Ultra-violet air treatment in a new light

UV air purifier
A UV air purifier arrangement designed to treat for both chemical and biological contaminants. The UVV section is smaller of the two units. (With permission of Sanuvox Ltd)
The benefits of ultra-violet air purification enjoyed in the UK are set to arrive in Europe. Bill Anderson explains.Now that the ASHRAE handbook includes a chapter on ultraviolet light air purification, the time is ripe for the European building-services market to benefit from UV. Ultra-violet air purification for preventing fouling on cooling coils and drip pans and for the treatment of odours and micro organisms in ducted air is commonplace in North America. Not only is the technology a specified requirement for air-handling units entering service in American public-sector buildings, it has also been tested and found to be effective by the US National Homeland Security Research Centre at defending building occupants against bio-terrorism. Now the technology is covered by a chapter in the ASHRAE handbook, it warrants closer examination in Europe. Technical rudiments Within the UV light spectrum, the wavelength 254 nm is known to be particularly effective at killing or inactivating micro-organisms. It is utilised by ultraviolet germicidal irradiation (UVGI) systems to penetrate the cell wall and damage the DNA of organisms such as bacteria, viruses and spores to prevent reproduction by cell division — certain doom, given their short lifespan. Another useful UV wavelength is 187 nm, which is sometimes known as UVV and is used for UV photo-oxidation of airborne chemicals and odours. UVV produces ozone, oxygen singlets and hydroxyls (O3, O and OH), which, although excellent for treating airborne odours and other chemicals, can leave harmful concentrations of residual ozone being supplied to occupied spaces. UVV is therefore only used if followed downstream by and interlocked with a UVGI system which acts as a catalyst to revert residual O3 safely back to O2. Within ducted systems, UV comes in two main guises. Coil cleaning First, UV lamps should be arranged to bathe the cooling coils and drain pan of an air-handling unit in UVC light to prevent the build-up of mould and slime. Left unchecked the cool, such wet surfaces present an ideal breeding ground for biomass — blocking fins and reducing thermal efficiency, and creating a source of biological contamination that is easily picked up and vented to rooms. UV coil cleaners prevent the formation of this biomass, with the economic benefit that the initial efficiency of the coils is maintained, and the costs and downtime associated maintenance and coil cleaning are eliminated. In-duct air purifiers Whist coil cleaners sterilise a ‘surface’, the second variant of UV found in ducted systems are purifiers for treating the air moving through the duct. Given that the airborne contaminants only have fractions of a second to pass a UV air purifier, the challenges to producing high efficiencies are far greater than those required to sterilise a surface. The result is that ‘in-duct’ purifiers are entirely different to coil cleaners because the UV energy and contact time have been engineered to maximise the UV dose given to passing air. To ensure that the purifier meets its design efficiency, UV lamps are arranged to be parallel to the air flow, in an outwardly radiating configuration, like the Sanuvox Biowall, to ensure that passing air gets both sufficient UV intensity and duration. So what? Organisations occupying buildings will have numerous concerns, particularly in the current lean times. Not only will they be looking for the lowest building operating cost, which is increasingly part and parcel of being ‘green’, they also need to maximise staff economies. The Lancet has reported on a study by McGill University in Montreal, that showed effective eradication of bio-aerosols using UV reduces infection rates of common ailments amongst building occupants of centrally ventilated buildings. In North America, at least, it is understood that this translates into lower absence rates and improved staff economies. UV air purifiers require the same pre-filtration as coils to prevent dust build up. However, where higher-efficiency filters and carbon filters have been traditionally used to catch biological and chemical contaminants, UV systems can achieve the same objective without the large pressure drop associated with filters. Leaving the inert fine dust, that is of little consequence, using UV rather than filters to treat the biological and chemical contaminants typically uses about 15 to 33% of the electrical energy than would be used by the system fan pushing air through the equivalent filters. With lamp life of two to three years, maintenance cost is also reduced. Assuming that the building-services sector can harness and repackage the benefits and savings from UV air purification to appeal to their customers (building users), it is inevitable that the technology will flourish here as it does in North America. Bill Anderson is with Adremit.

Duct-air purifier
In duct-air purifier. The lamp array is suspended centrally in the duct or plenum on adjustable struts, with the ballast attached to the duct wall. (With permission of Sanuvox Ltd)
UVGI section
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