, are arranged behind the U-shaped portion 24C. The lower end portion 42 of the first portion 22A enters the tank 12, and this lower end portion 42 functions as a suction nozzle.

[0038] The detection unit 24 detects the overflow of liquid nitrogen from the tank 12. The detection unit 24 has a temperature sensor 62 provided on the outer surface (bottom surface) of the bottom portion of the U-shaped section 24C. When liquid nitrogen enters the U-shaped section 24C, the temperature detected by the temperature sensor 62 drops sharply. This allows the control unit 34, described later, to determine that an overflow has occurred. The U-shaped section 24C is made of a material with good thermal conductivity (for example, copper).

[0039] Next, the pressure control equipment 16 will be described. The pressure control equipment 16 includes a relief valve 26, a flow control valve 28, an on-off valve (solenoid valve) 30, and a vacuum pump 32, and also includes a control unit 34 that controls the operation of these components.

[0040] The relief valve 26 is a safety valve. The relief valve 26 is normally in a closed state and automatically opens when the pressure in the suction piping 22, i.e., the pressure in the tank 12, reaches a predetermined pressure (upper limit). The predetermined pressure is, for example, 0.03 M Pascals (0.3 atmospheres). The relief valve 26 may be installed in other positions as long as its function is maintained. The flow control valve 28 is a valve for adjusting the flow rate of gas flowing through the suction piping 22, and in this embodiment, the flow control valve 28 functions during the suction process.

[0041] The vacuum pump 32 is a suction device that sucks gas from the tank 12 and reduces the pressure inside the tank 12 below atmospheric pressure. The vacuum pump may be operated only when suction is required, or it may be operated continuously. A buffer tank may be provided before the vacuum pump 32. Various pumps can be used as the vacuum pump 32. A pump used to evacuate the inside of a transmission electron microscope can also be used as the vacuum pump 32.

[0042] The control unit 34 is composed of a computer, microcomputer, etc. Specifically, the control unit 34 is composed of a processor that executes programs. The control unit 34 controls the operation of the on-off valve 30 and the operation of the vacuum pump 32. The operation of the flow control valve 28 may also be controlled by the control unit 34. Output signals from temperature sensors 44, 46, 56, and 58 are input to the control unit 34. The control unit 34 controls the operation of the cooling device 10 based on these output signals.

[0043] More specifically, the control unit 34 determines that liquid nitrogen is present in the tank 12 based on the output signal of the temperature sensor 44. The control unit 34 also determines that the liquid nitrogen level has reached a second height based on the output signal of the temperature sensor 46. As described later, the control unit 34 changes the pressure inside the tank from the first pressure (negative pressure) to the second pressure (atmospheric pressure) after a predetermined time has elapsed since the determination that the liquid level has been reached. The control unit 34 determines that the liquid nitrogen is overflowing based on the output signal of the temperature sensor 62. If the liquid nitrogen overflows, an alert is output.

[0044] The control unit 34 determines, based on the output signal from the temperature sensor 56, that there is a certain amount of liquid nitrogen in the external container 14. The control unit 34 supplies liquid nitrogen from the external container 14 to the tank 12 only when there is a certain amount of liquid nitrogen in the external container 14. The control unit 34 determines, based on the output signal from the temperature sensor 58, that the liquid nitrogen level has reached the fourth height (upper limit of liquid level). In that case, an alert is output.

[0045] The control unit 34 is connected to an input device, an operation panel 36, and also to a display unit 38. The operation panel 36 includes multiple switches, multiple buttons, etc. The display unit 38 is configured, for example, as a liquid crystal display.

[0046] The supply pipe 20 is relatively long, and its inner diameter is relatively small. Moreover, a porous material 54 is provided at the suction port of the supply pipe 20. Therefore, the flow resistance in the supply pipe 20 is large. An on-off valve 30 is provided on the suction pipe 22. When the on-off valve 30 is closed, the inside of the tank 12 is close to a closed space. In that state, a rapid pressure increase occurs in the tank 12 for a short period of time due to spontaneous bumping, which will be described later. Conversely, the length and inner diameter of the supply pipe 20, the porosity of the porous material 54, etc. are determined so that such a pressure increase occurs.

[0047] When moisture enters tank 12 and ice forms inside the tank, relatively large bubbles tend to form as a result. Therefore, a heater (not shown) is provided on the outside of the bottom of tank 12. Prior to supplying liquid refrigerant, the heater can be energized as needed to release moisture inside tank 12 to the outside as water vapor. A heater (not shown) is also provided on the outside of the top of the outer container 14. If frost or ice has accumulated on the top of the outer container 14, the heater can be energized to remove the frost or ice.

[0048] Figure 2 shows an example of the installation of a cooling device according to the embodiment. The transmission electron microscope 70 has a microscope tube 72 that extends vertically (up and down). Inside the microscope tube 72 are an electron gun, a focusing lens, an objective lens, a sample chamber, etc. The sample to be observed, 78, is held by a sample holder 80. A gas molecule trap 82 is provided near the sample 78. The gas molecule trap 82 is provided to trap contaminants to prevent contamination of the sample and to increase the vacuum level inside the sample chamber.

[0049] The cooling target of the cooling device 10 is the gas molecule trap 82. The tank 12 is connected to the gas molecule trap via a heat conductive member 18. The gas molecule trap 82 is fixed inside the sample chamber via a material with low thermal conductivity (e.g., glass).

[0050] The transmission electron microscope 70 has a base (horizontal plate) 74 supported by a pedestal 76. An external container 14 is mounted on the base 74. The supply piping between the external container 14 and the tank 12 has a long vertical extension. In Figure 2, the pressure control equipment is not shown.

[0051] Figure 3 illustrates the function of the cavity. (A) through (G) show the bubble growth process in stages. There is a very small depression (cavity) 102 on the inner surface 100 of the tank. Small bubbles 104 are generated within the cavity. These bubbles gradually grow (see symbols 106, 108, 110, and 112). As shown in (G), the grown bubbles 104 leave the cavity and float to the surface. At this time, small bubbles 116 remain in the cavity. These bubbles 116 become the source of the next larger bubbles. Once bubbles are generated in the cavity, the process repeats.

[0052] Figure 4 shows an example of the operation of the cooling device according to the embodiment as a flowchart. Figure 5 shows the changes in the temperature of the liquid refrigerant and the pressure inside the tank during the cooling preparation process. First, the example of operation will be explained, and then the changes in temperature and pressure will be explained in detail.

[0053] In the initial state, the tank is either empty or contains an unspecified amount of liquid refrigerant. The pressure inside the tank in the initial state (initial pressure) is atmospheric pressure. For the sake of explanation, the following will assume that the tank is empty in the initial state.

[0054] In Figure 4, S10 shows the cooling preparation process, and S28 shows the cooling process. First, in S12, the operator injects a specified amount of liquid nitrogen into the external container. If the liquid nitrogen level exceeds the upper limit (or if a predetermined time has elapsed since it exceeded the limit), an alert (sound, light, image, etc.) is output in S14. After the specified amount of liquid nitrogen has been injected into the external container, in S16, the operator operates the supply start button on the control panel.

[0055] When the vacuum pump is operating and the supply start button is pressed, the control unit opens the on / off valve. This changes the pressure inside the tank from the initial pressure (atmospheric pressure) to the first pressure (negative pressure). The first pressure can be adjusted by changing the amount of the flow control valve. Note that if the liquid nitrogen level in the external container is lower than the first liquid level (lower limit), the liquid nitrogen supply will not begin. If a similar situation occurs during the liquid nitrogen supply process, the supply will be stopped.

[0056] When the pressure inside the tank changes from atmospheric pressure to the first pressure, the liquid refrigerant in the outer container begins to move into the tank. If the supply piping and tank are not sufficiently cooled, the liquid nitrogen will first vaporize rapidly in the supply piping, and then any liquid nitrogen that enters the tank will also vaporize rapidly. If there is air in the tank, the nitrogen gas produced by vaporization will push out the air, replacing the air inside the tank with nitrogen gas. This will cause the water vapor in the air to be released to the outside. If the pressure inside the tank exceeds the specified pressure due to the vaporization of liquid nitrogen, the relief valve will operate automatically. Once the supply piping and tank have cooled to a certain extent, liquid nitrogen will begin to accumulate in the tank.

[0057] In S20, it is detected that the liquid nitrogen level in the tank has reached a second height (specified height). In S22, a predetermined time has elapsed since the detection of reaching the specified height, and the control unit stops the supply of liquid nitrogen. Specifically, it closes the on / off valve. As a result, the pressure inside the tank changes from the first pressure to the second pressure (atmospheric pressure).

