COVID-19 | UVGI passes over the UV lamp might be cleaned and disinfected, but not much air is going through, says Beggs, who adds that many manufacturers claim such units achieve a 99.9% disinfection rate. This is true, but these claims only relate to the air that passes through the UV device, and not the effect the unit will have on the room space. If youve got a very big room and a small device, youre not going to get much impact. Nathan Wood, managing director at ventilation firm Farmwood M&E, and chair of BESAs Health & Wellbeing in Buildings Group, says UVGI room air cleaners are most effective in contained spaces, including non-ventilated meeting rooms or classrooms, used by a lot of people sequentially. He has been installing UVGI Radic8 Virus Killer units for several years. The freestanding or wall-mounted devices use technology, developed with the South Korean government in response to the SARS outbreak in 2003. They comprise course, Hepa and carbon air filters and multiple UV tubes, each surrounded by a reactor chamber that reflects and refracts the light, magnifying the reaction. Wood says a unit can process up to 18 cubic metres of air per minute, eliminating 99.9999% of pathogens known as a Log 6 reduction. Since lockdown in March, the two MVHR systems at Farmwoods office have been recommissioned to supply more outside air, while its UVGI systems have been repositioned to ensure the best airflow, following the offices new socially distanced layout. The grill configuration needed to be changed as the high-level acrylic screens reduced the airflow in the room, says Wood. We plan on further revamping the ducting layout to allow for high-level supply and low-level extract. With additional UVGI units on desks were able to zone each working area and reduce the risk of any potential aerosolised particles leaving it. When installing such units, design engineers must do their research on suppliers, adds Wood. They should have independent testing from specialised labs, history of real life application and results, and time served experience with the technology, he says. In-duct UVGI These systems use UV-C lamps mounted in return or supply air ducts of mechanical HVAC systems, to disinfect air to or from the room space. In the US, UV-C lamps have been installed in return-air ductwork to mitigate the spread of tuberculosis. As Covid-19 is also a disease spread by aerosols, this system would, in theory, be equally applicable to the coronavirus, says Beggs. Upper-room installations rely on natural convection currents to carry aerosol particles through the UV field, and can be particularly effective, often achieving very high equivalent air-change rates JenActs UV Torpedo KEY QUESTIONS THAT ENGINEERS SHOULD ASK Professor Clive Beggs says the first thing to look for on a manufacturers website is scientific evidence, for a measure of how much knowledge they have. Have they done any microbiological testing? If they have results, this shows they are a company that has put some investment into this. If its an in-duct UV system, ask for supporting calculations of the dose that the average particle is going to get, he adds. I would expect a manufacturer to be able to produce robust calculations for the UV field they are proposing to use, so that it meets your specification. Redundancy is an issue too, so it is also important to ask what would happen if a lamp goes out. Infectious droplets, which start at 100 microns (m), evaporate down to less than 50m diameter rapidly, so they can float around in air currents and get taken into return air ducts. Central HVAC systems can recirculate as much as 50-80% of the room air during the winter, so the virus on aerosol particles could be drawn up into the return air duct and recirculated round, potentially resulting in the spread of Covid-19, says Beggs. A possible retrofit solution, he suggests, might be to put UV lamps in the return air duct, disinfecting the air before it passes filters and coils. This way, not only is the recirculated air disinfected, but the maintenance staff are also protected from contamination and the exhaust air is cleaned. Its easy to retrofit such devices into ducts, says Beggs. However, if the air velocities are too high or the lamps are too weak the aerosol particles carrying the virus may not be irradiated for long enough to inactivate them, which is a potential problem. If you have big ducts and fast-moving air, you may have to have huge lamps which means youll have electrical loads of several kilowatts, he says. This can be a substantial additional load in some buildings. Design is another issue, as manufacturers can place lamps in different geometrical arrangements. As a result, sophisticated modelling technology such as computational fluid dynamics is needed to calculate the UV field required for a particular duct size, air velocity and lamp arrangement. 34 November 2020 www.cibsejournal.com CIBSE Nov 2020 pp33-35 UVGI.indd 34 23/10/2020 16:12