Coaxial sleeve of ground source heat pump and preparation method of coaxial sleeve

Abstract

The invention discloses a coaxial sleeve of a ground source heat pump and a preparation method of the coaxial sleeve, and belongs to the field of ground source heat pumps. The coaxial sleeve of the ground source heat pump comprises an outer pipe and an inner pipe which are coaxially arranged in a sleeved mode, the inner wall of the outer pipe is coated with a hydrophilic coating, and the outer wall and the inner wall of the inner pipe are both coated with hydrophobic coatings; the hydrophilic coating is a hydrophilic coating prepared from silicon dioxide sol; the hydrophobic coating is a hydrophobic coating prepared from a suspension containing hydrophobic silica nanoparticles. The coaxial sleeve can strengthen heat extraction from a stratum, weaken heat transfer between the inner pipe and the outer pipe, reduce heat loss and enable the water outlet temperature to be increased by 0.5-3.0 DEG C compared with the water inlet temperature in application.

Description

Technical Field

[0001] The invention belongs to the technical field of ground source heat pumps, and in particular relates to a coaxial sleeve of a ground source heat pump and a preparation method thereof. Background Art

[0002] The ground source heat pump uses ground source energy (soil, groundwater, etc.) as the cooling source for summer cooling and the low temperature heat source for winter heating. It is also a system that realizes heating, cooling and domestic hot water. The coaxial shell and tube heat exchanger is a commonly used device for underground heat exchange in ground source heat pumps. The coaxial shell and tube heat exchanger used in ground source heat pumps takes heat from the ground, such as Figure 1 As shown, when taking heat from underground, low-temperature water is injected from the annular cavity 5 between the outer tube 1 and the inner tube 2. The water continuously absorbs heat during its downward flow, and the heat from the rock layer 6 is transferred to the water through the outer tube wall. At the bottom of the tube, the water temperature reaches the highest. After reaching the bottom of the tube, the water goes up through the inner tube 2 to the ground. The water in the inner tube 2 is high-temperature water, and is used for heating after reaching the ground.

[0003] However, the water temperature in the inner tube is higher than that in the annular cavity. Therefore, it is inevitable that the heat in the inner tube will be transferred to the annular cavity. After the water in the annular cavity obtains the heat from the inner tube, the temperature rises, and the temperature difference between the water in the annular cavity and the external rock and soil decreases, the driving force for absorbing heat from the outside decreases, and the amount of heat absorbed decreases. In engineering applications, it is always hoped that the water in the annular cavity absorbs more heat from the outside, that is, to obtain a larger amount of heat; at the same time, it is hoped that the water in the inner tube transfers less heat to the annular cavity, that is, to obtain a higher water outlet temperature. In the prior art, the inner wall of the outer tube is usually added with a structure that can enhance heat transfer (such as fins, spiral structures, etc.), which can increase the amount of heat taken from the outside, but the flow resistance of the water will increase, thereby increasing the power consumption of the water pump; or the inner tube is insulated, or the inner tube is made of a material with a small thermal conductivity coefficient, which can reduce the heat loss of the inner tube, but it will greatly increase the cost of the inner tube.

[0004] Therefore, a method with low cost investment and no additional increase in heat transfer structure and water pump power consumption is needed to solve this technical problem. Summary of the invention

[0005] In order to solve the problem that the coaxial sleeve of the existing ground source heat pump is provided with a heat extraction structure on the outer pipe to obtain more heat, but the heat extraction structure will increase the water flow resistance and the modification cost of the inner pipe is relatively high, the present invention provides a coaxial sleeve of a ground source heat pump and a preparation method thereof, which can enhance heat extraction and reduce heat loss.

[0006] In order to achieve the above object, the present invention provides the following technical solutions: The present invention provides a coaxial sleeve of a ground source heat pump, comprising an outer tube and an inner tube coaxially sleeved, wherein the inner wall of the outer tube is provided with a hydrophilic coating, and the outer wall and the inner wall of the inner tube are both provided with a hydrophobic coating; The hydrophilic coating is a hydrophilic coating made of silica sol; the hydrophobic coating is a hydrophobic coating made of a suspension containing hydrophobic silica nanoparticles. The thickness of the hydrophobic coating is 5 to 50 μm. The thickness of the hydrophilic coating is 0.5-20 μm.

