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Clean Coils = Clean Air

Vern Klein of Advanced Engineering Limited looks at how microscopic organisms in air-conditioning systems can lead to big problems…

A basic air-conditioning system, like the one pictured, consists of indoor cooling coils (evaporators) and outdoor condensing coils (either air or water cooled). There are many industrial variations on the theme, but the principle is still the same.

To achieve optimum cooling, it is vital that every part of the operating system functions correctly. Even minor fluctuations in certain elements can have major effects on the efficiency, and therefore the running costs, of a system. For example, a 1°C rise in the condensing temperature will increase costs by 2% to 4%, as will a 1°C reduction in evaporating temperature.

These minor changes in temperature are usually caused by the build-up of dirt on the coils. Because the coils are the place in an air conditioning system where heat transfer takes place, any dirt there forms a barrier to the process. This effectively insulates the coil. Heavy contamination can also clog the coil, reducing airflow.

Indoor Coils
The function of the indoor coil is to remove heat from mechanically circulated air. All the air in the building will eventually pass through these coils and, possibly, a filter of some description. The filter is designed to remove particulate matter from the air stream.

Unfortunately, the filters commonly used in standard air conditioning systems are by no means 100% efficient and anything small enough will get through, in particular, dust and any airborne microbes. Eventually, due to the large volumes of air passing through the coil on a daily basis, there will be an accumulation of dirt and, possibly, bacteria on the fins and tubes. Where no filter is used, the problem of dust and contamination is inevitably much greater and the need for regular cleaning is even more important.

Indoor Air Quality (IAQ)
With the growing concerns over Sick Building Syndrome (SBS) and IAQ, controlling airborne microbes is a major challenge. However, by examining the pollutants, we can understand how best to combat them.

Chemical Pollutants – These can come from outdoor air, human bodies and activities, emissions from building materials (insulation materials, adhesives), furnishings (carpets, soft furnishings etc), appliances (photocopiers, laser printers etc) and consumer products (cleaning fluids, colognes etc).

Biological Pollutants – Again, these can come from outdoor sources and thrive indoors where humidity can be higher. Individuals carrying infectious diseases also increase the level of biological contamination, especially in environments characterised by reduced ventilation and increased circulation of untreated air by an air-conditioning system. Many infectious diseases are contracted through inhalation.

Poor indoor air quality can lead to a number of health problems for the occupants of the building, including:

• Respiratory Infections – inhalation of microbial pathogens
• Allergic Asthma – outdoor allergens, dust mites and moulds in an indoor environment
• Irritation of eyes, nose and throat, odour perception and behavioural effects – due to volatile organic compounds
• Lung Cancer – passive inhalation of tobacco smoke and glass fibre
• Sick Building Syndrome – not believed to be due to a single contaminant but to a variety of factors within the building environment

The HVAC system, especially the evaporator coil, can be a major source of biological contamination. As air passes through the coil, moisture contained within it condenses onto it. Any particulate matter that escaped the filter is effectively washed out with plenty of organic debris – an ideal breeding ground for bacteria, mould and algae.

Under such favourable conditions, these microbes multiply rapidly, to form large colonies. Agitation of air by fans then circulates microbes, their reproductive spores, the waste products from their metabolic activity and any toxins they produce to protect themselves. These flow around the building and into the spaces occupied by human residents. Here they can make the environment uncomfortable for the humans and cause a wide variety of health problems. These biological contaminants include:

Viable Microbes – many of the airborne microbial contaminants are disease-causing (pathogenic) organisms

Spores & Moulds – can cause allergic responses in some people leading to respiratory problems such as asthma

Mycotoxins, produced by moulds, toxins and metabolic by products (gaseous emissions) – these have been found to contain a cocktail of volatile organic compounds (VOC) including dead and dying bacteria thought to be responsible for the “Dirty Socks” or “Tom Cat” smell emitted by some indoor coils

Outdoor Coils (Air-cooled and Water-cooled Condensers)
Air-cooled condensers present a completely different set of problems. They operate at a higher temperature than indoor coils and the air entering them is rarely, if ever, filtered. They are often exposed to extremes of environmental conditions. For example, in coastal areas, aluminium-finned condensers, particularly, are subject to corrosion by the saline atmosphere. The dirt on air-cooled condensers generally comes from airborne particles, insects, leaves etc.

These contaminants, meeting a hot coil, are literally baked on, making them extremely difficult to remove if maintenance is not undertaken on a regular basis.

Water cooled condensers – where water is supplied by a cooling tower – as well as generating their own set of problems, must be treated on a regular basis to avoid Legionnaires Disease.

Before you resign yourself to illness and even death, take comfort from the fact that a wide range of solutions are on the market which, when used as part of a regular maintenance programme, can help combat the numerous problems facing coils.