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Inertial forces
Due to inertia, a particle moving in a gas stream can strike slowly moving or stationary obstacles (targets) in its path. As the gas stream deflects around the obstacle, the particle continues toward the object and impacts it. The obstacle may be a liquid droplet or a solid particulate as shown in the figure below.
Two primary factors affect the probability of an impaction occurring: (1) aerodynamic particle size and (2) the difference in velocity between the particle and the obstacle. Larger particles are collected more easily than smaller particles due to their greater inertia.
Also, collection efficiency increases as the difference in velocity between the particle in the gas stream and the obstacle (or target) increases. The efficiency of impaction is directly proportional to the impaction parameter. As the value of this parameter increases, the efficiency of inertial impaction increases. This parameter is related to the square of the Stokes particle diameter and the difference in velocity between the particle and the target droplet.