JP2023140782A - Cooling down method of liquefied gas storage tank - Google Patents

Abstract

To reduce the time needed to cool down a multiple heat-insulating structure tank for storage of a liquefied gas to reduce costs.SOLUTION: A method for cooling a tank for storing a liquefied gas, which includes an inner tank (3) and an outer tank (5), before the tank is filled with the liquefied gas, an object to be stored, includes: introducing a cooling liquefied gas (CH) into an inner tank internal space (7); and supplying a temperature adjustment gas (TG) adjusted to an adjusted temperature range which is a predetermined temperature or higher to a space (9) between the inner and outer tanks through a temperature adjustment device (31).SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a method for cooling down a liquefied gas storage tank.

BACKGROUND ART Conventionally, it has been proposed to use a double-shell tank including an inner tank and an outer tank as a tank for storing liquefied gas, such as cryogenic liquefied hydrogen (see, for example, Patent Document 1).

Generally, when storing low-temperature liquefied gas in a tank, in order to avoid rapid cooling of the tank by filling a room-temperature tank with a large amount of liquefied gas to be stored at once, Before filling the tank, the tank is cooled at a relatively low speed (hereinafter referred to as "cool down").

Japanese Patent Application Publication No. 2019-151291

However, in the case of tanks with multiple heat insulation structures such as double-shell tanks for liquefied gas, they have high insulation properties, so cooling only the inner tank takes a long time to cool the entire tank, and a large amount of water is required. This will require liquefied gas for cooling. Therefore, the cost required for cooldown increases.

An object of the present disclosure is to shorten the time required to cool down a multi-layer heat-insulated structure tank for storing liquefied gas, and to suppress costs, in order to solve the above-mentioned problems.

In order to achieve the above object, a method for cooling down a liquefied gas storage tank according to the present disclosure includes:
A method for cooling a tank including an inner tank and an outer tank for storing liquefied gas before filling it with the liquefied gas to be stored, the method comprising:
Introducing a cooling liquefied gas into the inner tank space;
Supplying a temperature adjustment gas adjusted to a adjustment temperature range that is a predetermined temperature or higher to a space between the inner and outer tanks via a temperature adjustment device;
including.

According to the method for cooling down a liquefied gas storage tank according to the present disclosure, it is possible to shorten the time required to cool down a tank with multiple heat insulation structure for storing liquefied gas, and to suppress costs.

1 is a cross-sectional view showing a schematic configuration of a liquefied gas storage tank to which a cool-down method according to an embodiment of the present disclosure is applied. FIG. 2 is a schematic diagram showing an initial state before the start of a cool-down method according to an embodiment of the present disclosure. FIG. 3 is a schematic diagram showing a state during cooling of the inner tank in a cool-down method according to an embodiment of the present disclosure. FIG. 7 is a schematic diagram showing a state during cooling of the inner tank in a cool-down method according to a modified example of an embodiment of the present disclosure. FIG. 7 is a schematic diagram showing a state during cooling of the inner tank in a cool-down method according to another modification of the embodiment of the present disclosure. FIG. 2 is a schematic diagram showing a state in which a cool-down method according to an embodiment of the present disclosure has been completed. FIG. 2 is an enlarged cross-sectional view schematically showing a part of the liquefied gas storage gas tank of FIG. 1. FIG.

Hereinafter, preferred embodiments of the present disclosure will be described with reference to the drawings. FIG. 1 shows a liquefied gas storage tank (hereinafter simply referred to as "storage tank") 1 to which a cool-down method according to an embodiment of the present disclosure is applied. This storage tank 1 is a tank for storing liquefied gas, and is configured as a double shell tank including an inner tank 3 and an outer tank 5. Note that in this specification, "cool down" means cooling the storage tank 1 before filling the storage tank 1 with the liquefied gas to be stored.

In the present embodiment described below, liquefied hydrogen at an extremely low temperature (approximately -250° C.) will be explained as an example of the liquefied gas to be stored. However, liquefied gas can also include other types of gas, such as liquefied petroleum gas (LPG, approximately -45°C), liquefied ethylene gas (LEG, approximately -100°C), liquefied natural gas (LNG, approximately -160°C), It may be helium (LHe, about −270° C.) or the like.

The storage tank 1 is installed, for example, on a ship such as a liquefied hydrogen carrier. However, the liquefied hydrogen storage facility in which the storage tank 1 is installed is not limited to this example as long as it has a structure and function capable of storing liquefied hydrogen. The liquefied hydrogen storage facility in which the storage tank 1 is installed may be, for example, a ship that uses liquefied hydrogen as a propulsion fuel, a land-based liquefied hydrogen storage facility other than a ship, or a plant that uses liquefied hydrogen. It's fine.

The storage tank 1 is configured as a double shell tank with an inner tank 3 and an outer tank 5. Specifically, the inner tank 3 has an inner tank shell that forms a storage space (hereinafter referred to as "inner tank interior space 7") for liquefied hydrogen to be stored, and an outer circumferential surface of the inner tank shell. It has an inner tank heat insulation layer that covers the inner tank. The outer tank 5 has an outer tank shell that forms an inter-inner/outer tank space 9 that is a heat insulating layer between it and the inner tank 3, and an outer tank heat insulating layer that covers the outer peripheral surface of the outer tank shell. Note that the locations where the heat insulating layers of the inner tank 3 and the outer tank 5 are installed are not limited to this example, but are arbitrary. For example, the heat insulating layers may be installed so as to cover the inner circumferential surface of the outer tank shell. Moreover, one or both of the heat-insulating layers of the inner tank 3 and the outer tank 5 may be omitted. This storage tank 1 is normally operated with low-temperature hydrogen gas sealed in the space 9 between the inner and outer tanks, which is a heat insulating layer.

In this embodiment, a communication passage 11 is provided that communicates the inner tank space 7 with the inner and outer tank space 9. The communication path 11 is configured to be openable and closable. In the illustrated example, specifically, a vaporized gas discharge passage 13 that discharges vaporized gas of liquefied hydrogen (hereinafter simply referred to as “vaporized gas”) G1 generated in the inner tank space 7 to the outside of the storage tank 1 and a hydrogen gas introduction passage 15 that introduces hydrogen gas (hereinafter referred to as "external hydrogen gas") G2 from a hydrogen gas source (not shown) provided outside the storage tank 1 into the space 9 between the inner and outer tanks. A connecting passage 17 connecting the vaporized gas discharge passage 13 and the hydrogen gas introduction passage 15 is provided outside the storage tank 1. A communication passage 11 is formed by the vaporized gas discharge passage 13, the connection passage 17, and the hydrogen gas introduction passage 15. Further, an on-off valve 19 is provided in the communication passage 11, and the communication passage 11 is configured to be openable and closable by this on-off valve 19. In this example, the on-off valve 19 is provided in a portion of the hydrogen gas introduction passage 15 downstream of the connection point with the connection passage 17, but the position and number of the on-off valves 19 are not limited to this example. Further, the on-off valve 19 may be a valve that can be opened and closed manually, or may be a valve that is automatically opened and closed according to a set differential pressure.

Note that the specific configuration of the communication path 11 between the inner tank interior space 7 and the outer and outer tank space 9, and the specific configuration that allows the communication path 11 to be opened and closed are not limited to this example. Further, the above-mentioned "hydrogen gas source" may have any configuration as long as it can serve as a supply source of hydrogen gas, and is typically a tank that stores hydrogen gas, but for example, it may be a tank that stores liquefied hydrogen. It may also be a combination of a tank and a vaporizer.

In this embodiment, the communication path 11 is provided with a temperature adjustment device 31 that adjusts the temperature of the gas supplied to the space 9 between the inner and outer tanks. In the illustrated example, a device such as a compressor that forcibly feeds gas to the space 9 between the inner and outer tanks (hereinafter simply referred to as a "gas feeding device") is installed downstream of the temperature adjustment device 31 in the communication path 11. .) 33 are provided. The gas supply device 33 is a device that moves gas by applying pressure to the gas, such as a turbo or positive displacement compressor, blower, or fan. In the illustrated example, the connection passage 17 is provided with a temperature adjustment device 31 and a gas supply device 33. In this example, the temperature adjustment device 31 is installed upstream of the gas supply device 33, but the arrangement of the temperature adjustment device 31 is not limited to this example. For example, the temperature adjustment device 31 is installed upstream of the gas supply device 33. It may be installed on the downstream side, or it may be installed on both the upstream side and the downstream side.

