A discharge pit for ceramic raw materials

Abstract

This invention relates to a discharge pit for dry powder production of ceramic bricks. The discharge pit is used to transport raw materials to grinding equipment under low-dust or dust-free conditions. The raw materials include non-plastic materials, weakly plastic materials, and strongly plastic materials. The discharge pit includes a negative pressure discharge pit for discharging non-plastic and weakly plastic materials and a strongly plastic material discharge pit for discharging strongly plastic materials. The negative pressure discharge pit and the strongly plastic material discharge pit are respectively connected to the grinding equipment through a raw material storage bin.

Description

Technical Field

[0001] This invention belongs to the field of dry-pressed ceramic brick production technology, and particularly relates to a discharge pit for dry powder preparation of ceramic raw materials. Background Technology

[0002] Tiles are thin sheet materials made from raw materials such as clay. They are usually formed by dry pressing of powder. The powder preparation process includes wet pressing powder and dry pressing powder.

[0003] In the preparation process of wet-process pressed powder (hereinafter referred to as wet powder preparation), the wet-milled slurry (with a water content of up to 40% by weight) needs to be spray-dried to produce the powder required for subsequent pressing. Spray drying requires drying the slurry at high temperature (700℃~900℃) until the moisture content is reduced to 7% by weight. The amount of water evaporated per ton of powder (dry basis) is about 600 kg, and the evaporation process requires a lot of energy.

[0004] In the preparation of pressed powder using the dry process (hereinafter referred to as dry powder preparation), the dry powder after grinding the raw materials is moistened and bound with water for granulation. The amount of water added during granulation is controlled to be below 12% by weight. The moisture content of the granulated particles is reduced to 7% by weight using a fluidized bed dryer. The amount of water required to evaporate per ton of powder (dry basis) is approximately 60 kg, far less than the amount of water required to prepare the same dry basis using the wet process. Therefore, the energy consumed, especially thermal energy, in the dry powder preparation process is far lower than that in the wet process, which can reduce carbon emissions by more than 40%.

[0005] Furthermore, since dry milling uses dry grinding, the debinding properties of the slurry produced by wet milling are not considered, significantly expanding the range of raw material choices. To further reduce the drying heat consumption of dry milling, raw materials with lower moisture content are generally preferred. Currently, in actual production, for raw materials with lower moisture content, the raw material stockpiling methods used in wet milling are still employed, i.e., the open-ground grid stockpiling mode of the wet milling process is still followed. At the stockpiling site, loaders and transport vehicles operate alternately to transport raw materials, resulting in significant dust in the raw material stockpiling and unloading areas, severely impacting the production environment. Summary of the Invention

[0006] To overcome the disadvantages of the prior art, the present invention provides a discharge pit for ceramic raw materials. The discharge pit is used to transport raw materials to grinding equipment under low-dust or dust-free conditions. The raw materials include non-plastic raw materials, weakly plastic raw materials, and strongly plastic raw materials. The discharge pit includes a negative pressure discharge pit for discharging non-plastic and weakly plastic raw materials and a strongly plastic raw material discharge pit for discharging strongly plastic raw materials. The negative pressure discharge pit and the strongly plastic raw material discharge pit are respectively connected to the grinding equipment through a raw material storage bin.

[0007] According to one embodiment of the present invention, the negative pressure unloading pit includes an unloading chamber (1), the unloading chamber (1) is a closed space, the bottom of the unloading chamber (1) is provided with an unloading hopper (2), and the inner wall of the unloading hopper (2) is provided with an ultra-high molecular weight polyethylene lining (3) where it contacts the raw material.

[0008] According to one embodiment of the present invention, the inner wall of the unloading chamber (1) is provided with an ultra-high molecular weight polyethylene lining (3).

[0009] According to one embodiment of the invention, the lining comprises, or is made of, ultra-high molecular weight polyethylene. As described above, the ultra-high molecular weight polyethylene lining is disposed on the inner wall side facing the raw material.

