These works we to be carried out through their Cavendish, Clipstone, Littetichfield, Regents Street and Wells Street Central London Campus, and their slightly further afield Harrow Campus.
The Challenge
We were required to carried out a site surveys to ascertain condition and configuration of the University of Westminster AHU’s, in order to provide recommendations for system improvements to ensure good system operation and measures to mitigate the spread of COVID-19 via the ventilation systems.
Measures to lessen the spread of COVID-19 were to be on the scale of temporary, quick to action measures (i.e. BMS scheduling and operation), to physical works to existing systems (i.e. Altering of ductwork to AHU supply-extract air by-pass).
The Solution
Recommendations were to be inline with CIBSE and REVHA guidance as current at time of investigations, noting guidance set out by CIBSE and REHVA for COVID-19 good practice, was based upon the confirmed COVID-19 data at the time. In general, the CIBSE and REHVA guidance details mitigation measures to stop cross contamination between intake and exhaust airstreams where heat recovery/retention system are used (i.e. exhaust to intake air by-passes and thermal rotary wheels). To understand the extent of necessary measures we firstly needed a full and better understanding of how the COVID-19 pathogen is transmitted.
The at the time for COVID transmission was;
The pathogen can be spread by sneezing in biological aerosols. The size of sneezing droplets range between 0.1-900microns.
The COVID-19 pathogen itself is estimated at 0.125microns.
Where the COVID-19 pathogen is held within an aerosol at =<2.5microns there are various studies concluding the pathogen lingers (dwell time) within the airstream for a period of between 0.5-3hours. Note: this is not conclusive, but an estimate. The actual results could be more or less.
We did not have a conclusive answer to what quantity of COVID-19 aerosol can cause infection in a person, nor did we have a conclusive answer to the quantity of breath or sneeze contains COVID-19 when expelled by an infected person.
Knowing that aerosols containing COVID-19 at =<2.5microns can have an estimated dwell time up to 3hours, we need to understand the quantity of COVID-19 particles it takes to infect a person. Note: this requires ongoing specialist research.
We needed to understand the time taken to recirculate air in systems with exhaust to supply air by-passes and thermal rotary wheels. Note: buildings can generally range between 3-15ach across occupied areas, with specialist areas being excessive in this (i.e. fume rooms), and these rates the pathogens could be recirculated between 9-45 times over a 3hour period.
We were in summary detailing and answer to does the plant setup offer a level of risk management? for example;
A building ventilation serving a whole building has the potential to recirculate air from a single enclosed room from a single person sneezing throughout multiple areas within the building due to interconnecting ductwork. – High risk of spread from the ventilation system alone.
A building ventilation system serving a single room, only has the potential to recirculate air within that single space. The second question is how large is this room, and how dense is the occupancy. – Low-medium risk of spread from the ventilation system are all factors.
Your work again is impeccable. Thank you again for dealing with it as professionally as you did.
