Cooler 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 member to be cooled. 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. A cooling device generally 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 occur 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 occur in the cavity, the bubbles will repeatedly occur.

[0005] The repeated occurrence of bubbles in the tank vibrates the tank. In particular, the repeated occurrence 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. The vibration becomes a major obstacle to sample observation.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to reduce the vibration of a tank storing a liquid refrigerant. Alternatively, an object of the present invention is to deactivate cavities on the inner surface of a tank storing a liquid refrigerant.

Means for Solving the Problems

[0008] A cooling device according to the present invention is a pressure control facility including a tank connected to a member to be cooled via a heat conducting member and a pump for sucking gas in the tank, and the pressure in the tank is set to a first pressure in a first period when injecting a liquid refrigerant into the tank, and the pressure in the tank is set to a second pressure higher than the first pressure in a second period following the first period, thereby causing spontaneous boiling in the liquid refrigerant in the tank in the second period, and is characterized by including the pressure control facility.

[0009] A cooling method according to the present invention includes a first step of reducing the pressure in a tank connected to a member to be cooled via a heat conducting member from an initial pressure to a first pressure, thereby reducing the boiling point of the liquid refrigerant injected into the tank; a second step of increasing the pressure in the tank from the first pressure to a second pressure after the first step, thereby increasing 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 bubbles in the liquid refrigerant in the tank, and is characterized by including the steps.

Effects of the Invention

[0010] According to the present invention, the vibration of a tank storing a liquid refrigerant can be reduced. Alternatively, according to the present invention, it becomes possible to deactivate cavities on the inner surface of a tank storing a liquid refrigerant.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments will be described with reference to the drawings.

[0013] (1) Outline of the Embodiment The cooling device according to the embodiment includes a tank and pressure control equipment. The tank is connected to a member to be cooled via a heat conducting member. The pressure control equipment has a pump that sucks gas in the tank. The pressure control equipment sets the pressure in the tank to a first pressure in a first period when injecting a liquid refrigerant into the tank. Further, the pressure control equipment sets the pressure in the tank to a second pressure higher than the first pressure in a second period following the first period, thereby causing spontaneous boiling in the liquid refrigerant in the tank in 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 (in the boiling state) is lowered. In the subsequent second period, the pressure in the tank is increased, thereby increasing the boiling point of the liquid refrigerant in the tank. That is, the boiling of the liquid refrigerant is temporarily suppressed. Through such a temporary calming process, as the temperature of the liquid refrigerant rises due to the heat flowing in from the outside, the temperature of the liquid refrigerant in the tank reaches the boiling point again. However, since the inflow of heat is relatively slow, the boiling of the liquid refrigerant does not occur immediately, and a part of the liquid refrigerant becomes in a superheated state. Then, triggered by a small vibration or the like, a part of the liquid refrigerant that has been heated vaporizes all at once, that is, violent boiling occurs. That is the above-mentioned spontaneous nucleate boiling.

[0015] Spontaneous nucleate boiling greatly increases the pressure in the tank in a short time. The rapid increase in pressure liquefies the bubbles in the liquid refrigerant. That is, the bubbles in the cavity disappear, and the repeated generation of bubbles in the cavity is stopped. That is, the cavity is deactivated.

[0016] After spontaneous nucleate boiling occurs, if a large amount of heat flows into the liquid refrigerant, the cavity will be reactivated. Therefore, it is desirable to form a heat balance state with the cooled member before the last spontaneous nucleate boiling. After that, even if heat flows into the liquid refrigerant from the outside, if the amount of heat is small, or if the heat flows into the upper part of the liquid refrigerant in the tank, usually, the vibration caused by the boiling of the liquid refrigerant is small, and basically no problems are caused by the vibration. Note that the above first pressure and second pressure are the steady-state pressures set by the pressure control equipment respectively, and correspond to the baseline. Due to the boiling (including nucleate boiling) of the liquid refrigerant, a pressure peak is superimposed on the baseline.

[0017] In an embodiment, the first pressure is a pressure lower than atmospheric pressure. The second pressure is atmospheric pressure. In this configuration, first, the pressure in the tank is made negative (negative pressure), and then the pressure in the tank is returned to atmospheric pressure. By using atmospheric pressure, the configuration of the pressure adjustment facility 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 the inflow of heat 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 becomes important. Therefore, the first temperature may be determined as an arbitrary temperature, and the second temperature may be determined based on that.