[0010] According to one embodiment of the present invention, the ultra-high molecular weight polyethylene has a relative molecular weight of 5,000,000 to 15,000,000 g / mol, preferably 8,000,000 to 12,000,000 g / mol, and more preferably 9,000,000 to 10,000,000 g / mol. Preferably, the polyethylene has a relative molecular weight of 0.920 to 0.960 g / cm³. 3 More preferably 0.925~0.950g / cm 3 The optimal value is 0.930–0.940 g / cm³. 3 The density is specified. Preferably, the ultra-high molecular weight polyethylene has a water absorption rate of less than 0.01% by weight. Preferably, the dynamic friction coefficient between the ultra-high molecular weight polyethylene and the raw material is less than 0.1, more preferably equal to or less than 0.08, and more preferably less than 0.05.

[0011] According to one embodiment of the present invention, the ultra-high molecular weight polyethylene liner (3) can be disposed on the inner wall of the corresponding equipment or compartment in any suitable manner. For example, the ultra-high molecular weight polyethylene liner (3) can be made into one or more sheets adapted to the internal shape of the equipment or compartment and fixed to the inner wall of the equipment or compartment, for example by form-fitting or by other elements, or by adhesive bonding. Alternatively, the polymer forming the liner can be sprayed onto the corresponding portion of the inner wall of the equipment or compartment, so that the polymer forms a coating on the inner wall of the equipment or compartment.

[0012] The ultra-high molecular weight polyethylene lining (3) ensures that the raw materials entering the unloading chamber (1) and / or unloading hopper (2) adhere to the inner walls of the unloading chamber (1) and / or unloading hopper (2) as little as possible, avoiding cross-mixing between different types of raw materials, thereby ensuring the stability of the ceramic tile raw material composition transported to the corresponding raw material storage silo.

[0013] According to one embodiment of the present invention, at least one side of the unloading room (1), preferably two sides, more preferably two opposite sides, is provided with a space for accommodating a transport vehicle, preferably a transport vehicle entry room (4) and a transport vehicle exit room (5), and the unloading room (1), the space for accommodating the transport vehicle or the transport vehicle entry room (4) and the transport vehicle exit room (5) are provided with a closed door (6) for the passage of the transport vehicle.

[0014] According to one embodiment of the invention, preferably, at least one side of the unloading chamber (1) is provided with a space for accommodating a transport vehicle, and a closed door (6) for the passage of the transport vehicle is provided between at least one side of the unloading chamber (1) and the space for accommodating the transport vehicle. In this case, the transport vehicle drives into the space for accommodating the transport vehicle, then enters the unloading chamber (1) to unload, and drives out of the unloading chamber (1) and the space for accommodating the transport vehicle after completing the operation; during this process, the closed door (6) is opened to allow the transport vehicle to pass through. In this arrangement, the space for accommodating the transport vehicle serves as both the transport vehicle entry chamber (4) and the transport vehicle exit chamber (5). Preferably, the number of the closed doors (6) is at least two; in this case, the first closed door (6) is located at the transport vehicle entry / exit point of the space for accommodating the transport vehicle, and the second closed door (6) is located at the connection between the space for accommodating the transport vehicle and the unloading chamber (1).

[0015] According to another embodiment of the present invention, preferably, spaces for accommodating transport vehicles are provided on two sides of the unloading bay (1), preferably on two opposite sides, for example, a transport vehicle entry bay (4) and a transport vehicle exit bay (5), respectively. Enclosed doors (6) for the passage of transport vehicles are provided at the unloading bay (1), the transport vehicle entry bay (4), and the transport vehicle exit bay (5). Preferably, the number of enclosed doors (6) is at least four: the first is located at the entrance of the transport vehicle entry bay (4), the second is located at the connection between the transport vehicle entry bay (4) and the unloading bay (1), the third is located at the connection between the unloading bay (1) and the transport vehicle exit bay (5), and the fourth is located at the exit of the transport vehicle exit bay (5).

[0016] Preferably, the closed door (6) is a quick-opening and closing door; preferably, the closed door (6) is a gate-type door or a roller shutter-type door.

[0017] By setting up a space to accommodate transport vehicles, a vehicle entry area (4), a vehicle exit area (5), and their corresponding closed doors (6), the unloading room (1) is always kept in a relatively closed space. This ensures the closure of the unloading room (1) and avoids the problem of a large amount of air flowing into and / or out of the unloading room (1) when the transport vehicle leaves after unloading and / or the next transport vehicle enters. In particular, when the closed door (6) is a gate-type door, it can also avoid the problem of excessive pressure difference between the inlet and outlet sides of the unloading room (1), which would be detrimental to the opening and closing of the closed door (6) of the unloading room (1).

