Abstract
In containing the contagion of viral diseases transmitted by air, it is essential to reduce the concentration of elementary dose of airborne nuclei present in inhabited rooms, that is reduced by increasing the renewal outdoor air flow, with the same other factor. The increase in the outdoor air flow, however, finds limits due to energy consumption, the maximum generators power required, the ducts size, which are not always compatible with the spaces available, especially in the case of the renovation of existing buildings. The alternative is the filtration of the recirculated indoor air flow. The paper describes possible strategies. Good filtration can always be possible in all-air systems for cinemas, theatres or supermarkets, or in VAV systems, because the AHUs have dimensions and characteristics such as to allow the housing of any type of filter. VAV systems suffer from the defect of spreading the virus in all rooms, while primary air systems leave it confined only where there is the presence of the infected subject, but filtration is necessary through filters installed directly in the fan coil units. In particular, the paper focuses on the studies carried out in the laboratories of the University of Medicine of Padua for photocatalytic filters and shows the results. The most interesting aspect of this type of filter is that they are effective not only on the finer particles that make up the aerosol, but also on the heavier particles deposited on the surfaces of the room.
Highlights
The main infection risk models assume that an infectious person constantly generates a number of infectious quanta over time, with a quantum defined as the dose of airborne droplet nuclei required to cause infection in 63% of susceptible people [7]
The virus removal rate value is given by three factors [3]: with: ne nd nrf external fresh air rate through ventilation, h-1; sum of the the deposition of droplets on the surfaces by sedimentation, 0.24 h-1 [3], and viral inactivation, 0.63 h-1
The movement of the virus in a room changes depending on whether: - the virus moves only within a single room: in this case there are differences depending on the type of system, primary air systems or total air systems; - the virus moves between different rooms: this is the case of VAV systems
Summary
The main infection risk models assume that an infectious person constantly generates a number of infectious quanta over time, with a quantum defined as the dose of airborne droplet nuclei required to cause infection in 63% of susceptible people [7]. The virus removal rate value is given by three factors [3]: with: ne nd nrf IVRR n n n (2). External fresh air rate through ventilation, h-1; sum of the the deposition of droplets on the surfaces by sedimentation, 0.24 h-1 [3], and viral inactivation, 0.63 h-1 [13] The total is 0.87 h-1. It is necessary to increase the ne and nrf sum to reduce Nt. The increase in ne generally produces an increase in energy expended for the treatment of the external fresh air and the generators capacity: ne must not exceed the values required for the desired IAQ. With: rf filter efficiency on the aerosol particles; nric air recirculation rate inside the room, h-1. Equation (3) shows that the filter efficiency is not important to minimize IVRR, but the product of the filter efficiency and the recirculation rate.
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