Bed slag recycling device of circulating fluidized bed boiler

Abstract

The invention discloses a circulating fluidized bed boiler bed slag recycling device which comprises a mounting base, a slag elevator and a mounting frame are fixedly arranged at the top of the mounting base, a slag bin slag sliding pipe is fixedly arranged at the discharging end of the slag elevator, and a discharging pipe is fixedly arranged at the bottom of the slag bin slag sliding pipe. A connecting pipe is arranged below the discharging pipe, and a pneumatic opening and closing assembly is arranged between the top of the connecting pipe and the bottom of the discharging pipe. The mounting frame is driven by the vibration assembly to vibrate, so that the mounting frame drives the first-stage screen and the second-stage screen to vibrate, the first-stage screen and the second-stage screen are used for screening slag, the slag removed through screening enters the slag bin, screened fine materials are conveyed into the collecting box on the ground through the recycling pipe, and therefore the slag can be recycled conveniently; the waste of bed materials such as sand, mud powder, pebbles, stones and iron blocks in the furnace slag is avoided, and the problem that the cost is increased due to the purchase of new bed materials is avoided.

Description

Technical Field

[0001] The invention relates to the technical field of circulating fluidized bed boiler bed slag recovery, and in particular to a circulating fluidized bed boiler bed slag recovery and utilization device. Background Art

[0002] The circulating fluidized bed boiler uses the most industrialized clean coal combustion technology. The circulating fluidized bed boiler uses fluidized combustion, and its main structure includes two parts: the combustion chamber and the circulating furnace. The biggest difference from the bubbling fluidized bed combustion technology is that the operating wind speed is high, which strengthens the heterogeneous reaction processes such as combustion and desulfurization. The boiler capacity can be expanded to a large capacity acceptable to the power industry. The circulating fluidized bed boiler has well solved the basic problems of thermodynamics, mechanics, materials science, and engineering problems such as expansion, wear, and overheating, becoming an advanced technology for the utilization of flame-retardant solid fuel energy.

[0003] The coal, biomass and other fuels put into the circulating fluidized bed boiler contain a certain proportion of ash. The mineral components carried by the fuel are complex, such as oxides or salts of silicon, aluminum, iron, calcium, magnesium, etc. Under high-temperature combustion conditions, some minerals will react chemically to form new stable compounds, further increasing the amount of solid residues, which are discharged together with the unreacted ash and become slag. Therefore, the composition of these slags is diverse and complex, including sand, mud powder, gravel, stone, iron blocks, etc.

[0004] At the initial start-up of the boiler, sand is first filled into the bottom of the furnace. When the primary wind is blown, the sand can quickly form a uniform fluidized state to ensure that the fuel can be fully suspended and mixed in it, starting a smooth combustion process. When there is no suitable sand on hand, mud powder, gravel, etc. are screened and the part with suitable particle size is selected. The iron block in the fluidized state constantly shuttles between the high-temperature combustion zone and the relatively low-temperature heating surface, efficiently transferring heat and improving the overall thermal efficiency of the boiler. Therefore, bed material needs to be added frequently.

[0005] The slag itself contains a large amount of bed materials such as sand, mud powder, gravel, stone, and iron blocks. Moreover, the sand and stone that have been subjected to complex working conditions such as high temperature and fluidization inside the boiler often have particle shape and surface characteristics that are more in line with the fluidization requirements of the boiler. If the slag is directly transported to the slag bin, it will cause waste of bed materials such as sand, mud powder, gravel, stone, and iron blocks. At the same time, purchasing new bed materials will increase costs. Based on the above problems, the present application proposes a circulating fluidized bed boiler bed slag recycling device. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a circulating fluidized bed boiler slag recovery and utilization device, which is used to solve the problems raised in the above background technology.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A circulating fluidized bed boiler slag recycling device comprises a mounting base, a slag elevator and a mounting frame are fixedly arranged on the top of the mounting base, a slag bin slag chute is fixedly arranged at the discharge end of the slag elevator, a discharge pipe is fixedly arranged at the bottom of the slag bin slag chute, a connecting pipe is arranged below the discharge pipe, a pneumatic opening and closing assembly is arranged between the top of the connecting pipe and the bottom of the discharge pipe, a slag bin is fixedly arranged inside the mounting frame, a mounting seat is fixedly arranged on the top of the slag bin, a compression spring is fixedly arranged on the top of the mounting seat, a mounting frame is fixedly arranged on the top of the compression spring, a mounting assembly is arranged inside the mounting frame, a primary screen and a secondary screen are installed inside the mounting frame through the mounting assembly, and vibration assemblies are arranged on both the front and rear sides of the mounting frame.

