1、Joseph Firrantello is a doctoral candidate in the Department of Architectural Engineering, The Pennsylvania State University, University Park, PA. William Bahnfleth is a professor in the Department of Architectural Engineering and director of the Indoor Environment Center, The Pennsylvania State Uni
2、versity. Ross Montgomery is owner of Quality Systems and Technologies, Parrish, FL. Paul Kremer is a Research Associate in the Department of Architectural Engineering, The Pennsylvania State University. Field Study of Energy Use-Related Effects of Ultraviolet Germicidal Irradiation of a Cooling Coil
3、 Joseph Firrantello, PE Member ASHRAE Ross Montgomery, PE Paul Kremer Fellow ASHRAE ABSTRACT The energy use-related effects of ultraviolet germicidal irradiation (UVGI) to mitigate biological fouling (biofouling) of a chilled water cooling coil are investigated via a field study. A visibly bio-foule
4、d cooling coil in an air-handling unit serving an operational building in a hot, humid climate is monitored for 5 months to establish a fouled coil baseline. Parameters monitored include air flow rate, airside pressure drop, air temperature and humidity upstream and downstream of the coil, chilled w
5、ater flow rate, entering and leaving chilled water temperature, and waterside pressure drop. A UVGI coil irradiation system is installed on the downstream side of the coil following typical manufacturer guidelines, and the system is then passively monitored over a period of 14 months. The change in
6、operation is estimated by comparing data from the baseline and post-irradiation periods. The 95% confidence intervals for average improvement of coil airside pressure drop and heat transfer coefficient are 11.07% to 11.13%, and 14.5% to 14.6%, respectively. Complexities of the physical phenomena inv
7、olved, in particular, the combined effect of airflow and latent load on airside pressure drop, are taken into account. INTRODUCTION Finned tube cooling coils play a key role in the operation of air-conditioning systems. Coils are susceptible to fouling by particulate matter impinging on their closel
8、y spaced fins. Condensate that wets coil surfaces during operation helps to capture particles and also promotes microbial growth. Fouling increases airside pressure drop across a coil and decreases the air to water or refrigerant heat transfer coefficient. Both effects can increase energy use of an
9、HVAC system significantly. This investigation considers the biofouling of chilled water coils and its mitigation by low power ultraviolet germicidal irradiation (UVGI) systems. BACKGROUND Airside Biofouling of Cooling Coils Heat exchanger fouling is the buildup of organic and/or inorganic matter on
10、the heat transfer surfaces. Cooling coils, due to the close spacing of fins on the air side (10 to 15 fins per inch or 4 to 6 per cm), can act as particulate William Bahnfleth, PhD, PE Fellow ASHRAE filters and trap material such as dust, hair, debris, and microbes. Coil surfaces, by design, become
11、wet during operation in many applications, thereby presenting growth opportunities for impacted microbes. A number of studies quantify the benefits of cleaning a fouled coil. Most of these studies consider mechanical or chemical coil cleaning and do not distinguish between different types of fouling
12、. Montgomery and Baker (2006) describe a coil cleaning case study performed on two air-handling units (AHUs) serving part of a 34-story office building in New York City. Cleaning of coils resulted in a 14% decrease in pressure drop across the coils, an increase in ability to transfer sensible loads
13、of 25%, and an increase of 10% for latent loads. Overall, coil cleaning appeared to have the potential to save 10%-15% in HVAC system energy consumption. Yang, Braun, and Groll (2004; 2007a; 2007b) describe the energy use effects of coil fouling as measured in a laboratory study. The authors found t
14、hat the energy penalty from the increased pressure drop across the cooling coil was more significant than that from the change in heat transfer coefficient. In some cases with lower amounts of fouling, the heat transfer coefficient was found to increase due to an increase in air velocity, but this w
15、as offset by the increased thermal resistance as fouling accumulates. Biological particles that deposit and grow on a cooling coil contribute to increased energy use and IAQ problems (Siegel and Walker 2001; Siegel and Carey 2001). Single pass deposition in these studies ranged from 1% for 1.1 m par
16、ticles at low velocities of around 200 fpm (1.02 m/s), to 30% for 8 m particles at high velocities of 1024 fpm (5.2 m/s). Ali and Ismail (2008) collected fouling material from room air conditioners, classified its biological and non-biological components, deposited it in increasing amounts on a DX c
17、ooling coil in a laboratory apparatus, and measured the resulting degradation in performance. The organic component of the fouling material comprised 18.4% of the mass on the upstream face of the coil and 1.2% on the downstream face. The organic component consisted of masses of Aspergillus fungal co
18、lonies. The coefficient of performance (COP) of the unit dropped from a clean value of 2.82 to fouled values of 1.89, 1.73, and 1.23 after the injection of 100g, 200g, and 300g (0.22 lbm, 0.44 lbm, and 0.66 lbm ) of fouling material, respectively. Pu et al. (2010) seeded a cooling coil with biologic
19、al material and recorded the airside pressure drop and heat transfer coefficient resulting from different levels of fouling after 28 days of growth. They found a range of -15.6% to 13.1% for the heat transfer coefficient and 19.8% to 43.1% for the air-side pressure drop fouling factors. UVGI for Con
