How Does the Coal Roller Screen Shaft Number Aaffect Screening Results
Introduction
Roller screen is a kind of equipment that uses the gaps between the sieve discs on multiple parallel arranged sieve shafts rotating in the same direction to screen materials. It is widely used in industries such as mining, coal, building materials, and power for the classification treatment of medium and coarse particle size materials. The screening effect is usually measured by multiple indicators such as processing capacity, screening efficiency, screening accuracy, and material adaptability. The number of sieve shafts, as a basic structural parameter of the equipment, is closely related to these indicators.
The influence of roller shaft number on roller screen screening effect
Processing capacity
The more sieve shafts there are, the longer the sieve surface is usually. The material has a longer travel distance on the sieve surface and more opportunities for screening. In a unit of time, the roller sieve can accommodate and process more materials, and the theoretical maximum processing capacity will increase.
However, increasing the number of sieve shafts will increase the equipment volume and weight, raise the manufacturing cost and occupy more space. At the same time, driving more sieve shafts requires greater power.
Screening efficiency
The number of sieve shafts is small and the sieve surface is short. As a result, the materials will pass through the roller sieve very quickly. This may cause some fine particles to be discharged before they have a chance to pass through the sieve, becoming the sieve residue and resulting in incomplete sieving and a decrease in the yield of the sieved material.
By increasing the number of sieve shafts and extending the sieve surface, it is equivalent to prolonging the time that the materials stay on the sieve, providing more opportunities for fine particle materials to pass through the sieve aperture multiple times, enabling them to undergo more thorough sieving. This significantly improves the sieving efficiency and reduces the contamination of qualified particles in the sieve residue.
Screening accuracy
The longer sieve surface (with multiple sieve axes) allows the materials to have a longer “loosening and stratification” period during the transportation process. Under the rolling effect of the sieve axes, fine particles can settle at the bottom of the material layer and come into contact with the sieve surface, while large particles float on the upper layer. The more thorough this process is, the higher the “probability of passing through the sieve” for particles near the sieve holes will be.
The product size separated by the roller sieve will be more uniform and the grading effect will be better. However, if the sieve surface is too short, it is prone to “immediate sieving”, meaning that the materials are discharged without sufficient stratification, resulting in mixed particle sizes, high powder content in the sieve top material, and possible inclusion of individual large pieces in the sieve bottom material.
Material adaptability
For “difficult-to-sieve materials” with high moisture content, high viscosity, or containing a large amount of particles close to the sieve holes, a longer screening path and more tumbling times are required to break adhesion and promote screening. More sieve shafts provide this possibility and enhance the equipment’s adaptability to complex materials.
By setting different numbers of sieve shafts in different sections, for example, the gap between the shafts at the front section is large for pre-screening, and the gap at the rear section is small for fine screening, multi-level screening can be achieved. This increases the flexibility of the roller screen process design.
Determination of the number of sieve plates on the shaft
Screen hole size
The gap between the screen plates is the size of the screen holes. It determines which particles can pass through the screen (becoming the undersize material) and which cannot (becoming the oversize material).
When the diameter of the screen shaft and the width of the screen surface are fixed, the more screen plates there are on the roller screen, the smaller the gap between the plates, and the finer the separation particle size. Conversely, the fewer the number of screen plates, the larger the gap, and the coarser the separation particle size.
Usually, the separation particle size required by the process is determined first, and then the gap between the screen plates is calculated based on this particle size to determine the number of screen plates.
Screen surface width and processing capacity
When the number and diameter of the rollers in a roller screen are fixed, the wider the screen surface, the greater the amount of material passing through the screen per unit time, and the processing capacity will accordingly increase. This is because a wider screen surface provides a larger passage space for the materials, reduces the accumulation and blockage of materials on the screen, and enables the materials to pass through the screening area more smoothly.
The increase in screen surface width requires stronger structural support to ensure the stability of the screen surface, and possibly requires greater driving power to drive the wider screen surface to rotate.
Material Characteristics
For materials with a wide particle size distribution, if the number of sieve shafts and the gap between sieve plates are not set appropriately, it may cause fine particles to pass through the sieve prematurely or coarse particles to block the sieve holes, thereby affecting the efficiency and accuracy of sieving. High-humidity or viscous materials are prone to adhere to the sieve shafts and sieve plates, forming blockages, which reduce the sieve-through rate. At this time, it is necessary to increase the rotational speed of the sieve shafts or adopt specially designed sieve plate structures to reduce adhesion.
When the proportion of particles near the sieve holes in the material is high, more precise sieving settings are required, such as increasing the number of sieve shafts and reducing the gap between sieve plates, to improve the sieving accuracy. Therefore, when designing a roller screen, it is necessary to fully consider the characteristics of the material, reasonably set the number of sieve shafts and the gap between sieve plates, in order to achieve the best sieving effect.
Screen shaft strength and driving power
When the number of sieve shafts increases, the pressure and friction force exerted by the materials on each sieve shaft will decrease accordingly, which to some extent helps to extend the service life of the sieve shafts. However, an increase in the number of sieve shafts also means an increase in the overall weight of the roller screen, which places higher requirements on the supporting structure and drive system of the sieve shafts.
Analysis of reasons roller screens are less prone to clogging
The rotational movement of the screen shafts
A roller screen is a type of mechanical equipment mainly used for screening and separating materials of different sizes. Its core component consists of multiple parallel arranged screen shafts, which rotate in the same direction and are equipped with screen discs on top. Unlike traditional vibrating screens, the working principle of a roller screen is not to make the materials “jump” over the screen surface through vibration, but to utilize the slow rotation of the screen shafts, causing them to roll, move and gradually pass through the gaps of the screen discs. This enables it to handle some special materials more effectively, such as damp or highly viscous materials.
The special shape of the sieve plate
The sieve plates of the roller screen are usually not circular but have irregular shapes such as Plum Blossom-shaped or hexagonal. When these sieve plates rotate， the gaps between them and their adjacent counterparts will change periodically. This changing gap will produce a slight axial “shear” and “combing” effect. Any flexible or fibrous debris caught between the sieve plates will be forcibly “pushed” out or “cut” off by this action， rather than remaining stuck there and causing blockage.
The relative motion between the sieve shafts
The two adjacent sieve shafts rotate in opposite directions or have a speed difference. When the tip of the sieve disc on one sieve shaft approaches the sieve disc of the other sieve shaft, they act like two combs combing each other, which can remove the materials stuck in the gaps of each other and achieve mutual self-cleaning.
The sieve holes are not easily clogged
Due to the continuous rotation of the sieve plate, the effective sieve separation gap (i.e., the sieve holes) between the sieve plates are always in a moving state. This is completely different from the static sieve holes on a fixed sieve plate. For wet fine materials, the static sieve holes are easily clogged (i.e., the materials stick to the sieve wall), while the dynamic sieve holes are much less likely to be stably attached and blocked.
Conclusion
The number of sieve shafts is a key parameter in the design of a roller screen. In actual selection and design, it is necessary to comprehensively determine the number of sieve shafts based on specific material characteristics (particle size, moisture content, viscosity), processing volume requirements, screening efficiency indicators, as well as site and energy consumption restrictions.
