ASHRAE Journal - July 2023 - 63

TECHNICAL FEATURE
lunch break after 4 h in the building in the morning
and come back for 4 h in the afternoon. An exhaust
fan in the service rooms extracts the transfer air,
while the air-handling unit supplies the full 100%
outdoor fresh air to the single offi ces only; there is
no direct supply air to the corridor or to the service
rooms.
The system in this example case has hydronic
local terminals and primary air supplied by a
ducted handling unit to the individual offi ces, with
heat recovery from the ducted exhaust air without
recirculation. To prevent the propagation of offensive
odors or other pollutants from certain rooms (toilet
service, copier room, etc.), a fraction of the total
ventilation air is expelled outside directly from the
rooms, independently of the general extraction
system, thus generating an appropriate pressure
gradient in the building.
Online Figure 3 shows a timeline of the infectious
quanta concentration in the room of the infected
individual CI, while Figure 11 shows the timeline of
quanta concentration in the well-mixed common
spaces (corridor, service rooms) CC. It is assumed
that the infected person and the other workers spend
20 min during the whole working time in the common
spaces (20 min in two 4 h periods); the plots
refer to a total number of people involved NS = 10,
an air renewal rate rn = 1 h−1, and 10% ventilation air
exhausted from the common rooms.
Figure 12 shows how the presence of the infected
individual in the common spaces and the fraction
of ventilation air exhausted from the service rooms
affect the number of susceptible people expected to
be infected during the working day R*. Figure 13 shows
how increasing the fraction of air extracted from the
common spaces can actively reduce the number of
infected individuals R*.
Conclusions
The aim of this work was to calculate, by means of
concentration balances and the Wells-Riley infection
model, the infection probability in an all-air ducted
HVAC system with air recirculation and in a primary
air system. After a brief analysis of a simple model
(already considered in the specifi c literature),
several improvements to the model were added. The
results highlight an important outcome related to
HVAC plants with air recirculation: dilution with
recirculation in multiple rooms is not enough to
compensate for the increased number of involved
susceptible individuals. High air renewal rates
can strongly reduce the risk of infection at a given
recirculation rate; furthermore fi ltration (or other
equivalent technologies, such as UV-C irradiation or
ionization, to inactivate the virus from recirculated
airfl ows) is a very powerful tool to reduce the
infection probability, especially if coupled with high
recirculation rates.
Energy consumption for virus inactivation/removal
can't be neglected in the design of operating plants
and can vary widely, strongly depending on the
equipment. The impact on energy consumption will
likely be quite high for traditional physical fi ltration,
and lower for ionization: the designer should consider
this issue in detail.
Finally, it is very important to consider, in realworld
applications, the management of airfl ows in
both segregated and common spaces, which can
heavily modify the performance of the system w ith
respect to infection probability. For online fi gures and
equations, visit https://tinyurl.com/JournalExtras.
References
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J U LY 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
63
https://tinyurl.com/JournalExtras https://www.doi.org/10.1016/j.envint.2020.105730 https://www.doi.org/10.1111/ina.12465 https://www.doi.org/10.1111/ina.12465 https://www.doi.org/10.1111/j.1600-0668.2009.00621.x https://www.doi.org/10.1016/j.envint.2020.105794 https://www.doi.org/10.1016/j.envint.2020.106112 https://www.doi.org/10.1016/j.envint.2020.106112 https://tinyurl.com/covid-estimator http://ashrae.org
ASHRAE Journal - July 2023

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