The cost of healthy air

Adiabatic humidification
Adiabatic humidification such as the Condair Fast Fog system have low energy costs, but allowance should be made for the cost of compressed air.
There’s more to air quality than filtering out airborne particles. The relative humidity of the air also makes a big difference but it needs to be controlled cost-effectively, says Roger Palamarczuk.When we talk about indoor air quality (IAQ) the emphasis is generally on levels of pollutants, particles and micro-organisms that can affect the health of people in the space. However, there is another aspect of IAQ that also needs to be addressed — relative humidity (RH). Just as polluted air can make people feel unwell, so too can low RH — plus it can also have an effect on the machinery that now dominates the modern workplace. So building-services systems need to address RH as well as filtering the air to remove pollutants. Further, RH needs to be controlled with minimum energy consumption and ongoing maintenance requirements. RH makes an incredible difference to the way we feel. If it is too high (above 60%) we feel hot and sweaty out of all proportion to the actual temperature. If it is too low (below 40%), the eyes become dry and itchy, especially for contact-lens wearers. At the same time, the respiratory surfaces start to dry out, leading to dehydration and reduced resistance to airborne pathogens. At low RH we also feel colder than is justified by the actual temperature, so people often turn the heating up to compensate which, ironically, lowers the RH and exacerbates the problem. Extensive research has shown that the optimum RH range for offices and other commercial premises is 40 to 60%. This is the recommendation of the humidity group of the Hevac Association, the World Health Organisation and the National Association of Optometrists. It is also the range recommended by BS EN 29241 as the optimum for visual-display terminals. In our experience, maintaining RH at 50% allows the temperature of the building to be reduced by a couple of degrees, saving a considerable amount on heating costs. It is also very clear that designing humidification into the scheme from the beginning ensures the most efficient system is deployed, as compared to a compromise ‘add on’ at a later date. Minimising cost of ownership The real question is not whether humidification should be provided but what form of humidification will provide the best and most cost-effective solution for the end client. For example, electrode boilers are often the first choice because of cost. However, during their life they can use as much as 200 times more electricity than an equivalent evaporative humidifier. At the other end of the running-cost spectrum are adiabatic, or evaporative, humidifiers because the water is not heated at all. However, when specifying adiabatic humidifiers it is important to consider other aspects of cost of ownership. For instance, some systems use compressed air to achieve a spray, which can add to overall running costs. Similarly, designs with minimum absorption distances will help reduce water consumption. Also, as the water is not being heated to kill bacteria, it is essential to maintain acceptable hygiene standards and to ensure such hygiene measures do not negate the savings achieved on running costs. For example, adding biocides is an expensive option, and many other forms of treatment have proved to offer mixed success. From an engineering perspective it makes sense to design the hygiene management into the units themselves, ensuring that all risks are eliminated and maintenance requirements are not too onerous. One option is to introduce silver ions to the water by electrolysis before it is evaporated. The silver ions will kill any micro-organisms on contact, ensuring the water is safe. At a hospital in Zurich, for example, engineers tested the Condair Dual 2 adiabatic humidifier and found bacteria levels well below the 100 bacteria/m3 specified for humidified supply air in operating theatres. Silver ionisation can also be combined with air filtration to address other aspects of air quality. Designing to scale Of course, there will always be situations where steam humidification is more appropriate than adiabatic methods and in hard water areas presents the challenge of managing limescale formation. Any build up of limescale will reduce water flows and heat-transfer efficiency (as much as 62% reduction with just a 0.5 mm limescale layer), shorten plant life and increase maintenance costs, while softening the water adds significantly to running costs. This issue can be addressed through the innovative application of sound engineering principles. The Defensor Mk5, for instance, incorporates a patented cold-water zone to ensure that water temperatures in key areas remain below 55ºC (scale forms above 55ºC). In addition, a special pulsing mechanism keeps the heating elements free of scale. Maintenance is therefore very straightforward, simply requiring occasional emptying of the easy-to-remove scale collection tanks. At a time when every organisation is looking to reduce overheads, yet cannot afford to skimp on health standards in the workplace, it is vital to offer cost-effective ways for managing the RH aspects of air quality. Considering the issues raised makes it relatively straightforward to arrive at the most cost-effective design through the life of the plant. Roger Palamarczuk is head of the humidification division of Walter Meier (Climate UK).

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