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What are the factors that affect the filtration efficiency of the dust collector?
The bag filter is to separate and collect the solid particles in the flue gas by filtering. The dust collection process mainly includes screening, inertial collision, interception, diffusion and electrostatic adsorption. The dust collection process in a short time after the new filter bag or cleaning mainly includes inertial collision, interception, diffusion and electrostatic adsorption. After the dust adhesion layer is formed on the filter cloth, the collision, interception, diffusion and electrostatic adsorption will gradually become smaller (Figure 1 ). The factors affecting the filtration efficiency of the bag filter include dust characteristics, flue gas properties, filter bag characteristics, unit operating conditions, the thickness of the dust layer on the filter bag surface, the pressure loss of the filter bag and the filtration speed. The commonly used filter bag treatment processes for bag filter in thermal power plants include PPS+PTFE film coating, PPS+15% P84, PPS+surface treatment, the resistance of the dust removal system is all <1500Pa, the emission concentration of the filtered flue gas is <50mg/m3, and the filter bag life is >30000h. 1. Filtration speed The filter wind speed v of the bag filter refers to the average speed of the flue gas passing through the surface of the filter bag, v=Qt/60F. Among them, Qt is the flue gas flow through the filter bag, m3/h; F is the total area of the filter bag, m2. The choice of filtration speed should consider factors such as economy, filter bag service life and filtration efficiency. When the same dust quality is accumulated on the surface of the filter bag, the cumulative dust collection rate decreases significantly with the increase of the filtration speed. In addition, the selection of filtration speed is closely related to the cleaning method, cleaning system, dust characteristics, inlet dust concentration and other factors. Generally, for low-pressure pulse jet cleaning and dust removal, the filtration speed is≤1.0m/min, filter speed after adding dust pretreatment device≤1.1m/min; filter speed by backflushing≤0.8m/min; when the dust particles are small, or the inlet dust concentration is high, and the dust viscosity is large, the filtration speed should be reduced. Reducing the filtration speed can improve the filtration efficiency and prolong the service life of the filter bag. 2. Resistance of the bag filter The resistance of the bag filter determines the energy consumption, filtration efficiency and cleaning cycle of the dust removal system. factors such as gas viscosity. The resistance Δp of the bag filter is composed of the structural resistance Δpc, the resistance of the cleaning filter bag Δpf and the resistance of the dust layer deposited on the filter bag Δpd: Δp = Δpc + Δpf + Δpd The flow resistance caused by plates, etc., usually Δpc<300Pa. Cleaning filter bag resistance Δpf is: Δpf = ξfμυ (2) where: ξf is the filter bag resistance coefficient, m-1;μ is the viscosity of the gas, Pa·s;υis the filtered wind speed, m/s. Generally, Δpf<300Pa, the dust layer resistance Δpd is related to the filter bag, dust particle size distribution, chemical composition, flue gas temperature, moisture content and other factors, which need to be determined through experiments. The resistance of the dust layer under specific conditions is shown in Figure 2. Under the same filtration speed, with the increase of the thickness of the dust deposition, the resistance of the dust layer increases; when the filtration speed is less than 0.5m/min, the thickness of the dust layer has nothing to do with the resistance; when When the filtration speed is less than 1.2m/min and greater than 0.5m/min, after the thickness of the dust layer increases to a certain extent, the resistance changes linearly when the thickness of the dust layer continues to increase. The increase of the resistance of the bag filter can improve the filtration efficiency, but the increase of the operating resistance will increase the energy consumption. Therefore, there is an optimal balance between the resistance of the bag filter and the filtration efficiency. It is recommended to control the resistance of the bag filter between 1200 and 1500Pa. 3. The influence of dust particle size on filtration efficiency. The size of flue gas dust particle size directly affects the filtration efficiency of bag filter. The classification efficiency of dust particle size under different conditions is shown in Fig. 3 [1]. It can be seen from Figure 3 that the filtration efficiency increases with the increase of particle size, but the particle size is 0.2-0.4μThe filtration efficiency of m dust is the lowest under different filter bag conditions. Compared with the filter bag after dust collection and cleaning, the filtration efficiency of the clean filter bag is the lowest, and its filtration efficiency mainly depends on the filter bag.μm dust filtration efficiency is significantly improved. 4. Dust pretreatment Dust pretreatment is mainly to reduce the dust concentration and change the dust particle size distribution to reduce the dust load of the filter bag, thereby reducing the number of cleaning times and prolonging the service life of the filter bag. The dust concentration can be appropriately reduced by setting up electrostatic precipitators, cyclone dust collectors, shutters, humidification towers, etc. in front of the bag filter. Changing the particle size distribution of dust is an important way to solve the problem of ultra-fine dust collection, that is, through electrocoagulation, liquid coagulation, and chemical coagulation, small particles of dust can be turned into larger particles of dust for utilization and collection. More about : Filter Material Testing Instruments
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