Filtration Mechanisms


As air passes through a filter, the trajectories of particles deviate from the streamlines due to various mechanisms, most important of which are interception, inertial im-paction, diffusion, gravity, and electrostatic forces. The radiometric phenomena such as thermophoresis and diffu-siophoresis are usually negligible. The single-fiber efficiency %ber is approximated as the sum of the efficiencies due to each of the preceding mechanisms acting individually as follows:

Vfiber = Winter + ^impaction + ^diffusion + Vgravity + Velec 5.16(23)

The maximum value of the single-fiber efficiency is 1.0; so if several of the mechanisms have large efficiencies, their sum cannot exceed unity.

Cc = the Cunningham slip correction factor

U = the free-stream velocity (cm/sec)

/ = the viscosity of the medium (poise)

pp and pm = the densities of the particle and medium, respectively (gm/cc)

The slip correction factor is given by the equation Cc = 1 + [2A(1.257)/dp] where A is the mean free path of gas molecules and is equal to 0.071 /m for air molecules at 25°C and dp > 2A.

Stechkina, Kirsch, and Fuch (1969) calculated the in-ertial impaction efficiency for particles using the Kuwabara flow field as follows:

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