[0018] According to the present invention, one or more negative pressure devices are provided in the unloading room (1), preferably dust extraction mechanism (7); when multiple dust extraction mechanisms (7) are provided, they are preferably arranged in a distributed manner.

[0019] Preferably, the dust extraction mechanism (7) can be distributed in the upper part of the unloading room (1), preferably on the ceiling, and more preferably evenly distributed.

[0020] Preferably, the dust extraction mechanism (7) includes a negative pressure dust extraction hood (71), a cyclone separator (72) and a negative pressure fan (73) connected in sequence; preferably, the negative pressure fan (73) is connected to the dust collector (8); preferably, the cyclone separator (72) is in the form of a cyclone tube and / or the dust collector (8) is a bag dust collector.

[0021] The negative pressure fan (73) draws the dust from the upper part of the unloading room (1) into the dust collector (8), effectively collecting and discharging the dust in the unloading room (1), thus avoiding the environmental pollution caused by the direct discharge of dust from the working area to the external environment.

[0022] According to the present invention, the cyclone separator (72) includes a cylindrical part (711) and a conical part (712) connected to each other. A volute is provided inside the cyclone separator (72). An air inlet pipe and an exhaust pipe are provided at the top of the cylindrical part (711). An inner cylinder is provided inside the cylindrical part (711). An expansion chamber (713) is provided in the conical part (712). A pressure reducing cone (714) is provided at the top of the expansion chamber (713). The volute surrounds the outer side of the inner cylinder and the expansion chamber (713).

[0023] Preferably, the inner wall of the cone (712) has an angle greater than 80° with the horizontal, and the inner wall of the expansion chamber (713) has an angle greater than 60° with the horizontal.

[0024] Preferably, the inner walls of the air inlet pipe and the cylindrical section (711) are provided with alumina ceramic composite lining plates; the inner walls of the cone section (712), the inner walls of the expansion chamber (713), and the top surface of the decompression cone (714) are provided with ultra-high molecular weight polyethylene lining plates.

[0025] According to the present invention, the bottom of the unloading hopper (2) is provided with a feeder (9) and a raw material conveying mechanism (10), the raw material conveying mechanism (10) being used to convey the raw material in the feeder (9) to the grinding equipment via the raw material storage bin.

[0026] The raw material conveying mechanism (10) includes one or more cyclone dust collectors (11) distributed outside the feeder (9). The cyclone dust collectors (11) are connected to the main dust collector (12). The cyclone dust collectors (11) and the main dust collector (12) are connected to corresponding fans to perform dust collection operations.

[0027] The dust collector (8) and the main dust collector (12) can convey the powder into the raw material storage silo through a conveying device, such as a pipeline.

[0028] Preferably, the unloading room (1) is further provided with a loader turning area (14), which is used to accommodate the loader, which is used to unload the raw materials in the transport vehicle and put them into the unloading hopper (2).

[0029] According to the present invention, the strong plastic raw material unloading pit includes a pit chamber (15), and a plurality of self-cleaning spiral assemblies (16) are provided at the bottom of the pit chamber (15). The self-cleaning spiral assembly (16) includes two rotating shafts (17), and umbrella-shaped blades (18) and full blades (19) are provided on the rotating shafts (17). The area covered by the umbrella-shaped blades (18) is larger than the area covered by the full blades (19).

[0030] Preferably, the inner wall of the pit (15) in contact with the raw material is provided with an ultra-high molecular weight polyethylene lining (3). Preferably, the inner wall comprises or is made of ultra-high molecular weight polyethylene, or the ultra-high molecular weight polyethylene is disposed on the side of the inner wall facing the raw material. Here, the ultra-high molecular weight polyethylene has the definition described above.

[0031] According to the present invention, the area covered by the umbrella-shaped blade (18) of the rotation axis (17) is greater than 2 / 3, preferably, the area covered by the umbrella-shaped blade (18) of the rotation axis (17) is greater than 3 / 4, preferably, the area covered by the umbrella-shaped blade (18) of the rotation axis (17) is greater than 4 / 5. For example, the area covered by the umbrella-shaped blade (18) of the rotation axis (17) is 3 / 4, and the area covered by the full blade (19) of the rotation axis (17) is 1 / 4.

[0032] Preferably, the umbrella-shaped blades (18) on the two rotating shafts (17) are arranged in an alternating manner. The inventors have found that, especially when conveying highly plastic materials, the alternating arrangement of the umbrella-shaped blades (18) can achieve mutual material clearing. At the same time, the umbrella-shaped blades (18) also reduce the resistance of the screw rotation when pushing the highly plastic material. At the discharge end of the screw, the full blades (19) are used to push the plastic material to the outside after extrusion.

[0033] Preferably, the rotating shaft (17) is driven by a drive mechanism, which is a hydraulic drive or a motor drive, preferably a hydraulic drive.

[0034] According to the present invention, the rotating shaft (17) includes a feeding end (20) and a discharging end (21), the feeding end (20) is located on the unloading side of the pit (15), and a conveying mechanism is provided at the bottom of the discharging end (21).

[0035] Preferably, the umbrella-shaped blade (18) is disposed away from the discharge end (21), and the full blade (19) is disposed close to the discharge end (21).

[0036] Preferably, a belt scale (22) is provided at the bottom of the discharge end (21), which has a weighing / weighing function and can feed back the weighing value to the controller in real time.

[0037] Preferably, the surface of the belt scale (22) is a smooth polyvinyl chloride tape.

[0038] Preferably, the belt scale (22) and the driver are further connected to a controller, which is used to adjust the speed of the driver according to the difference between the weighing value of the belt scale (22) and the set value.

[0039] Beneficial effects of the present invention

[0040] This invention solves the dust problem in the unloading area of ​​easily dusty raw materials by using a negative pressure unloading pit; at the same time, it effectively solves the metering and feeding problem of viscous raw materials by using the combination of a self-cleaning spiral assembly and a belt scale in the unloading pit of highly plastic raw materials. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the unloading pit in an embodiment of the present invention;

[0042] Figure 2 for Figure 1 A cross-sectional view of the unloading pit;

[0043] Figure 3 for Figure 1 A top view of the unloading pit;

[0044] Figure 4This is a schematic diagram of the negative pressure dust extraction hood in an embodiment of the present invention;

[0045] Figure 5 This is a schematic diagram of the structure of the unloading pit for highly plastic raw materials in an embodiment of the present invention;

[0046] Figure 6 This is a schematic diagram of the self-cleaning spiral assembly.

[0047] Among them, 1-unloading room, 2-unloading hopper, 3-ultra-high molecular weight polyethylene lining, 4-vehicle entry room, 5-vehicle exit room, 6-closed door, 7-dust extraction mechanism, 71-negative pressure dust extraction hood, 711-cylinder section, 712-cone section, 713-expansion chamber, 714-pressure reducing cone, 72-cyclone separator, 73-negative pressure fan, 8-dust collector, 9-feeder, 10-raw material conveying mechanism, 11-cyclone dust collector, 12-dust collector, 14-loader turning area, 15-pit, 16-self-cleaning spiral assembly, 17-rotating shaft, 18-umbrella-shaped blade, 19-full blade, 20-feeding end, 21-discharge end, 22-belt scale. Detailed Implementation

[0048] The following detailed description, in conjunction with specific embodiments, illustrates the general formula compounds of the present invention, their preparation methods, and applications in further detail. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0049] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0050] Example 1

[0051] Four quick-opening and closing gate-type (or roller shutter-type) sealing doors are installed within the enclosed passageway of the transport vehicles, thus forming a space including an unloading chamber and two spaces for accommodating transport vehicles. This ensures that the unloading area remains relatively enclosed compared to the external environment, guaranteeing its enclosure and preventing large amounts of air from flowing into and / or out of the unloading area when the next transport vehicle enters or leaves after unloading. Furthermore, the smaller pressure difference on both sides of the gate-type sealing doors facilitates their opening and closing. Figure 1As shown, the transport vehicle passes through the unloading pit from right to left to complete the unloading operation. Specifically, in this process, firstly, the closed door 6 is opened, and the transport vehicle (vehicle) drives into the space accommodating the transport vehicle, i.e., the transport vehicle entry compartment 4, through the closed door 6. Then, the closed door 6 is closed, and the closed door located on the side of the unloading compartment 1 and connected to the transport vehicle entry compartment 4 is opened. The transport vehicle continues to drive into the unloading compartment 1, and the closed door is closed to complete the unloading. Then, the closed door on the side of the unloading compartment 1 and connected to the space 5 accommodating the transport vehicle is opened, and the transport vehicle drives out to the space accommodating the transport vehicle, i.e., the transport vehicle exit compartment 5, and the closed door is closed. Finally, the leftmost closed door of the space 5 accommodating the transport vehicle is opened to allow the transport vehicle to drive out.

