1、 Priscilla A. Guerrero is a graduate research assistant and Richard L. Corsi is a professor in the Department of Civil, Architectural, and Environmental Engineering at the University of Texas at Austin, Austin, Texas. Effects of HVAC Operations on Sorptive Interactions Priscilla A. Guerrero Richard
2、L. Corsi, PhD PE Student Member ASHRAE Member ASHRAE ABSTRACT Decreasing building energy demands will necessitate changes in HVAC design and operation. Such changes may have a wide spectrum of effects on indoor air quality. Human exposure to a wide range of air pollutants, particularly toxic contami
3、nants, is dominated by what is breathed indoors. Two contaminants, p-dichlorobenzene (p-DCB) and naphthalene are both classified as Group C carcinogens according to the US EPA, and these chemicals exist in nearly pure form in consumer products within which they are used. Products that contain p-DCB
4、include moth crystals, closet air fresheners, and toilet bowl deodorizers. Moth balls, which serve as a moth repellent, are the main products that contain naphthalene. Emission rates of p-DCB and naphthalene from these products have been ascertained from a previous study and range from 50 mg/hr to 3
5、00 mg/hr for p-DCB products and 5 mg/hr to 12 mg/hr for naphthalene products. Emission rates were employed in a well-mixed reactor model to estimate indoor concentrations and the extent of dynamic sorptive interactions between these chemicals and two indoor materials: gypsum wallboard and carpet. In
6、 particular, we explore the effects of HVAC operation on sorptive interactions between these pollutants and the two indoor materials. HVAC cycling enhances the adsorption and desorption mechanisms that occur when indoor pollutants are present. INTRODUCTION Human activity patterns show that Americans
7、 spend 90% of their time indoors (Klepeis et al. 1999). Inside buildings many hazaroudous air pollutants (HAPs) can persist due to sorptive interactions with indoor materials. Many can contaminate building materials through sorption processes, which can be sequestered and slowly desorbed for months
8、to years after a source is terminated (Popa and Haghighat 2003, Colombo et al. 1993, Van der Wal et al. 1998). During source activities, chemicals such as p-DCB and naphthalene are removed from the indoor air by adsorbing to materials, thus decreasing occupant exposures. However, when a source is re
9、moved desorption from contaminated materials prolongs the overall period of exposure. Heating, ventilating, and air conditioning (HVAC) operations can affect the extent of adsorption and desorption. This paper will focus on indoor concentrations of p-DCB and naphthalene, specifically with varying ai
10、r exchange rates and the presence of sorptive building materials, i.e., carpet and painted gypsum board. BACKGROUND p-Dichlorobenzene is a white crystal at room temperature with a distinct aromatic odor recognized as moth repellent. Severe symptoms of p-DCB inhalation are chronic cough, rhinitis, an
11、d difficulty breathing (Hill et al., 1995, Ashley et al., 1994). It is used in fragranced products such as toilet bowl deodorizers and room air fresheners. In addition to odor control, p-DCB is primarily used as a fumigant to remove moths from interior spaces (U.S. Dept. of Health and Human Services
12、, 1998). LV-11-C051 2011 ASHRAE 4192011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital fo
13、rm is not permitted without ASHRAES prior written permission.Naphthalene is typically a solid cake moth repellent. Naphthalene is considered a ubiquitous environmental toxicant that is released by many industrial, domestic and natural burning processes, leading to a background burden for the general
14、 population (Preuss et al. 2003). Naphthalene fumes can irritate the eyes, skin and the respiratory tract (Lu et al. 2005). Adgate et al. (2004) measured childrens exposure to fifteen different volatile organic compounds (VOCs) inside classrooms, homes, breathing zones and outside schools. Median co
15、ncentrations of p-DCB were 0.08ppb (part per billion) (0.5 g/m3) (classrooms), 0.12 ppb (0.7 g/m3) (homes), 0.16 ppb (1.0 g/m3) (breathing zones), and 0.02 ppb (0.1 g/m3) (outdoor air). Indoor air concentrations were observed to be significantly higher than outdoor concentrations. The higher concent
16、rations in breathing zone measurements suggest that the subjects spend time in locations of close proximity to p-DCB sources. Children are likely to be in closer proximity or more frequently exposed to sources of p-DCB than adults. Adgate et al. (2004) found that children had two times the amount of
17、 p-DCB in their blood relative to adults, while 10% of the children had ten times the amount relative to adults in the national Health and Nutrition Examination Survey (NHANES) study. Hill et al. (1995) analyzed data reported for 1,000 adults in the NHANES III study. They observed that 98% of the su
18、bjects had detectable concentrations of p-DCB in their urine, and 96% of them had detectable p-DCB in their blood. Cigarette smoke contributes significantly to human exposure to naphthalene (Preuss et al. 2005). According to the US National Toxicology Program, exposure of humans to naphthalene has b
19、ecome an environmental and occupational health concern since clear evidence for carcinogenic activity was associated with exposure during a long-term inhalation study in rats. Elovaara et al. (2007) investigated enzyme induction in rat liver and lung, and the activities in five human lung specimens.
