Page 80 - Air Pollution Control Engineering

02_chap_wang.qxd 05/05/2004 12:39 pm Page 60
60 Lawrence K. Wang et al.

Although fabric filtration is suitable for removing solids from both gases and liquids,
it is often important that the filter remain dry when gases are filtered, and likewise, it
may be desirable to prevent the filter from drying out when liquids are filtered. In the
gas system, many solids are deliquescent, and if moisture is present, these materials will
have a tendency to pick up moisture and dissolve slightly, causing a bridging or blind-
ing of the filter cloth. The result is a “mudded” filter fabric. In such cases, it is often
impossible to remove this material from the cloth without washing or scraping the filter.
If the cake on the cloth is allowed to dry during liquid filtration, a reduction in the
porosity of the cake as well as a partial blinding of the filter could result, which could
then reduce the rate of subsequent filtration.
2. PRINCIPLE AND THEORY

In section 1, it was stated that the fabric itself provides the support, and true filtering
usually occurs through the retained solid cake that builds up on the fabric. This is
especially true for woven fabrics; however, felts themselves actually can be considered
as the filtering media. It has also been stated that the cake must be removed periodically
for continued operation. The resistance to fluid flow through the fabric therefore con-
sists of cloth resistance and cake resistance and is measured as a pressure drop across the
filter. Cleaned cloth resistance is often reported, although this in itself is not the new or
completely clean cloth resistance. Once the filter has been used and cleaned a few times,
a constant minimum resistance is achieved, which consists of the clean cloth resistance
and the residual resistance resulting from deposited material that remains trapped in the
cloth pores. This resistance may remain constant for the life of the fabric. Changes in this
resistance usually indicate either plugging of the pores or breaking of the filter. Clean
cloth resistances may be obtained from suppliers. However, it is best to obtain the steady-
state values by empirical measurements. An example of clean cloth resistance, expressed
according to the American Standards of Testing and Materials (ASTM) permeability tests
2
3
3
2
for air, ranges from 10 to 110 ft /min-ft (3–33.5 m /min-m ) with a pressure differential
of 0.5 in. (1.27 cm) H O. In general, at low velocities, the gas flow through the fabric
2
filter is viscous, and the pressure drop across the filter is directly proportional to flow:
∆P = K v
1 1 (1)
where ∆P is the pressure drop across fabric (inches of water [cm H O]), K is the resis-
1 2 1
tance of the fabric [in. H O/ft/min (cm H O/m/min)], and v is gas flow velocity [ft/min
2 2
(m/min)].
In practice, the fabric resistance K is usually determined empirically. It is possible
1
to estimate a theoretical value of this resistance coefficient from the properties of cloth
media. Darcy’s law states that
∆P =−( vK ) + ρ g (2)
µ
1
where K is the Kozeny permeability coefficient, µ is viscosity, ρ is density, and g is
gravitational acceleration. Note that necessary constants need to be applied to make the
equation dimensionally consistent. Values of the permeability coefficient K found in
2
2
literature range between 10 −14 and 10 −6 ft (10 −15 and 10 −8 m ). Values of K may also
be estimated using the relation