[0007] The present invention also provides a method for preparing a coaxial casing of a ground source heat pump, the method being used to prepare the above coaxial casing, the method comprising: coating or spraying the silica sol on the inner surface of the outer tube to obtain the outer tube coated with a hydrophilic coating; Adding hydrophobic silica nanoparticles into anhydrous ethanol and dispersing them evenly to form a suspension; Spraying a resin adhesive on the outer surface and the inner surface of the inner tube, and then immersing the inner tube in the suspension, and after the immersion, standing the inner tube to dry at room temperature to obtain an inner tube coated with a hydrophobic coating; The inner pipe coated with a hydrophobic coating is placed in the outer pipe coated with a hydrophilic coating, and the pipe sections are connected by a connecting unit to obtain a ground source heat pump placed in a coaxial casing.

[0008] The silica sol is prepared by adding tetraethoxysilane and anhydrous ethanol into a mixture of deionized water and ammonia water, and stirring the mixture at room temperature.

[0009] The mass ratio of tetraethoxysilane to water is 1:(1-2). The mass concentration of the hydrophobic silica particles in the suspension is 2% to 5%.

[0010] The inner tube coated with the resin adhesive is immersed in the suspension for not less than 1 hour.

[0011] The resin adhesive is one of hydrocarbon resin, epoxy resin, polyurethane resin and phenolic resin.

[0012] After the inner tube is taken out from the suspension, it is allowed to stand at room temperature for not less than 24 hours.

[0013] Compared with the prior art, the present invention has the following beneficial effects: The coaxial sleeve of the ground source heat pump provided by the present invention can strengthen the heat extraction from the stratum, weaken the heat transfer between the inner tube and the outer tube, and reduce heat loss. The present invention arranges a hydrophilic coating on the inner wall surface of the outer tube, so that the cold water has a larger contact area with the inner wall surface of the outer tube when flowing in the annular cavity, thereby strengthening the heat exchange and absorbing more heat from the external rock and soil. Moreover, the method of coating the hydrophilic coating can greatly reduce the resistance to water flow compared with the use of a special structure; a hydrophobic coating is arranged on the outer wall surface and the inner wall surface of the inner tube, so that the contact area of ​​the hot water with the corresponding wall surface when flowing is small, thereby weakening the heat transfer, reducing the heat transfer, and reducing the flow resistance of the water in the inner tube and the annular cavity. In addition, by arranging the coating on the surface of the outer tube and the inner tube, not only the heat transfer of the coaxial sleeve of the ground source heat pump is improved, but also the cost investment can be reduced. The coaxial sleeve of the ground source heat pump provided by the present invention can increase the outlet water temperature by 0.5-3.0℃ compared with the inlet water temperature in application.

[0014] The preparation method of the coaxial sleeve provided by the present invention is to coat the surface of the pipeline with a hydrophilic coating or a hydrophobic coating, the source of the hydrophilic coating or the hydrophobic coating is simple and easy to obtain, the overall preparation process is relatively simple, easy to implement in an industrial environment, and low in cost. The method can also be extended to pipelines in other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solution of the embodiment of the present invention, the following briefly introduces the drawings required for use in the embodiment: Figure 1 It is a working principle diagram of a coaxial shell-and-tube heat exchanger of a prior art ground source heat pump; Figure 2 Schematic diagram of surface wettability, wherein (a) is a schematic diagram of the contact angle of a superhydrophilic surface, (b) is a schematic diagram of the contact angle of a hydrophilic surface, (c) is a schematic diagram of the contact angle of a hydrophobic surface, and (d) is a schematic diagram of the contact angle of a superhydrophobic surface; Figure 3 It is a structural schematic diagram of the coaxial sleeve of the present invention; Figure 4 It is a schematic diagram of the method for preparing the outer tube coating of the present invention; Figure 5 It is a schematic diagram of the method for preparing the inner tube coating of the present invention; Among them, 1. outer tube; 2. inner tube; 3. hydrophilic coating; 4. hydrophobic coating; 5. annular cavity; 6. rock and soil. DETAILED DESCRIPTION

[0016] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention described and shown in the drawings here can usually be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0017] like Figure 3 As shown, the present invention provides a coaxial sleeve of a ground source heat pump, comprising: an outer tube 1 and an inner tube 2 which are coaxially sleeved, the inner wall of the outer tube 1 being solution coated or sprayed with a hydrophilic coating 3, and the inner and outer walls of the inner tube 2 being solution coated or sprayed with a hydrophobic coating 4.

