ASHRAE Journal - August 2021 - 28

COLUMN IEQ APPLICATIONS
total dose mathematically integrated over time and distance
derived from geometry, reflectance and velocity.
Vendor A told us orally that they use a " rule of thumb " of
15 W/ft2 to 20 W/ft2 (162 W/m2 to215 W/m2) of coil area.
While this may be a useful number for budgeting purposes,
and may be based on coil cleanliness, it does not
readily translate to kill rate of the virus.
We performed a calculation check based on one air
handler in the Vendor A proposal, using the above survival
rate formula. We used manufacturer literature
UV intensity data 3.3 ft (1 m) from the lamp, design air
velocity, the lamp centered in a 2 ft (610 mm) plenum
length and 21 in. (533 mm) lamp spacing. Even at the
farthest point from the lamp (1 ft [305 mm]) and worstcase
conditions of highest velocity and lowest k value, we
found that the UV intensity was more than sufficient to
achieve the 90% kill rate. We extrapolated this calculation
check to both vendors' allowances for lamp intensity
and found all allowances to be sufficient.
UVGI lamps can be installed either upstream or downstream
of the cooling coils, and there are varying opinions
which is better. Downstream installation, as proposed
by Vendor A, has the advantages of killing mold
and other biological growth on the wetter face of the
coil, exposing the lamps to filtered air; the heat added to
the airstream is essentially reheat, which will generally
result in better humidity control. Upstream installation,
as proposed by Vendor B, has the advantage of higher
lamp output due to higher temperature.
Cooler air at coil discharge will reduce performance
of lamps. Actual ambient temperature must be taken
into account, not just peak performance. Vendor B listed
temperature and humidity conditions and duct/plenum
UV reflectance, but did not describe how these were
used in the calculation to get average I = irradiance.
Therefore, we recommended that detailed submittals
and calculations should be provided prior to installation,
showing:
* Actual air handler dimensions;
* Lamp placement in the air handler;
* Lamp output corrections factor for temperature and
air velocity, and source of the factors;
* Lamp output correction factor for lamp age and age
of lamp assumed;
* Value used for k (species inactivation rate) and biological
basis for it;
28
ASHRAE JOURNAL ashrae.org A U G UST 2021
TABLE 2 Sample calculation of annual electric cost for Building 2.
ELECTRICITY COST
TOTAL WATTS, BUILDING 2
PER VENDOR A PROPOSAL
OPERATION
ANNUAL ELECTRIC COST
ADDITIONAL COOLING COP
TOTAL COST
ANNUAL COST
$0.1056/kWh
10,260 W
3,000 Hours/Year
$3,300 (Lamps Alone)
3 (Estimated Average Including Parasitic
Distribution and System Losses)
$4,400 (Lamps and Additional Cooling)
$0.0460/ft2
Electricity cost source: https://www.electricitylocal.com/states/
* Reflectance of air handler surfaces assumed if credit
is taken for reflection;
* Any other values used in the calculations; and
* Detailed calculations based on air handler geometry
and the other values above.
Maintenance Cost
Prior to contracting for the installation, we recommended
that a proposal for system maintenance be procured
from the installer(s), including lamp replacement
cost based on useful lamp life and hours of operation.
Lamp life is on the order of 10,000 hours, and intensity
declines toward the end of life. Hours of operation may
vary with each air handler.
Table 2 shows a sample calculation for estimated annual
electric cost for running in-duct UVGI lamps at Building
2 based on $0.1056/kWh and 3,000 operating hours
per year. Since the lamps add heat to the airstream,
this adds cost in the cooling-dominated climate of the
buildings.
Based on this, the average annual cost of energy for
lamps and the air conditioning to counteract heat from
the lamps comes to about $0.045/ft2 ($0.48/m2) of building
area. This is two to four times higher than the estimated
operating cost of $0.01/ft2 to $0.02/ft2 ($0.11/m2
to $0.22/m2) given in the 2019 ASHRAE Handbook-HVAC
Applications1 based on a similar electricity cost.
Alternate Recommendations
Other ventilation-related actions that are likely to be
more cost-effective and have other benefits for indoor
air quality, thermal comfort and, in some cases, equipment
life and energy efficiency include:
1. Improve filtration to MERV-13 with good edge seals.
2. Check for adequate airflow to each occupied space.
https://www.electricitylocal.com/states/ http://www.ashrae.org
ASHRAE Journal - August 2021

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