A supercritical CO 2 Phase change energy gathering test device

Abstract

The present invention discloses a supercritical CO2 phase change energy gathering test device, including a test box with a hollow interior, in which a sleeve and a clamp for clamping the sleeve are arranged; the sleeve is a metal cylinder with openings at both ends, and the fracturing tube extends into the sleeve during the test; a plurality of strain assemblies are fixedly connected to the sleeve wall; the strain assemblies and the clamp are electrically connected to the control module. The present invention replaces the blasting object rock commonly used by researchers with a metal sleeve, and transforms the problem of rock blasting into a research model of thin-walled cylinder pressure blasting, which can extend the data collection time and facilitate direct observation of the deformation of the sleeve caused by the energy gathering effect. In addition, the radial pressure applied by the clamp to the sleeve can simulate the radial pressure in different burial depths and different ground stresses, making the test closer to the actual working conditions.

Description

Technical Field

[0001] The invention relates to the technical field of industrial automatic detection devices, and in particular to a supercritical CO2 phase change energy gathering testing device. Background Art

[0002] Traditional blasting methods such as explosives often produce a large amount of toxic gases, dust and other pollutants during the blasting process, which pose a serious threat to the environment and human health. Supercritical CO2 phase change is a new type of blasting technology. This technology uses a heating tube (or fracturing tube) to quickly change the liquid-gas phase of liquid CO2 at a certain pressure and temperature, and the volume expands 600 times instantly, thereby generating a strong physical pressure (above 200MPa). In contrast, CO2 itself is non-toxic and harmless, and will not produce any harmful substances after blasting, and will not pollute the environment, which is completely in line with the modern society's pursuit of green environmental protection.

[0003] Supercritical CO2 phase change is now often used in a variety of projects such as foundation pit excavation, coal seam permeability enhancement, tunneling, and mining. This technology still has the following problems in blasting application and experimental research:

[0004] 1. The fracturing tube has an obvious energy gathering effect (hereinafter referred to as the energy gathering effect) due to its own structural reasons. At present, there is a lack of test equipment that can directly observe the rock strain caused by the energy gathering effect, and the law of the energy gathering effect with practical application significance has not yet been explored. Limited by theoretical research, this technology often cannot consider the influence of energy gathering characteristics in practical applications, resulting in the inability to improve blasting efficiency.

[0005] 2. In actual blasting projects, the fracturing tube will be buried several meters or even hundreds of meters underground depending on the actual working conditions. The rock mass is subjected to various ground stresses underground. Different borehole burial depths will directly affect the stress field of the rock mass hole wall after blasting, which will make the blasting effect very different. The current test devices all insert the fracturing tube into a through hole in a test rock, and there is a lack of test devices that take into account the influencing parameter of the borehole burial depth. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention provides a supercritical CO2 phase change energy-gathering test device, which solves the problem in the prior art that there is a lack of a test device that can intuitively observe the energy-gathering effect strain and can consider the influence of the drilling depth.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is:

[0008] A supercritical CO2 phase change energy gathering test device is provided, which comprises a test box with a hollow interior, wherein a sleeve and a clamp for clamping the sleeve are arranged in the test box; the sleeve is a metal cylinder with openings at both ends, and a fracturing tube is inserted into the sleeve during the test; a plurality of strain assemblies are fixedly connected to the sleeve wall; and the strain assemblies and the clamp are both electrically connected to a control module.

[0009] The beneficial effects of the present invention are as follows: conventional CO2 explosion test devices all use rocks as explosion objects, but the strength of rocks is relatively low, and rocks will break quickly under the action of shock waves, so researchers can only use the strain data collected before the rocks break as valid data. This means that the time window for collecting test data is extremely short, and the time for collecting valid strain data is very limited.

[0010] Based on the above considerations, the inventor replaced the rock blasting object commonly used by researchers with a metal sleeve, and transformed the rock blasting problem into the problem of thin-walled cylinder compression blasting, thereby establishing a research model. Due to the structural characteristics of the metal cylinder, its destruction process is slower than that of the rock, with a fairly long observation time, and more data about the blasting process can be recorded.