[0009] Preferably, the pneumatic opening and closing assembly includes an opening and closing box fixedly installed between the facing surfaces of the discharge pipe and the connecting pipe, cylinders are provided on the front and rear sides of the opening and closing box, a movable plate is fixedly provided at the output end of the cylinder, an opening and closing plate is fixedly provided on the left side of the movable plate, the opening and closing plate is slidably provided inside the opening and closing box, a support roller is overlapped at the bottom of the opening and closing plate, and the support roller is rotatably provided inside the opening and closing box.

[0010] Preferably, the mounting assembly includes a mounting shell fixedly mounted on the right side of the mounting frame, the mounting shell being rotatably provided with a rotating screw, one end of the rotating screw being fixedly provided with a knob, and the other end of the rotating screw being rotatably provided with a wedge block, the wedge block being slidably arranged inside the mounting shell, a T-shaped slider being fixedly arranged on the inclined surface of the wedge block, a trapezoidal block being overlapped on the inclined surface of the wedge block, the trapezoidal block being slidably arranged inside the mounting shell, a T-shaped slot being provided on the inclined surface of the trapezoidal block, and the T-shaped slider being slidably arranged inside the T-shaped slider, a locking plate 1 being fixedly arranged on the left side of the trapezoidal block, a connecting plate being fixedly arranged on the left side of the locking plate 1, a locking plate 2 being fixedly arranged on the left side of the connecting plate, the surfaces of the locking plate 1 and the locking plate 2 being overlapped with mounting plates, the surface of the mounting plate being overlapped on the L-shaped plate, and the L-shaped plate being fixed inside the mounting frame.

[0011] Preferably, the number of the mounting plates is four, and the four mounting plates are fixedly mounted on the bottom of the primary sieve and the bottom of the secondary sieve respectively, and the mounting plates are in a mountain-shaped structure, and the mounting plates are made of metal material.

[0012] Preferably, the vibration component includes a fixing seat fixedly mounted on the front and rear sides of the mounting frame, a driving motor is fixedly arranged inside the fixing seat, a fixing stud is fixedly arranged at the output end of the driving motor, an eccentric wheel is overlapped on the surface of the fixing stud, and a fixing nut for fixing the eccentric wheel is threadedly arranged on the surface of the fixing stud.

[0013] Preferably, the installation frame is composed of a discharge plate and two U-shaped frames, a buckle lock is connected between the two U-shaped frames, and a recovery pipe is fixedly provided on the right side of the discharge plate.

[0014] Preferably, a coarse slag outlet pipe is fixedly provided on the left side of the installation frame, the coarse slag outlet pipe is fixed to the U-shaped frame below, a limiting pipe is overlapped on the surface of the coarse slag outlet pipe, and the limiting pipe is fixedly installed on the top of the slag bin.

[0015] Preferably, there are several supporting rollers, and the several supporting rollers are symmetrically distributed.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The circulating fluidized bed boiler slag recycling device drives the installation frame to vibrate through a vibration component, so that the installation frame drives the primary screen and the secondary screen to vibrate, so that the primary screen and the secondary screen screen screen the slag, wherein the slag removed by screening enters the slag bin, and the screened fine material is transported to the inside of the collection box on the ground through a recovery pipe, thereby facilitating the recycling of the slag, avoiding the waste of bed materials such as sand, mud powder, gravel, stone, iron block, etc. in the slag, and avoiding the problem of increased cost due to the purchase of new bed materials.

