Collection Efficiency

The collection efficiency of a fabric filter depends on the interactions between the fabric, the dust, and the cleaning method. However, a clear understanding of these factors is not available, and existing models cannot be generalized beyond the data sets used in their development.

Impaction, diffusion, and interception in a dust cake are effective. Essentially all incoming dust is collected through a pulse-jet filter, with or without a dust cake. Nevertheless, some dust penetrates a fabric filter. It does so because gas bypasses the filter by flowing through pin-holes in the dust cake or because the filter fails to retain the dust previously collected (seepage).

Pinholes form in woven fabrics at yarn intersections. With dust loading, some pinholes are bridged, while the gas velocity increases through open pinholes. Incoming particles bounce through pinholes rather than collect. A disproportionately high fraction of gas flows through un-bridged pinholes due to their lower resistance. As the fabric flexes during cleaning, some dust particles dislodge and recollect deeper in the fabric. After several cycles of dis-lodgment and recollection, the particles completely pass through the filter. Particle penetration by this mechanism is called seepage.

Dennis et al. (1977) and Dennis and Klemm (1979) developed a model for predicting effluent fly ash concentration with fabric loading for woven glass fabrics. However, their models contain empirical constants that may be inappropriate for other fabrics. Leith and Ellenbecker (1982) showed that seepage is the primary mechanism for penetration through a pulse-jet-cleaned felt fabric. They developed a model (Leith and Ellenbecker 1980a) for outlet flux N assuming that all incoming dust is collected by the filter and that seepage of previously collected particles through the fabric accounts for all the dust emitted once the filter is conditioned. Seepage occurs as the bag strikes its supporting cage at the end of a cleaning pulse. The impact dislodges particles from the filter; the particles are then carried into the outlet gas stream. The following equation calculates the outlet flux:

kw2v t

where:

N = the outlet flux w = the areal density of the dust deposit v = the filtration velocity t = the time between cleaning pulses to each bag k = a constant that depends on factors such as dust characteristics, fabric types, and length of filter service

Figure 5.16.13 plots the outlet flux measured in laboratory experiments against the flux predicted using Equation 5.16(48) with k = 0.002 m — s/kg. These data are for different felt surfaces with different filtration velocities and different dusts. However, all data cluster about the same line.

Figure 5.16.14 plots the mass outlet flux against particle diameter for a pulse-jet filter. Comparatively little flux results from the seepage of small particles because of their small mass. The flux due to large particles is also little because they do not pass through the filter. Intermediate size particles contribute the most to the flux since they are large enough to have appreciable mass and small enough to seep through. However, no theory exists to predict outlet flux as a function of particle size.

However, Christopher, Leith, and Symons (1990) evaluate mass penetration as a function of particle size. They developed the following empirical equation:

N =k where the exponent n is given by: n = 0.916(dp)a:

where dp is the particle diameter in /m.

The following equation gives model constant k:

where a = 4.45 X 10—6 for Ptfe-laminated fabric and a = 2.28 X 10—5 for untreated polyester felt fabric.

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