[0011] Moreover, the sleeve will not break into pieces like rock, but will leave a clear destruction pattern. By collecting and recording parameters such as the deformation and damage area of ​​the blasting metal tube with an industrial camera, the impact of the energy gathering effect on the stress field can be more accurately evaluated, providing strong support for subsequent blasting design and optimization.

[0012] Secondly, after the clamp clamps the sleeve, on the one hand, it can prevent the sleeve from moving under force, and on the other hand, it can apply radial pressure to the sleeve to simulate radial ground stress. Different clamping forces can simulate ground stress at different depths.

[0013] Furthermore, the clamp includes two clamping cylinders arranged opposite to each other, and a clamping hole penetrating the clamping cylinders is provided at the edge of the clamping cylinders. When the two clamping cylinders are close together, the two clamping holes are enclosed to form a columnar structure; a push rod is fixedly connected to the side of the clamping cylinder away from the clamping hole, the push rod is connected to the driver A, and the driver A is electrically connected to the control module.

[0014] The beneficial effects of the above technical solution are: the clamping hole is opened to fit the curved surface of the sleeve, and the clamping hole can be in close contact with the sleeve during the test. The control module drives the push rod to move by starting the driver, thereby controlling the distance between the two clamping cylinders.

[0015] Furthermore, the two opposite sides of the clamping cylinder are open ends, and the two sides of the open ends are covered with elastic diaphragms, which are sealed and connected to the clamping cylinder; a piston is arranged inside the clamping cylinder to divide the internal cavity into two independent chambers, and the chamber close to the elastic diaphragm is a working chamber filled with liquid, and a piston rod fixedly connected to the piston extends from the other chamber of the clamping cylinder and is connected to the driver B, and the driver B is electrically connected to the control module.

[0016] The beneficial effect of the above technical solution is that the elastic diaphragm under pressure will produce elastic deformation and tightly wrap the sleeve, which can not only clamp the sleeve to prevent the sleeve from being displaced by shock waves, but also simulate the confining pressure effect in the ground stress, so that the sleeve is subjected to uniform or non-uniform radial pressure.

[0017] Furthermore, the clamping hole is a cylindrical structure with a semi-elliptical cross section, and the push rod and the piston rod both extend out of the test box. The cross section of the clamping hole is set to be semi-elliptical instead of circular because it can improve the adaptability and facilitate clamping of sleeves of different diameters to study the factors affecting the energy-gathering effect of sleeves of different diameters.

[0018] Furthermore, a pressure gauge is provided on the clamping cylinder. When the pressure gauge displays a reading When , the control module controls driver B to close.

[0019] Among them, a r is the preset radial clamping force of the clamp on the sleeve; π is the pi ratio; r is the outer radius of the sleeve; L is the length of the elastic diaphragm wrapped around the sleeve; P is the reading of the pressure gauge.

[0020] Furthermore, several strain assemblies are evenly arranged along the length direction of the sleeve, and one strain assembly includes several strain gauges evenly distributed along the circumferential direction of the sleeve wall. The strain gauges are all pasted on the inner wall of the sleeve, and the position where the strain gauges are pasted does not exceed the range of the elastic diaphragm wrapping the sleeve. The strain gauge is pasted on the inner wall of the sleeve to prevent the clamp from touching the strain gauge during clamping, causing damage or falling off of the strain gauge, resulting in data collection failure. The installation position of the strain gauge can ensure that the strain collection point coincides with the ground stress simulation point.

[0021] Furthermore, a fixing column with a threaded outer wall is fixedly connected to the bottom plate of the test box, the fixing column passes through the bottom plate of the sleeve and extends into the sleeve, a threaded hole matching with the fixing column is provided at the bottom of the fracturing tube, and the fixing column is threadedly connected with the fracturing tube during the test. Connecting the fracturing tube with the fixing column can limit the displacement of the fracturing tube, and prevent the impact force of the fracturing tube from causing the fracturing tube to detach from the test box and fly off the tube, thereby affecting the test results or causing safety accidents.

[0022] Furthermore, a through hole is provided on the top plate of the test box for the fracturing tube to extend into; the test device also includes a limit support, which is fixed to the outer wall of the fracturing tube and contacts the lower surface of the top plate when in use; the surface area of ​​the limit support is larger than the cross-sectional area of ​​the through hole. The limit support can further prevent the fracturing tube from moving upward.

