Page 179 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

154 6 Separation of Particles from a Gas

6.2 Gravity Settling Chambers

A gravity settling chamber was mainly used for the separation of large particles
from the air stream. In practice, a gravity settling chamber is only effective for
particles with a diameter of 50 μm or larger. As seen in Fig. 6.1, it classifies the
particles by gravitational force. A consequential drawback of these devices is the
large footprint. Depending on the gas velocity, a chamber can be designed to
operate at laminar flow or turbulent flow.

6.2.1 Laminar Flow Model

Consider a gravity settling chamber with dimensions of height H, length, L and
width W. A particle having diameter d p enters the chamber as shown in Fig. 6.2.
Under laminar flow condition, the trajectory of any particle should be a straight line.
As it moves from left to the right together with the airflow at a speed of U, it settles
at a terminal speed of v TS . The concept of terminal settling speed was introduced
above in Sect. 4.2.1.
If the particle barely touches the lower right edge of the chamber when it enters
the chamber at a height H c , then all the particles of the same size entering the
chamber above H c will penetrate through the chamber; those below H c will be

Fig. 6.1 A schematic diagram of a gravity settling chamber

L

Q
U
H
H c v TS

Fig. 6.2 A laminar flow gravity settling model