Further, in this embodiment, an inner tank temperature detection device 21 that detects the temperature of the inner tank 3, an inner tank space pressure detection device 23 that monitors the pressure of the inner tank space 7, and a temperature detection device 23 that detects the temperature of the space 9 between the inner and outer tanks. The inner and outer tank space temperature detection device 25 detects the pressure in the inner and outer tank space 9, and the inner and outer tank space pressure sensor 27 detects the pressure in the inner and outer tank space 9. These detection devices consist of a sensor element that detects the physical quantity to be detected (temperature, pressure), various circuits that perform necessary processing such as signal conversion processing and arithmetic processing on the obtained detected quantity, and information necessary for these processing. The device is equipped with a memory for storing the data, a power source circuit such as a power supply element such as a battery or a power supply circuit for receiving power supply from the outside, a transmission circuit for transmitting the output signal to the outside by wire or wirelessly, and the like. Note that as such a temperature sensing device and a pressure sensing device, a device that measures parts other than those described above, such as an inner tank temperature sensing device or an outer tank temperature sensing device, may be provided. Moreover, only the necessary temperature sensing devices and pressure sensing devices may be provided depending on the embodiment of the cool-down method described later.

A method for cooling down the storage tank 1 configured as described above will be described in detail below.

In this embodiment, in the storage tank 1 in the initial state at the time of starting the cool-down shown in FIG. In each of the spaces 9, hydrogen gas exists at room temperature and atmospheric pressure (0 kPaG), for example. However, the temperature and pressure of the hydrogen gas in the initial state are not limited to room temperature and atmospheric pressure. From this state, as shown in FIG. 2B, liquefied hydrogen for cooling (hereinafter simply referred to as "hydrogen for cooling") CH is introduced into the inner tank space 7. In this example, cooling hydrogen CH is sprayed into the inner tank space 7 using the sprayer 29 .

By continuing to spray the cooling hydrogen CH in this state, the temperature of the inner tank 3 is lowered. As the temperature of the inner tank 3 decreases, the temperature of the space 9 between the inner and outer tanks also decreases. Further, in the inner tank space 7, vaporized gas G1 in which the cooling hydrogen CH is vaporized is generated, and the pressure increases, while in the inner tank space 9, the pressure decreases due to the temperature drop. Since the communication passage 11 is opened as described above, the vaporized gas G1 generated in the inner tank inner space 7 flows into the inner and outer tank space 9 via the communication passage 11 due to the pressure difference between both spaces 7 and 9. do.

As the cool-down progresses in this way, the vaporized hydrogen gas G1 may reach an extremely low temperature of about 20K, so the extremely low-temperature vaporized gas G1 flows through the inner tank 3, which forms the space 9 between the inner and outer tanks, and the outer tank. Preferably, rapid local cooling of the bath 5 is avoided. In addition, since the set temperature of the outer tank 5 during steady operation is higher than the set temperature of the inner tank 3, the design temperature of the outer tank 5 is higher than the design temperature of the inner tank 3, so even during the cool-down period, the It is preferable to prevent the temperature of the tank 5 from becoming lower than the design temperature. Therefore, in the present embodiment, after starting the introduction of the cooling gas CH, the temperature adjusting gas TG adjusted to the adjusting temperature range that is a predetermined temperature or higher is introduced into the space 9 between the inner and outer tanks via the temperature adjusting device 31. supply Specifically, the vaporized gas G1 is supplied to the space 9 between the inner and outer tanks via the temperature adjustment device 31 as the temperature adjustment gas TG.

More specifically, in this embodiment, when the temperature of the vaporized gas G1 is below a predetermined temperature, the vaporized gas G1 is supplied to the space 9 between the inner and outer tanks as the temperature adjustment gas TG. Specifically, the vaporized gas G1 is heated by the temperature adjustment device 31 to a temperature equal to or higher than a predetermined temperature. On the other hand, when the temperature of the vaporized gas G1 exceeds the predetermined temperature, the vaporized gas G1 is directly supplied to the space 9 between the inner and outer tanks. For example, when the cooling of the inner tank 3 has not progressed, the vaporized gas G1 is at a relatively high temperature, so the vaporized gas G1 is directly supplied to the space 9 between the inner and outer tanks, and when the cooling of the inner tank 3 has progressed, Since the temperature of the vaporized gas G1 is decreasing, the vaporized gas G1 is heated by the temperature adjustment device 31.

