Particulate Emission Control - Mathematics and Statistics for the Environment - page 490

FIGURE 17.1 Particle collection of a stationary object. (Adapted from USEPA, APTI Course SI:412C: Wet
Scrubber Plan Review: Self-Instructional Guidebook , EPA 450/2-82-020, U.S. Environmental Protection
Agency Air Pollution Training Institute, Research Triangle Park, NC, 1984.)

The mean free path , kinetic theory's most critical quantity, is the average distance a molecule
travels in a gas between collisions with other molecules. The mean free path increases with increas-
ing temperature and decreases with increasing pressure (Hinds, 1986).
The Reynolds number characterizes gas flow, which is a dimensionless index that describes the
flow regime. The Reynolds number for a gas is determined by the following equation:

Re = pU D

g
η

(17.1)

where
Re = Reynolds number.
p = Gas density (lb/ft 3 , kg/m 3 ).
U G = Gas velocity (ft/s, m/s).
D = Characteristic length (ft, m).
η = Gas viscosity (lb m /ft⋅s, kg/m⋅s).

The Reynolds number helps to determine the flow regime, the application of certain equations,
and geometric similarity (Baron and Willeke, 1993). Flow is laminar at low Reynolds numbers, and
viscous forces predominate. Inertial forces dominate the flow at high Reynolds numbers when mix-
ing causes the streamlines to disappear.

17.1.2 p artiCulate C olleCtion
Particles are collected by gravity, centrifugal force, electrostatic force, and by impaction, intercep-
tion, and diffusion. Impaction occurs when the center of mass of a particle that is diverging from
the fluid strikes a stationary object. Interception occurs when the particle's center of mass closely
misses the object, but because of its size, the particle strikes the object. Diffusion occurs when
small particulates happen to “diffuse” toward the object while passing near it. Particles that strike
the object by any of these means are collected if short-range forces (chemical, electrostatic, and so
forth) are strong enough to hold them to the surface (Copper and Alley, 1990). Different classes
of particulate control equipment include gravity settlers, cyclones, electrostatic precipitators, wet
(Venturi) scrubbers, and baghouses (fabric filters). In the following sections we discuss many of the
calculations used in particulate emission control operations. Many of the calculations presented are
excerpted from USEPA (1984a).