[0052] The inner wall of the elongated unloading hopper 2 is lined with ultra-high molecular weight polyethylene (UHMW-PE / PE1000, molecular weight: 500-1000 million g / mol, dynamic friction coefficient: 0.08, density: 0.93 g / cm³). 3 Lining 3 (water absorption rate <0.01%). Thus, in the case of different types of ceramic raw materials being transported successively by two or more transport vehicles, the raw materials unloaded first adhere as little as possible to the inner wall of the long strip unloading hopper 2, thereby avoiding cross-mixing of different types of ceramic raw materials and ensuring the stability of the ceramic tile raw material composition entering the corresponding raw material storage silo.

[0053] A dust extraction mechanism 7 is installed on the non-discharge side of the unloading pit, such as multiple (e.g., 6) negative pressure dust extraction hoods 71. Each dust extraction hood is connected to a cyclone separator 72, and each cyclone separator 72 is connected to a negative pressure fan 73. This multi-point decentralized dust collection can solve the problem of dust collection in a large space area and avoid environmental pollution caused by dust in the work area.

[0054] The exhaust gas, after being removed by dust collector 8, such as a cyclone separator, is uniformly collected into the main pipeline, and then discharged after being removed by a bag filter, or sent to the air inlet pipeline of the raw material grinding equipment as air for powder selection.

[0055] The cyclone separator 72 can be designed to include an inlet, a volute, and an inner cylinder lined with an alumina ceramic composite liner, while the straight cylinder, the inner wall of the cone, the inner wall of the expansion chamber, and the top surface of the decompression cone are made of ultra-high molecular weight polyethylene (UHMW-PE); the angle between the inner wall of the separator and the horizontal is greater than 80°, the cone is provided with an expansion chamber, the top of the expansion chamber is provided with a decompression cone, and the angle between the inner wall of the expansion chamber and the horizontal is greater than 60°.

[0056] Example 2

[0057] For example, see Figure 5 The pit for unloading highly plastic raw materials adopts a square cylindrical structure with side walls lined with ultra-high molecular weight polyethylene.

[0058] The bottom of the unloading pit for highly plastic raw materials includes multiple sets of hydraulically or electrically driven, dual-shaft self-cleaning screws, with hydraulic drive being preferred. Umbrella-shaped blades are distributed along the 3 / 4 length of the dual-shaft self-cleaning screws closest to the feed side, while full-length blades are distributed along the remaining 1 / 4 length. These umbrella-shaped blades are used to rake the highly plastic raw materials and reduce the resistance to the rotation of the dual-shaft screws. The umbrella-shaped blades on the two shafts are arranged in a staggered pattern, allowing for mutual cleaning during operation.

[0059] The full-length blades of the twin-shaft self-cleaning screw, extending approximately one-quarter of its length from the discharge end 21, are used to expel the highly plastic raw material through the conveying channel to the discharge port. Belt scales 22 are arranged below the outlets of multiple sets of twin-shaft self-cleaning screws. The speed of the multiple sets of twin-shaft self-cleaning screws is adjusted by a controller based on the difference between the weighing value and the set value of the belt scales 22, thereby ensuring stable material supply. Here, the weighing belt of the belt scales is made of smooth PVC tape, and the cleaning device includes flexible brushes.

