Modern Building Services

30 MODERN BUILDING SERVICES APRIL 2023 FEATURE INDOOR AIR QUALITY Ella’s Law passed successfully through the House of Lords, but is has stalled in the house of commons. Recent government data showed that the country exceeded its own legal limits for PM2.5 in 2021, so this is going to be a challenge for a few years while the current approved documents are in force. Despite the challenges faced because of Part L’s lack of leniency with specific fan power, there are innovative solutions that can be deployed to reduce the amounts of pollutants entering buildings without increasing energy consumption. In the industry, there is a rigidly held assumption that filter grades cannot be increased without increasing pressure drop and therefore energy consumption. It is true that the type of low-cost basic bag filter typically provided by AHU manufacturers in new units will typically have a high pressure drop relative to its filtration efficiency. As an example; a low cost 8 pocket ePM1 60% filter with an “E” energy rating has an initial pressure drop of 120Pa, and will consume 2189kWh per annum. A higher filtration grade 10 pocket A+ energy ePM1 85% filter has an initial pressure drop of 65Pa and an annual energy consumption 882kWh. Specifiers do not have to accept the filters suggested by the AHUmanufacturer, specification of high-performance particulate filters needs to become more commonplace. When it comes to molecular filtration, to limit the ingress of nitrogen dioxide and sulphur dioxide in order to comply with Part F, activated carbon filters will be required. Conventional carbon filters can add a significant pressure drop (~90Pa clean) to an HVAC system. Molecular and particle combined filters are available, saving on total pressure drop, but the long-term performance of these is limited. Typical final stage filters will take many months to reach their efficient dust loading, at which point they should be changed. Conversely, the small weight of carbon contained in combined filters, will quickly saturate and will no longer remove gaseous contaminants. For this reason, we recommend utilising pre-filters with combined ePM10 particle filters and gaseous removal, as the dust loading and saturation of these filters will occur at a similar time. As pre-filters are more regularly changed than final filters, the relatively low weight of carbon in the pre filters becomes less of an issue. The pressure drop of a combined molecular and ePM10 particulate filter is similar to that of a standard particulate-only pre filter, so the overall pressure drop of the system is not increased. What is possible In some ventilation systems like small MVHR units, it is not possible to provide the full range of filtration options that are available in full or half sized filters typically used in AHUs. In this instance, electronic air cleaners become the best option. Photocatalytic oxidation (PCO) systems utilize a UV light shining on a titanium dioxide catalyst. These can be fitted in the air discharge of an AHU or MVHR to break-down benzene and other volatile organic compounds in the supply air. Bipolar ionization (BPI) systems can be fitted into the supply air discharge. These systems give opposing electrical charges to any particles that have passed through the filters. These ultrafine particles then form clusters of larger particles that either drop out of the breathing zone or are captured in the low- grade filters fitted to fan coils or air conditioning systems. Both PCO and BPI systems create negligible additional pressure drop, and use very little electrical power. It is important that specified electronic air cleaning devices carry the UL 2998 ozone free certification. “Specifiers do not have to accept the filters suggested by AHU manufacturers” More information can be found at www.spirebuildingservices. co.uk