[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 facility 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 becomes 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 a pressure 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 the 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 high-altitude work, the safety of the operator can be enhanced.

[0022] In an embodiment, a porous member is provided at the suction port in the supply pipe. According to this configuration, several advantages can be obtained. First, the porous member can function as a filter. That is, it is possible to prevent foreign substances such as ice from entering the tank. Second, when the porous member is exposed, when the gas generated in the tank flows out from the porous member, the surface of the porous member will be covered with the gas. Thereby, the entry of air into the supply pipe is blocked. Third, since the flow path resistance is increased by the porous member, it is possible to cause a large pressure increase in the tank at the time of spontaneous boiling without providing a on-off valve in the middle of the supply pipe and closing it.

[0023] In an embodiment, the pressure control facility includes a suction pipe provided between the pump and the tank, and a detector that detects the temperature at a detection position on the suction pipe. Based on the temperature of the detector, the overflow of the liquid refrigerant from the tank is determined. According to this configuration, the overflow of the liquid refrigerant can be determined with a simple configuration. In the embodiments described later, the control unit functions as an overflow determination device.

[0024] In an embodiment, the pressure control facility includes a relief valve that operates when the pressure in the tank reaches a predetermined pressure. When the pressure in the tank exceeds a predetermined value (upper limit value) beyond the pressure at which the cavity can be deactivated, the relief valve operates. According to this configuration, the safety can be enhanced.

[0025] The cooling preparation method according to the embodiment includes a first step, a second step, and a third step. In the first step, the pressure in the tank connected to the member to be cooled via the heat conduction member is reduced from the initial pressure to the first pressure. As a result, the boiling point of the liquid refrigerant injected into the tank decreases. In the second step, after the first step, the pressure in the tank is increased from the first pressure to the second pressure. As a result, the boiling point of the liquid refrigerant in the tank increases. Thereby, the boiling of the liquid refrigerant in the tank is temporarily suppressed. In the third step, after the second step, as the temperature of the liquid refrigerant in the tank rises, spontaneous nucleate boiling occurs in the liquid refrigerant in the tank. As a result, the bubbles in the liquid refrigerant in the tank 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 generated in the tank can be reduced in the cooling state of the member to be cooled.

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

[0027] In FIG. 1, the cooling device 10 includes a tank 12, an outer container 14, a pressure control facility 16, a supply pipe 20, and a suction pipe 22. The tank 12 is a container for storing liquid nitrogen. The side wall of the tank 12 is made of, for example, stainless steel, and the bottom wall of the tank 12 is made of, for example, copper. The tank 12 is provided 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 composed of, for example, a copper plate, copper wire mesh, or the like.

[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. Based on the height of the inner bottom surface of the tank 12, 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 of judging the stop of the supply of liquid nitrogen. The temperature sensor 44 is provided for detecting the presence or absence of liquid nitrogen in the tank 12. The control unit 34 described later determines that the liquid level of the liquid nitrogen has reached or exceeded the second height based on the output signal of the temperature sensor 46.

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

[0031] The temperature sensor 56 is provided for determining that a certain amount or more of liquid nitrogen exists in the outer container 14. More specifically, it is for confirming that the liquid level of the liquid nitrogen is at a position higher than the upper surface of a porous member 54 described later. The temperature sensor 58 is provided for preventing the overflow of nitrogen gas from the outer container 14.

[0032] An insulating member is provided on the outside of the supply pipe 20, but its illustration is omitted. The supply pipe 20 is composed of a first portion 20A extending along the vertical direction, a second portion 20B extending along the horizontal direction, and a third portion 20C extending along the vertical direction.

[0033] The lower end opening (suction port) of the first part 20A is covered by the porous member 54. The porous member 54 has a large number of micropores, and each micropore allows the passage of liquid and gas. The porous member 54 is made of, for example, sintered metal. The porous member 54 may be made of ceramic or the like.

