Discussion on the selection of bag filters

I’m also looking for this. I have some related materials here; you can take a look first to see if they can be of any help to you. FEATURE Bag filters: To meet the various requirements posed by different industries, a range of solid-liquid separation filtration devices have been developed. Regardless of their design, these devices operate based on one of two filtration mechanisms: one involves filtration taking place on the surface of the filtering medium, where the filtering medium is thin, and solid particles are trapped on its surface while the filtrate passes through the pores in the medium ； In one case, the filtering medium has a certain thickness, and the solid-phase particles are mainly retained within the filtering medium. Both of these filtration mechanisms rely on the pressure difference across the filtering medium, and there is no absolute boundary between them; they often exist simultaneously. Its filtration rate is generally related to factors such as the pores of the filtering medium, the filtration pressure, the solid-liquid concentration of the suspension, the particle size distribution of the solid phase, the compressibility of the solid phase, and the nature of the suspended colloids. Currently, solid-liquid separation processes in fields such as environmental protection and the chemical industry require a high degree of precision; the materials to be processed are often difficult to separate. Therefore, filtration equipment needs to have a simple structure, allow for accurate and easy operation, produce filtrates that are as clear as possible, have a short settling time at the beginning of filtration, and possess sufficient mechanical strength and stiffness. Additionally, economic considerations and quality control must be taken into account, as well as the need to carry out the entire process within a sealed system. The bag filter is a new type of filter that meets the aforementioned requirements, and is suitable for the solid-liquid separation of suspensions that are difficult to filter. Structure and working principle: As is well known, in filtration that takes place on the surface of a filtering medium, the fresh filtering medium allows for high filtration efficiency at the beginning. The larger particles in the suspension first form a filter cake layer on the surface of the filtering medium, while the smaller particles are carried away with the filtrate (the filtering medium itself cannot perform precise filtration). At this stage, the filtrate is not clear, as it contains many tiny particles. As filtration proceeds and the filter cake layer thickens, these tiny particles get trapped within the filter cake layer, resulting in increasing clarity of the filtrate. Meanwhile, the filtration resistance also increases (during filtration, the resistance of the filtering medium can be considered constant, but the resistance of the filter cake layer increases as its thickness increases). As the filtration time increases, tiny particles and gel-like substances clog the flow channels in the filtering medium through which the liquid passes, resulting in a decrease in the filtering rate of the medium, until those flow channels become completely blocked. A bag filter consists of two main components: the filter housing and the filter bags. The filtration process takes place on the surface of the filtering medium. A very convenient backwashing technique is used to clean the filter bags during filtration; it is a relatively innovative type of pressure filtration device. Its working principle is as follows: the slurry (suspension) is pumped into a pressure vessel, where it is filtered through the filter bags. During filtration, since the diameter of the filter bag is larger than that of the cage frame, the filter bag presses tightly against the cage frame during pressure filtration. The intercepted solid particles adhere to the surface of the filter bag to form a filter cake, while the filtrate that has passed through the filter bag is discharged from the housing. Filtering is stopped when the filter cake layer adhering to the surface of the filter bag reaches the optimal thickness for operation; the residual liquid is drained, and compressed air is used to reverse-blow the filter bag to remove the residue. During backwashing, the filter bags expand and return to their original diameter; the filter cake on the outer surface of the filter bags cracks into small pieces that are not connected to each other. The backwash air causes the filter bags to vibrate continuously, causing the small pieces of filter cake to fall off, thereby cleaning them thoroughly and quickly. After removing the filter cake, the filter bag is rinsed with washing water to regenerate it. Bag filters utilize this back-blowing and reverse-washing technique to keep the filter bags clean at all times, ensuring they are in an optimal condition for filtration. Therefore, bag filters are particularly suitable for the separation of difficult-to-filter suspensions containing many small particles and colloids. 2. Experimental setup and results The experimental setup consists of a single-tube filter that uses filter bags as the filtering medium. The filtering medium is a cage-type support made up of 12 round steel bars with a diameter of 6 mm and a length of 1,200 mm, made of 1Cr18Ni9Ti material, which are welded together along with ring-shaped struts; filter bags made of 20# polyester fabric are then placed over this structure (the actual filtering area is 0.38 m²). The filter slurry used in the experiments was taken from the circulating wastewater tank of the boiler water film dust collector. The analysis of solid particle sizes and the solid-liquid ratio are shown in Table 1 and Table 2, respectively. The pressure of the filter slurry remained constant throughout the experiment. Based on the analysis of the curves in the graph, the filtrate contains tiny solid particles; at the beginning of filtration the filtrate is turbid due to a high concentration of these particles. However, once a filter cake of a certain thickness forms on the surface of the filter bag, the filtrate becomes clear. The curves show that 10 minutes after starting the filtrate pump, the solid-to-liquid ratio in the filtrate stabilizes at around 0.35 mg/m³, with the solid particle content in the filtrate remaining below 3,173. The solid phase concentration in the filtrate is high, so the thickness of the filter cake increases rapidly; moreover, the solid-liquid ratio in the filtrate reaches a stable value more quickly as well. The filtration rate is related to the filtration pressure; as the pressure increases, the filtration rate increases as well. When the driving force remains constant, the filtration rate gradually decreases over time and also decreases as the solid content ratio in the filter cake increases, which reflects changes in the thickness of the layer of solid particles formed on the surface of the filtration medium. Product features:  The filtering elements are installed vertically inside the housing, resulting in a very compact structure and a large filtering area per unit volume ；  The filter bag has low filtration resistance, high filtration speed, fast regeneration, and is inexpensive ；  Simple structure, easy to operate and maintain, lightweight, convenient for loading and unloading; backwashing for slag removal results in clean results ；  It has excellent filtering performance, and is particularly suitable for separating difficult-to-filter suspensions containing many small particles and colloids.

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