[0018] The hydrophilic coating 3 is a hydrophilic coating made of silica sol, and the thickness of the hydrophilic coating 3 is 0.5-20 μm. Silica sol is usually hydrophilic because there are a large number of silanol groups (-SiOH) on the surface of silica, and these silanol groups can form hydrogen bonds with water molecules, thereby showing hydrophilicity. The hydrophilic coating can make the cold water flow in the annular cavity more closely fit the inner wall of the outer tube, reducing the air gap between the water film and the wall, thereby effectively increasing the heat exchange area. Since the hydrophilic coating reduces the thermal resistance between water and the wall, heat can be transferred from the rock and soil through the outer tube wall to the cold water faster, improving the overall heat exchange efficiency of the system. Strengthening the heat exchange effect enables the system to absorb more heat from the rock and soil with lower energy consumption, thereby improving energy utilization efficiency. The thickness of the hydrophilic coating 3 is set at 0.5-20 μm. The appropriate coating thickness helps to improve the adhesion between the coating and the inner wall of the pipe and prevent the coating from falling off or peeling off during use.

[0019] The hydrophilicity or hydrophobicity of silica sol depends on the chemical properties of the silica particle surface. Silica nanoparticles are inherently hydrophilic. This is because the silica surface is rich in silanol groups (-SiOH), which can form hydrogen bonds with water molecules, making the silica particles easy to disperse in water and showing hydrophilicity. However, the surface properties of silica nanoparticles can be adjusted by chemical modification, thereby converting hydrophilic silica into hydrophobic. This modification usually involves reacting the silanol groups on the silica surface with hydrophobic compounds (such as silanes) to replace the original hydroxyl groups with hydrophobic groups (such as long-chain alkyl groups), so that the treated silica nanoparticles can better disperse in non-polar or organic solvents and show hydrophobicity. Therefore, unmodified silica nanoparticles are hydrophilic, while surface-modified ones can become hydrophobic, depending on their specific application requirements.

[0020] The hydrophobic coating 4 is a hydrophobic coating made of a suspension containing hydrophobic silicon dioxide nanoparticles, and the thickness of the hydrophobic coating 4 is 5 to 50 μm. The hydrophobic coating can reduce the contact area between water and the wall during the flow process, weaken the heat transfer by reducing the contact area between the hot water and the pipe wall, and reduce the heat transfer from the water in the inner pipe to the water in the outer pipe, thereby reducing heat loss. The thickness of the hydrophobic coating 4 is set at 5 to 50 μm, which can not only ensure the effect of the hydrophobic coating on weakening heat transfer, but also ensure the adhesion of the coating to the inner pipe wall.

[0021] The present invention provides a coaxial sleeve of a ground source heat pump, wherein a hydrophilic coating is used on the inner wall of an outer pipe, which not only improves the absorption of heat from the outside by water in the pipe, but also eliminates the need to provide a mechanical structure for enhancing heat transfer, thereby avoiding obstruction of water flow. A hydrophobic coating is provided on both the inner and outer walls of the inner pipe to reduce heat transfer and reduce the cost of inner pipe modification.

