In a wet scrubbing system, particulate matter tends to follow the streamlines of the exhaust system.When liquid droplets are introduced into the exhaust stream, however, particulate matter cannot always follow these streamlines as they diverge around the droplet. Instead, because of the particle’s mass, the particles break away from the streamlines and impact on the droplet. When gas stream velocity exceeds 0.3 m/sec (1 ft/sec), impaction is the predominant collection mechanism. Most scrubbers do operate with gas stream velocities well above 0.3 m/sec, allowing particles with diameters greater than 1.0μm to be collected by this mechanism (USEPA-84/03, p. 1-4). Impaction
increases as the velocity of the particles in the exhaust stream increases relative to the liquid droplet’s velocity. In addition, as the size of the liquid droplet decreases, impaction also increases.
In impaction, after the design, the key parameter is known as the impaction parameter
ψ (USEPA-81/10, p. 9-5) and is expressed by: