During filtration, as shown in Figure 5.16.10, dusty gas passes upward into tubular or pocket-shaped bags that are closed at the top. Tubular bags are typically 10 m tall and 300 mm in diameter. A dust cake builds on the inside bag surface during filtration. Clean gas passes out through the filter housing. Filtration velocities are about 10 mm/sec (2 cfm/ft2). Many bags in a compartment act in parallel, and a fabric filter usually comprises several compartments.
Two cleaning techniques are used with inside collectors: reverse flow and shaking. In reverse flow, as shown in Figure 5.16.10, filtered gas from the outlets of other compartments is forced backward through the bags. Between the support rings, this reverse flow causes the bags to partially collapse causing the dust cake on the inside to deform, crack, and partially dislodge. A framework of rings keeps the bags open during cleaning. After a few minutes, the reverse gas stops, and filtration resumes. Cleaning is usually performed offline. Typical air-to-cloth ratios are 2
cfm/ft2, and dust cake weights range from 0.5 to 1.5 lb/ft2. The pressure drop across the fabric and dust cake with reverse flow is 0.5 to 1.0 in w.g. Some inside collectors used for oil mist recovery are not cleaned but are replaced when they become saturated with oil.
When equilibrium is established, the forces acting to remove the residual dust cake must equal those tending to retain it (Carr and Smith 1984). The removing forces are mechanical flexing and deformation, aerodynamic pressure, gravity, erosion, and acceleration by snapping the bags. Forces acting to retain the dust cake are adhesion and cohesion. Adhesion refers to the binding forces between particles and fibers when they contact each other; whereas cohesion refers to the bonding forces that exist between the collected particles. Forces acting to remove the residual dust cake increase with thickness or weight of the dust cake, while those tending to retain it decrease or remain the same. Thus, when equilibrium is achieved, a residual dust cake is established.
In shaking, the bag tops are connected to an oscillatory arm, causing the dust on the inside bag surfaces to separate from the fabric and fall into the hopper. Shaking is usually used in conjunction with reverse-gas cleaning. Although bags in shake and deflate units contain no anti-collapse rings, they retain a nominally circular cross section because the reverse-gas flow is low. The shaking force is applied to the tops of the bags causing them to sway and generating traveling waves in them. Deformation of the bags is significant in dislodging the dust cake.
Bags made from woven fabrics are generally used for these filters. Most large filters cleaned by reverse flow collect fly ash at coal-fired plants (Noll and Patel 1979). Bags are usually made of glass fiber to withstand the hot flue gases. Bag lifetime has exceeded 20,000 hr for woven bags. The long life is due to the low filtration velocity and infrequent cleaning (Humphries and Madden 1981). Filter problems in the utility industry are caused by improper bag specifications, installation, or tensioning. Other problems are related to bag blinding, a gradual, irreversible increase in pressure drop.
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