[0022] like Figure 4 and Figure 5 As shown, the present invention also provides a method for preparing a coaxial casing of a ground source heat pump, and the preparation method is used for the above coaxial casing, comprising the following steps: Step 1: preparing silica sol by hydrolyzing tetraethoxysilane in water, wherein the mass ratio of tetraethoxysilane to water is 1: (1-2); Step 2: coating the silica sol prepared in step 1 on the inner surface of the outer tube 1 to obtain a hydrophilic coating 3; Step 3: Add hydrophobic silica nanoparticles into anhydrous ethanol, and use ultrasound to evenly distribute the nanoparticles in the suspension, so that the mass concentration of the hydrophobic silica particles in the suspension is 2% to 5%; Step 4: spraying resin adhesive on the outer surface and inner surface of the cleaned inner tube 2; Step 5: immerse the inner tube 2 sprayed with the resin adhesive in step 4 in the suspension prepared in step 3 and let it stand for no less than 1 hour; Step 6: Take out the inner tube 2 prepared in step 5, and after standing and drying at room temperature, a hydrophobic coating 4 can be obtained on the inner and outer wall surfaces of the inner tube 2; Step 7: Place the inner tube 2 prepared in step 6 into the outer tube 1 prepared in step 2, install the connecting unit between the pipe sections, and complete the production of the coaxial casing of the ground source heat pump.

[0023] In some embodiments, in step 1, the silica sol is prepared by adding tetraethoxysilane and anhydrous ethanol to a mixture of deionized water and ammonia water, and stirring at room temperature, and the mass ratio of tetraethoxysilane to water is 1: (1-2). The method for preparing the silica sol is simple, and the hydrophilic coating obtained by this method has good hydrophilic properties.

[0024] In some embodiments, in step 2, when the silica sol is coated on the outer tube 1, the silica sol repeatedly flows through the inner wall of the outer tube 1, so that the inner wall of the outer tube 1 obtains a hydrophilic coating. The silica sol is repeatedly flowed through the inner wall of the outer tube 1, so that the silica sol can be evenly distributed on the inner wall of the outer tube 1, and a hydrophilic coating with a uniform texture is obtained.

[0025] In some embodiments, the mass concentration of the hydrophobic silica particles in the suspension is between 2% and 5%; appropriate particle concentration ensures uniform distribution of the particles in the coating, avoids performance degradation caused by particle agglomeration, can improve the hydrophobic properties of the hydrophobic coating, and also improves the mechanical properties of the hydrophobic coating.

[0026] In some embodiments, in step 6, after the inner tube 2 is taken out from the suspension, it is left to stand at room temperature for not less than 24 hours, which can ensure the formation of a hydrophobic coating with uniform texture and excellent effect.

[0027] In some embodiments, the resin adhesive is one of hydrocarbon resin, epoxy resin, polyurethane resin and phenolic resin. As a bottom layer, the resin adhesive can enhance the adhesion between the coating and the inner tube surface, prevent the coating from falling off or being damaged, and thus improve the durability of the coating.

[0028] It should be noted that the inner and outer pipes of the coaxial casing of the ground source heat pump are not a whole long pipe, but are segmented. After the geothermal well is drilled, the coaxial casing is put in segments, and there is a connecting structure between the pipe segments. When the coating is made, each section of the pipe is made separately.

[0029] It should be noted that the wettability of a solid surface refers to the ability of water to spread on the solid surface. Specifically, a drop of water is dropped on the solid surface. Figure 2As shown in the figure, the angle between the solid, liquid and gas phases is the static contact angle, or contact angle. When the contact angle is less than 90°, the surface is hydrophilic, and when it is greater than 90°, it is a hydrophobic surface. Among them, contact angles less than 30° (or less than 10°) and greater than 150° are called superhydrophilic surfaces and superhydrophobic surfaces, respectively. When water flows on a hydrophilic surface, water tends to spread on the surface, and the contact area with the surface is large. A larger contact area is conducive to heat transfer; when it flows on a hydrophobic surface, water tends to stay away from the surface, and the contact area with the surface is small. A smaller contact area will hinder heat transfer, but the water flow resistance is small. The hydrophilic and hydrophobic properties of the surface are defined and distinguished by the static contact angle of the water droplet, and the contact angle ranges from 0° to 180°. Some literatures may also have different definitions of superhydrophilic and superhydrophobic, but as long as the wettability of the tube wall is changed to achieve the purpose of strengthening / weakening heat transfer and reducing drag, they are all alternatives to this solution.