[0034] The porous member 54 functions as a filter. That is, the passage of foreign matters such as ice is blocked by the porous member 54. Also, the porous member 54 contributes to an increase in flow path resistance in combination with the small inner diameter of the supply pipe 20. After liquid nitrogen is injected into the tank 12, nitrogen gas from the tank flows out to the outside from the surface of the porous member 54. 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 wrapped in nitrogen gas, the entry of air into the supply pipe 20 is blocked. At the same time, dew 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 position lower than the tank 12. Specifically, the bottom wall of the outer container 14 is installed at a position lower than the bottom wall of the tank 12. Although it is also possible to pour liquid nitrogen into the tank 12 from a position higher than the tank 12, in that case, work at a high place by an operator is required. According to the configuration according to the embodiment, the operator only needs to perform work at a low place, so the burden on the operator can be reduced and the safety of the operator can be enhanced.

[0036] The lower end portion 40 of the third part 20C in the supply pipe 20 enters the tank 12. The lower end portion 40 functions as a discharge nozzle.

[0037] The suction pipe 22 has a first portion 22A extending along the vertical direction, a second portion 22B extending in the horizontal direction, a U-shaped portion 24C belonging to the detection unit 24, and the like. A heat insulating member is also provided outside the suction pipe 22 as necessary. The U-shaped portion 24C consists of a portion extending downward, a curved portion, and a portion extending upward. A plurality of devices described in detail below are arranged in the subsequent stage of the U-shaped portion 24C. The lower end portion 42 of the first portion 22A enters the tank 12, and the 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 in the U-shaped portion 24C. When liquid nitrogen enters the U-shaped portion 24C, the temperature detected by the temperature sensor 62 drops rapidly. Thereby, an overflow is determined in the control unit 34 described later. The U-shaped portion 24C is made of a material with good thermal conductivity (for example, copper).

[0039] Next, the pressure control facility 16 will be described. The pressure control facility 16 has a relief valve 26, a flow rate adjustment valve 28, an on-off valve (solenoid valve) 30, and a vacuum pump 32, and also has a control unit 34 for controlling their operations.

[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 pipe 22, that is, the pressure in the tank 12, reaches a predetermined pressure (upper limit value). The predetermined pressure is, for example, 0.03 MPa (0.3 atm). The relief valve 26 may be provided at other positions as long as its function is exerted. The flow rate adjustment valve 28 is a valve for adjusting the flow rate of the gas flowing through the suction pipe 22, and in the embodiment, the flow rate adjustment valve 28 functions during the suction process.

[0041] The vacuum pump 32 is a suction device for sucking the gas in the tank 12 and reducing the pressure in the tank 12 below atmospheric pressure. The vacuum pump may be operated only during suction, or the vacuum pump 32 may be operated constantly. A buffer tank may be provided in front of the vacuum pump 32. Various pumps can be used as the vacuum pump 32. A pump for evacuating the inside of the lens barrel of a transmission electron microscope can also be used as the vacuum pump 32.

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

[0043] Specifically, the control unit 34 determines that liquid nitrogen exists in the tank 12 based on the output signal of the temperature sensor 44. Further, the control unit 34 determines that the liquid level of the liquid nitrogen has reached the second height based on the output signal of the temperature sensor 46. As will be described later, the control unit 34 changes the pressure in the tank from the first pressure (negative pressure) to the second pressure (atmospheric pressure) at the timing when a predetermined time has elapsed from the timing of the determination of the liquid level reaching. The control unit 34 determines the overflow of the liquid nitrogen based on the output signal of the temperature sensor 62. When the liquid nitrogen overflows, an alert is output.

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

[0045] The control unit 34 is connected to an operation panel 36 which is an input device, and a display 38 is also connected thereto. The operation panel 36 includes a plurality of switches, a plurality of buttons, etc. The display 38 is constituted by, for example, a liquid crystal display.

[0046] The supply pipe 20 is relatively long, and the inner diameter of the supply pipe 20 is relatively small. Moreover, a porous member 54 is provided at the suction port of the supply pipe 20. Therefore, the flow path 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 in the closed state, the inside of the tank 12 is close to a closed space. In that state, due to the spontaneous nucleate boiling described later, a rapid pressure rise occurs in the tank 12 in a short time. Conversely, the length and inner diameter of the supply pipe 20, the porosity of the porous member 54, etc. are determined so that such a pressure rise occurs.