[0018] 2. The circulating fluidized bed boiler slag recycling device can conveniently control the slag to enter the connecting pipe or not enter the connecting pipe through the pneumatic opening and closing components, thereby controlling the slag to enter the installation frame for screening, or not enter the installation frame for screening, so as to facilitate the screening of the slag according to needs.

[0019] 3. The circulating fluidized bed boiler slag recovery and utilization device facilitates the replacement of the primary screen and the secondary screen by installing components, thereby facilitating the replacement of the specifications of the primary screen and the secondary screen, so that the primary screen and the secondary screen can be suitable for different screening needs, and the replacement process of the primary screen and the secondary screen is convenient, so that the time consumed in replacing the primary screen and the secondary screen is reduced, thereby facilitating the rapid replacement of damaged primary screens and secondary screens. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a positive isometric drawing of the structure of the present invention;

[0021] Figure 2It is a top cross-sectional view of the opening and closing component structure of the present invention;

[0022] Figure 3 It is a right sectional view of the opening and closing assembly structure of the present invention;

[0023] Figure 4 It is a schematic diagram of the local structure of the present invention;

[0024] Figure 5 This is a schematic diagram of the structure of the installation assembly of the present invention;

[0025] Figure 6 It is a schematic diagram of the local structure of the installation assembly of the present invention;

[0026] Figure 7 It is a schematic diagram of the structure of the vibration component of the present invention.

[0027] In the figure: 1. Mounting base; 2. Slag elevator; 3. Slag slag chute; 4. Discharge pipe; 5. Connecting pipe; 6. Mounting frame; 7. Slag bin; 8. Mounting seat; 9. Compression spring; 10. Mounting frame; 11. Primary screen; 12. Secondary screen; 13. Opening and closing box; 14. Cylinder; 15. Moving plate; 16. Opening and closing plate; 17. Support roller; 18. Mounting shell; 19. Rotating screw; 20. Knob; 21. Wedge block; 22. T-shaped slider; 23. Trapezoidal block; 24. T-shaped slide; 25. Locking plate one; 26. Connecting plate; 27. Locking plate two; 28. Mounting plate; 29. ​​L-shaped plate; 30. Fixed seat; 31. Driving motor; 32. Fixing stud; 33. Eccentric wheel; 34. Fixing nut; 35. Coarse slag outlet pipe; 36. Limiting pipe; 37. Recovery pipe. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] Reference Figure 1-7A circulating fluidized bed boiler slag recycling device comprises a mounting base 1, a slag elevator 2 and a mounting frame 6 are fixedly arranged on the top of the mounting base 1, a slag bin slag pipe 3 is fixedly arranged on the discharge end of the slag elevator 2, a discharge pipe 4 is fixedly arranged on the bottom of the slag bin slag pipe 3, a connecting pipe 5 is arranged below the discharge pipe 4, a pneumatic opening and closing component is arranged between the top of the connecting pipe 5 and the bottom of the discharge pipe 4, the pneumatic opening and closing component comprises an opening and closing box 13 fixedly installed between the facing surfaces of the discharge pipe 4 and the connecting pipe 5, a cylinder 14 is arranged on both the front and rear sides of the opening and closing box 13, a moving plate 15 is fixedly arranged on the output end of the cylinder 14, an opening and closing plate 16 is fixedly arranged on the left side of the moving plate 15, the opening and closing plate 16 is slidably arranged inside the opening and closing box 13, a supporting roller 17 is overlapped at the bottom of the opening and closing plate 16, the number of the supporting rollers 17 is several, and the several supporting rollers 17 are symmetrically distributed, so that the opening and closing plate 1 is conveniently opened and closed by the supporting rollers 17 6 is supported to prevent the opening and closing plate 16 from being deformed by the heavy pressure of the slag, so that the opening and closing plate 16 cannot be opened. The supporting roller 17 is rotatably arranged inside the opening and closing box 13, and the output end is driven by the cylinder 14 to drive the movable plate 15 to move to the right, and the opening and closing plate 16 is driven to move to the right by the movable plate 15, so that the opening and closing plate 16 is opened, thereby facilitating the control of whether the slag enters or does not enter the interior of the connecting pipe 5. A slag bin 7 is fixedly arranged inside the mounting frame 6, and a mounting seat 8 is fixedly arranged on the top of the slag bin 7. A compression spring 9 is fixedly arranged on the top of the mounting seat 8, and a mounting frame 10 is fixedly arranged on the top of the compression spring 9. The mounting frame 10 consists of a discharge plate and two U-shaped frames, and a buckle lock is connected between the two U-shaped frames. A recovery pipe 37 is fixedly arranged on the right side of the discharge plate, and a mounting assembly is arranged inside the mounting frame 10. A primary screen 11 and a secondary screen 12 are installed inside the mounting frame 10 through the mounting assembly, and vibration assemblies are arranged on both the front and rear sides of the mounting frame 10.