[0060] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A discharge pit for dry powder production of raw materials for ceramic tiles, characterized in that, The discharge pit is used for conveying raw materials to a grinding device for grinding under low-dust or dust-free conditions, and the raw materials include non-plastic raw materials, weakly plastic raw materials and strongly plastic raw materials, and the discharge pit includes a negative pressure discharge pit for non-plastic raw materials and weakly plastic raw materials and a strongly plastic raw material discharge pit for strongly plastic raw materials, and the negative pressure discharge pit and the strongly plastic raw material discharge pit are connected with the grinding device through a raw material storage bin respectively. The negative pressure discharge pit includes a discharge chamber (1), the discharge chamber (1) is a closed space, the bottom of the discharge chamber (1) is provided with a discharge hopper (2), and the inner wall of the discharge hopper (2) is provided with an ultra-high molecular weight polyethylene lining (3) at the position in contact with the raw materials. Opposite two sides of the discharge chamber (1) are provided with spaces for accommodating transport tools: a transport tool driving-in chamber (4) and a transport tool driving-out chamber (5), and the discharge chamber (1), the transport tool driving-in chamber (4) and the transport tool driving-out chamber (5) are provided with closed doors (6) for the transport tools to pass through. One or more negative pressure devices are arranged in the discharge chamber (1), and the negative pressure device is a dust extraction mechanism (7); when a plurality of dust extraction mechanisms (7) are arranged, a decentralized arrangement is adopted; the dust extraction mechanism (7) includes a negative pressure dust extraction cover (71), a cyclone separator (72) and a negative pressure fan (73) connected in sequence; the negative pressure fan (73) is connected with a dust collector (8), and the negative pressure fan (73) collects dust in the upper part of the discharge chamber (1) into the dust collector (8). The strongly plastic raw material discharge pit includes a pit bin (15), the bottom of the pit bin (15) is provided with a plurality of groups of self-cleaning spiral assemblies (16), the self-cleaning spiral assembly (16) includes two rotating shafts (17), the rotating shaft (17) is provided with umbrella-shaped blades (18) and full blades (19), the area covered by the umbrella-shaped blades (18) is greater than the area covered by the full blades (19), and the umbrella-shaped blades (18) on the two rotating shafts (17) are arranged in a staggered manner; the inner wall of the pit bin (15) is provided with an ultra-high molecular weight polyethylene lining (3) at the position in contact with the raw materials.

2. The unloading pit according to claim 1, characterized in that The ultra-high molecular weight polyethylene lining (3) comprises or is made of ultra-high molecular weight polyethylene.

3. The unloading pit according to claim 2, characterized in that The ultra-high molecular weight polyethylene has a relative molecular weight of 5,000,000 to 15,000,000 g / mol; the polyethylene has a density of 0.920 to 0.960 g / cm 3 .

4. The unloading pit according to claim 2, characterized in that The ultra-high molecular weight polyethylene has a relative molecular weight of 8,000,000 to 12,000,000 g / mol, and the polyethylene has a density of 0.925 to 0.950 g / cm 3 .

5. The unloading pit according to claim 2, characterized in that The ultra-high molecular weight polyethylene has a relative molecular weight of 9,000,000 to 10,000,000 g / mol, and the polyethylene has a density of 0.930 to 0.940 g / cm 3 .

6. The surge bin of claim 1, wherein, The closed door (6) is selected from a shutter door or a roller shutter door.

7. The surge bin of claim 1, wherein, The inside of the discharge chamber (1) is further provided with a loader rotation area (14) for accommodating a loader, and the loader is used for unloading the raw materials in the transport tools and placing the raw materials into the discharge hopper (2).

8. The surge bin of claim 1, wherein, The rotating shaft (17) includes an inlet end (20) and an outlet end (21), the inlet end (20) is located on the discharge side of the pit bin (15), and the bottom of the outlet end (21) is provided with a conveying mechanism.

9. The dump pit of claim 8, wherein, The umbrella-shaped blades (18) are arranged away from the outlet end (21), and the full blades (19) are arranged close to the outlet end (21).

10. The surge bin of claim 1, wherein, The umbrella-shaped blades (18) cover more than 2 / 3 of the area of the rotating shaft (17).

11. The surge bin of claim 1, wherein, The umbrella-shaped blades (18) cover 3 / 4 of the area of the rotation axis (17) and the full blades (19) cover 1 / 4 of the area of the rotation axis (17). The umbrella-shaped blades (18) cover 3 / 4 of the area of the rotation axis (17) and the full blades (19) cover 1 / 4 of the area of the rotation axis (17). The umbrella-shaped blades (18) cover 3 / 4 of the area of the rotation axis (17)

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