[0047] When moisture enters the tank 12 and ice is formed inside the tank, relatively large bubbles are likely to be generated due to this. Therefore, a heater (not shown) is provided outside the bottom of the tank 12. Prior to the supply of the liquid refrigerant, if necessary, by energizing the heater, the moisture inside the tank 12 can be discharged to the outside as water vapor. A heater (not shown) is also provided outside the upper part of the outer container 14. When frost or ice adheres to the upper part of the outer container 14, by energizing the heater, the frost and ice adhering to the upper part of the outer container 14 can be removed.

[0048] FIG. 2 shows an installation example of the cooling device according to the embodiment. The transmission electron microscope 70 has a lens barrel 72 extending in the vertical direction (up and down direction). Inside the lens barrel 72, an electron gun, a focusing lens, an objective lens, a sample chamber, etc. are provided. A sample 78 to be observed 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, prevent sample contamination, and increase the degree of vacuum in the sample chamber.

[0049] The object to be cooled by the cooling device 10 is the gas molecule trap 82. The tank 12 is connected to the gas molecule trap via the heat conduction member 18. The gas molecule trap 82 is fixed in the sample chamber via a member 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 installed on the base 74. The supply pipe provided between the external container 14 and the tank 12 has a portion that extends vertically for a long distance. In FIG. 2, the illustration of the pressure control facility is omitted.

[0051] FIG. 3 shows the action of the cavity. (A) to (G) show the growth process of the bubbles step by step. There is a very small depression (cavity) 102 on the inner surface 100 of the tank. Small bubbles 104 are generated in the cavity. The bubbles gradually grow (see reference numerals 106, 108, 110, 112). As shown in (G), the grown bubbles 104 detach from the cavity and float up. At that time, small bubbles 116 remain in the cavity. These bubbles 116 become the source of the next large bubble. Once bubbles are generated in the cavity, it will be repeated.

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

[0053] In the initial state, the inside of the tank is empty, or an indefinite amount of liquid refrigerant remains in the tank. The pressure (initial pressure) inside the tank in the initial state is the atmospheric pressure. In the following, for the convenience of explanation, it is assumed that the tank is empty in the initial state.

[0054] In FIG. 4, S10 indicates the cooling preparation process, and S28 indicates the cooling process. First, in S12, a prescribed amount of liquid nitrogen is injected into the external container by the operator. When the liquid level height of the liquid nitrogen exceeds the upper limit height (or when a predetermined time has elapsed since the timing of the exceeding), an alert (sound, light, image, etc.) is output in S14. After a prescribed amount of liquid nitrogen is injected into the external container, in S16, the operator operates the supply start button in the operation panel.

[0055] When the supply start button is operated while the vacuum pump is operating, the control unit causes the on-off valve to open. Then, the pressure in the tank changes from the initial pressure (atmospheric pressure) to the first pressure (negative pressure). The first pressure can be adjusted by changing the operation amount of the flow rate adjustment valve. Note that when the liquid level height of the liquid nitrogen in the external container is lower than the first liquid level height (lower limit), the supply of liquid nitrogen is not started. If the same situation occurs during the supply of liquid nitrogen, the supply of liquid nitrogen is stopped.

[0056] When the pressure in the tank changes from atmospheric pressure to the first pressure, the liquid refrigerant in the external container begins to move to the tank. In a state where the supply pipe and the tank are not sufficiently cooled, first, the liquid nitrogen violently vaporizes in the supply pipe, and then, even if the liquid nitrogen enters the tank, it violently vaporizes. If the tank contains air, the nitrogen gas generated by vaporization pushes out the air, and the air in the tank is replaced by the nitrogen gas. As a result, the water vapor contained in the air is discharged to the outside. When the pressure in the tank exceeds the specified pressure due to the vaporization of the liquid nitrogen, the relief valve automatically operates. When the supply pipe and the tank are cooled to a certain extent, the liquid nitrogen begins to accumulate in the tank.

