ASHRAE Journal - June 2023 - 49

TECHNICAL FEATURE
zone, has been employed successfully in schools to
reduce the spread of measles and other contagious
childhood diseases and in health-care facilities to
inactivate tuberculosis (TB) bacteria in isolation
rooms and TB wards and to control transmission of
nosocomial infections.2,3 The COVID-19 pandemic
created a renewed interest in upper-room UVGI systems,
which were introduced in the 1930s, to control
the transmission of the SARS-CoV-2 virus. Wall or
ceiling-mounted UV-C fi xtures using, in the current
generation of source technology, mercury vapor or
amalgam lamps generate a UV-C fi eld with 254 nm
UV-C. Mixing airfl ow patterns in
the space helps carry infectious
aerosols into this UV-C radiation
fi eld where microorganisms are
inactivated. The rate of inactivation
depends on type of microorganism,
UV-C intensity and the
duration of exposure to the UV-C
fi eld.3,4 To reduce the occupants'
exposure to hazardous UV-C
radiation, the UV-C radiation is
directed horizontally or toward
the ceiling. Microbial susceptibility
to UV-C and the delivered
UV-C dose, which is the product of UV-C fl uence and
the residence time of the microbes in the UV-C fi eld
are the most important factors affecting the effi cacy
of upper room UVGI systems. Several factors can
affect the performance of upper-room UVGI systems.
The location and type of UV-C fi xtures, fi xture
UV-C output and the UV-C refl ective characteristics
of adjacent objects such as walls, ceiling, and other
ceiling-mounted diffusers can affect the UV intensity
distribution in the space. Additionally, indoor airfl ow
patterns play a crucial role in determining the fl ow
path of airborne pathogens and their residence time
in the upper UV-C fi eld, which in turn determines the
effective kill rate of microorganisms.
Airfl ow patterns and their effects on the distribution
of aerosols depend on several factors including
the number, location and type of supply diffusers
in space; supply airfl ow rates (air change rates) and
associated diffuser throws; supply air temperature;
number, size and locations of return/exhaust grilles;
the location and strengths of various heat sources
Return
in a room; an arrangement of furniture and other
obstructions to airfl ow; location, type and capacity of
air mixing devices such as ceiling fans; and importantly,
the relative positions of sources of aerosols
in space. Strategic selection and layout of supply
diffusers and exhaust grilles combined with appropriate
size and capacity of UV-C fi xtures can help
optimize the UVC dose for effective inactivation of
microorganisms.5
This study employed CFD simulations to systematically
evaluate the impact of UV-C intensities on the
effectiveness of an upper room UVGI system. The
FIGURE 1 Schematic of the office CFD models with and without UV-C fixtures.
Supply 4-Way
Infected Person
UV-C Fixtures
Without UV
With UV
survival fraction of microorganisms and the resulting
probability of infection was computed for a representative
offi ce space by keeping the ventilation rate and
HVAC confi guration (the location and type of supply
diffuser and return grille) fi xed. Finally, the overall
effectiveness of the UV-C system combined with the
ventilation effectiveness for cases was evaluated by
using a newly developed Spread Index method.
Virtual Offi ce Space
Three-dimensional, steady state, isothermal CFD
models for a typical small offi ce space with two
cubicles were developed. The space had a total fl oor
area (including the corridor) of about 300 ft2 (27.9
m2) with 9 ft (2.74 m) ceiling height and an occupancy
of 6 persons. As shown in Figure 1, the cubicles are
separated by 5 ft (1.5 m) tall dividing partitions. The
area of each cubicle was approximately 100 ft2 (9.3
m2). There was a common corridor adjacent to these
cubicles leading to the room door. Each cubicle has
three occupants seating around a table. In Figure 1, an
J U N E 2 0 2 3 ashrae .o rg ASHRAE JOURNAL
49
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ASHRAE Journal - June 2023

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