Pt APr APc 686

KR KC

where APr is the conditioned residual pressure drop; APc is the dust cake pressure drop; KR and KC are the filter and dust cake permeabilities, respectively; V is the superficial velocity; ^g is the gas viscosity; and xr and xc are the filter and dust cake thicknesses, respectively. The permeabilities Kr and Kc are difficult quantities to predict with direct measurements as they are functions of the properties of the filter and dust such as porosity, pore size distribution, and particle size distribution. Therefore, in practice APr is usually measured after the bags are cleaned and APc is determined using the equation:

where ci is the dust loading and, along with V, is assumed constant during the filtration cycle; t is the filtration time; and K2, the dust resistance coefficient, is estimated from:

where dg is the geometric mass median diameter (m), ^g is the gas viscosity (kg/m/sec), pp is the particle density (kg/m3), and V is the superficial velocity (m/sec).

Basic Types of Fabric Filters The three basic types of baghouses are reverse-gas, shake-deflate, and pulse-jet. They are distinguished by the cleaning mechanisms and by their A/C ratios. The A/C ratios for fabric filters range from a low of 1.0 to 12.0 ft/min depending on the type of cleaning mechanism used and characteristics of the fly ash [6]. Ash that accumulates on the bags in excess of the desired residual dust cake must be removed by periodic bag cleaning to reduce the gas flow resistance (and, hence, induced draft fan power requirements) and to reduce bag weight. In U.S. utility bag-houses, cleaning is done off-line by isolating individual compartments for cleaning.

Reverse-Gas Fabric Filters Reverse-gas fabric filters are generally the most conservative design of the fabric filter types. They typically operate at low A/C ratios ranging from 1.5 to 3.5 ft/min [6,68]. Fly ash collection occurs on the inside of the bags, because the flue gas flow is from the inside of the bags to the outside, as illustrated in Figure 6-20 [67]. Reverse-gas baghouses use off-line cleaning, where compartments are isolated and cleaning air is passed from the outside of the bags into the inside, causing the bags to partially collapse and release the collected ash. The dislodged ash falls into the hopper. A variation of the reverse-gas cleaning method is the use of sonic energy for bag cleaning. With this method, low-frequency (<250-300 Hz), high-sound-pressure (0.3-0.6 inH2O) pneumatic horns are sounded simultaneously, and the normal reverse-gas flow adds energy to the cleaning process. Reverse-gas fabric filters are widely used in the United States; approximately 90% of the utility baghouses employ this reverse-gas cleaning process [67].

Reverse Gas Valve purge

Outlet Valve AirVaive

Bypass Valve

Outlet Manifold

J_ Gas Outlet

Purge Gas

Tubesheet

Dirty Flue Gas Inlet

Bypass Valve

Outlet Manifold

Tubesheet

Dirty Flue Gas Inlet

J_ Gas Outlet

Purge Gas

Clean Flue

FIGURE 6-20. Schematic diagram of the compartments in a reverse-gas baghouse illustrating the flue gas and cleaning air flows during the various cycles of operation. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

Clean Flue

Null \ Cleaning Purging Filtering Disposal

(Reverse Gas) Inlet Manifold Thimble

FIGURE 6-20. Schematic diagram of the compartments in a reverse-gas baghouse illustrating the flue gas and cleaning air flows during the various cycles of operation. (Source: Bustard, C. J. et al., Fabric Filters for the Electric Utility Industry, Vol. 1, General Concepts, Electric Power Research Institute, Palo Alto, CA, 1988.)

Deflation Valve

Purge Air Valve

Outlet Manifold

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