[0023] Furthermore, an industrial camera electrically connected to the control module is provided on one side of the test box, and a transparent plate is provided on the side of the test box close to the industrial camera. The industrial camera can collect images of the conditions in the test box in real time and upload them to a computer, so that the test personnel can observe the test conditions. After the blasting, three-dimensional scanning can be used to measure the deformation of the sleeve.

[0024] Furthermore, the test box includes a rectangular frame structure formed by angle steels, a bottom plate and side panels fixed on the angle steels, and a top plate is detachably connected to the angle steels through fixings to facilitate installation or removal of the fracturing tube with a limited support. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of a supercritical CO2 phase change energy testing device;

[0026] Figure 2 It is a top view when the clamp is not clamping the sleeve in the non-test state;

[0027] Figure 3 This is a top view of the cross section when the clamp is clamping the sleeve during the test;

[0028] Figure 4 This is a front cross-sectional view of the clamp clamping the sleeve during the test;

[0029] Figure 5 It is a schematic diagram of the arrangement of strain gauges on the inner wall of the sleeve;

[0030] Figure 6 A schematic diagram showing the fixed connection between the fracture-causing pipe and the limiting support;

[0031] Figure 7 It is the structural diagram of the test box;

[0032] Figure 8 Schematic diagram of the contact between the top of the clamping hole and the sleeve during the test.

[0033] Among them, 1. test box; 11. top plate; 12. through hole; 13. fixing part; 14. angle steel; 15. transparent plate; 2. cracking tube; 3. limit support; 4. sleeve; 5. strain gauge; 6. industrial camera; 7. clamp; 71. clamping cylinder; 72. clamping hole; 73. push rod; 74. piston; 75. elastic diaphragm; 76. piston rod; 77. working chamber; 8. fixing column. DETAILED DESCRIPTION

[0034] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0035] like Figure 1 As shown, the supercritical CO2 phase change energy gathering test device of this scheme includes a test box 1 with a hollow interior, in which a sleeve 4 and a clamp 7 for clamping the sleeve 4 are arranged. After the clamp 7 clamps the sleeve 4, on the one hand, it can prevent the sleeve 4 from moving under force, and on the other hand, it can provide radial pressure for the sleeve 4 to simulate radial ground stress. The sleeve 4 is a metal cylinder with openings at both ends. During the test, the fracturing tube 2 extends into the sleeve 4; a number of strain assemblies are fixedly connected to the wall of the sleeve 4; the strain assemblies and the clamp 7 are both electrically connected to the control module.

[0036] The control module includes an electrically connected single chip microcomputer and a computer, and the data collected by the strain gauge 5 is transmitted to the computer for subsequent processing. The drive module in the clamp 7 is electrically connected to the single chip microcomputer, and can output instructions for controlling the opening and closing of the drive module of the clamp 7 through the computer end.

[0037] Preferably, an industrial camera 6 electrically connected to the control module is provided on one side of the test box 1, and a transparent plate 15 is provided on the side of the test box 1 close to the industrial camera 6. The industrial camera 6 can collect images of the situation in the test box 1 in real time and upload them to the computer, so that the test personnel can observe the test situation. After the blasting, three-dimensional scanning can be used to measure the deformation amount, damage area and other parameters of the sleeve 4, which can more accurately evaluate the influence of the energy gathering effect on the stress field, and provide strong support for the subsequent blasting design and optimization.

[0038] The traditional CO2 blasting test device is as follows: a strain gauge 5 is fixedly connected to the rock wall, and a hole is drilled in the rock to allow the fracturing tube 2 to be inserted. After the fracturing tube 2 is placed in the drill hole, the fracturing tube 2 is detonated to blast the rock. Strain data is collected through the strain gauge 5 to explore the stress field during the blasting. However, the strength of the rock is low, and the rock will break quickly under the action of the shock wave. Therefore, researchers can only use the strain data collected before the rock breaks as effective data.

[0039] This means that the time window for test data collection is extremely short, and the effective strain data that can be collected is very limited. This fast process may not be able to fully capture the strain data or other data of the entire process of rock from elastic deformation to plastic deformation and final fracture, thus limiting the in-depth analysis of the strain-stress relationship.

