Basic Knowledge of Coal Screening Machine Selection

　　The coal screening machine plays a very critical role in the coal conveying system of coal-fired power plants, especially the fine coal screening machine in circulating fluidized bed units. To achieve better particle size distribution and minimize over-crushing, users have higher demands for the screening machine, such as higher screening efficiency, less maintenance, and lower maintenance costs. Therefore, the choice of screening type becomes very important.

　　Well-known academic institutions in China conducting research on screening machines include Northeastern University, China University of Mining and Technology, Beijing Iron and Steel Research Institute, etc. These institutions have academicians, professors, and supervisors conducting research projects on the theory and equipment manufacturing of screening, crushing, and feeding in various basic industries, contributing to the advancement of China's screening technology. Research on screening machines should not only be done in China but also involve cooperation with major international screening and crushing companies to introduce cutting-edge technology and follow a path with Chinese characteristics.

　　However, regardless of the type of screening machine chosen, the fundamental parameters cannot be arbitrarily interpreted, such as screening efficiency, the material distribution form of the screening machine, the material stratification mechanism, screening time, screening area, screen opening rate, screen utilization rate, screen thickness, and anti-clogging technology of screen holes. For vibrating screens, parameters like vibration frequency, vibration intensity, vibration acceleration, amplitude, and vibration direction must also be considered. For roller screens and drum screens, factors such as circumferential speed, screen surface angle, and vibration must be taken into account. For fluctuating screens and swinging screens, mechanical synchronization of the shaft and mechanical strength of the crank must also be considered.

　　Screening efficiency is a key factor and the fundamental requirement of users. Losing focus on screening efficiency means losing the fundamental purpose of using the screening machine.

　　If a screening machine lacks a distribution structure and the screen surface cannot be fully covered, the utilization of the screening area will decrease, directly affecting screening efficiency. Incomplete coverage also easily leads to localized wear, increasing local maintenance. Without an effective material stratification mechanism, materials move flatly, and fine raw coal cannot quickly stratify and sink. Thus, the chance for fine materials in the middle of the material group to contact the screen holes will not exist, making it impossible to achieve high screening efficiency.

　　Screening time is also an extremely important factor. The speed of the coal belt conveyor is fixed, so the screening and conveying speed of the screening machine must keep up with or exceed the belt speed; otherwise, instantaneous blockage of the screening machine will occur. Theoretically, the longer the screening time, the higher the screening efficiency, but this must be controlled within a certain range.

　　If a certain type of screening machine has a very small screening area, low screen opening rate, and low screen utilization rate, yet achieves very high screening efficiency, this contradicts basic scientific knowledge and is impossible.

　　A very thick screen will cause the screen holes to be parallel and narrow, prolonging the passage time of fine materials, which easily accumulates and gradually clogs the narrow screen holes. Therefore, if the screening machine lacks effective anti-clogging measures for screen holes, it will clog when encountering wet sticky raw coal or coal slime. Once the entire screen surface is clogged, screening efficiency drops to zero.

　　If choosing a vibrating screen, vibration acceleration must be considered. The higher the acceleration of the screen, the stronger the ability of wet sticky materials to break free from the adhesion to the screen, improving the cleaning effect. The larger the screen angle, the faster the material movement speed, theoretically reducing screening efficiency. Therefore, the angle must be reasonable; too large or too small is unfavorable for screening. Higher vibration frequency means smaller amplitude, which is also a regular pattern. For handling wet sticky raw coal, larger amplitude and lower frequency are generally chosen. For extremely fine dry raw coal, smaller amplitude and higher frequency are usually selected, which can multiply screening efficiency.

　　When selecting a vibrating screen, it is essential to properly handle the interrelated issues of vibration frequency, vibration intensity, vibration acceleration, amplitude, and vibration direction.

　　If choosing drum screens or roller screens, in addition to following the above regular factors, attention must be paid to the moderate rotational speed of the screen drum and shaft. The higher the speed, the shorter the material screening time, causing a sharp decline in screening efficiency. Conversely, if the speed is too low, blockage of the screening machine will occur. Excessive speed also causes strong vibration. Vibrating screens use vibration principles and are not afraid of vibration, but drum and roller screens should avoid mechanical fatigue caused by vibration to prevent rapid bearing damage, shaft breakage, and risks to downstream equipment. Additionally, attention should be paid to issues caused by iron wires, iron blocks, detonating cords, wood, woven materials, etc., which increase resistance such as shaft jamming and entanglement, causing shutdowns.

　　If choosing fluctuating or swinging screens, mechanical synchronization of the shaft and mechanical strength of the crank must be considered. The principle of crank connection and multi-shaft mechanical synchronization to achieve reciprocating swinging is very simple; historically, steam engines were driven by this principle. However, with multiple shafts connected, machining precision increases exponentially with the number of shafts, sometimes making it difficult to assemble before leaving the factory. This type of screening machine cannot be made very long or wide; screening area and opening rate are limited. It can be applied in coarse particle screening processes.

　　In summary: screening machine selection must follow the basic principles of high screening efficiency, reliable operation, and easy maintenance. The basic elements and their purposes of screening machines are briefly listed.

　　1: Uniform distribution measures: It is recommended to add distribution measures to improve screening efficiency.

　　2: Screening area: Adhere to the principle of choosing larger rather than smaller to improve screening efficiency.

　　3: Screen utilization rate: Choose a higher screen utilization rate to improve screening efficiency.

　　4: Screen opening rate: Choose a larger screen opening rate to improve screening efficiency.

　　5: Screen thickness: Choose thinner screen materials to reduce passage time and improve screening efficiency.

　　6: Screen angle: Choose a reasonable angle to improve screening efficiency.

　　7: Amplitude: Choose a moderate amplitude to improve screening efficiency.

　　8: Vibration frequency: Choose a moderate vibration frequency to improve screening efficiency.

　　9: Vibration acceleration: Choose higher vibration intensity to improve screening efficiency.

　　10: Rotational speed: Choose a reasonable rotational speed to improve screening efficiency.

　　11: Rotational bearing lubrication method: Choose a simple and effective lubrication method to improve equipment reliability.

　　12: Equipment sealing: Choose a method that is easy to disassemble and reliably sealed to meet environmental protection requirements.

　　13: Energy consumption: Choose equipment with lower energy consumption to reduce operating costs.

　　14: Maintenance amount: Choose equipment with less maintenance to reduce labor intensity and lower usage costs.

　　15: Program control: Choose practical and reliable program control to improve system safety.

　　The above indicators are not exhaustive; some factors are interrelated and exhibit mutually extreme mechanisms. Additionally, the geographical location of the equipment, the characteristics of the process it is involved in, and the nature of the materials all contribute to the confusion in selecting screening machines.

　　Therefore, mastering basic screening theory is essential. Communicating with experts and scholars from various fundamental industries will greatly improve screening and crushing capabilities, making the selection process for screening equipment clearer. After rigorous scientific investigation and verification, a correct choice is believed to be achievable.