20、trol of Biofouling The UVC (or UV-C) wavelength band of ultraviolet light inactivates biological organisms by disrupting their DNA and rendering them unable to reproduce. UVC generated by low pressure mercury vapor lamps is used both for air and surface disinfection in HVAC systems. The basics of UV
21、GI coil treatment systems are described by ASHRAE (2008) and Kowalski (2009), which both review numerous sources on the subject. The devices are installed upstream, downstream, or on both sides of the coil. Bahnfleth (2011) provides a recent review of the technology used in air handlers and summariz
22、es published reports of its effectiveness. At the time of its publication, there were no peer-reviewed studies of the energy use impacts of UVGI for coil treatment applications. Several published reports describe the ability of coil surface UVGI to mitigate or prevent coil fouling but without quanti
23、tatively assessing the impact on system energy use. For example, Shaughnessy, Rogers, and Levetin (1998) documented the effectiveness of UVC in reducing contaminant concentrations on various AHU surfaces, and did extensive microbe classification, but did not measure energy impacts. Levetin et al. (2
24、001) also demonstrated the effectiveness of UVGI in reducing surface contamination without reporting energy use-related data. In a brief trade magazine article (Steril-Aire 2000a, 200b), a manufacturer describes the installation of UVC surface treatment technology in a facilitys twenty AHUs to addre
25、ss IAQ-related problems, and the resultant energy benefit. The article claims that the facility experienced a 28% reduction in HVAC energy use. Unfortunately, the reporting of energy savings is missing many details. Keikavousi (2004) describes the results of a number of coil cleaning UVGI installati
26、ons throughout a Florida hospital system. An AHU with a visible buildup of mold (estimated 50% of coil face area) was selected for initial testing. Following installation of UVGI, static pressure drop across the coil decreased from 1.8 to 0.7 in wg (448 to 174 Pa), face air velocity increased from 2
27、30 to 520 fpm (1.17 to 2.64 m/s), and leaving wet bulb temperature decreased from 57 to 53F (13.9 to 11.7C). A California Energy Commission study (Arent 2006) found a 1-2% airflow improvement due to the installation of coil surface UVGI, which was noted as positive but not statistically significant.
28、 No efficiency improvements were found. The study was performed on 54 schools: an 18 school control group and two study groups. They note, importantly, that fouled coils were not targeted for this study and little coil fouling was observed pre-intervention, so the results were perhaps predictable. B
29、latt, Okura, and Meister (2006) report on a number of UVGI installations in schools (including the aforementioned California Energy Commission study) and other commercial buildings that showed energy savings through improvement in coil operation. Recent conference papers on research in progress by L
30、uongo and Miller (2014) and Yi et al (2014) present promising preliminary results of coil fouling studies. OBJECTIVEThe objective of the research is to quantify the energy benefit of using UVGI for mitigation and prevention and biofouling on a chilled water coil. This is accomplished by monitoring t
31、he performance of a visibly fouled cooling coil in an operational AHU, installing generic UVGI equipment per manufacturer design recommendations, and then monitoring performance after turning the lamps on. The two measures examined are change in airside pressure drop, and change in heat transfer coe
32、fficient. METHODOLOGYConceptually, the research plan is a simple before/after experiment, but with a number of complexities involved. Experimental Site The experimental site is an AHU in an occupied laboratory classroom building on a college campus in Tampa, FL. The variable air volume (VAV) AHU has
33、 a design supply air flow rate of approximately 6000 cfm (2.832 m3/s), which was estimated based on coil face area and typical design velocity due to lack of design documentation. The chilled water coil is 60 in. wide by 33 in. high (152.4 cm by 83.8 cm) and 6 rows deep. Outdoor air flow rate varies
34、 from 500 to 1500 cfm (0.236 to 0.708 m3/s) due to an interlock with fume hoods in the classrooms. The site was chosen based on a combination of factors: visibly fouled cooling coil, owner interest, researcher access, and climate. Climate aids in acquiring a broad range of airflow, sensible load, an
35、d latent load. The relatively long cooling season also aids with accruing sufficient data. Two rows of UV lamps were installed 12 in. (30.5 cm) downstream of the cooling coil. The lamp power and arrangement design resulted in an average surface irradiance of 327 W/cm2and minimum of 180 W/cm2(0.304 W
36、/ft2and 0.167 W/ft2, respectively). To verify the design, irradiance measurements were taken 3 inches (0.0762 m) from the coil face after 6 months of operation and roughly extrapolated to values at the coil surface using the analytical solution for an infinite line source. The extrapolated measureme
37、nts predicted an average irradiance of approximately 300 W/cm2and a minimum of about 200 W/cm2(0.279W/ft2and 0.186 W/ft2). Measurement The data acquisition setup measures all points necessary to characterize cooling coil operation, as well as other items of interest, at 1 minute intervals (Table 1).