[0030] Specifically, the mounting assembly includes a mounting shell 18 fixedly mounted on the right side of the mounting frame 10, a rotating screw 19 is rotatably arranged inside the mounting shell 18, a knob 20 is fixedly arranged at one end of the rotating screw 19, a wedge block 21 is rotatably arranged at the other end of the rotating screw 19, the wedge block 21 is slidably arranged inside the mounting shell 18, a T-shaped slider 22 is fixedly arranged on the inclined surface of the wedge block 21, a trapezoidal block 23 is overlapped on the inclined surface of the wedge block 21, the trapezoidal block 23 is slidably arranged inside the mounting shell 18, a T-shaped slide groove 24 is opened on the inclined surface of the trapezoidal block 23, the T-shaped slider 22 is slidably arranged inside the T-shaped slider 22, and the trapezoidal block A locking plate 25 is fixedly arranged on the left side of 23, a connecting plate 26 is fixedly arranged on the left side of the locking plate 25, a locking plate 27 is fixedly arranged on the left side of the connecting plate 26, and mounting plates 28 are overlapped on the surfaces of the locking plates 15 and the locking plates 27. There are four mounting plates 28, which are respectively fixedly arranged on the bottom of the primary screen 11 and the bottom of the secondary screen 12. The mounting plates 28 are in a mountain-shaped structure, and the mounting plates 28 are made of metal. The surface of the mounting plates 28 is overlapped on the L-shaped plate 29, and the L-shaped plate 29 is fixed inside the mounting frame 10. When the primary screen 11 needs to be replaced with a screen with a different aperture, the rotating The rotating knob 20 drives the rotating screw 19 to rotate, so that the rotating screw 19 shrinks into the installation housing 18, and the rotating screw 19 pushes the wedge block 21 to move to the left, so that the wedge block 21 drives the T-shaped slider 22 to slide inside the trapezoidal block 23, and the two trapezoidal blocks 23 are pushed away from each other by the T-shaped slider 22, and the locking plate 1 25 is driven away from each other by the trapezoidal block 23, and the locking plate 1 25 drives the locking plate 27 away from each other through the connecting plate 26, so that the locking plate 1 25 and the locking plate 27 are moved out of the mounting plate 28, and the first-level screen 11 is pulled to drive the mounting plate 28 to move out of the L-shaped plate 29, and the first-level screen 11 is replaced by the first-level screen 11. The first-level screen 11 inserts the mounting plate 28 into the L-shaped plate 29, and rotates the rotating screw 19 in the opposite direction, so that the knob 20 pulls the trapezoidal blocks 23 closer to each other through the T-shaped slider 22, so that the locking plate 1 25 drives the locking plate 2 27 closer to each other through the connecting plate 26, so that the locking plate 1 25 and the locking plate 2 27 are inserted into the mounting plate 28, and the replacement of the first-level screen 11 is completed. If the secondary screen 12 needs to be replaced, open the buckle lock, remove the U-shaped frame on it, and repeat the above operation to replace the secondary screen 12, so that it is convenient to replace the first-level screen 11 and the secondary screen 12 according to usage requirements.