[0030] Therefore, changing the surface wettability of different pipe walls can increase the heat intake and outlet water temperature of the coaxial tube heat exchanger for ground source heat pumps, while reducing flow resistance. Specifically: The use of the hydrophilic coating 3 on the inner wall of the outer tube 1 can make the water have a larger contact area with the inner wall of the outer tube 1 when flowing in the annular cavity, thereby enhancing heat exchange and absorbing more heat from the external rock and soil. Compared with the use of special structures, this method has a smaller increase in water flow resistance.

[0031] The use of the hydrophobic coating 4 on the outer wall of the inner tube 2 and the inner wall of the inner tube can reduce the contact area between water and the corresponding wall when flowing, thereby weakening heat transfer and reducing heat transfer, while reducing the flow resistance of water in the inner tube 2 and the annular cavity.

[0032] In the following examples, unless otherwise specified, all materials used can be obtained through common channels; and the test methods adopted are conventional methods in the art.

[0033] Example 1 This embodiment is a deep ground source heat pump with a well depth of 2500 meters. The outer diameter of the outer pipe 1 is 177.8 mm, the wall thickness is 9.19 mm, and the inner diameter is 159.4 mm. The outer diameter of the inner pipe 2 is 110 mm, the wall thickness is 10 mm, and the inner diameter is 90 mm. The water inlet and outlet method of the coaxial shell and tube heat exchanger for the ground source heat pump is: water is injected from the annular cavity of the outer pipe 1 and the inner pipe 2, the water injection temperature is 10°C, and water is discharged from the inner pipe 2. The outer pipe 1 with the hydrophilic coating 3 and the inner pipe 2 with the hydrophobic coating 4 increase the wellhead temperature by 0.5-3.0°C compared with the outer pipe 1 and the inner pipe 2 without the coating.

[0034] In this embodiment, a hydrophobic coating 4 is provided on the inner wall surface and the outer wall surface of the inner tube 2 of the coaxial shell-and-tube heat exchanger, and a hydrophilic coating 3 is provided on the inner wall surface of the outer tube 1. The preparation of the hydrophilic coating 3 and the hydrophobic coating 4 is as follows: like Figure 4 As shown, the hydrophilic coating is prepared by using tetraethoxysilane (also called tetraethyl silicate, English Tetraethoxysilane or Tetraethyl orthosilicate, molecular formula C8H 20 O4Si) is hydrolyzed in an aqueous medium to prepare a silica sol, tetraethoxysilane and anhydrous ethanol are added to a mixture of deionized water and ammonia water, the mass ratio of tetraethoxysilane to water is about 1:1, and the silica sol can be prepared by stirring at room temperature, and the silica sol can be coated on the inner surface of the outer tube 1 to obtain a hydrophilic coating 3 (the hydrophilic effect of the coating is related to the concentration of the silica sol), and the silica sol is repeatedly passed through the inner surface of the outer tube 1, and the inner wall of the outer tube 1 obtains the hydrophilic coating 3.

[0035] like Figure 5 As shown, the preparation of the hydrophobic coating is as follows: hydrophobic silica nanoparticles are added to anhydrous ethanol, and ultrasound is used to evenly distribute the nanoparticles in the suspension. The mass concentration of the hydrophobic silica particles in the suspension is 5%. A layer of hydrocarbon resin adhesive is sprayed on the outer surface and inner surface of the clean inner tube 2, and then the inner tube 2 is immersed in the suspension, taken out after 2 hours, and left to stand at room temperature for 24 hours, and a hydrophobic coating 3 can be obtained on the inner and outer wall surfaces of the inner tube 2.

[0036] In this embodiment, the surface wettability of both the inner and outer tubes is changed. In some embodiments, the wettability of only the inner or outer tube wall surface may be changed.

[0037] In some embodiments, the coating material added to the tube wall may be different, but the material should have hydrophilic / hydrophobic properties, or the tube itself may be made of hydrophobic / hydrophilic material.

[0038] In the drawings of this embodiment, the thickness of the coating is for illustration only, and the coating thickness should be changed according to actual needs.

[0039] This embodiment does not limit the size of the coaxial tube heat exchanger (such as the diameters of the inner and outer tubes, the length of the tubes, etc.).

[0040] In summary, the present invention sets a hydrophilic coating on the inner wall of the outer tube so that the cold water has a larger contact area with the inner wall of the outer tube when flowing in the annular cavity, thereby enhancing heat exchange and absorbing more heat from the external rock and soil. Compared with the use of special structures, this method has a small increase in water flow resistance. Setting a hydrophobic coating on the outer wall of the inner tube and the inner wall of the inner tube can reduce the contact area between the hot water and the corresponding wall when flowing, thereby weakening heat transfer and reducing heat transfer. At the same time, the flow resistance of water in the inner tube and the annular cavity can be reduced. Therefore, the present invention has a wide range of application prospects.

[0041] The endpoints and any values ​​of the scope disclosed in the present invention are not limited to the precise scope or value, and these scopes or values ​​should be understood to include values ​​close to these scopes or values. For numerical ranges, the endpoint values ​​of each scope, the endpoint values ​​of each scope and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in this article. Hereinafter, in principle, each technical solution can be combined with each other to obtain a new technical solution, which should also be regarded as specifically disclosed in this article.

[0042] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art can still modify or make equivalent substitutions to the specific implementations of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the scope of protection of the claims of the present invention to be approved.

Claims

1. A coaxial casing of a ground source heat pump, characterized in that: It comprises an outer tube (1) and an inner tube (2) which are coaxially sleeved, the inner wall of the outer tube (1) being provided with a hydrophilic coating (3), and the outer wall and the inner wall of the inner tube (2) being provided with a hydrophobic coating (4); The hydrophilic coating (3) is a hydrophilic coating made from silica sol; and the hydrophobic coating (4) is a hydrophobic coating made from a suspension containing hydrophobic silica nanoparticles.

2. The coaxial casing of a ground source heat pump according to claim 1, characterized in that: The thickness of the hydrophobic coating (4) is 5 to 50 μm.

3. The coaxial casing of a ground source heat pump according to claim 1, characterized in that: The thickness of the hydrophilic coating (3) is 0.5 to 20 μm.

4. A method for preparing a coaxial casing of a ground source heat pump, characterized in that: The preparation method is used to prepare the coaxial sleeve according to any one of claims 1 or 2, and the preparation method comprises: Coating or spraying silica sol on the inner surface of the outer tube (1) to obtain an outer tube (1) coated with a hydrophilic coating (3); Adding hydrophobic silica nanoparticles into anhydrous ethanol and dispersing them evenly to form a suspension; Spraying a resin adhesive on the outer surface and the inner surface of the inner tube (2), then immersing the inner tube (2) in the suspension, and after the immersion, leaving the inner tube (2) to stand and dry at room temperature to obtain an inner tube (2) coated with a hydrophobic coating (4); An inner pipe (2) coated with a hydrophobic coating (4) is placed in an outer pipe (1) coated with a hydrophilic coating (3), and the pipe sections are connected via a connecting unit to obtain a ground source heat pump placed in a coaxial casing.

5. The method for preparing a coaxial casing of a ground source heat pump according to claim 4, characterized in that: The silica sol is prepared by adding tetraethoxysilane and anhydrous ethanol into a mixture of deionized water and ammonia water, and stirring the mixture at room temperature.

6. The method for preparing a coaxial casing of a ground source heat pump according to claim 5, characterized in that: The mass ratio of tetraethoxysilane to water is 1:(1-2).

7. The method for preparing a coaxial casing of a ground source heat pump according to claim 4, characterized in that: The mass concentration of the hydrophobic silica particles in the suspension is 2% to 5%.

8. The method for preparing a coaxial casing of a ground source heat pump according to claim 4, characterized in that: The resin adhesive is one of hydrocarbon resin, epoxy resin, polyurethane resin and phenolic resin.

9. The method for preparing a coaxial casing of a ground source heat pump according to claim 4, characterized in that: The inner tube (2) coated with the resin adhesive is immersed in the suspension for not less than 1 hour.

10. The method for preparing a coaxial casing of a ground source heat pump according to claim 4, characterized in that: After the inner tube (2) is taken out from the suspension, it is left to stand at room temperature for not less than 24 hours.

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