Blog

As shown in Figure 1.1 , airborne particles may range in size from 1 nm (0.001 um) up to 1 mm or more. The larger particles, such as might come from heavy foundry dust or ground limestone, would need updrafts of the order of 4 or 5m/s to maintain them in suspension. For the very smallest, however, the settling velocities are so low as for them to remain in suspension all the time – with the Brownian motion of the molecules of the fluid ensuring that they do so remain.

Particles in air suspension will range downwards from 100 um. Down to 20 um they will be visible to the naked eye, while on down to 0.1–0.2 um they can be observed with a conventional microscope. The major problem particles that are viruses are smaller than this, and so that much more difficult to remove.

Typically, some 90% by weight of all airborne particulate impurities range from 0.1 to 10 μm in size, although this range and the actual concentration of solids will vary markedly, dependent upon the immediately local environment and wind activity – a desert sandstorm is far worse than any industrial activity.

If the air is, indeed, in motion, then quite large particles will be held in suspension, particularly if the flow is turbulent. Thus 10 um particles will be held up by quite gentle air movements, while velocities of 0.3 m/s in a vertical direction, a quite common flue gas regime, will keep 100 um particles suspended.

A somewhat different picture of contaminant size ranges is given in Figure 1.2 which also shows the capture ranges for several different types of separation equipment.

Effect on humans

The average person breathes in about 12.5 m3 or 16kg of air each day. This is normal atmospheric air, which is far from clean. The breathing of contaminated air, which may well contain particularly aggressive substances, is a frequent cause of
ill health, contributing to the common cold and influenza, emphysema, headaches, eye irritation, coughing, dizziness, and the build-up of toxins in the bloodstream.

Impurities enter the body through the mouth and nose, and gradually becoming deposited on the bronchial mucous membrane. At the initial stage of deposition, the bronchial membrane itself is able to provide protective counteractions in the
form of sputum and coughing. This physical adjustment becomes impossible as a result of repeated inhalation of contaminants and their inevitable deposits over an extended period. As a consequence, the functions of the bronchi become abnormal. In the lungs, congestion of the alveoli and irreversible fibrinous changes occur, leading perhaps to pulmonary emphysema. Functional deterioration of the lungs will affect the heart adversely, leading to heart disease.

This somewhat over-dramatic picture nevertheless highlights the need for clean air for all human activity. Legislation such as the Health and Safety at Work Act is taking care of the industrial environment, but only slow progress is being made to
protect the general ambient air. Steps towards alleviation of diesel engine pollution are certainly in the right direction.

Effect on machines

Most engines take in air at a considerable rate: the factor of interest here is the weight of solids that could be taken to the engine. The average automobile engine inducts something like 9 or 10m3 of air for each litre of petrol burnt in its cylinders,
which corresponds to an annual intake of between 30,000 and 60,000 m3 of air per year. The solid particle content of this air is likely to be 0.3 to 0.6 kg, which, in the absence of good filtration, would all deposit in the engine where its abrasive nature would soon do serious damage. These figures correspond to a dust concentration in the air of 10mg/m3, and much worse conditions are readily identified: a mineral processor ’ s rock crusher could be working in a dust concentration up to 20 times that figure. From another perspective, a land-based gas turbine could easily be taking in tens of thousands times the air volumes of an automobile.

Inducted solids will tend to accumulate as deposits on pistons, cylinder heads, valves and other internal components, with the strong possibility of their being bonded in place by the oil film normally on these surfaces. The oil film will thus accumulate an abrasive content. Meanwhile, the dust still in the air will be washed out by the oil mist, to collect in the lubricant and then be circulated through the machine with dire consequences. In the drier interior of a gas turbine, the suspended solids will quickly wear away the turbine blades.

Clearly such dust intake levels cannot be tolerated, and filtration systems must be installed to remove the dust. Fitting a filter involves a compromise; firstly as to its cut point, and secondly as to its energy needs. The finer the filter, the more dust  will be removed but the more energy will be expended in achieving that removal. It will clog faster and so need cleaning or changing more quickly. So only the finest necessary filter should be fitted, but it must remove particles that are likely to jam
in the narrow spaces of the engine. Particles of sizes down to about 10m can do damage, and this is probably the level of particle cut-point that should be sought.

Obviously, the filtration requirements for an engine will depend markedly upon the environment in which it is to be used. A stationary location in country air will be less demanding than the use in an urban area, less again than in a heavy industrial use.

It must not be forgotten that all machinery is capable of producing contaminants by internal wear or from degradation of the materials being handled. Wear products will be at their peak when the machine is new and is being bedded down, and at the same time residues from its manufacture (such as grains of foundry sand) could also be present. Filtration must also be provided to protect the machinery from such materials, and the type will depend very much on the nature of the machine being protected.

Liquid contaminants

Whilst the presence of liquid water droplets from rain or marine spray cannot be avoided, the more serious situation with water drops comes in the provision and use of compressed air systems. If the intake air is close to saturation before compression it takes but a small increase in pressure for the water vapor to condense into drops. These drops can be a problem in the subsequent usage of the compressed air, and quite complex chains of filters and filter/dryers, together with intercoolers, are used to reduce the water content to an acceptable level.

The compressor may also be a source of oil spray such that oil droplets are formed in the product air stream. A major problem here is that oil droplets are a health hazard, as well as becoming taken up as an oil/water emulsion. Here again filtration is necessary to deal with the problem, and separation of droplets at least in the 0.4 to 4 um size range may be required from the health point of view alone.

Applications

It will have become apparent that the decontamination applications for gaseous systems are very important, and can involve very large flow volumes. The applications include:
● cleaning of inlet air entering living spaces
● cleaning of air entering work spaces, and protection from work space exhausts
● machinery and engine air intakes
● respirators and breathing apparatus
● compressed air treatment and pneumatic systems
● engine exhausts
● process and boiler furnace exhausts.

For more information, please check here.