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

[0058] After that, after a certain period of calming, in S26, in the liquid refrigerant in the tank, spontaneous boiling typically occurs multiple times. The mechanism of spontaneous boiling will be described in detail later. Due to the spontaneous boiling, typically the relatively large final spontaneous boiling, the pressure in the tank temporarily rises rapidly, and the bubbles contained in the liquid refrigerant liquefy. In experiments, it has been confirmed that the bubbles liquefy due to a pressure increase of 0.005 MPa (0.05 atm) or more. As a result, the cavities existing on the inner surface of the tank are deactivated. The repeated generation of bubbles caused by the cavities no longer occurs, or the repeated generation of bubbles decreases.

[0059] In S28, the member to be cooled is constantly cooled by the cooling device. In addition, in case there is a defect in the temperature sensor installed at the second height and the liquid level at the second height cannot be detected, a detection unit for detecting overflow is provided. When the elapsed time from the point when the output signal of the temperature sensor installed at the second height falls below the threshold exceeds a certain time, a supply error may be determined in S24 and the supply of the liquid refrigerant may be stopped. Also, when the liquid level is not detected at the second height even after a predetermined time has elapsed since the start of the supply of liquid nitrogen, a supply error may be determined in S24 and the supply of the liquid refrigerant may be stopped.

[0060] Note that even when an indefinite amount of liquid refrigerant remains in the tank in the initial state, by performing the above control, spontaneous boiling can be caused.

[0061] In the upper part of FIG. 5, the temperature change during the cooling preparation process is shown. 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) provided at the first height. Reference numeral 86 indicates the temperature detected by the temperature sensor (upper temperature sensor) provided at the second height. Ta indicates the boiling point of the liquid refrigerant under atmospheric pressure.

[0062] In the lower part of FIG. 5, the change in the pressure inside the tank during the cooling preparation process is shown. The horizontal axis is the time axis, and the vertical axis is the pressure axis. D1 indicates the injection period (first period), and D2 indicates the subsequent period (second period) following the injection period D1. pa indicates the atmospheric pressure.

[0063] In a state where the vacuum pump is operating, the on-off valve starts to open at time t0. Then, the pressure inside the tank changes from the initial pressure (atmospheric pressure) to the first pressure (negative pressure). As a result, the supply of the liquid refrigerant into the tank is started. That is, in a state where the atmospheric pressure is applied to the liquid refrigerant in the external container, when the pressure inside the tank becomes the first pressure lower than the atmospheric pressure, due to the pressure difference, the liquid refrigerant in the external container starts to move into the tank through the supply pipe. The pressure inside the tank is maintained at the first pressure. The gas suction force, the weight of the liquid nitrogen in the supply pipe 20, and the flow path resistance of the supply pipe 20 contribute to the formation and maintenance of the first pressure.

[0064] When the supply pipe and the tank are not sufficiently cooled, first, the liquid refrigerant violently boils in the pipe, and subsequently, the liquid refrigerant violently boils in the tank. After the liquid nitrogen or nitrogen gas from the external container starts to enter the tank, the temperature detected by the lower temperature sensor rapidly decreases, and the temperature detected by the upper temperature sensor rapidly decreases with a slight delay therefrom.

[0065] In period t1, as described above, violent boiling occurs inside the tank (see reference numeral 88). In period t1, as a result of the superheated state, the initial boiling over 90 occurs. That is, film boiling occurs beyond nucleate boiling. When liquid nitrogen is supplied to an empty tank, the initial boiling over 90 is likely to occur. On the other hand, when liquid nitrogen remains in the tank and the tank is cooled, such an initial boiling over 90 is less likely to occur.

[0066] After a large amount of liquid nitrogen is stored in the tank, especially after it reaches the full-tank state, if initial boiling occurs, 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 the full-tank state.

[0067] After the occurrence of the initial boiling 90, the amount of heat flowing into the liquid nitrogen in the tank is relatively large, and the boiling (nucleate boiling) of the liquid nitrogen will continue. In that case, the initial boiling may occur again.

[0068] At time t2, following the lower temperature sensor, the detected temperature of the upper temperature sensor also drops below the boiling point Ta of the liquid refrigerant under atmospheric pressure. The detected temperatures of the two temperature sensors respectively indicate the actual boiling points of the liquid refrigerant at each height. When the liquid refrigerant level is detected by the upper temperature sensor at time t2, at the timing t3 after a predetermined time has elapsed from the detection timing, the on-off valve is closed, and the pressure in the tank is switched from the first pressure (negative pressure) to the second pressure (atmospheric pressure). As a result, the supply of the liquid refrigerant to the tank is stopped. The bubbles in the liquid refrigerant liquefy when the pressure rises.

