Shine a UV light on air quality
Widely accepted and used in North America, ultraviolet (UV) air purification, its use for preventing fouling on cooling coils and drip pans, treating odours and for inactivating or killing micro organisms in ducted air, is common place. In fact, the General Services Administration (GSA), who are responsible for large swaiths of US public sector assets, now requires UV lamps to be fitted to all air conditioning coils in new build projects.
In-duct mounted UV systems have even been tested and assessed on behalf of the United States Environmental Protection Agency (EPA), by the National Homeland Security Research Centre, to quantify the technology’s capability of defending building occupants against bioterrorism attack. In this year’s ASHRAE handbook, the technology is investigated and explained in considerable detail. Perhaps now that the technology has gained broad acceptance in North America, the time is right for the European markets.
UV light has wavelengths that fall within the range from 100nm to 400nm, and this range further divides to UVA, UVB and UVC. The UVC wavelength is on the shorter end of the spectrum between 100 and 280nm, and within UVC it is well recognised that 254nm is particularly effective at killing or inactivating micro-organisms. Ultraviolet germicidal irradiation (UVGI) systems generally use UV lamps that produce this wavelength. The effect of the UV light on micro-organisms, including bacteria, viruses and spores, is that the light penetrates the cell wall of these organisms and disrupts their DNA. The organism may still be alive, but it cannot reproduce. As individual micro organisms are short lived and reliant on fast reproduction for their spread, the inability to multiply spells certain doom.
The 187nm wavelength falls within the UVC band, but is confusingly known as UVV and is used for UV photo oxidation of airborne chemicals and odours. On its own UVV can produce ozone, oxygen singlets and hydroxyls (O3, O and OH) , which are excellent for treating airborne odours and other chemicals, however residual ozone can lead to harmful concentrations of the gas in occupied spaces. For this reason UVV can only be used if followed downstream by and interlocked with, a UVGI system which acts as a catalyst to revert the residual O3 safely back to O2.
Applications & payback
Although UV air purification comes in numerous guises there are really only two main applications in ducted systems. Firstly UV can be used to prevent the build up of mould and slime in air handling units, which left unchecked, can build up on the cool, wet surfaces of coils and drain pans. UV lamps arranged to bathe the coils and pan in UVC light ensure that the coils do not need subsequent cleaning and that clean state thermal efficiency and pressure drop are maintained. Moreover this elimination of a biomass source reduces the biological contamination (spores and bacteria) that would otherwise be picked up and vented to rooms. UV coil cleaners bring economic benefits because they maintain the initial efficiency of the coils and also because the costs and down-time associated with maintenance and coil cleaning are eliminated.
The second application for UV is to purify the moving air. Whether this is for reducing sick building syndrome, absenteeism and odours, or as part of the fight against hospital acquired infections, it is an entirely different proposition to coil cleaning which is merely sterilising a surface. Removing odours, viruses, bacteria and spores from an air stream requires far more UV energy. For this reason UV air sterilisers look quite different from coil cleaners. The UV lamps are typically arranged to be parallel to the air flow, in an outwardly radiating configuration, like the Sanuvox Biowall, which ensures that passing air gets both sufficient UV intensity and duration to achieve its design efficiency. The benefits to this type of system are numerous. Several studies, like that undertaken by McGill University in Montreal and reported in the Lancet, testify that effective eradication of bio-aerosols using UV reduces the infection rates of common ailments amongst building occupants of centrally ventilated buildings. For workplace buildings this translates into lowered absence rates and therefore improved staff economies.
Where there are sensitive indoor environments that require biologically and chemically clean air, the traditional solution is filters of higher efficiency that would typically be required for protecting coils, or carbon filters in the case of odours and chemical contaminants. Correctly specified filter based solutions can certainly solve these problems, however, in the case of high efficiency particle filters, they bring significant additional pressure drop to a system which is not only costly in electrical energy to overcome, but which increases through the life of the filters. Carbon filters also bring a pressure penalty, however both types of filters also have a significant labour and materials cost associated with filter replacement. In these lean times, working out the energy cost associated with pushing air through higher efficiency filters and carbon filters produces uncomfortable results and investigating more cost efficient solutions is a must. At: www.adremit.co.uk/PressureCostCalculator.php, there is a calculator tool which can work out the energy cost to your filter application.
A system based on UV requires the pre-filtration efficiency that is required in any case to protect the coils, but it is more selective. It can be designed to target problematical biological and chemical contaminants, whilst leaving the very fine inert dust particles that that are of no consequence. In this type of application, economies are reaped through energy savings, which vary depending on the particular parameters, but would typically be expected to be about 15 – 33% of the energy cost of an equivalent filter system. With better UV systems, the lamp life can easily be as long as three continuous years, which, considering typical filter change frequencies brings a further source of savings.
There is no doubt that there are now some excellent UV air purification and coil cleaning systems entering the European market backed by plenty of North American experience and credible independent research. It is only a matter of time before the building services professionals develop an appreciation of the benefits that UV air purification brings, and how they can then repackage those benefits and savings to appeal to their customers – the building users. However, buyers should be cautious of UV marketing material with bold claims such as, 99.99% efficiency – these type of claims are totally meaningless until applied to your particular set of parameters. The key is to work with those who can design a bespoke system to deal with each unique set of contaminants within financial and mechanical constraints.