Coronavirus, Covid-19 and Ventilation System Filtration
Extract ventilation systems requiring the addition of HEPA filtration to stop the spread of the coronavirus and Covid-19.
The Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) has issued guidance on the control of the virus as it pertains to ventilation services in buildings. The current version of their guidance document can be found here (The document will be updated and re-issued as necessary.):
In the UK, CIBSE (Chartered Institution of Building Service Engineers) have issued their own guidance here:
Ventilation systems perform a significant role in in the transmission mechanism of the Coronavirus which leads to the development of Covid-19. The entrainment of virus-containing water droplets (<5um) by extract ventilation systems and redistribution to other areas via the supply ventilation system is cited as an issue for recirculation systems. Full fresh air systems are better in this regard since the extract and supply streams don’t mix and the energy from the exhaust stream is recovered and re-used in the supply air system via any number of heat recovery schemes. However, when the air from the extract system is exhausted outside, it may still contain those contaminated droplets. In which case it is necessary to filter the exhaust stream before rejection to the local ambient to avoid either being re-entrained in the fresh air intakes or falling to ground level and infecting passers-by. However, standard filtration (G or F grade to EN779 or ePM Coarse 60% or ePM1 65% to ISO16890 ) will not capture droplets of this size. Only HEPA (High Efficiency Particle Arrestor) filters are sufficiently fine to stop the droplets.
Typical HEPA Filter for Covid-19
One issue with retrospectively installing HEPA filters on exhaust ventilation systems is that they have a relatively large pressure drop even when clean. It is unlikely that any existing ventilation systems have sufficient spare capacity in terms of pressure to be able to overcome the resistance of the HEPA filters. In which case, the installation of the HEPA filters will entail the addition of a complementary fan sized to just overcome the filter pressure drop leaving the remainder of the system to operate at the same volume flow rate as it did previously. The HEPA filter is so fine that when challenged by normal room air, which contains millions of particles smaller than are visible with the naked eye, the HEPA filters will block up in a short space of time and require to be replaced at significant cost. To extend the lifespan of the HEPA filters, pre-filtration is required which adds further pressure drop. Therefore, to add filtration to an exhaust air ventilation system requires a degree of engineering to ensure that the effectiveness of the ventilation system is not impaired as a result of the addition.
Callidus Design have the necessary engineering skills to design and specify the required components to permit HEPA filtration to be added to extract or exhaust ventilation systems and have carried this out already for NHS installations.
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