38、 Data collection is passive, i.e., the data collected is all from the buildings normal operation over a period of many months, not from a controlled set of experiments. Table 1. Measurement Points Measurement Accuracy Airside Supply Air Flow Rate 2% of reading Airside Pressure Drop 0.14% of reading
39、Entering and Leaving Air Temp 0.11F (0.2C) Entering & Leaving RH 1% RH Waterside Chilled Water Flow Rate 1% of reading Entering and Leaving Water Temp 0.11F (0.2C) Waterside Pressure Drop 0.14% of reading Power Fan Power 0.1% of reading UV Ballast Power 0.1% of reading Most of the measurement points
40、 pertain to the two previously mentioned measures of interest: airside pressure drop and heat transfer coefficient. The pressure drop is primarily influenced by airflow, but also by the amount of water on the coil (ASHRAE 2012). One way to quantify this is latent load, which is calculated from the a
41、irflow and the upstream and downstream air conditions. The heat transfer coefficient necessitates the use of air and water flow rates, as well as the upstream and downstream conditions. Fan power is measured in an attempt to relate it to airflow, though the effect of other parts of the air system ma
42、kes this not as useful as one might hope. UV power is measured for future energy analysis. Data Analysis Pressure Drop. Accurate comparison of the coil pressure drop before and after application of UVGI requires controlling for airflow and latent load. One cannot simply perform a t-test between the
43、“before” and “after” data pressure drop data. The comparison method adopted employs regression analysis to quantify the effect of the UVGI intervention. The form of the equation chosen is based on the Darcy-Colebrook equation: 221vCDLfPh+=(1)A key consideration is the primary influence of velocity p
44、ressure. For a dry coil, the constants inside the parentheses of Eqn. 1 are dependent on the construction geometry, e.g., fin spacing, tube size and placement. However, for a wet coil the amount of water on the coil alters the dry values of several key parameters. The construction of a regression eq
45、uation involves art, trial, and error, as well as theoretical considerations, which will not be reviewed in their totality. The regression form adopted in this case is: tqQtQqQQPll24232221 += tsqQtsQsqQsQll28272625 + (2) Eqn. 2 reflects the primary influence of velocity pressure (using airflow as a
46、surrogate), while also including the effects of latent load and time. The form of Eqn. 2 is such that pressure drop is zero when airflow is zero, as must be the case. The term s is a categorical variable, coded 0 for all data points before the lamps are turned on and coded 1 for all points after. Th
47、is means that the entire second half of the equation is 0 before the lamps are turned on. When s=1, the second half of the equation represents the difference between the lamps off and lamps on states. For example, 1Q2is the influence of velocity squared alone on the baseline data, 5Q2is the change i
48、n influence resulting from the lamps being turned on, and (1+5)Q2is the influence of airflow squared on the lamps-on data. Changes to the data and regression equation are made using formal methods: regression outliers are identified and deleted using one pass of a Bonferroni outlier test (=0.05), an
49、d adjustment of the coefficients uses weighted least squares to reduce predictor influence on variance of residuals. The before/after pressure drop are compared in two ways. The first is by using Eqn. 2 to construct the family of pressure drop vs. flow curves with latent load as a parameter. The second is to use Eqn. 2 to predict the pressure drop values corresponding to a year of measured airflow and latent load data. In each case, the pressure drop before UV (s=0) and after UV (s=1) is compared. Heat Transfer. Heat transfer effectiveness
