Applying Ultraviolet Germicidal Irradiation to HVAC Heat
Exchangers to Reduce Biofouling and Improve Heat Transfer
Capability
Julia C Luongo and Shelly L Miller
Department of Mechanical Engineering, University of Colorado, Boulder, CO
Key words: UVC, UVGI, Coil fouling, Biofilm, Cooling coil
Introduction
Ultraviolet Germicidal Irradiation (UVGI) has a long history of being used for the disinfection of
air streams, primarily in environments with higher risk of airborne pathogen transmission such as
healthcare facilities, schools, and prisons (Reed, 2010). Using UVGI as a coil cleaning
technology in air-handling units (AHUs) has recently gained popularity. While air disinfection
may still occur as air passes by the UVGI system, the primary focus of UVGI on cooling coils is
surface disinfection and, in turn, energy savings, maintenance cost savings, and increased or
prolonged system capacity. The buildings sector accounted for about 41% of primary energy
consumption in the US in 2010 (DOE, 2011). More than half of the energy used in buildings is
for heating, ventilating and/or air-conditioning (HVAC) the indoor environment (EIA, 2003) so
energy savings for HVAC systems could have large implications for total building energy
consumption.
Heat exchanger surfaces are an ideal site for biofilms due to the presence of adequate nutrients
(debris inherent on coil surfaces) and moisture (Morey, 1988). High bacterial and fungal
concentrations have been documented within AHUs, specifically on cooling coils and drain pans
(Hugenholtz & Fuerst, 1992; Levetin et al., 2001; Menzies et al., 2003).
Anecdotal evidence exists reporting energy savings from cooling coil UVGI (Keikavousi, 2004;
ACHR News, 2007). An increase in energy efficiency of 10-15% from coil cleaning has also
been reported, but not specifically using UVGI (Montgomery & Baker, 2006). To the author’s
knowledge, few peer-reviewed studies have measured changes in heat transfer coefficients, flow
resistance, or energy consumption resulting from UVGI coil cleaning. The objective of this study
is to report detailed measurements of heat transfer and flow characteristics for an irradiated coil
versus a non-irradiated coil in a laboratory setting. Biological samples of the air and coil surface
were analysed using various techniques, including culturing and epifluorescent microscopy.
Methodologies
System Parameters
An HVAC test apparatus was built in the Air Quality Laboratory at the University of Colorado,
consisting of two parallel ducts, each with its own cooling coil, but supplied by the same
temperature and relative humidity controlled airstream (Figure 1). The test apparatus is equipped
with sensors to measure duct velocities, static pressure drops, entering and exiting water
temperatures, and entering and exiting air temperatures and relative humidities for each branch.
Fan
Electric Heating
Element
Humidifier
Length that allows appropriate mixing
Tin Tout
Damper
Pitot Tube
Tout , RHout
!
Damper
Pitot Tube
Cooling Coil
Tin , RHin
Meshing to
eliminate water
droplets
Static Pressure
Taps
Tout , RHout Cooling Coil
UVC lamp
Tin , RHin
Static Pressure
Taps
Tin Tout
Figure 1. Schematic of HVAC test apparatus.
The test apparatus uses indoor air from the room as the inlet air. The room HVAC system
supplies 100% outdoor air filtered with MERV 14 filters. Air enters each cooling coil, on
average, at 72 oF and 45% relative humidity and chilled water enters at 45 oF, satisfying
conditions for condensation onto the coils. These conditions, however, are mild compared to the
condensing conditions of cooling coils in very humid climates. The system mimics a constant
volume HVAC system, meaning the volumetric flow rate is held constant. The flow rates
through each coil are held equal to one another using dampers since the static pressure drop
across the coils may not be equal given equivalent flow rates. Air and water inlet temperatures,
inlet relative humidity, and water flow rate are all held constant.
The system ran undisturbed for 10 months without UV on either coil to ensure that both coils
“fouled” at an equivalent rate and to establish a robust baseline dataset. After 10 months of
operation, the UV lamp was turned on, irradiating the downstream side of one of the cooling
coils (labeled “Top” coil). The control coil is labeled “Bottom” coil. The irradiance at the surface
of the Top coil was 200 μW/cm2 at the center and 150 μW/cm2 at the corners. The UV lamp had
been constantly irradiating the Top Coil for 3 weeks at the time of submittal of this paper.
Assessment of Heat Exchanger Flow Characteristics and Effectiveness
One of the main challenges in assessing changes in flow characteristics and effectiveness in two
heat exchangers over time is that all variables affecting these qualities are never exactly the same
and cannot be held completely constant. For this reason, small fluctuations in temperature,
relative humidity, or flow rate will affect static pressure drop and the calculated value of heat
transfer, making it difficult to compare. To remedy this, comparisons between the control and
UV-treated coils were only made with dimensionless quantities, including heat exchanger
effectiveness and normalized static pressure drop at a reference flow rate of 350 cubic feet per
minute (CFM).
Heat exchanger effectiveness is the ratio of the actual airside heat transfer to the maximum heat
transfer theoretically possible. Equation 1 shows how the effectiveness is independent of the
flow rate through the heat exchanger.
,
,
,
@
(1)
,
where
is the mass flow rate through the heat exchanger, , is the enthalpy of the air
entering the heat exchanger, , is the enthalpy of the air leaving the heat exchanger, and
@ , is the saturation enthalpy at the temperature that the water enters the heat exchanger.
