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68 Lawrence K. Wang et al.

b. Pressure: Probably not a factor except that increase of pressure after part of the dust cake
has formed can fracture it and greatly reduce efficiency until the cake reseals.
c. Cleaning: Relatively unstudied but discussed in the following sections.
It is important to stress that all of the considerations discussed thus far can be opti-
mized only when the system is properly operated and maintained. Several of the factors
mentioned earlier under operational variables are significant enough to merit further
discussion in the following section.

4. ENGINEERING DESIGN
4.1. Pretreatment of an Emission Stream
The temperature of the emission stream should remain 50–100°F above the stream
dew point. An emission stream too close to its dew point can experience moisture con-
densation, causing corrosion and bag rupture. Acid gases (e.g., SO ) exacerbate this
3
problem. Procedures for determining the dew point of an emission stream are provided
in Chapter 1. If the emission stream temperature does not fall within the stated range,
pretreatment (i.e., emission stream preheating or cooling) is necessary, as discussed in
Chapter 1. Pretreatment alters emission stream characteristics, including those essential
for baghouse design: emission stream temperature and flow rate. Therefore, after select-
ing an emission stream temperature, the new stream flow rate must be calculated. The
calculation method depends on the type of pretreatment performed and should use appro-
priate standard industrial equations. Also, emission streams containing appreciable
amounts of large particles (20–30 µm) typically undergo pretreatment with a mechanical
dust collector. Chapter 1 also describes the use of mechanical dust collectors.
All fabric-filter systems share the same basic features and operate using the principle
of aerodynamic capture of particles by fibers. Systems vary, however, in certain key
details of construction and in the operating parameters. Successful design of a fabric
filter depends on key design variables (7–26).

• Filter bag material
• Fabric cleaning method
• Air-to-cloth ratio
• Baghouse configuration (i.e., forced or induced draft)
• Materials of construction

4.2. Air-to-Cloth Ratio
The filtration velocity, or air-to-cloth (A/C) ratio, is defined as the ratio of actual volu-
metric air flow rate to the net cloth area. This superficial velocity can be expressed in
units of feet per minute or as a ratio. A/C ratios of 1:1 to 10:1 are available in standard
fabric-filter systems. Low-energy shaker and reverse-flow filters usually operate at A/C
ratios of 1:1–3:1, whereas the high-energy reverse-pulse units operate at higher ratios.
Particulate collection on a filter fabric occurs by any or all mechanisms of inertial
impaction, interception, and diffusion, as shown in Fig. 2. Inertial impaction occurs for
particles above about 1 µm in diameter when the gas stream passes around the filter fiber,
but the solid, with its high mass and inertia, collides with and is captured by the filter.
Interception occurs when the particle moves with the gas stream around the filter fiber,