ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 7

Duct/Plenum Surface

relatively harder to kill than more rapidly replicating

UV-C Multiplier

and non-environmental microbes and most bacteria. But
Stainless steel

1.40

even fungi are killed effectively with high-dose UV.7 For

Galvanized steel

1.50

example, studies have demonstrated that viruses are more

Aluminum

1.75

susceptible to UV-C inactivation than typical bacteria
are (Figure 3).15

Use of reflective materials can increase germicidal-UV
disinfection dosage/fluence.

Lamp Placement

TABLE 2. Reﬂectivity of different metals.

When designing a germicidal-UV disinfection system,

Some UV-C lamps are encapsulated with an anti-

engineers should consider the impact AHU location has

shatter ﬂuorinated-ethylene-propylene (FEP) coating

on performance. Because cold air affects the output of

or outer sleeve that helps to insulate the lamp surface

UV-C lamps and high relative humidity affects pathogen

from changes in temperature and/or air volume. This

susceptibility to UV-C, air-stream disinfection can be more

protection can be beneﬁcial when the temperature is

effective when lamps are installed upstream of a coil. In

low and/or the air-stream velocity is high, but it also

fact, moving UV-C lamps from the typical downstream

can reduce UV output by up to 10
to 12 percent.
■

1

The reﬂectivity of the plenum-
reﬂective metals boost UV-C dose,
as t he ger micida l waveleng t h

2

being absorbed by the surfaces.
Different metals have different
reflectance multipliers that can
signiﬁcantly increase UV-C ﬂuence
levels (Table 2).

Pathogen Susceptibility
Although environmental factors

UVGI removal rate
RR = 1 - e - klm Et

"bounces" throughout the plenum
and remains "in play" instead of

Dimensions of duct and exposure time
Vol
WHL
-
Et = -
Q = Q

3

cation in a predictive-modeling
formula when designing an airor surface-disinfection strategy.
In Figure 2, we can calculate

0.37700

Base Pairs kb
30.378

Fungal
Spores

Bacterial
Spores

Mycobacteria

Vegetative
Bacteria

Viruses

Aspergillus
versicolor

Bacillus
anthracis

Mycobacterium
tuberculosis

Staphylococcus
aureus

Influenza
viruses

Penicillium
chrysogenum

Bacillus
cereus

Mycobacterium
bovis

Streptococcus
pyogenes

Measles

Stachybotrys
chartarum

Bacillus
subtilis

Mycobacterium
leprae

Escherichia
coli

Coronavirus
Smallpox

Pseudomonas
aeruginosa

the amount of time a pathogen is
exposed to UV energy to deter-

Serratia
marcescens

mine the total exposure time.
Bacteria and viruses vary in

RR = removal rate, fraction or %
e = efficiency (desired removal rate)
k = UV rate constant, m 2/J
lm = mean irradiance, W/m 2
Et = exposure time, seconds

FIGURE 2. Air-stream disinfection.15

inﬂuence UV-C dosing, it is best
to consider all aspects of an appli-

UV k m 2 /J

Coronavirus
(Walker 2007)

Vol = volume of UV chamber, m 3
Q = airflow, m 3/s
W = width, m
H = height, m
L = length, m

Removal rate

Rate constant
(K or Z value)
Microbe

Exposure time

LEAST SUSCEPTIBLE

253.7 nm

MOST SUSCEPTIBLE

susceptibility to UV energy, with
environmental organisms, fungal
spores, and mycobacteria being

w w w. a m c a .o r g

FIGURE 3. Microorganisms susceptible to germicidal UV-C.12

2020 A M CA i n m o t i o n

7
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ASHRAE Journal Supplement - AMCA InMotion - October 2020

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