[0040] Secondly, since the rock will experience severe stress and deformation during the blasting process, the strain gauge 5 attached to the rock wall is likely to fall off or be damaged during the blasting process, resulting in data loss or distortion. This not only affects the integrity of the data, but may also introduce errors, making the analysis results unreliable.

[0041] Based on the above considerations, the inventors completely changed the structure of the CO2 blasting test device, changed the blasting object to a thin-walled metal sleeve 4, transformed the rock blasting problem into a thin-walled cylinder pressure blasting problem, and established a research model based on this. Due to the structural characteristics of the metal cylinder, its destruction process is slower than that of the rock, with a fairly long observation time, and more data about the blasting process can be recorded.

[0042] Moreover, the sleeve 4 will not break into pieces like a rock, but will produce radial deformation, leaving clear traces of damage. Researchers can more intuitively observe the damage mode of the sleeve 4 under the focused energy impact, and can more intuitively and deeply study and analyze the strain-stress relationship.

[0043] like Figure 2 As shown, the clamp 7 includes two clamping cylinders 71 arranged opposite to each other, and a clamping hole 72 penetrating the clamping cylinder 71 is provided at the edge of the clamping cylinder 71. When the two clamping cylinders 71 are pressed against each other, the two clamping holes 72 can be enclosed to form a columnar structure; a push rod 73 is fixedly connected to the side of the clamping cylinder 71 away from the clamping hole 72, and the push rod 73 is connected to the driver A, and the driver A is electrically connected to the control module.

[0044] The clamping hole 72 is provided to fit the curved surface of the sleeve 4 . During the test, the clamping hole 72 can be in close contact with the sleeve 4 . The control module drives the push rod 73 to move by starting the driver, thereby controlling the distance between the two clamping cylinders 71 .

[0045] like Figure 3-4 As shown, the two opposite sides of the clamping cylinder 71 are open ends, and the two sides of the open ends are covered with elastic diaphragms 75, and the elastic diaphragm 75 is sealed and connected to the clamping cylinder 71; the interior of the clamping cylinder 71 is provided with a piston 74 that divides the internal cavity into two independent chambers, and the chamber close to the elastic diaphragm 75 is a working chamber 77 filled with liquid, and a piston rod 76 fixedly connected to the piston 74 extends from the other chamber of the clamping cylinder 71 and is connected to the driver B, and the driver B is electrically connected to the control module.

[0046] Driver A and driver B can be any driving mechanism capable of driving an object to perform linear motion, such as hydraulic driving or electric push rod driving. The connection node where the piston rod 76 passes through the clamping cylinder 71 and the piston 74 and the inner wall of the clamping cylinder 71 are sealed.

[0047] First, the clamping cylinder 71 is pushed by the push rod 73 to move so that the elastic diaphragm 75 can contact the curved surface of the sleeve 4, and then the driver B is controlled to start to move the piston rod 76, thereby pushing the piston 74, so that the volume of the working chamber 77 is reduced, and the liquid therein is squeezed and moves toward the elastic diaphragm 75, squeezing the elastic diaphragm 75. By controlling the displacement of the piston rod 76, the clamping force on the sleeve 4 can be controlled.

[0048] The elastic diaphragm 75 under pressure will produce elastic deformation and tightly wrap the sleeve 4, which can not only clamp the sleeve 4 to prevent the sleeve 4 from being displaced by the shock wave, but also simulate the confining pressure effect in the ground stress, so that the sleeve 4 is subjected to uniform or non-uniform radial pressure, making the test closer to the working conditions during the actual blasting. Compared with the hard contact clamping, this flexible contact clamping method does not affect the deformation of the sleeve 4 during blasting.

[0049] refer to Figure 8 The clamping hole 72 is a cylindrical structure with a semi-elliptical cross section, and the push rod 73 and the piston rod 76 both extend out of the test box 1. The cross section of the clamping hole 72 is set to be semi-elliptical instead of circular because it can improve the adaptability and facilitate clamping of sleeves 4 with different diameters, so as to study the influence factors of sleeves 4 with different diameters on the energy-gathering effect.