Here, "the vaporized gas is directly supplied to the space between the inner and outer tanks" means that the vaporized gas G1 is supplied to the space between the inner and outer tanks 9 without forcibly adjusting the temperature. That is, for example, the vaporized gas G1 may be passed through the temperature adjustment device 31 with the temperature adjustment function of the temperature adjustment device 31 turned off, or the temperature adjustment device 31 may be bypassed through the communication path 11 as shown in FIG. 2C. A bypass passage 35 and a flow path switching device 37 may be provided to allow the vaporized gas G1 to pass through the bypass passage 35. In the example shown in the figure, the bypass passage 35 is provided so as to bypass the temperature adjustment device 31 and the gas supply device 33; may be provided so as to pass through.

The temperature adjustment gas TG may be forcibly supplied to the space 9 between the inner and outer tanks by the gas supply device 33. In that case, the temperature adjustment gas TG forcibly supplied to the space 9 between the inner and outer tanks is discharged to the outside of the storage tank 1 via the discharge passage 39, for example. For example, the discharge passage 39 has an entrance near the bottom of the space 9 between the inner and outer tanks, passes through the space 9 between the inner and outer tanks, and discharges the storage tank 1 from an outlet located at the upper part of the space 9 between the inner and outer tanks, for example, near the top. The cooling gas CG is discharged to the outside. The discharge passage 39 may be appropriately installed with a gas compressor or an exhaust pump for discharge.

The space 9 between the inner and outer tanks may be provided with upper and lower safety valves to prevent the pressure in the storage tank 1 from exceeding an allowable range. Further, by controlling the gas supply device 33, the discharge passage 39, the on-off valve 19, etc. by the control device, the gas supply to the space 9 between the inner and outer tanks and the exhaust from the space 9 between the inner and outer tanks are controlled according to the pressure. It's okay.

Instead of or in addition to supplying the vaporized gas G1, external hydrogen gas G2 may be supplied from the hydrogen gas introduction passage 15 to the space 9 between the inner and outer tanks as the temperature adjustment gas TG. In this case, as shown as a modification in FIG. 2D, the temperature adjustment device 31 may be provided on the hydrogen gas introduction passage 15, and a bypass passage 15a may be provided for when the temperature adjustment device 31 is not used. Furthermore, when supplying the external hydrogen gas G2 to the space 9 between the inner and outer tanks as the temperature adjustment gas TG, the supply of the temperature adjustment gas TG is performed before the introduction of the cooling hydrogen CH into the inner tank space 7. Good too.

In this embodiment, the above-mentioned "predetermined temperature" is the set temperature (for example, 110 K) of the space 9 between the inner and outer tanks during steady operation of the storage tank 1. Further, the above-mentioned "adjusted temperature range" is set within a range that does not impede cooling of the space 9 between the inner and outer tanks, and is, for example, 110K or more and 120K or less. As the temperature of the vaporized gas G1, which serves as a reference for temperature adjustment, a value obtained by directly measuring the temperature of the vaporized gas G1 flowing through the communication path 11 may be used, or a value obtained by measuring the temperature of the inner tank 3 may be used. Good too.

Note that if the temperature of the vaporized gas G1 and/or the external hydrogen gas G2 introduced into the space 9 between the inner and outer tanks is not low enough, the temperature difference between the inner tank and the outer tank will become too large during the cool-down process, causing damage to the components. The difference in the amount of heat shrinkage may become excessive. In order to avoid this, the cooling rate of the outer tank 5 is adjusted by adjusting the temperature of the temperature adjustment gas TG and the flow rate of the temperature adjustment gas TG using the temperature adjustment device 31, and the temperature of the inner tank 3 and the outer tank 5 is adjusted. Cooldown may be performed while controlling the difference to be less than or equal to a predetermined value. The predetermined value of the temperature difference between the inner tank 3 and the outer tank 5 is, for example, a value set as the temperature difference at the start of normal operation of the storage tank.

The temperature adjustment device 31 used in this embodiment may be any device as long as it has a function of adjusting the temperature of the liquefied gas. For example, the temperature adjustment device 31 includes a temperature sensor that detects gas temperature, an electrically driven heater, a switch that turns on and off the heater, and a control circuit that controls these. However, the temperature adjustment device 31 may be a device having a configuration other than the above, such as a heat exchanger or a device that adjusts the temperature by mixing gases having different temperatures.

