www . refrigerationandaircon . co . za RACA Journal I January 2024 37
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Size in microns of some small particles .
Dusts are solid aerosols generated from the reduction of larger solid materials .
David , the size of particles is often stated in µ ( micrometers ), i . e . 1 µ = one thousandth of a millimetre .
The particles in the atmosphere may vary in size from less than 0.01 µ up to the sizes of fibres , leaves and insects . Almost every conceivable shape and size is represented . Dust is normally taken to refer to particles of less than 100 µ. The smaller aerosol particles are produced by condensation or sublimation of the smoke from processes of combustion , or direct from gases in physical or chemical processes .
Larger particles are formed by the erosion of the earth ' s surface and spread by the wind . They may , however , also be formed in the actual atmosphere by the agglomeration of smaller particles . Similar phenomena occur in clouds , where the particles form the condensation for water droplets .
Natural cleaning of the atmosphere is affected by rain . Opinions differ , as there is a balance between dust generated and returned . Certain measurements of solar radiation , for example , suggest that the total dust concentration in the atmosphere has increased during the last few decades .
Particles smaller than 0.1 µ in size begin to behave like gas molecules , with a Brownian motion and have no definite or measurable setting velocity .
Particles in the 0.1 - 1 µ range have a setting velocity , which can be calculated , but is so low as to be negligible for practical purposes . Normal air currents counteract the precipitation tendency .
Particles in the range 1 - 10 µ settle with a constant and calculable velocity . Normally , air current tends to keep them floating ; however , particles which are larger than 10 µ fall fairly rapidly and can float only in the vicinity of the source under certain wind conditions . Exceptions to this include cotton and other light fibrous materials such as parts of certain weed seeds , which may float for long periods .
Some particles of 10 µ can be seen with the naked eye under favourable lighting conditions . Smaller particles become visible in high concentrations . Cigarette smoke with an average particle size of 0.5 µ is an example of this .
The term ‘ smoke ’ is usually used for a mixture of solid , liquid and gaseous products . Smoke consists of extremely small particles – solid or in fluid form – which arise from incomplete combustion of organic substances such as tobacco , wood , coal , oil and the like . Smoke particles vary considerably in size . Most of them are less than 1 µ and are often between 0.1 - 0.3 µ. Viruses vary in size between 0.005 µ and 0.1 µ
The size of most bacteria is between 0.4 and 5 µ. They are usually dust-borne on larger particles ; however , the size of fungus spores is between 10 and 30 µ, while that of pollen is between 10 and 100 µ.
Mist and fog consist of small airborne droplets , usually formed by condensation of vapour , or fine dispersion and liquid spraying , or vapourisation .
As we can see , David , air pollution is a growing problem , mainly because of the increase in population and industries . Thus , cleaning air of foreign matter has become an important part of air-conditioning . The total distribution and concentration of atmospheric dust varies greatly , depending on factors such as the place , season and time of day . The air of industrial areas and cities normally contains soot and other products of combustion , and the dust concentration is higher than in the country .
Efficient air-conditioning systems should remove between 75 % and 95 % of contaminants out of the air . Such contaminants could be :
• Pollen , mold and dust carried by the wind , or dust created by mining activities etc .
• Fumes from motor exhausts , or from industrial or chemical processing etc .
• Smoke and soot caused by fire and the use of tobacco etc .
There are many more air-contaminants such as mists that are mechanically ejected into the air by splashing or atomising and bacteria ( micro-organisms ) that are responsible for the transfer of many diseases .
Air may be cleaned in many ways , depending on the contaminants to be removed . The following are a few examples :
• Large air-conditioning systems use water sprays to remove liquid contaminants , water soluble gas contaminants and water absorbent solid contaminants . Some of the gasses that water can remove are sulphur dioxide , nitrogen oxides and carbon monoxide . Water will not remove soot . This process of cleaning the air is known as ' washing the air ’.
www . refrigerationandaircon . co . za RACA Journal I January 2024 37