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Filtration, filtration, filtration...
Published:  11 April, 2007
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Modern air-conditioning units can do much more than cool the air, they can also remove all manner of impurities. DAVID MacRAE reviews the technologies used.

Silver, gold, titanium, gingko, bamboo, carbon, zeolites, vitamin C, apple, grapefruit and pineapple — what do these substances have in common? Answer — they are all used by air-conditioning manufacturers in air-filtration systems in high-wall-mounted room air conditioners. Well, all except the pineapple that is, which is only found in domestic air fresheners available at the supermarket.

Consumer emphasis

How much air filtration do we need? The question depends on how much cleanliness do we need? Toshiba believes we need a lot. Trends in filtration of indoor air have varied over the years. We seem to be at a point in time where it is receiving much greater consumer emphasis, and some manufacturers have responded. Certainly, looking around at the recent RAC show, there was some evidence of increasingly interesting developments.

Correlations are claimed between the cleanliness of the air, its level of ionisation and the performance of people working in the conditioned space. When we think of how well we appear to breathe on visits to the countryside or the seaside, it is possible to believe that this may have some validity. However important all this is becoming within the home and the workplace, it is also vital not to underestimate the effect of airborne pollutants on the efficiency of modern electronic equipment — although detailed examination of this issue and the specification implications would require a separate article.

extrapic
— Multi-stage filtration systems remove particles from the air, deactivate bacteria and viruses and disinfect the air.

Dust, metal particles, smoke, bacteria, viruses, mould, fungus, mildew and gases are all airborne. Most of the dust comes from skin, hair, clothes and shoes. Mildew, fungus and moulds can come in along with the dust, and gases can be given off by cleaning chemicals, building materials, carpets, dyes, paints, varnishes and adhesives.

The way houses and buildings are increasingly constructed to conserve energy has necessarily reduced natural ventilation from fresh air. This means that buildings are warmer, and are consequently likely to need air conditioning — even in homes. It also means that the constantly re-circulating air is dirtier, so there is a need for the best possible air filtration. Indeed, there is evidence of an increasing number of air-conditioning units being sold purely on the basis of their superior filtration capability.

An especially high level of interest in air filtration is, of course, most important for applications in sensitive locations — particularly residential, but importantly also professional services such as dentists, doctors, vets, podiatrists, chiropractors and physiotherapists, other therapists using oils and smokes, some shops such as pet shops, and small offices with high levels of occupant expectation or sensitivity.

A Toshiba system was recently sold to provide heating for a cottage because the tenant did not want to use smoky coal any more and was conscious of the problems that the couple’s heavy cigarette smoking was causing to the air that they and their friends and neighbours were breathing. They were delighted by the ability of the air-conditioning equipment to rapidly clear the air of all smoke and, apparently, all smells associated with it. They are recommending it to everyone. Dentists are also frequent purchasers, for the same simple reason that the units are very effective at removing all the smells associated with drilling, cleaning etc. and any particles circulating in the air.

Manufacturers are divided on the benefits of ionisers. The market is even more fractured, some might say sceptical, but in some areas ionisation is seen to have huge benefits. In Japan ionisers have been incorporated into air conditioners for several years, and now the trend is being accepted in the UK by anyone who has already experienced this equipment. The Toshiba Daiseikai range, which was the first commercially successful range in the UK to integrate an ioniser, has won three environmental awards in Japan alone.

Multi-stage filtration

The efficiency of filtration is measured by the effectiveness of arrestance of particles. Newly introduced low-resistance filters catch more particles. Particles as small as viruses can now be captured.

Some manufacturers also put great store by the way in which the air can be disinfected and deodorised.

extrapic
— To disinfect air, some manufacturers of air-conditioning units offer filters based on silver nano-particles held in a plasma.

The latest 7-stage, and earlier 6-stage, filtration process offered by some Toshiba high-wall units begins with a pre-filter to traps large particles and dust. Further trapping takes place at the next stage to ensure that airborne viruses and bacteria are held for elimination.

A new filter uses a combination of Sasa (a bamboo extract to deactivate bacteria and viruses), zeolite (TiO2, which traps particles and removes chemical pollutants) and impregnation with Vitamin C to act as an anti-oxidant. This filter is washable and regenerates its properties after drying in sunlight.

Another filter used combines a bio-enzyme and gingko extract, which has anti-bacterial and anti-allergy properties.

In some filters, silver nano-particles held in a plasma are also used to disinfect the air.

Most filters have a life of two or three years, or even longer, and can be cleaned using a vacuum cleaner or washing and exposure to sunlight. All materials used in Toshiba filters have been tested at the Japanese food-research laboratories.

Another feature of Toshiba’s indoor is a self-cleaning function using ozone — designed to reduce humidity, a common cause of mould inside units.

Fresh air

Some other types of indoor unit offer a degree of improved filtration, but most are used when fresh air ventilation is also specified. The best ceiling cassette systems offer a fresh-air inlet box option to facilitate the additional input of fresh air. Ducted systems can be fitted with fresh air input boxes and/or air-to-air heat exchangers.

The incorporation of fresh air into air-conditioning systems, its impact on filtration and other options for the control of air quality, such as demand-controlled ventilation according to the level of carbon dioxide, merit further detailed examination in another article. Air filtration is certainly a topic that will grow in importance as our buildings become more and more sealed to the unpredictable outdoor environment.

David MacRae is UK commercial director with Toshiba Air Conditioning.