[0031] Specifically, the vibration component includes a fixing base 30 fixedly installed on the front and rear sides of the mounting frame 10, a driving motor 31 is fixedly arranged inside the fixing base 30, a fixing stud 32 is fixedly arranged at the output end of the driving motor 31, an eccentric wheel 33 is overlapped on the surface of the fixing stud 32, and a fixing nut 34 for fixing the eccentric wheel 33 is threadedly arranged on the surface of the fixing stud 32. The driving motor 31 drives the fixing stud 32 to rotate, and the fixing stud 32 cooperates with the fixing nut 34 to drive the eccentric wheel 33 to rotate. The centrifugal force generated by the high-speed rotation of the eccentric wheel 33 will cause the entire driving motor 31 to vibrate, and the vibration direction is the same as the rotation direction of the eccentric wheel 33. Since the driving motor 31 itself is installed together with the mounting frame 10, the vibration of the driving motor 31 will be transmitted to the mounting frame 10, so that the primary screen 11 and the secondary screen 12 also generate corresponding vibrations.

[0032] Specifically, a coarse slag outlet pipe 35 is fixedly provided on the left side of the installation frame 10, and the coarse slag outlet pipe 35 is fixed to the U-shaped frame below. A limiting pipe 36 is overlapped on the surface of the coarse slag outlet pipe 35, and the limiting pipe 36 is fixedly installed on the top of the slag bin 7. The coarse slag outlet pipe 35 and the limiting pipe 36 are used to facilitate the transportation of the slag removed by screening to the inside of the slag bin 7. The coarse slag outlet pipe 35 is fixed to the U-shaped frame below, which is convenient for removing the upper U-shaped frame and the primary screen 11, thereby facilitating the replacement of the secondary screen 12.

[0033] When in use: when the slag elevator 2 transports the circulating fluidized bed boiler bed slag to the discharge end, the slag moves downward through the slag bin slag chute 3 and enters the interior of the slag bin 7 through the slag bin slag chute 3. If the slag transported by the slag elevator 2 needs to be recycled, the cylinder 14 drives the output end to drive the movable plate 15 to move to the right, and the movable plate 15 drives the opening and closing plate 16 to move to the right, so that the opening and closing plate 16 opens, allowing the slag to enter the interior of the connecting pipe 5, and enter the interior of the installation frame 10 through the connecting pipe 5, and fall to the top of the primary screen 11, and the fixed stud 32 is driven to rotate by the driving motor 31, and the eccentric wheel 33 is driven to rotate by the fixed stud 32 and the fixed nut 34, so that the installation frame 10 The compression spring 9 vibrates, and the mounting frame 10 drives the primary screen 11 and the secondary screen 12 to vibrate, and the secondary screen 12 vibrates to screen the slag. After screening by the primary screen 11, the powdered particles enter the coarse slag outlet pipe 35, and enter the limiting pipe 36 through the coarse slag outlet pipe 35, and are finally led back to the slag bin 7 through the limiting pipe 36, and the screened slag falls to the top of the secondary screen 12, and is screened again by the secondary screen 12. The screened fine particles fall onto the discharge plate, and slide through the discharge plate to the recovery pipe 37, and are transported to the collection box on the ground through the recovery pipe 37. The slag with too large particles is transported to the limiting pipe 36 through the coarse slag outlet pipe 35, and enter the slag bin 7 through the limiting pipe 36.

[0034] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0035] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A circulating fluidized bed boiler slag recycling device, comprising a mounting base (1), characterized in that: A slag elevator (2) and a mounting frame (6) are fixedly arranged on the top of the mounting base (1); a slag bin slag chute (3) is fixedly arranged at the discharge end of the slag elevator (2); a discharge pipe (4) is fixedly arranged at the bottom of the slag bin slag chute (3); a connecting pipe (5) is arranged below the discharge pipe (4); a pneumatic opening and closing component is arranged between the top of the connecting pipe (5) and the bottom of the discharge pipe (4); a slag bin (7) is fixedly arranged inside the mounting frame (6); a mounting seat (8) is fixedly arranged on the top of the slag bin (7); a compression spring (9) is fixedly arranged on the top of the mounting seat (8); a mounting frame (10) is fixedly arranged on the top of the compression spring (9); a mounting component is arranged inside the mounting frame (10); a primary screen (11) and a secondary screen (12) are installed inside the mounting frame (10) through the mounting component; and vibration components are arranged on both the front and rear sides of the mounting frame (10).

