840 PARTICULATE REMOVAL
cause poor gas flow patterns and allowing trials of baffle, dif-
fusers, and turning vanes to reduce the problems.Fabric FiltrationThe process of particulate removal by filtration is one of
the oldest methodologies in the gas cleaning field. Fabric
filters possess several advantages, the most important one
being that dust collection efficiencies of better than 99% are
obtainable with sub-micron particles. Fabric filters are reli-
able, and relatively simple to operate, require moderate capi-
tal and operating costs and can be designed to handle a wide
range of particulate loadings and gas flows. On the other
hand, the choice of a fabric material limits the temperature,
and corrosivity characteristics of the gas and high humidities
must be avoided in order to prevent cake formation.
Removal of particulates from a gas stream by fabric
filtration involves passing the gas through a filter medium
made of a porous fabric. The solid is deposited on the fabric
with a resultant increase in the flow resistance of the gas.
A means of removing the solid matter which is built up on
the filter is thus necessary to avoid excessive pressure drop
of the gas through the filter cake.
Industrially, the filtration medium is contained in equip-
ment known as a baghouse. These baghouses function to
hold a large area of the filter exposed to the gas flow. The
filter medium itself is generally in a tubular shape with gas
flowing from the inside, through the medium and exiting
outside the tubes.
This section will deal with the mechanisms of particulate
collection on fabric filters, some of the design parameters
of baghouses, performance characteristics of baghouses, and
some data on the cost of baghouse equipment and the cost of
running a fabric filtration.Mechanisms of Particle CollectionParticulate collection by fabric filters essentially involves the
capture of a particle by a collecting body. The gas stream
carries the particles close to the collecting body, where a
variety of mechanisms are involved in the actual capture.
The mechanism or mechanisms which predominate depend
on the size and velocity of the particles, and on the charac-
teristics of the filter element itself.
The three major collection mechanisms involved in
fabric filtration are (1) Interception, (2) Inertial Impaction,
and (3) Diffusion. In addition to these three mechanisms,
gravity, electrostatic, and thermal forces may act to modify
filtration performance in special cases, but in general, these
effects are negligible.
The mechanism of interception can be visualized as fol-
lows. The particle is assumed to follow the streamlines of the
gas flow. As the gas flows by the collector, the streamlines
are bent. Interception occurs when the streamlines on which
the particle travels come close enough to the collecting body
such that the particles comes into contact with the body. The
efficiency of interception is a function of the ratio of the
diameter of the particle to the diameter of the target body
and to some extent to the flow characteristics of the stream.
Ranz^36 has developed an expression for the efficiency in the
case of a cylindrical targeth
1
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⎦⎥ (1)The acrylics (for example Orlon ® , Acrilan ® , Creslan ® , and
Crylon ® ) can be used at temperature up to about 275F, but
have relatively poor resistance to alkalis. Their resistance to
acids and moist gases leads to application in the metallurgi-
cal industries as well as others such as carbon black, cement,
fertilizers, solvents, and galvanizing.
The polyesters (for example Dacron ® , Fortrel ® , Vycron ® ,
and Kodel ® ) can be used at temperatures of 250F and have
good resistance to alkalis as low temperatures.
Glass, Nomex ® , and Teflon ® are the fibers of choice of
high temperature application (400–500F). Glass is superior
to the others in terms of high chemical and thermal resistance
but suffers from very poor resistance to abrasion. Abrasion
resistance can be improved by coating the glass fibers with a
lubricant such as silicon oil or graphite. Applications include
cement, gray iron cupolas, and electric arc steelmaking.
Nomex ® , an aromatic polyamide from duPont, can be used
at temperatures up to about 400F and excellent abrasion and
flexural resistance.
Cloth Cleaning A large amount of effort in the develop-
ment of fabric filters has gone into methods of removing the
deposited cake from the fabric. Several considerations are
important with regard to cloth cleaning.1) The cleaning time should be much smaller than
the filtering time, otherwise effective use is not
being made of the filter.0.6560.4920.3280.16401 23
Sulfur Content of the Fuel, %Precipitation Rate Parameter, wft/sec2015105Temp – 300° Fcm/secFigure 11 Effect of sulfur in fuel on precipitation
velocity.^27C016_002_r03.indd 840C016_002_r03.indd 840 11/18/2005 1:06:47 PM11/18/2005 1:06:47 P