In this embodiment, an example has been described in which the communication passage 11 is opened from the start of cool-down, that is, from the start of spraying of the cooling hydrogen CH, but the timing of opening the communication passage is not limited to this. That is, spraying of the cooling hydrogen CH is started with the communication passage 11 closed, and if necessary, such as when the pressure in the space 9 between the inner and outer tanks falls below a predetermined value, the communication passage 11 is opened to spray the vaporized gas G1. may be supplied, and at that time, the vaporized gas G1 may be supplied to the space 9 between the inner and outer tanks as the temperature adjustment gas TG.

In this way, by supplying the temperature-adjusted temperature-adjusted gas TG to the space 9 between the inner and outer tanks while cooling the inner tank 3 with the cooling hydrogen CH, the inner tank 3 and the outer tank 5 can be locally and rapidly cooled. Cooling of the space 9 between the inner and outer tanks and the outer tank 5 is promoted while avoiding cooling and the temperature of the outer tank 5 from becoming lower than the design temperature. Therefore, compared to the case where only the inner tank 3 is cooled, the time required for cooling down the entire storage tank 1 can be shortened and costs can be suppressed.

Thereafter, as shown in FIG. 2E, when the temperature of the inner tank 3 and the temperature of the space 9 between the inner and outer tanks have respectively decreased to the target temperatures, if the communication passage 11 is open, it is closed, and the cooling The spraying of hydrogen CH is stopped to complete the cool-down.

In addition, in this embodiment, as shown in FIG. 1, a deflection plate 41 that deflects the temperature adjustment gas TG introduced into the space 9 between the inner and outer tanks is provided. Specifically, as shown in FIG. 3, the deflection plate 41 is arranged at a position facing the supply port 43 of the temperature adjustment gas TG in the space 9 between the inner and outer tanks so as to be substantially perpendicular to the outflow direction of the temperature adjustment gas TG. It is located. The deflection plate 41 is provided from the outer circumferential surface of the inner tub 3 or the inner circumferential surface of the outer tub 5 via a supporting member (not shown) protruding from the outer circumferential surface of the inner tub 3 or the inner circumferential surface of the outer tub 5. supported at a distance. The temperature adjusting gas TG flowing out from the supply port 43 collides with the deflection plate 41, is dispersed in each direction along the surface of the deflection plate 41, and then diffuses into the space 9 between the inner and outer tanks. In this way, by providing the deflection plate 41, the temperature adjusting gas TG flowing out from the supply port 43 collides with a concentrated portion of the inner tank 3 or the outer tank 5 constituting the storage tank 1, and the said part is locally This prevents the product from being cooled down. The number of supply ports 43 to the space 9 between the inner and outer tanks is not limited to the one illustrated, but may be plural. When the number of supply ports 43 is plural, the deflection plate 41 may be provided for all the supply ports 43 or some of the supply ports 43 of the plurality of supply ports 43, respectively. Further, the shape of the deflection plate 41 may be any shape that can disperse the temperature adjustment gas TG flowing out from the supply port 43, and is not limited to the illustrated flat shape. However, the deflection plate 41 may be omitted. Furthermore, the gas blown onto the deflection plate 41 is not limited to the cooling gas CG, and may be any temperature gas introduced into the space 9 between the inner and outer tanks.

In this embodiment, an example has been described in which cool down is performed using liquefied hydrogen for cooling, but cool down may be performed using liquefied gas other than liquefied hydrogen. For example, the cooldown may be performed in stages, such as introducing liquefied nitrogen from a state in which air is present in the inner tank 3, and then replacing the nitrogen with hydrogen.

Although FIG. 1 shows an independent double-shell tank formed independently of the ship's hull as an example of the storage tank 1, the cool-down method according to the present embodiment is not limited to this example. , can be applied to any type of storage tank. For example, the cool-down method according to the present embodiment can also be applied to a type of storage tank that is formed integrally with the hull. Further, the multiple structure of the storage tank may be a triple structure or more, and the cool-down method according to the present embodiment can be applied to the internal tank space and any other inter-tank space of such a multiple structure. can.