2. A circulating fluidized bed boiler slag recycling device according to claim 1, characterized in that: The pneumatic opening and closing assembly comprises an opening and closing box (13) fixedly mounted between the facing surfaces of the discharge pipe (4) and the connecting pipe (5); cylinders (14) are arranged on both the front and rear sides of the opening and closing box (13); a movable plate (15) is fixedly arranged at the output end of the cylinder (14); an opening and closing plate (16) is fixedly arranged on the left side of the movable plate (15); the opening and closing plate (16) is slidably arranged inside the opening and closing box (13); a supporting roller (17) is overlapped at the bottom of the opening and closing plate (16); and the supporting roller (17) is rotatably arranged inside the opening and closing box (13).

3. The circulating fluidized bed boiler slag recycling device according to claim 1, characterized in that: The mounting assembly comprises a mounting shell (18) fixedly mounted on the right side of the mounting frame (10); a rotating screw (19) is rotatably arranged inside the mounting shell (18); a knob (20) is fixedly arranged at one end of the rotating screw (19); a wedge block (21) is rotatably arranged at the other end of the rotating screw (19); the wedge block (21) is slidably arranged inside the mounting shell (18); a T-shaped sliding block (22) is fixedly arranged on the inclined surface of the wedge block (21); a trapezoidal block (23) is overlapped on the inclined surface of the wedge block (21); the trapezoidal block (23) is slidably arranged inside the mounting shell (18) The trapezoidal block (23) has a T-shaped sliding groove (24) on its inclined surface, the T-shaped slider (22) is slidably arranged inside the T-shaped slider (22), a locking plate (25) is fixedly arranged on the left side of the trapezoidal block (23), a connecting plate (26) is fixedly arranged on the left side of the locking plate (25), a locking plate (27) is fixedly arranged on the left side of the connecting plate (26), a mounting plate (28) is overlapped on the surface of the locking plate (25) and the locking plate (27), the surface of the mounting plate (28) is overlapped on the L-shaped plate (29), and the L-shaped plate (29) is fixed inside the mounting frame (10).

4. A circulating fluidized bed boiler slag recycling device according to claim 3, characterized in that: The number of the mounting plates (28) is four. The four mounting plates (28) are respectively fixedly mounted on the bottom of the primary screen (11) and the bottom of the secondary screen (12). The mounting plates (28) are in a mountain-shaped structure and are made of metal.

5. The circulating fluidized bed boiler slag recycling device according to claim 1, characterized in that: The vibration component comprises a fixing seat (30) fixedly mounted on the front and rear sides of the mounting frame (10); a driving motor (31) is fixedly arranged inside the fixing seat (30); a fixing stud (32) is fixedly arranged at the output end of the driving motor (31); an eccentric wheel (33) is overlappedly arranged on the surface of the fixing stud (32); and a fixing nut (34) for fixing the eccentric wheel (33) is threadedly arranged on the surface of the fixing stud (32).

6. The circulating fluidized bed boiler slag recycling device according to claim 1, characterized in that: The installation frame (10) is composed of a discharge plate and two U-shaped frames, a buckle lock is connected between the two U-shaped frames, and a recovery pipe (37) is fixedly arranged on the right side of the discharge plate.

7. The circulating fluidized bed boiler slag recycling device according to claim 6, characterized in that: A coarse slag outlet pipe (35) is fixedly arranged on the left side of the installation frame (10), the coarse slag outlet pipe (35) is fixed to the U-shaped frame below, and a limiting pipe (36) is overlapped and arranged on the surface of the coarse slag outlet pipe (35), and the limiting pipe (36) is fixedly installed on the top of the slag bin (7).

8. The circulating fluidized bed boiler slag recycling device according to claim 2, characterized in that: The number of the supporting rollers (17) is multiple, and the multiple supporting rollers (17) are symmetrically distributed.

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