Heat exchanger effectiveness was monitored throughout the entire experiment for both coils.
The static pressure drop at a reference flow rate of 350 CFM was also monitored throughout the
experiment for both coils. Since both control and treatment coils did not have identical static
pressure drops at the start of the experiment, the static pressure drop throughout the experiment
was normalized by the initial pressure drop.
Biological Assessment
Coil surface samples were taken with sterile BBL CultureSwabs (BD, Sparks, MD). A 10-cm2
area of the coil surface was swabbed and extracted into HPCL water. Both the upstream and
downstream side of each coil was swabbed. Samples were stained using SYTO BC Green
Fluorescence stain (Life Technologies, Carlsbad, CA) and deposited on a 0.2-μm black
polycarbonate membrane (Millipore, Billerica, MA). SYTO BC is a nucleic acid stain that
penetrates both Gram-negative and Gram-positive bacteria, yielding total cell counts. Samples
were directly counted using an epifluorescent microscope (Nikon E600).
Results and Discussion
Assessment of Heat Exchanger Flow Characteristics and Effectiveness
The effectiveness of both coils remained fairly constant for the first 6 months of operation and,
in fact, rose slightly over the first 4 months. We hypothesize that a clean coil sees a slight
increase in effectiveness in the early stages of cooling due to the condensed water slightly
increasing the heat transfer coefficient. After the initial 5-6 months of operation, the
effectiveness began to decrease rapidly, approximately 15% over 3-4 months. Figure 3 displays
the effectiveness of both cooling coils over the 10-month period of “fouling” as well as 3 weeks
of UV exposure for the treatment coil. The arrow indicates when the UV lamp was turned on for
the treatment coil (Top coil).
1
Top Coil (UV)
Bottom Coil
0.9
Coil Effectiveness (%)
UV turned on (Top Coil)
0.8
0.7
0.6
0.5
0.4
10/01
01/01
04/01
07/01
10/01
Time
Figure 3. Effectiveness of both cooling coils throughout the baseline data collection of 10
months plus 3 weeks of irradiating the Top coil. The vertical line indicates when the UV lamp
was turned on for the treatment coil (Top Coil).
Both coils began the experiment with a normalized static pressure drop of 1 (normalized by the
initial static pressure drop of 0.167 inches of H2O for the Top coil and 0.158 inches of H2O for
the Bottom coil) at the reference flow rate of 350 CFM. After 10 months, the normalized static
pressure drops for the Top and Bottom coils were 1.11 and 1.18, respectively, meaning that static
pressure drops at 350 CFM increased 11% and 18%, respectively. After 3 weeks of irradiating
the downstream side of the Top coil, the normalized pressure drop was 1.06, a reduction of 5%.
The UV-irradiated coil began looking visually cleaner after 1 week of irradiation.
Biological Assessment
Results from epifluorescent microscopy show higher total microbial counts on the downstream
surface samples versus upstream surface samples on both coils prior to being exposed to UV
(two sets of bars on the left side in Figure 4). This result is consistent with evidence of higher
viable microbial loads on the downstream side of cooling coils when culturing surface samples
(Hugenholtz & Fuerst, 1992). No change was observed in the total surface cell counts for the
irradiated coil after 3 weeks even though the coil appeared visually cleaner. Previous studies on
UVGI coil cleaning have seen drastic declines in viable bacterial counts when culturing coil
surface samples from UV-irradiated coils (Moyer et al., 2010; Menzies et al., 2003). We
experienced difficulty culturing bacteria from surface samples due to mild condensing conditions
and low nutrient loads in the inlet air but preliminary results show lower viable bacterial counts
on the irradiated coil. A larger dataset of culturable bacteria is being collected. Our preliminary
results may indicate that while UVGI inactivates surface microbes and cleans the coil via
oxidation, the inactive DNA of previously viable biofilms may still remain on the coil.
300
Microbial Counts per Field (410x330 um)
Downstream
Upstream
250
200
150
100
50
0
Bottom Coil Baseline Top Coil Baseline Top Coil (1 wk UV) Top Coil (2 wks UV) Top Coil (3 wks UV)
Figure 4. Total microbial surface cell counts per field of view using epifluorescent microscopy.
The two sets of bars on the left show average counts for both sides of both coils prior to UV
irradiation (baseline). The following three sets of bars on the right show the UV-irradiated coil
(Top Coil) cell counts for each week of the 3 weeks of UV exposure. No significant change was
observed. Error bars represent the standard error.
Conclusions
After 10 months of baseline (“fouling”) operation, the effectiveness of both cooling coils
dropped roughly 15% and the static pressure drop at the reference flow rate of 350 CFM
increased between 11-18%. Microbial surface sampling indicates a higher degree of biofouling
on the downstream side of the cooling coils prior to UV irradiation. While the UV-irradiated coil
appeared visually cleaner after 1 week of irradiation and saw a 5% reduction in static pressure
drop at 350 CFM after 3 weeks, the total surface cell counts from epifluorescent microscopy
remained the same. Preliminary data from culturing surface samples, however, indicates higher
counts of viable microbes on the non-irradiated coil but a more robust dataset is currently being
collected. Data collection is ongoing and further results will be reported.
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