Page 210 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

6.5 Filtration 185

2500
Davies Model, 5mm thick Filter
Davies Model, 10mm Thick Filter
2000
Davies Model, 20mm Thick Filter
Yeh's Model, 5mm Thick Filter
Pressure Drop (Pascal) 1500 Yeh's Model, 10mm Thick Filter

Yeh's Model, 20mm Thick Filter

1000

500

0
0 0.02 0.04 0.06 0.08 0.1
Filter Solidity

Fig. 6.15 Comparison between Yeh’s and Davies models

ln aðÞ 3 a 2
Y ¼ þ a
2 4 4
evaluates to zero, and the pressure drop is infinite because the filter is completely
blocked; a situation that Davies’s model cannot predict.
Actually both models overestimate the pressure drop of a fibrous filter, but
Davies’s model is closer to experiments and therefore more widely used.

6.5.4 Particle Accumulation

A factor that is not considered so far in the preceding analysis is the effect of
particle buildup on the fiber surface as the particle collection proceeds. Particles
buildup on the fiber surface and it can be within the filter or on the surface of the
filter, depending on the solidity of the filter and the particle sizes. The smaller the
particle sizes, the deeper it goes into the bulk filter, and these particles are captured
by internal fibers. This type of filter is also called internal filter. Larger particles are
captured on the surface and these particles form a layer of dust, which are com-
monly called dust cake in industry. Since surface filters are designed to collect
particles on the surface, they are usually as thin as a piece of fabric cloth. Surface
filters can be reused after removal of the dust cake.