[0069] Generally speaking, the detected temperature of the upper temperature sensor is higher than that of the lower temperature sensor because when the agitation of the liquid nitrogen in the tank is not very large, the liquid nitrogen with a lower temperature is heavier than the liquid nitrogen with a higher temperature.

[0070] During the period t4, although the inflow of heat from the outside continues, since the temperature of the liquid refrigerant in the tank is lower than the boiling point of the liquid refrigerant under atmospheric pressure, the liquid refrigerant basically does not boil and becomes calm. 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 the liquid nitrogen under atmospheric pressure. At time t5, the temperature detected by the upper temperature sensor reaches the boiling point of the liquid nitrogen under atmospheric pressure.

[0071] Thereafter, at time t5, on the inner surface of the upper part in the tank, the temperature of the liquid nitrogen reaches its boiling point. However, since the inflow of heat is relatively gentle, the liquid nitrogen does not boil immediately, and a part of the liquid nitrogen becomes superheated. Under such circumstances, triggered by a small vibration or the like, a part of the superheated liquid nitrogen vaporizes all at once. Specifically, in the illustrated example, multiple spontaneous nucleate boiling 92 occurs from period t6 to period 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 nucleate boiling 94 occurs. It occurs after the temperature detected by the lower temperature sensor reaches the temperature of liquid nitrogen under atmospheric pressure. The last spontaneous nucleate boiling 94 is a relatively large nucleate boiling. After the last spontaneous nucleate boiling 94 occurs, no spontaneous nucleate boiling occurs. Each individual spontaneous nucleate boiling, especially the last large spontaneous nucleate boiling 94, causes a large pressure increase in the tank in a short time. Each individual spontaneous nucleate boiling, especially the last large spontaneous nucleate boiling 94, causes the bubbles in each cavity on the inner surface of the tank to liquefy. That is, each cavity becomes deactivated. Pb in FIG. 5 indicates the pressure threshold. When a pressure increase exceeding pb occurs, each cavity is effectively deactivated. As described above, pb is 0.005 M Pascal (0.05 atm), but its value can vary depending on various conditions. For example, pb may be set to 0.003 M Pascal (0.03 atm), or pb may be set to 0.007 M Pascal (0.07 atm).

[0073] In order to deactivate each cavity by the last major spontaneous boiling 94 and maintain that state, it is desirable that thermal equilibrium with the cooled member occurs before the recurrence of the spontaneous boiling 94. For example, if thermal equilibrium occurs between the cooled member and the liquid nitrogen supply starting 45 minutes after the start of the liquid nitrogen supply, various conditions should be determined so that the last spontaneous boiling 94 occurs after 45 minutes. The flow rate may be adjusted by a flow rate control valve so that such conditions are met. For example, increasing the flow rate, i.e., increasing the exhaust speed, increases the flow path resistance of the supply pipe, thereby lowering the first pressure and further lowering the temperature of the liquid nitrogen in the tank. In that case, after the liquid nitrogen supply is stopped, the time for the temperature of the liquid nitrogen to reach the boiling point of the liquid nitrogen under atmospheric pressure can be delayed. However, if the temperature of the liquid nitrogen is lowered arbitrarily, the liquid nitrogen may freeze when the pressure in the tank returns to atmospheric pressure. It is desirable to adjust the temperature of the liquid nitrogen so that such problems do not occur.

[0074] Even after the period t8, the inflow of heat into the tank continues and the boiling of the liquid nitrogen in the tank continues. However, after thermal equilibrium occurs between the cooled member and the liquid nitrogen, the heat flowing into the upper part of the tank becomes dominant. Therefore, the liquid nitrogen will boil exclusively in the upper part of the tank, specifically in the part near the liquid level inside the inner surface of the tank. The vibration caused by the bubbles generated near the liquid level is smaller than the vibration caused by the bubbles generated in the lower part of the tank, and the mechanical influence on the tank is small. In addition, since heat is taken away by evaporation at the liquid level of the liquid refrigerant, the boiling stop state may continue for a certain period.

