Filter elements, that separate solid and liquid particles from compressed air, function on the basis of a combination of differing physical mechanisms, depending on the mode and type of filtration. Filtration makes use of
- Brownian molecular motion (intermolecular forces)
- Forces of inertial
- Direct interception/sieving
- Electrostatic effects
Should the particles settle on a fibre in the course of flowing past it, all the mechanisms described below may become effective simultaneously. To provide a universally valid mathematical relationship is very difficult. For this reason, only empirical solutions of individual mechanisms, or several of these in combination, are available.
The entire range of Reynolds numbers of interest to filter technology incorporates as well the range of viscous flow around cylinders (Re < 1) as the so-called transition range (Re > 1-103) ie. 0.001 right up to potential flow (Re > 103).
An important range for depth filtration is among the smaller Reynolds numbers (Re < 1). Flow around a cylinder in the range of creeping flow is such that the speed in the immediate area of the fibre is significantly lower than the approach flow velocity in the undisturbed range of the fibre. This applies equally to fluid as well as to entrained particles. It has to be added that particle impingement velocity onto the fibre surface depends on the point of impingement and thus on the boundary point of the undisturbed flow zone outside the cylinder, thus undergoing a range of velocities. This causes the effective speed of impingement in fact to be only a fraction of the approach flow and to increase evenly with growing distance from the point of contact.
The process of separation is explained here in an idealised manner, envisaging a circular cylinder being met by flow normal to its axis. The separation of particles at the cylinder surface is a consequence of two continuously occurring phenomena in the course of the filtration effect: The transport of particles to the cylinder surface.
The adhesion of these particles at the cylinder surface. Essential transportation effects are based on inertia forces, sedimentation16, Brownian motion or electrical forces. Four conditions are necessary in order that a particle adheres to a fibre: The elastic energy after impingement must be smaller or, at most, equal to the adhesion energy capable of bringing about retention. There must be no break-up or atomising of the aerosol. The drag forces of the gas acting upon the separated particles must not be stronger than the adhesion forces. Particles already deposited must not be torn free again through the impact of subsequent aerosols collected.
