Bag filters are used for the clarification/filtration of fluids that have a relatively small loading of particles to be removed. The particle suspension passes through and the particles settle in the bag. Filtration occurs from inside to outside with a delivery speed of about 100 m3/m2 h. Therefore bag filters are normally used with a supporting vessel. In situations where no support is used, the pressure drop across the filter needs to be lower. In situations where bag filters are used, industry standards normally define the size of the bag house. The supporting vessel provides mechanical support for filters with high throughput rates and prevents the filter material from elongating. Bag filters are normally made from needle punched nonwovens and act as depth filters. The pore volume varies between 70–90%. The filter bags are normally stitched at the edges. Bypass filtration can be avoided by welding the seams, however, this is only possible if the material used is thermoplastic e.g. polypropylene or polyester.
Bag filters, similar to cartridge filters, have various configurations and materials of construction, yet the flow in this process is inside-to-outside. A bag filter normally has a connection for a high-pressure inlet on the top and filtrates exit at the sides and bottom. The solids stay inside the bag. A metal or plastic cage (perforated basket) holds the bag in place during operation. The dirt-holding capacity is the important parameter for the filter design. Depending upon the bag construction—whether it is mesh or felt or single, multilayer, or pleated—the dirt-holding capacity will increase. For instance, a pleated bag has the largest filter area. Bag filters are used for noncritical aqueous applications where the bags and solids can be easily disposed.
Bag filters have now been almost entirely superseded for liquid filtration by other types of filter, although one of the few remaining types is the Taylor bag filter which has been widely used in the sugar industry. A number of long thin bags are attached to a horizontal feed tray and the liquid flows under the action of gravity so that the rate of filtration per unit area is very low. It is possible, however, to arrange a large filtering area in the plant of up to about 700 m2. The filter is usually arranged in two sections so that each may be inspected separately without interrupting the operation.
Bag filters are still extensively used for the removal of dust particles from gases and can be operated either as pressure filters or as suction filters.
Bag filters are used for a number of liquid filter applications including bulk chemicals; food industry applications such as vegetable oil, coolants, cleaning fluids, paints, varnishes, waxes, and plastisols. Bag filters may be as simple as a filter medium bag attached to the end of a pipe. It will probably be more secure if it is attached by means of an adapter as shown in Figure 7.15. For higher-pressure and larger volume applications, the bag is inserted into a housing and supported by a mesh basket as displayed in Figure 1.1.
Figure1.1.Filter bag housing
Flow of contaminated liquid is usually from the inside of the bag to its outside, with filtration occurring on the inner face of the bag, predominantly by surface filtration (followed by cake formation), with a degree of depth filtration with felted media. Figure 4.54 shows the principle of operation of the liquid bag filter. (There is no strong reason why filtration should not occur in the opposite direction, with the bag supported on a cage – as is mostly the case with gas filtration in bag filters. The layer of collected contaminants now forms on the outside of the bag, from which it can be blown, by a reverse flow of liquid, to accumulate in the base of the housing, from which it is blown out periodically.)
Modern filter media technology has enabled filter bags to be manufactured in a variety of materials offering ratings of between 1 and 1200 µm. These materials include nylon, polypropylene, polyester, porous PTFE film and other fluoropolymers, viscose, aromatic polyamides, felts and woven wool. The development of seamless bags in these various materials has helped to maintain the popularity and ubiquity of liquid bag filters. One piece glass-reinforced polypropylene all-plastic bag filters are extremely corrosion resistant, and can replace more expensive filters with plastic-lined housings in many applications.
Most liquid bag filters are of the single bag in a single housing type. Flow through such a unit must be stopped when the pressure drop across it indicates that the maximum dirt-holding capacity has been reached. Bag filters can also be used, in this polishing role, in a duplex housing, with two bags side-by-side, and piped so that each is either online, filtering, or offline, being cleaned (as shown in Figure 1.2).
Figure 1.2. Single bag filter arrangement.