[0075] In the experiment, taking the start time of the liquid nitrogen supply as the reference time, thermal equilibrium occurred 45 minutes after the start, and then the last spontaneous boiling occurred 65 minutes after the start. After that, a low vibration state occurred. The low vibration state was maintained for a long time until the liquid nitrogen in the tank disappeared.

[0076] A comparative example is shown in FIG. 6. A 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. A connection portion 122 between the funnel 124 and the tank 120 is connected via a tube. In that state, liquid nitrogen is injected into the funnel 124. When using the cooling device according to the comparative example, the injection operation of liquid nitrogen has to be performed at a high place. Also, the work load is large.

[0077] On the other hand, according to the cooling device and the cooling preparation method according to the embodiment, the injection operation at a high place is not required, so the safety of the operator can be enhanced. The burden during work can also be reduced. However, the deactivation of the cavity due to spontaneous boiling can occur regardless of the installation position of the outer container. In the embodiment, the cooling device is connected to the member to be cooled in the transmission electron microscope, but the cooling device may be connected to the member to be cooled in other devices. After reducing the pressure in the tank during or after the injection of liquid nitrogen, the pressure in the tank may be increased to cause spontaneous boiling in the liquid refrigerant.

Explanation of Signs

[0078] 10 Cooling device, 12 Tank, 14 Outer container, 16 Pressure adjustment facility, 20 Supply pipe, 22 Suction pipe, 24 Overflow detection unit, 26 Relief valve, 28 Flow rate adjustment valve, 30 On-off valve, 32 Vacuum pump, 34 Control unit, 54 Porous member.

Claims

1. A tank connected to a cooled member via a heat conducting member, and A pressure control facility including a pump for sucking gas in the tank, wherein in a first period of injecting a liquid refrigerant into the tank, the pressure in the tank is set to a first pressure, and in a second period following the first period, the pressure in the tank is set to a second pressure higher than the first pressure, thereby causing spontaneous boiling in the liquid refrigerant in the tank in the second period. A pressure control facility, A cooling device characterized by including the above.

2. In the cooling device according to Claim 1, The first pressure is a pressure lower than atmospheric pressure, A cooling device characterized by this.

3. In the cooling device according to Claim 2, The second pressure is atmospheric pressure, A cooling device characterized by this.

4. In the cooling device according to Claim 1, Including a sensor for detecting the liquid level height of the liquid refrigerant in the tank, The pressure control facility determines the timing to change the pressure in the tank to the second pressure based on the output signal of the sensor. A cooling device characterized by this.

5. In the cooling device according to Claim 1, An external container into which a liquid refrigerant is injected, and A supply pipe for sending the liquid refrigerant in the external container to the tank, Including, The first pressure is a pressure lower than atmospheric pressure, In the first period, the liquid refrigerant in the external container is taken into the tank through the supply pipe. A cooling device characterized by this.

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

7. In the cooling device according to Claim 5, A porous member is provided at the suction port in the supply pipe. A cooling device characterized by this.

8. In the cooling device according to Claim 1, The pressure control facility A suction pipe provided between the pump and the tank, and A detector for detecting temperature at a detection position on the suction pipe, Including, Based on the temperature of the detector, overflow of the liquid refrigerant from the tank is determined. A cooling device characterized by this.

9. In the cooling device according to Claim 1, The pressure control facility includes a relief valve that operates when the pressure in the tank reaches a predetermined pressure. A cooling device characterized by this.

10. A first step of reducing the pressure in a tank connected to a member to be cooled via a heat conductive member from an initial pressure to a first pressure, thereby reducing the boiling point of a liquid refrigerant injected into the tank; After the first step, a second step of increasing the pressure in the tank from the first pressure to a second pressure, thereby increasing the boiling point of the liquid refrigerant in the tank; After the second step, a third step in which spontaneous nucleate boiling occurs in the liquid refrigerant in the tank as the temperature of the liquid refrigerant in the tank rises, whereby the bubbles in the liquid refrigerant in the tank are liquefied; A cooling preparation method characterized by including the above steps.

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