Particle sizes

Atmospheric contaminants fall into four classes: solid, liquid, gaseous and organic. Distinction between some members of these classes is not clear cut and not particularly important, but recognition of their existence is relevant. For this reason the following broad statements are made; they should not be regarded as a set of definitions.

In speaking of small particles—which all atmospheric contaminants are—it is customary to use the micrometre as a unit of measurement of their size. To give a perspective, the diameter of human hair is in the range 30 to 200 (im.

(a) Dusts (<100 ™

These are solid particles by natural or man-made processes of erosion, crushing, grinding or other abrasive wear. Dusts do not agglomerate, except under the influence of electrostatic forces, but settle on the ground by the force of gravity.

(b) Fumes (<1 |J. m)

These also are solid particles but formed in a different way from dusts. Fumes are produced by the sublimation, or by the condensation and subsequent fusion, of gases which are solids at normal temperature and pressure. Metals can be made to produce fumes. The term is often misused to indicate merely a gaseous substance which has a pungent smell. Fumes agglomerate into large clusters with ageing.

(c) Smokes (< 1 (im)

Smokes may be regarded as small solid particles which are the product of incomplete combustion or, more truly, as a mixture of solid, liquid and gaseous particles resulting from partial combustion. Excluding the gaseous particles, which are molecular in size, industrial and domestic furnaces produce particles which vary in size from 0.1 |im to 1.0 |im, but tobacco smokes are much smaller, existing in the range 0.01 to 1.0 |im. Hence the difficulty in their effective removal from air streams.

(d) Mists and fogs (<100 fim)

The distinction between mists and fogs is somewhat blurred. However, they are both airborne droplets which are liquid at normal temperature and pressure. Their normal range of size is 15 to 35 Jim.

(e) Vapours and gases

A distinction between the two has already been drawn in section 2.2. From a filtration point of view, they are substances which are in the gaseous phase at normal temperature and pressure, but whereas a vapour may be removed by cooling to below the dew point, a gas cannot. Both gases and vapours diffuse uniformly throughout an enclosing space. Separation by inertia is not possible. The common atmospheric gases have molecule sizes from 0.0003 |im to 0.000 45 (im and mean free paths at atmospheric pressure of 0.06 to

0. 2 Jim, implying that only a very small percentage of the volume of a gas is occupied by the molecules themselves.

(/) Organic particles

The commonest of these are: bacteria (0.2-5.0 (j. m), pollen (5.0-150 (im), the spores of fungi (1.0-30 Jim) and viruses (much less than 1.0 |xm in size). Bacteria are generally larger in size than 1.0 (Am and rely on dust particles as a mode of transport. Hence the importance of dust filtration in controlling the spread of infection by bacteria. Viruses are very small indeed (0.03 to 0.06 (im), and for this reason many have never been identified. Some are transported by airborne liquid droplets.

(g) Raindrops

These vary in size from 500 to 5000 |im.

(h) Aerosols

This is the name given to a semi-stable dispersion of small liquid droplets, or solids, in a gas, with a range of sizes from less than 0.01 to 100 |im. Although they follow a general pattern of movement in the parent gas they may coagulate or be deposited on surfaces by normal gravitational, inertial and other forces acting on them.

Individual particles, other than viruses, of size less than 0.1 (im are not thought to be of great importance. They may, however, become permanent atmospheric impurities, particularly as smells. Other very much larger particles, such as insects and even birds, must be kept out of air-conditioning systems but, except in the case of insects and electric filters, they do not usually constitute a special filtration problem.

Posted in Engineering Fifth Edition