[0058] Subsequently, after a certain period of calming, in S26, multiple spontaneous bumps typically occur in the liquid refrigerant within the tank. The mechanism of spontaneous bumping will be described in detail later. Spontaneous bumping, typically the last and relatively large spontaneous bump, causes a rapid and temporary increase in pressure within the tank, liquefying the bubbles contained in the liquid refrigerant. Experiments have confirmed that bubbles liquefy with a pressure increase of 0.005 M Pascals (0.05 atm) or more. This deactivates the cavities present on the inner surface of the tank. Repeated bubble formation caused by the cavities ceases or decreases.

[0059] In S28, the cooling device continuously cools the component to be cooled. A detection unit is provided to detect overflow in case of a malfunction in the temperature sensor installed at the second height, which prevents detection of the liquid level at the second height. If the elapsed time since the output signal of the temperature sensor installed at the second height fell below a threshold exceeds a certain period, a supply error may be determined in S24, and the supply of liquid refrigerant may be stopped. Furthermore, if a predetermined time has elapsed since the start of liquid nitrogen supply and the liquid level is not detected at the second height, a supply error may also be determined in S24, and the supply of liquid refrigerant may be stopped.

[0060] Furthermore, even if an unspecified amount of liquid refrigerant remains in the tank in the initial state, spontaneous boiling can be induced by performing the above control.

[0061] The upper part of Figure 5 shows the temperature changes during the cooling preparation process. The horizontal axis is the time axis, and the vertical axis is the temperature axis. Reference numeral 84 indicates the temperature detected by the temperature sensor (lower temperature sensor) located at the first height. Reference numeral 86 indicates the temperature detected by the temperature sensor (upper temperature sensor) located at the second height. Ta indicates the boiling point of the liquid refrigerant under atmospheric pressure.

[0062] The lower part of Figure 5 shows the pressure changes inside the tank during the cooling preparation process. The horizontal axis represents time, and the vertical axis represents pressure. D1 represents the injection period (first period), and D2 represents the subsequent period following injection period D1 (second period). Pa represents atmospheric pressure.

[0063] With the vacuum pump operating, the on-off valve opens at time t0. This changes the pressure inside the tank from the initial pressure (atmospheric pressure) to the first pressure (negative pressure). This initiates the supply of liquid refrigerant to the tank. Specifically, when atmospheric pressure is acting on the liquid refrigerant in the external container, and the pressure inside the tank becomes the first pressure (lower than atmospheric pressure), the pressure difference causes the liquid refrigerant in the external container to begin moving into the tank via the supply piping. The pressure inside the tank is maintained at the first pressure. Gas suction force, the weight of the liquid nitrogen in the supply piping 20, and the flow resistance of the supply piping 20 contribute to the formation and maintenance of the first pressure.

[0064] If the supply piping and tank are not sufficiently cooled, the liquid refrigerant will first boil violently in the piping, followed by the liquid refrigerant boiling violently in the tank. After liquid nitrogen or nitrogen gas from an external container begins to enter the tank, the temperature detected by the lower temperature sensor will drop sharply, followed somewhat later by a sharp drop in the temperature detected by the upper temperature sensor.

[0065] During period t1, as described above, violent boiling occurs in the tank (see reference numeral 88). During period t1, as a result of the superheating condition, initial bumping 90 occurs. That is, film boiling occurs beyond nucleate boiling. Initial bumping 90 is likely to occur when liquid nitrogen is supplied to an empty tank. In contrast, such initial bumping 90 is less likely to occur when liquid nitrogen remains in the tank and the tank is cold.

[0066] If initial boiling occurs after a large amount of liquid nitrogen has been stored in the tank, especially after it has reached full capacity, the liquid nitrogen will overflow from the tank and be wasted. Therefore, the flow rate and other parameters are adjusted so that initial boiling occurs before the tank reaches full capacity.

[0067] After the initial bumping (90), a relatively large amount of heat flows into the liquid nitrogen in the tank, causing the liquid nitrogen to continue boiling (nucleated boiling). In this case, initial bumping may occur again.

[0068] At time t2, following the lower temperature sensor, the temperature detected by the upper temperature sensor also falls below the boiling point Ta of the liquid refrigerant at atmospheric pressure. The temperatures detected by the two temperature sensors each represent the actual boiling point of the liquid refrigerant at their respective heights. When the liquid level of the liquid refrigerant is detected by the upper temperature sensor at time t2, the on / off valve is closed at timing t3, a predetermined time after the detection, and the pressure in the tank is switched from the first pressure (negative pressure) to the second pressure (atmospheric pressure). This stops the supply of liquid refrigerant to the tank. Bubbles in the liquid refrigerant liquefy when the pressure rises.

[0069] Generally speaking, the reason the temperature detected by the upper temperature sensor is higher than that detected by the lower temperature sensor is that, when the agitation of liquid nitrogen in the tank is not very significant, lower temperature liquid nitrogen is heavier than higher temperature liquid nitrogen.

[0070] During period t4, although heat continues to flow in from the outside, the temperature of the liquid refrigerant in the tank is lower than the boiling point of the liquid refrigerant at atmospheric pressure, so the liquid refrigerant basically does not boil and remains still. However, the temperature of the liquid nitrogen gradually rises due to the heat flowing in from the outside, and its temperature approaches the boiling point of liquid nitrogen at atmospheric pressure. At time t5, the temperature detected by the upper temperature sensor has reached the boiling point of liquid nitrogen at atmospheric pressure.

[0071] Subsequently, at time t5, the temperature of the liquid nitrogen on the upper inner surface of the tank reaches its boiling point. However, because the heat inflow is relatively gentle, the liquid nitrogen does not boil immediately, and a portion of the liquid nitrogen becomes superheated. Under these conditions, a small vibration or other trigger causes a portion of the superheated liquid nitrogen to vaporize all at once. Specifically, in the illustrated example, multiple spontaneous boiling events occur between periods t6 and t8. At time t7, the temperature detected by the lower temperature sensor reaches the boiling point of liquid nitrogen under atmospheric pressure.

[0072] At time t9, the last spontaneous bump 94 occurs. This occurs after the temperature detected by the lower temperature sensor reaches the temperature of liquid nitrogen at atmospheric pressure. The last spontaneous bump 94 is a relatively large bump. No further spontaneous bumps have occurred since the last spontaneous bump 94. Each spontaneous bump, especially the last large spontaneous bump 94, results in a short-lived, large pressure increase in the tank. Each spontaneous bump, especially the last large spontaneous bump 94, liquefies the bubbles in each cavity on the inner surface of the tank. In other words, each cavity is deactivated. In Figure 5, pb indicates the pressure threshold. Each cavity is effectively deactivated when the pressure increase exceeds pb. As mentioned above, pb is 0.005 M Pascals (0.05 atm), but its value can vary depending on the conditions. For example, you could set pb to 0.003 M Pascals (0.03 atmospheres) or to 0.007 M Pascals (0.07 atmospheres).

[0073] In order to deactivate each cavity with the final large spontaneous bumping 94 and maintain that state, it is desirable that thermal equilibrium with the cooled components be achieved before another spontaneous bumping 94 occurs. For example, if thermal equilibrium with the cooled components is achieved 45 minutes after the start of liquid nitrogen supply, various conditions should be set so that the final spontaneous bumping 94 occurs after 45 minutes. The flow rate may be adjusted by a flow control valve to satisfy such conditions. For example, increasing the flow rate, i.e., increasing the pumping speed, increases the flow resistance of the supply piping, which lowers the first pressure and further lowers the temperature of the liquid nitrogen in the tank. In that case, after the liquid nitrogen supply is stopped, the time it takes for the liquid nitrogen temperature to reach the boiling point of liquid nitrogen at atmospheric pressure can be delayed. However, if the temperature of the liquid nitrogen is lowered arbitrarily, there is a possibility that the liquid nitrogen will freeze when the pressure in the tank is returned to atmospheric pressure. It is desirable to adjust the temperature of the liquid nitrogen so that such problems do not occur.

[0074] Even after period t8, heat continues to flow into the tank, and the boiling of liquid nitrogen inside the tank continues. However, after thermal equilibrium is achieved with the cooled component, the heat flowing into the upper part of the tank becomes dominant. Therefore, the liquid nitrogen boils mainly in the upper part of the tank, specifically on the inner surface of the tank, near the liquid surface. The vibrations caused by bubbles near the liquid surface are smaller than those caused by bubbles at the bottom of the tank, and the mechanical impact on the tank is small. Furthermore, since heat is removed by the evaporation of the liquid refrigerant at the liquid surface, it is possible that the boiling cessation state may continue for a certain period.

[0075] In the experiment, thermal equilibrium was reached 45 minutes after the start of liquid nitrogen supply, and the final spontaneous boiling occurred 65 minutes later. Subsequently, a low-oscillating state was established. This low-oscillating state was maintained for a long period until the liquid nitrogen in the tank was depleted.

[0076] Figure 6 shows a comparative example. The cooling device according to the comparative example is connected to a transmission electron microscope. The cooling device has a tank 120. When injecting liquid nitrogen into the tank 120, a funnel 124 is used. The connection part 122 of the funnel 124 and the tank 120 are connected via a tube. In this state, liquid nitrogen is injected into the funnel 124. When using the cooling device according to the comparative example, the liquid nitrogen injection work must be performed at a height. Furthermore, the workload is also high.

