Particle behaviour and collection

Air resistance is a major factor in the settlement of particles under gravitational forces, the one balancing the other at a terminal velocity approximately proportional to the square of the particle size (diameter) up to about 60 |i. m. Because of this, gravitational separation is only of use for particles exceeding roughly 1 |J. m in size, for which the terminal velocity is approximately 0.3 x 10-1 m s’1, and so the filters of interest in air conditioning rely on other forces.

There are four basic ways in which dust particles are collected:

(1) Diffusion. The natural (Brownian) motion of the molecules of air is sufficient to impart movement to very small particles of dust by collision with them. The particles deviate from the direction of flow of the mainstream to be collected at the filter surfaces. This is the major mode of filtration in high efficiency filters where air velocities are too low for the effective inertial separation of particles less than about 0.5 |im in size.

(2) Straining. Particles larger than the space between the fibres of the filter material are strained out of the airstream, largely at the upstream face.

(3) Interception. Also termed impingement and impaction this is an aspect of inertial separation: larger, heavier dusts are removed by collision with the filter material, the background airstream of lighter and smaller particles flowing around the collection surfaces.

(4) Electrostatic. Apart from the high voltage fields deliberately engineered in electrostatic filters similar effects may exist naturally within filter materials, particles being collected at surfaces of opposite sign. Dust can become charged by collision with ionised molecules or by friction, and coagulate. Acoustic coagulation at ultrasonic frequencies is also possible but exceedingly large sound intensities, of the order of 1 kW irf2 are needed according to Brandt et al. (1937).

As a general principle, inter-fibre distances should be large compared with fibre diameters in a filtration material, otherwise the filter will rapidly clog but, after the dust particles collide with a fibre they must adhere to it for effective collection.

The influences on surface adhesion are:

(1) London-van der Waals forces. Dorman (1975) has explained that these are apparently electrical in origin and are more important for very small particles sticking to surfaces.

(2) Electrostatic.

(3) Surface tension. This arises if films of moisture are present on the surface. Adhesion is increased by the presence of water vapour and humidity can play a significant part for larger dusts (>1 |im), adhesion diminishing as humidity falls. Ma (1965) has shown that so-called capillary condensation can occur within filters under suitable circumstances, when humidities exceed 70 per cent. Although this assists adhesion, it can favour bacterial growth.

(4) The nature of the surface. This affects the van der Waals forces and influences the extent to which vapours and gases are adsorbed.

(5) Surface contamination.

(6) Shape and size of the dust particles.

(7) Duration of contact. (Adhesive forces increase with time.)

(8) Temperature. It is believed by Com (1966) that, by altering the forces of surface tension, surface adhesion could be modified.

The extent to which particles penetrate a filter increases as the velocity rises and is greatest for particles of 0.1 to 0.3 |im diameter at velocities of a few cm s-1, although filter material and packing density have an effect. The presence of a pin-hole in a filter greatly reduces its effectiveness in preventing the penetration of microbes or toxic materials. The penetration measured at normal velocities (see section 17.3) is usually unaffected by the presence of pin-holes and their detection must be achieved by a test at very low velocity when the proportional penetration through a pin-hole is much greater than that through the rest of the filter.

Posted in Engineering Fifth Edition