[0050] After the clamping cylinder 71 clamps the sleeve 4, the elastic diaphragm 75 at the middle position can still tightly wrap the sleeve 4. The elastic diaphragms 75 at the top and bottom positions, due to the shape of the clamping cylinder 71, can be approximately regarded as point contact with the sleeve 4. The contact point a between the sleeve 4 and the top of the elastic diaphragm 75 is as shown in FIG. Figure 8 shown.

[0051] Preferably, a pressure gauge is provided on the clamping cylinder 71. When , the control module controls driver B to close.

[0052] Among them, a r is the preset radial clamping force of the clamp on the sleeve; π is the pi ratio; r is the outer radius of the sleeve; L is the length of the elastic diaphragm wrapped around the sleeve; P is the reading of the pressure gauge.

[0053] refer to Figure 5 Several strain components are evenly arranged along the length direction of the sleeve 4. One strain component includes several strain gauges 5 evenly distributed along the circumferential direction of the sleeve 4. The strain gauges 5 are all pasted to the inner wall of the sleeve 4, and the position where the strain gauges 5 are pasted does not exceed the range of the elastic diaphragm 75 wrapping the sleeve 4.

[0054] The strain gauge 5 is attached to the inner wall of the sleeve 4 to prevent the clamp 7 from touching the strain gauge 5 during clamping, causing damage or falling off of the strain gauge 5, which would result in data acquisition failure. The installation position of the strain gauge 5 can ensure that the strain acquisition point coincides with the ground stress simulation point. Since the strain gauge 5 on the inner wall will be directly affected by the blasting shock wave, the blasting force in the test can be appropriately reduced to avoid excessive blasting force affecting data acquisition and remove instantaneous large fluctuation data.

[0055] refer to Figure 4 and Figure 6 A fixing column 8 with a threaded outer wall is fixedly connected to the bottom plate of the test box 1. The fixing column 8 passes through the bottom plate of the sleeve 4 and extends into the sleeve 4. A threaded hole matching the fixing column 8 is provided at the bottom of the fracturing tube 2. During the test, the fixing column 8 is threadedly connected to the fracturing tube 2.

[0056] The fixing column 8 can be welded at the center of the bottom plate of the test box 1. Connecting the fracturing tube 2 to the fixing column 8 can limit the displacement of the fracturing tube 2. Firstly, the impact force of the fracturing tube 2 can be prevented from causing the fracturing tube 2 to escape from the test box 1 and fly away, thereby affecting the test results or causing safety accidents. Secondly, the installation position of the fracturing tube 2 can be adjusted so that the jet port of the fracturing tube 2 is located in the area where the elastic diaphragm 75 wraps the sleeve 4.

[0057] refer to Figure 7 The test box 1 includes a rectangular parallelepiped frame structure formed by angle steel 14, a bottom plate and a side plate fixed on the angle steel 14, and a top plate 11 is detachably connected to the angle steel 14 through a fixing member 13. The top plate 11 of the test box 1 is provided with a through hole 12 for the fracturing tube 2 to extend into; the test device also includes a limit support 3, which is fixed to the outer wall of the fracturing tube 2 and contacts the lower surface of the top plate 11 when in use; the surface area of ​​the limit support 3 is larger than the cross-sectional area of ​​the through hole 12 to further prevent the fracturing tube 2 from moving upward.

[0058] The assembly steps of the test box 1 are roughly as follows: first, the angle steel 14 is fixed into a rectangular frame by welding or other fixing connection methods, and then the bottom plate and four side panels of the rectangular parallelepiped are fixed respectively. Before fixing the top plate 11, the fixed fracturing tube 2 is first placed in the sleeve 4 and fixedly connected with the fixing column 8, and the installation position of the limit support 3 is measured so that the limit support 3 can just abut against the top plate 11, and then the limit support is fixed to the outside of the fracturing tube 2 by welding or other fixing methods.

[0059] Then the top plate 11 is fixedly connected to the angle steel 14 through the fixing piece 13. The top of the fracturing tube 2 is extended out of the top plate 11 from the through hole 12. After the test is completed, the top plate 11 can be disassembled by disassembling the fixing piece 13, and the fracturing tube 2 can be taken out. Among them, the transparent plate 15 is preferably a tempered glass plate, and the other side plates are preferably steel plates.