The cool down according to the present embodiment is typically performed, for example, after construction of the storage tank 1, after construction of liquefied gas storage equipment such as a ship in which the storage tank 1 is installed, or after the construction of the equipment or the storage tank 1. Maintenance is carried out after warming up the storage tank 1 and before loading it again. However, the cool-down method according to the present embodiment is also applied when carrying out an empty voyage (ballast voyage) after unloading the liquefied gas in the storage tank 1 when the storage tank 1 is installed on a ship. be able to. That is, during a ballast voyage, the temperature of the inner tank 3 may gradually rise, and in that case, the above-mentioned cool-down method can be applied. In addition, when cooling down the storage tank 1 during a ballast voyage, for example, the liquefied gas left in the inner tank 3 for cooling down without unloading is used to cool down the pumps etc. installed in the inner tank 3. A feeding device transports the liquefied gas to the top of the tank for cooling down. When a plurality of storage tanks 1 are installed, liquefied gas or vaporized gas for cooling down may be supplied from other storage tanks 1.

According to the cool-down method according to the present embodiment described above, while cooling the inner tank 3 with cooling hydrogen CH, the temperature-adjusted temperature gas TG is supplied to the space 9 between the inner and outer tanks. Cooling of the space 9 between the inner and outer tanks and the outer tank 5 is promoted while avoiding rapid local cooling of the inner and outer tank 3 and the outer tank 5 and preventing the temperature of the outer tank 5 from becoming lower than the design temperature. Therefore, compared to the case where only the inner tank 3 is cooled, the time required for cooling down the entire storage tank 1 can be shortened and costs can be suppressed.

In the cool-down method according to the present embodiment, vaporized gas generated in the inner tank space may be introduced into the temperature adjustment device 31 and supplied to the inner and outer tank space 9 as the temperature adjustment gas TG. Thereby, the temperature adjustment gas TG can be supplied to the space 9 between the inner and outer tanks with a simple structure and at low cost by utilizing the cooling liquefied gas.

In the cool-down method according to the present embodiment, when the temperature of the vaporized gas G1 is below the predetermined temperature, the vaporized gas G1 may be supplied to the space 9 between the inner and outer tanks as the temperature adjustment gas TG. This prevents the inner tank 3 and the outer tank 5 from being excessively cooled, for example, with respect to the designed set temperature.

In the cool-down method according to the present embodiment, a deflection plate may be provided in the space 9 between the inner and outer tanks, and the temperature adjustment gas TG may be injected toward the deflection plate. According to this configuration, the temperature adjustment gas TG is prevented from colliding in a concentrated manner with a part of the inner tank 3 or the outer tank 5 constituting the storage tank 1, thereby preventing the part from being locally cooled.

As described above, the preferred embodiments of the present disclosure have been described with reference to the drawings, but various additions, changes, or deletions can be made without departing from the spirit of the present disclosure. Accordingly, such are also included within the scope of this disclosure.

1 Liquefied gas storage tank 3 Inner tank 5 Outer tank 7 Inner tank space 9 Space between the inner and outer tanks 11 Communication path 29 Sprayer 31 Temperature adjustment device 41 Deflection plate CH Cooling liquefied gas G1 Vaporized gas G2 External hydrogen gas TG Temperature adjustment gas

Claims (4)

A method for cooling a tank including an inner tank and an outer tank for storing liquefied gas before filling it with the liquefied gas to be stored, the method comprising:
Introducing a cooling liquefied gas into the inner tank space;
Supplying a temperature adjustment gas adjusted to a adjustment temperature range that is a predetermined temperature or higher to a space between the inner and outer tanks via a temperature adjustment device;
including,
How to cool down a liquefied gas storage tank. The cool-down method according to claim 1,
Supplying the temperature adjustment gas includes introducing vaporized gas generated in the inner tank space into the temperature adjustment device and supplying it as the temperature adjustment gas to the space between the inner and outer tanks.
How to cool down. The cool-down method according to claim 2,
supplying the vaporized gas as the temperature adjustment gas to the space between the inner and outer tanks when the temperature of the vaporized gas is below the predetermined temperature;
How to cool down. The cool-down method according to any one of claims 1 to 3,
A deflection plate is provided in the space between the inner and outer tanks, and the temperature adjustment gas is injected toward the deflection plate.
How to cool down.

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