fabric filters Widely used in Europe, needled felts enjoy 85% of the market share of all fabrics used in dry filtration and are finding increasing use in the U.S. due to ever stringent federal regulations to control particulate matter
Lutz Bergmann Filter M e d i a C o n s u l t i n g LaGrange, G A 32041
Air pollution control in the U S .is being progressively enforced by the Environmental Protection Agency (EPA) and has created a fast-growing market for fabric filters. In general, particulate emissions are estimated to constitute 13% of total air pollution, coming from four different areas: air pollution control (as in public utilities) , dust collection (in cement plants, hot asphalt mixing), product recovery (carbon black, primary smelting), and pow der ed - mat e,ri a 1 ha nd 1i n g (spray drier, mining, food industry). The most versatile gas-stream cleaning systems involve the use of fabric filters (baghouses). The fabric-filter system selected depends on several variables such as: type of fumes, 1472
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cleaning system (pulse jet, shaker, reverse air, combinations), temperature of effluent, inlet concentration of particles, pressure drop limits, filter efficiency required, 6 chemistry of gas stream, nature of particles (size, surface area, shape), and space requirements and access to filter unit needed. In general, fabric-filter units are giant vacuum cleaners with basically one additional element. Both have in common the housing, the fan, and the “bag” doing the filtering. Fabric filters, however, also have a cleaning mechanism to ensure continuous operation. The fabric, often called the “bag”, is the heart of every filter, capturing solid particles while allowing air or gas to pass through. There are two basic, but important, differences: Collection: Collector:
Inside Reverse Air, Shaker Fabric Type: Woven
Outside Cage type (pulse jet) Needled
This article is devoted to “outside” collection units utilizing needled fabrics; however, the type of collector determines the kind of fabric being used. Incoming air or gas enters the collectors; all solid particles are trapped at the fabric’s surface, forming a cake but still permitting air to pass through. In such filter installations, it is essential to maintain a continuous process, thus avoiding a complete blinding or plugging of the filter media. For that purpose each filter bag is cleaned by methods shown in Figure 1. A certain amount of dust remains within the fabric; this is called a primary dust layer. It is important, however, in designing needled felts, to maintain the highest possible air handling capacity, while at the same time capturing the fine dust. A balance has to be found in these different parameters and always towards the maximum goal of the “ideal” filter media which provides the highest attainable efficiency with the greatest possible air flow at the lowest possible pressure drop across the fabric.
0013-936X/79/09 13-1472$01.00/0 @ 1979 American Chemical Society
Fabric filters are the most efficient type of filtration equipment. The performance of any fabric filter depends on the type of fabric chosen for a certain application. The filter media therefore must perform a t least three basic functions. They must: I . separate and collect particulate matter, 2. provide for cake-dust removal or cleaning operation, and 3 . have good durability-resistance to chemical, thermal and physical breakdown. Needled felts are three-dimensional filter media. The active element is the single filter and not the yarn as in woven fabrics. To improve dimensional stability, most needled fabrics have a scrim sandwiched i n the center of the felt between mechanically interlocked fiber layers. Felts made of highly shrinkable fibers-as increasingly used i n Europe-permit elimination of the scrim support, and thus are more economical. A unique feature of needled felts is fiber which is the distribution random in three dimensions (Figure 2). Fiber orientation is partly horizontal, partly vertical and a t many angles. Size, shape, and physical properties of the pores are of major importance for the
task of the filter function. The filter has to separate the submicron fine particles efficiently and also permit the passage of a large volume of air or gas. Thus, formation of the pores should be fairly uniform so that filtering pressure is distributed uniformly. The main objectives in designing the most useful needled felt for fabric filters are concentrated in three basic areas: I . highest obtainable efficiency, 2. lowest possible differential pressure, and 3 . sufficient life cycle for economy. The fibrous structure and porous surface of needled fabrics permit efficient collection of even submicron fine dust, adhesive fumes, mechanically abrasive particles, hygroscopic chemicals, or even oily substances provided the surface of the feit is properly treated, the right fiber is selected, the cleaning frequency is adjusted, and all of this is based on experienced A / C ratio relationship. Surface modification Capacity, cleanability, and efficiency are greatly influenced by the fabric surface. Therefore, surface
modifications have been shown to have an especially marked influence on the performance of filter fabrics-notably filter felts. Through controlled heat treatment, fabric surface alteration can be achieved to prevent serious penetration of particles into the inner structure of the felt. Instead, the dust is encouraged to form a protective primary layer on the outer surface. As a result, permeability at equilibrium can be maintained at a high level, and, therefore, provide better overall filter capacity. Often, the improved surface encourages formation of a more porous dust layer. The advantages of such treated felts are better cleaning and simpler dust removal. It is then immaterial whether the cleaning is carried out by reverse air, shaking, oscillating, or high-pressure air. Laboratory observations of deposits on felted-filter media indicate that an initial “depth” filtration period is followed by cake formation in constant velocity filtration. These observations suggest that classical liquid-solid filtration theory may be applicable to gas-solid systems; microscopic examination of these deposits confirm that cake filtration is the principal mode of separation.
FIGURE 1
Different cleaning devices
3 Plenum
cleaning 1 Venturi
cleaning
4 Compartment
LHigh energy (80-1 20 PSI)
cleaning
Low volume (.01 S) (Micro Pul, Flex Kleen, etc.)
2 Single-element
cieaning
5 Mtscellaneous
e system y, Carter Day, etc.)
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These observations are tremendously important. We must try to design the felt so that cake forms on the outside of the fabric rather than traveling into the center, thus making it more difficult to remove. This fact is still not fully recognized in our industry and, more importantly, not fully understood by original equipment manufacturers, as well a s end-users. The multifiher-layer design concept, as shown in Figure 3, is a successful step toward that direction. This concept incorporates different layers of fibers of different fineness all needled into a filter felt. The theoretical thesis behind this concept demands the ability to capture particles on the surface or not a t all. Very fine dust shall travel, if not trapped at the surface, all the way through. the felt, rather than halfway, and then settle.
Air permeability Air permeability, as commonly reported in cfm (cubic feet per square foot of cloth measured at Y-in.-W.G.), for the new fabric represents a close approximation of the starting airtransfer qualities of the filtration fabric. Generally, rapid changes occur in air flow early in filter use. The air flow rate is largely dependent upon the fabric structure and the filtrate characteristics, reaching a new and steady state known as “service permeability”. Thus, while the initial fabric structure and air permeability represent only part of the fabric specification, they are fundamental and influence the entire filtration operation including the type and rate of cake formation. Air permeability, under steady-state working conditions, will remain a t roughly 20-50% of the initial clean cloth throughout, at an operating differential pressure of 3-6 in. W.G. It is the target in fabric design to keep this value of equilibrium permeability as high as possible. Efficiency In looking at different factors, the type of fiber, its dullness, crimp, fineness, and generic heritage have the most dramatic effect on field performance. The kind of dust to he handled may vary from fine to coarse, dry to moist, light to heavy, chemically inert to aggressive, soft to abrasive; it basically has less effect on the fabric filter’s performance. With proper selection of the right fabric type, a fabric filter is the most versatile control equipment for effluent gases. Needled fabrics have contributed substantially to an excellent performance revutation. The main features 1474
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Guidelinesfor filter rates in ft/min Fine dusts
