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The type of filters we can expect to find on a central air conditioning plant.

Welcome to the Solutions page

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten.

We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.

David asks: Hi Grant. I would like to ask a question around all the type of filters we can expect to find on a central air conditioning plant. We are expanding our maintenance into this area only to find quite a diverse range of filters, nothing like one would see on a mid-wall split unit.

Hi David, I would imagine moving from a split unit environment to a central plant situation could be somewhat eye opening. Let us consider the air itself as a starting point.

Air is a mixture of gasses. Normal atmospheric air consists of 21 % oxygen, 78% nitrogen, 1 % argon and 0.03% carbon dioxide. There are also small quantities of other gases such as hydrogen, neon, helium, ozone and xenon and varying amounts of water vapour.

We depend on air to survive and any substantial differences in the percentage composition of normal air makes it unsuitable. Oxygen concentrations of less than 12% and carbon dioxide concentrations of less than 5%, even for short periods are dangerous. Over long periods, even smaller variations in the composition of the air may be dangerous.

A human being uses approximately 30 litres of oxygen per hour. The air requirement is thus in itself quite small, 150 litres, or about 0.15m³ per hour. The carbon dioxide produced by human’s means, however, that the air required in order to keep the carbon dioxide concentration below the danger level rises to about 5 m³ per person per hour.

Greater air flows may however be needed in order to control heat, cooling or contaminants.

The air contains various foreign materials, both from natural process such as wind erosion, evaporation from the sea, earthquakes, and volcanic eruptions and from industrial activities, such as products of combustion from industrial processes.

Atmospheric dust consists of a mixture of mist, fumes, dry granular particles and fibres. An analysis of the air usually reveals soot and smoke, quartz, clay, traces of decomposed animals and plants, organic material in the form of cotton and plant fibres and metallic fragments. It also contains organisms such as bacteria, spores and plant pollen. Such floating particles mixed in air or gas is often called “aerosols”.

Despite the fact that these impurities occur in small concentrations in ordinary air, they have a decisive effect on our environment. Electrical effects in the atmosphere, absorption of solar radiation and cloud formation are all affected to some degree by the impurities in the air. What is perhaps more evident is the effect of atmospheric air pollution on materials and living creatures.

The concerns with particulate matter and gases which influence our health or comfort and contaminate the spaces we occupy or which affect the products and components we manufacture are very real and not unique to specific areas but rather standard elements in the atmosphere irrespective of location. All that really changes are the concentration levels.

Although great strides have been made to identify the hazardous particulate matter in the atmosphere, the reality of it all is that the air we breathe is not very clean at all.

Contaminants originate from the outdoor air and from building contents such as furniture and furnishings and from processes and materials used within the building. Many of these are nuisances such as cooking odours but others are identified as causing discomfort and even illness for some if not all of the building occupants.

David let us move on to the contaminants. An aerosol is a suspension of solid or liquid particles in the air. The size of an aerosol is usually measured in microns. One micron is one millionth of a meter or one thousandth of a millimetre. Under the International System Units (SI) “micron” is being replaced by “micrometer”. However, we will use the term microns due to it being the most popular term in industry. The abbreviation for micron is “µ”.

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 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 effected by rain. Opinions differ, as there is a balance between dust generated and returned. Certain measurements of solar radiation etc 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 it 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 tend 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 etc. Smoke particles vary considerably in size. Most of them are less than 1 µ and they 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 vaporisation.

As we can see David, air pollution is a growing problem, mainly because of the increase in population and industries. Thus the cleaning of air of foreign matter has become an important part of air-conditioning. The total distribution and concentration of atmospheric dust varies greatly, depending on the place, season, time of day, etc. 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 percent of contaminants out of the air. Such contaminants could be:

  • Pollen, mould 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. 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”.
  • Adhesive filtering for removing dust and pollen etc. They are made of various fibres such as glass; cotton, synthetic material and aluminium. The fibres are coated with an adhesive liquid or oil. Air is forced to change direction and lose speed as it passes through the filter. This results in trapping the particles of dust, pollen, etc., as they make contact with the adhesive surfaces of the filter.
  • Electrostatic filtering for removing very small particles. This type of filtering electrically charges the particles to be removed and adheres them to a surface having an opposite charge. Electrostatic filters are normally used as secondary filters to screen or adhesive filtering to clean the air for computer rooms etc.
  • Ultraviolet lighting will kill most bacteria in a fraction of a second. The lamps are installed in the return air duct in such a way that the rays cover the full cross-section of the duct to make it effective.
  • Filters made of activated carbon will remove solid particles as well as certain gasses that cause bad smells. They will also remove a limited amount of bacteria.
  • Paper filters are sometimes used to remove finer dust or pollen particles.

More than one type of filter is sometimes used on one air conditioning system. For example, the return air in a hospital may first pass through a primary screen type filter made of a fibrous material to remove large particles. The air then passes through a secondary paper or electrostatic filter to remove fine particles. Finally, at the end of the duct where the air enters the room, the air passes through ultraviolet lighting to kill bacteria.

Air filtration supplies the means to obtain the level of particulate cleanliness required by any definition of “air-conditioning”: It extends from the simple task of prevent lint and other debris from blocking heating/cooling coils to remove particles as small as 0.1 micron which could cause a short circuit on a microchip.

In addition to the reasons given above, air filters are used for a wide variety of purposes, some of which include:

  • Protecting the general well-being of the occupants of spaces or buildings;
  • Protecting the decor of occupied spaces by removing the staining portion of airborne dust;
  • Reducing maintenance of building interiors by reducing the frequency of cleaning such as Venetian blinds, fluorescent bulbs and furnishings and equipment;
  • Protecting other contents of occupied spaces including paintings, tapestries and other items of historic value;
  • Elimination of fire hazards by removing lint and other materials which might accumulate in ductwork;
  • Extension of shelf life of perishable dairy products by removing airborne mould during processing operations and;
  • Removing airborne bacteria from operating rooms air to help prevent postoperative infection.

Outside air (fresh air) is in the ideal world free of all dust and gases which could affect a space’s occupants, decor or contents, in the real world, it is rarely deserving of the description “fresh” and must be filtered to remove the contaminants it would bring inside.

Return air contains the particulate material, which was generated within the space plus that which was not removed from the supply air by the filters, which are part of the conditioning system.

Supply air, as delivered to the space, should be of the cleanliness necessary to achieve the objectives of the space. However, once this supply air reaches the diffuser, its utilisation depends on the air diffusion system. If this clean air is not distributed properly throughout the space, comfort may not be achieved.

David I hope that this increases your understanding of air filtration.

Thanks to everybody for the overwhelming response. I receive on average of over sixty

questions per month and cannot publish all of them. But keep them coming, as I may answer you directly.

Looking forward to hearing from you.

Grant Laidlaw


SETA training