[0077] In contrast, the cooling device and cooling preparation method according to the embodiment eliminate the need for injection work at high altitudes, thereby increasing worker safety. The burden during work can also be reduced. However, deactivation of the cavity due to spontaneous boiling can occur regardless of the installation position of the external container. In the embodiment, the cooling device was connected to the component to be cooled inside a transmission electron microscope, but the cooling device may be connected to the component to be cooled inside other devices. Spontaneous boiling of the liquid refrigerant may be induced by lowering the pressure inside the tank during or after the injection of liquid nitrogen, and then raising the pressure inside the tank. [Explanation of Symbols]

[0078] 10 Cooling device, 12 Tank, 14 External container, 16 Pressure regulating equipment, 20 Supply piping, 22 Suction piping, 24 Overflow detection unit, 26 Relief valve, 28 Flow control valve, 30 On / off valve, 32 Vacuum pump, 34 Control unit, 54 Porous member.

Claims

1. A tank connected to the component to be cooled via a heat conduction member, A pressure control system including a pump for drawing gas from the tank, wherein the pressure inside the tank is set to a first pressure during a first period in which liquid refrigerant is injected into the tank, and the pressure inside the tank is set to a second pressure higher than the first pressure during a second period following the first period, thereby causing spontaneous boiling of the liquid refrigerant inside the tank during the second period. Includes, The member to be cooled and the heat conductive member are provided outside the tank. During the second period, the pressure inside the tank is increased to a second pressure, raising the boiling point of the liquid refrigerant inside the tank. Subsequently, heat flowing in from the outside through the tank causes the temperature of the liquid refrigerant inside the tank to reach the boiling point, and a portion of the liquid refrigerant inside the tank becomes superheated, causing the spontaneous bumping that deactivates the cavity inside the tank. A cooling device characterized by the following features.

2. In the cooling device according to claim 1, The aforementioned first pressure is a pressure lower than atmospheric pressure. A cooling device characterized by the following features.

3. In the cooling device according to claim 2, The second pressure is atmospheric pressure. A cooling device characterized by the following features.

4. In the cooling device according to claim 1, Includes a sensor for detecting the liquid level of the liquid refrigerant in the tank, The pressure control equipment determines the timing for changing the pressure in the tank to the second pressure based on the output signal of the sensor. A cooling device characterized by the following features.

5. In the cooling device according to claim 1, An external container into which liquid refrigerant is injected, A supply pipe for sending the liquid refrigerant in the external container to the tank, Includes, The aforementioned first pressure is a pressure lower than atmospheric pressure. During the first period, the liquid refrigerant in the external container is drawn into the tank via the supply piping. A cooling device characterized by the following features.

6. In the cooling device according to claim 5, The external container is installed at a lower position than the tank. A cooling device characterized by the following features.

7. In the cooling device according to claim 5, A porous member is provided at the suction port of the supply pipe. A cooling device characterized by the following features.

8. In the cooling device according to claim 1, The aforementioned pressure control equipment is A suction pipe is provided between the pump and the tank, A detector that detects temperature at a detection position on the suction pipe, Includes, Based on the temperature of the detector, it is determined whether the liquid refrigerant is overflowing from the tank. A cooling device characterized by the following features.

9. In the cooling device according to claim 1, The pressure control equipment includes a relief valve that operates when the pressure in the tank reaches a predetermined pressure. A cooling device characterized by the following features.

10. The first step involves reducing the pressure inside a tank connected to the component to be cooled via a heat conduction member from an initial pressure to a first pressure, thereby lowering the boiling point of the liquid refrigerant injected into the tank. After the first step, the second step involves increasing the pressure inside the tank from the first pressure to the second pressure, thereby raising the boiling point of the liquid refrigerant inside the tank. Following the second step, a third step occurs in which, as the temperature of the liquid refrigerant in the tank rises, spontaneous boiling occurs in the liquid refrigerant in the tank, causing the bubbles in the liquid refrigerant in the tank to liquefy. Includes, The member to be cooled and the heat conductive member are provided outside the tank. In the third step, the heat flowing in from the outside through the tank causes the temperature of the liquid refrigerant in the tank to reach the boiling point, and a portion of the liquid refrigerant in the tank becomes superheated, which causes the spontaneous bumping to occur. A cooling preparation method characterized by the following:

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