ASHRAE Journal - August 2021 - 27

COLUMN IEQ APPLICATIONS
* Lamp operation indicator and view
window;
* Airfl ow switches to " prove " airfl ow
prior to energizing lamps;
* Door switches to de-energize lamps
and caution signs for personnel safety;
* Run-hour meters to track use and
estimate lamp end of life; and
* Lamp encapsulation to protect
against breakage and to improve lamp
output by insulating them from cool
air.
TABLE 1 Price comparison in-duct UVGI.
BUILDING 1 THROUGH 4
1
2
3
4
BUILDING NUMBER
1 THROUGH 4 SUBTOTAL
BUILDING 1 THROUGH 4
PRICE/FT2
BUILDING NUMBER 5
AND 6
Comparison to ASHRAE Standards
ASHRAE Standard 185.2-20202
describes a method of test to generate
a performance report for a UVC
device but does not provide a pass-fail
criterion for the device. The test setup
includes 10 ft (3048 mm) of duct up
and downstream of the device, which is
likely to be larger than an actual air handler. Therefore,
the test report may be useful for comparing products,
but it does not predict performance in a particular fi eld
application. Neither vendor provided such test data.
Vendor B said, " while we meet Standard 185, the only
lab that was ever certifi ed to test for that (RTI) is closed
so no UV companies were ever tested. The design standards
for 185.1/185.2 are met. " However, as described
above, " meets " is not a criterion in Standard 185.2-2020 .
5
6
ALL BUILDINGS TOTAL
BUILDING 1 THROUGH 6
VENDOR PRICE/FT2
Effectiveness and Kill Rate
The approximate lamp intensity included in the proposals
from both vendors seems more than enough to
achieve a kill rate of 90% or more for SARS-CoV-2, based
on a calculation check we performed,* provided that there
is 2 ft (610 mm) in the direction of airfl ow in the plenum where
the UV lamps are located and the lamp is installed in the
middle. The longer the plenum length, the higher the
dose and the kill rate. Shorter plenums have a correspondingly
shorter residence time and require higher
*We performed this calculation check based on 2 ft (610 mm)
clearance between the coil and the end of the plenum, lamp
placement centered in this 2 ft (610 mm) clearance, 420 µW
lamp output at 3.3 ft (1 m), 500 fpm (2.5 m/s) air velocity at
the coil, and a k value (species-dependent inactivation rate) of
0.0025 cm2/µW-sec (0.25 m2/J) (low end of the range to be
conservative).
QUANTITY
OF AHUS
1
AREA (FT2)
8,490
10 95,700
16 16,236
15 181,811
42 302,237
-
QUANTITY
OF AHUS
-
AREA (FT2)
12 165,000
2 70,652
56 537,889
-
-
VENDOR A
PRICE ($)
8,144
154,519
73,672
214,627
450,962
1.49
VENDOR A
PRICE ($)
No Bid
No Bid
450,962
1.49
VENDOR B
PRICE ($)
10,260
92,357
80,580
196,008
379,205
1.25
VENDOR B
PRICE ($)
174,406
22,148
575,759
1.07
LOWER OF A AND B
PRICE FOR EACH
BUILDING ($)
8,144
92,357
73,672
196,008
370,181
1.22
LOWER OF A AND B
PRICE FOR EACH
BUILDING ($)
174,406
22,148
566,736
1.05
LOWER COST
IN $/FT2
0.96
0.97
4.54
1.08
1.22
-
LOWER COST
IN $/FT2
1.06
0.31
1.05
-
The competitively bid average low price of $1.22/ft2 ($13.13/m2) is fi ve to nine times
higher than the estimated cost of $0.13/ft2 to $0.25/ft2 ($1.40/m2 to $2.69/m2) in the
ASHRAE Handbook-HVAC Applications.1
lamp intensity to achieve the same kill rate as a longer
plenum.
The formula to calculate survival rate of a microbial
population is:
S = e-kIt
where
S = Survival rate, typically 10% if 90% deactivation is
the goal
k = A species-dependent inactivation rate constant.
Measured k values for coronaviruses have
been in the range of 0.0025 cm2/µW-sec to
0.0038 cm2/µW-sec (0.25 m2/J to 0.38 m2/J)
I = The average irradiance in µW/cm2
t = Time in seconds
Determining the dose " I × t " based on air handler
geometry, surface refl ectance and residence time is not
simple. According to ASHRAE, although calculating
the dose " ...appears quite simple, its application can
be complex (e.g., when calculating the dose received
by a microorganism following a tortuous path through
a device with spatial variability in irradiance). " See
Reference 1, page 62.1.
Vendor A gave us data on plenum dimensions, air temperature,
air velocity, average residence time and UV
dose, but neither vendor provided detailed dose calculations
that relate lamp output at a reference distance to
A U G U S T 2 0 2 1 ashrae.org ASHRAE JOURNAL
27
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ASHRAE Journal - August 2021

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