[0060] In summary, the present invention can prolong the data collection time and facilitate direct observation of the deformation of the sleeve caused by the energy gathering effect. The radial pressure applied by the clamp to the sleeve can simulate the radial pressure in different burial depths and different ground stresses, making the test closer to the actual working conditions.

Claims

1. A supercritical CO2 phase change energy testing device, characterized by: The test box comprises a test box (1) with a hollow interior, wherein a sleeve (4) and a clamp (7) for clamping the sleeve (4) are arranged inside the test box (1); the sleeve (4) is a metal cylinder with openings at both ends, and during the test, the fracturing tube (2) extends into the sleeve (4); a plurality of strain assemblies are fixedly connected to the wall of the sleeve (4); the strain assemblies and the clamp (7) are both electrically connected to a control module; The clamp (7) comprises two clamping cylinders (71) arranged opposite to each other, the edges of the clamping cylinders (71) are provided with clamping holes (72) penetrating the clamping cylinders (71), and when the two clamping cylinders (71) are pressed against each other, the two clamping holes (72) are enclosed to form a columnar structure; a push rod (73) is fixedly connected to the side of the clamping cylinder (71) away from the clamping holes (72), and the push rod (73) is connected to the driver A, and the driver A is electrically connected to the control module; Two opposite sides of the clamping cylinder (71) are open ends, and two sides of the open ends are covered with elastic diaphragms (75), and the elastic diaphragms (75) are sealed and connected to the clamping cylinder (71); A piston (74) is arranged inside the clamping cylinder (71) to divide the internal cavity into two independent chambers. The chamber close to the elastic diaphragm (75) is a working chamber (77) filled with liquid. A piston rod (76) fixedly connected to the piston (74) extends out of the other chamber of the clamping cylinder (71) and is connected to a driver B. The driver B is electrically connected to a control module. A pressure gauge is provided on the clamping cylinder (71). When , the control module controls the driver B to close; in, is the preset radial clamping force value of the clamp on the sleeve; is the ratio of pi; is the outer radius of the sleeve; The length of the sleeve wrapped by the elastic diaphragm; is the pressure gauge reading.

2. The supercritical CO2 phase change energy testing device according to claim 1 is characterized in that: The clamping hole (72) is a columnar structure with a semi-elliptical cross-section, and the push rod (73) and the piston rod (76) both extend out of the test box (1).

3. The supercritical CO2 phase change energy testing device according to claim 1 is characterized in that: A plurality of the strain assemblies are evenly arranged along the length direction of the sleeve (4), and one strain assembly comprises a plurality of strain gauges (5) evenly distributed along the circumferential direction of the sleeve (4) wall. The strain gauges (5) are all adhered to the inner wall of the sleeve (4), and the position where the strain gauges (5) are adhered does not exceed the range of the elastic diaphragm (75) wrapping the sleeve (4).

4. The supercritical CO2 phase change energy testing device according to claim 1 is characterized in that: A fixing column (8) with a threaded outer wall is fixedly connected to the bottom plate of the test box (1); the fixing column (8) passes through the bottom plate of the sleeve (4) and extends into the interior of the sleeve (4); a threaded hole matching the fixing column (8) is provided at the bottom of the fracturing tube (2); during testing, the fixing column (8) is threadedly connected to the fracturing tube (2).

5. The supercritical CO2 phase change energy testing device according to claim 1 is characterized in that: The top plate (11) of the test box (1) is provided with a through hole (12) for the fracturing tube (2) to extend into; the test device further comprises a limit support (3) which, when in use, is fixed to the outer wall of the fracturing tube (2) and contacts the lower surface of the top plate (11); the surface area of ​​the limit support (3) is greater than the cross-sectional area of ​​the through hole (12).

6. The supercritical CO2 phase change energy testing device according to claim 1 is characterized in that: An industrial camera (6) electrically connected to the control module is provided on one side of the test box (1), and a transparent plate (15) is provided on the side of the test box (1) close to the industrial camera (6).

7. The supercritical CO2 phase change energy testing device according to claim 5 is characterized in that: The test box (1) comprises a rectangular parallelepiped frame structure formed by enclosing angle steels (14), and a bottom plate and side panels fixed to the angle steels (14); the top plate (11) is detachably connected to the angle steels (14) via fixing members (13).

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