ASHRAE Journal - November 2023 - 30

TECHNICAL FEATURE
PHOTO 1 Microbial contaminated chilled water cooling coil in hospital AHU.
FIGURE 1 Leaving air conditions from cooling coil near saturation.
Air Leaving Cooling Coil:
55.00°F Dry Bulb
54.80°F Wet Bulb
98.78% Relative Humidity
.028
.024
.020
.016
.012
.008
.004
Mold, fungi and bacteria growing on HVAC system
components can be aerosolized and distributed by
ductwork to various spaces within the building,
resulting in considerable concern regarding occupant
well-being.1 One issue is wet and moldy air filters,
which create unsustainable operating conditions for
air handling in any HVAC application. In addition to
health consequences, this moisture may reduce filter
life or efficiency and add additional airside pressure
drop, which increases fan energy consumption and
operational costs.2
Filters become wet for various reasons, most of
which can be eliminated with proper AHU design,
selection and control. Ensuring cooling coils are
kept clean and air velocities through coils are kept
low enough to prevent moisture carryover (blowoff)
into the airstream is critical. If humidification
devices have been inadvertently installed upstream
of filters, their humidistat, sensors and valves must
be properly installed and functioning correctly. But
even with the most judicious AHU design, final filters
may still become wet for what appears to be no good
reason. Besides physical wetting
by condensate carryover, another
relatively simple explanation exists
as to how this can happen.
HVAC System Dynamics
Cold air leaving a refrigerantbased
direct-expansion (DX) or
chilled water cooling coil will
typically approach saturation (the
air is near 100% relative humidity)3
(Figure 1). When this airstream
30
40
50
60
70
80
90
Dry-Bulb Temperature (°F)
enters a final filter (Figure 2), or any component that
increases its velocity, the pressure and temperature
of the air will drop as the air accelerates. Even a slight
increase in air velocity accompanied by a small drop
in temperature may be enough to allow the airstream
to cool to a point where moisture is condensed. In
cooling systems that operate for many hours a day,
considerable amounts of moisture may collect on
filters or other internal surfaces within the AHU. If
there is not sufficient downtime that allows for this
moisture to evaporate, filters will remain wet and
microbial growth can proliferate.4
This wetting situation may be more evident with
blow-through fan configurations and when filters are
positioned immediately downstream of cooling coils
in what is considered the final-filter position.
When designing a critical-care AHU that includes
final filters, one may consider using a drawthrough
(cooling coil located before the fan) versus
FIGURE 2 Typical HVAC final filter location (in draw-through AHU).
Return Airflow
Return Air Dampers
Air-Handling Unit (AHU)
Supply Air Fan
Outside Airflow
Outside Air Dampers
ASHRAE JOURNAL ashrae.org N OVEMBER 2 0 2 3
Prefilters (MERV 10)
Final Filters (MERV 15)
Supply Airflow
100
120
Supply Air Dampers
Humidity Ratio (Lbv/Lba)
Wet Bulb (°F)
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ASHRAE Journal - November 2023

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