20、 Naphthalene was administered to rats for 3 days and found that after first dose the rats experienced nausea. Greigo et al (2008) studied indoor and outdoor naphthalene concentrations. Greigo et al (2008) reported exposure concentrations for background 0.000020.0006 ppb (0.00010.003 g/m3), ambient a
21、ir 0.00020.20 ppb (0.0011.0 g/m3), vehicles 0.00060.60 ppb (0.0033.0 g/m3), residences 0.022.0 ppb (0.110 g/m3), mothball use 0.2020 ppb (1100 g/m3), and occupational low: 1620 ppb high: 6.0200 ppb (low: 3 100 g/m3; high: 301,000 g/m3). METHODS Due to the nearly pure p-DCB and naphthalene compositio
22、n of each product, p-DCB and naphthalene emission rates were calculated gravimetrically. Guerrero and Corsi (Paper In Review) further describes gravimetric experiments in detail. Mean experimental emission rates are presented in Table 1. Table 1. ProductSpecific Mean Emission Rates Compound Product
23、Emission Rate p-Dichlorobenzene Closet Air Freshener* 3.3x10-4 lb/hr (150 mg/hr) Naphthalene Moth Box* 2.2x10-5 lb/hr (10 mg/hr) * Guerrero and Corsi (Paper In Review) Predicted Concentrations of p-DCB and Naphthalene in the Absence of Sorptive Interactions p-Dichlorobenzene and naphthalene emission
24、 rates were used to predict indoor concentrations in a 10590 ft3(300 m3) home with varying air exchange rates. Concentrations were calculated using Equation 1, which is based on the assumptions of steady-state conditions, well-mixed conditions (uniform concentration), no sorptive interactions betwee
25、n the compounds and indoor materials, and a negligible outdoor concentration of each compound. 420 ASHRAE TransactionsVECO (1) Using the conditions described above, in conjunction with mean emission rates, leads to the concentration profiles presented in Figure 1. Each curve in Figure 1 corresponds
26、to a single product emitting p-DCB or naphthalene. Figure 1 Predicted p-DCB and naphthalene concentrations based on a hypothetical 10590 ft3(300 m3) home. Calculation of p-DCB and Naphthalene Adsorption Sorptive interactions were modeled using a linear sorption model as presented by Tichenor et al.
27、(1991). Assuming well-mixed conditions with a constant emission rate and negligible outdoor concentrations leads to Equation 2: 11iiiiidiiacyclingrecircHVACVAMkVACkCVECdtdCDOGO(2) The air exchange rate was assumed to be 0.5 hr-1based on the median value of US homes (Murray and Burmaster, 1995). The
28、home volume was assumed to be 10590 ft3(300 m3). Adjustments were not made for reductions in volume due to internal furnishings. Product-specific mean emission rates were used for each compound. The materials selected as sinks for this model were carpet and latex-painted gypsum wall board. All walls
29、 and ceiling were assumed to be painted gypsum board 3765 ft2(350 m2) and the floor was assumed to be fully carpeted 1183 ft2(110 m2). HVAC efficiency parameters were estimated using Hanley et al. (1994) information for a house (min=0% and max=60%), in this study an average efficiency parameter of 3
30、0% was used. Recirculation air exchange rates were estimated using ASHRAE (1996) handbook on HVAC conditioning systems (min= 5 hr-1and max= 20 hr-1), an average recirculation air exchange rate of 12.5 hr-1was used. HVAC cycling frequency information was retrieved from a recent study of homes in the
31、Austin, TX area. Stephens et al. (2010) estimated HVAC cycling to be about (6.1 hr/day) 25% of the time. In order to calculate the p-DCB and naphthalene concentration adsorbed to each material the following mass balance was used: 2011 ASHRAE 421iidiaiMkCkdtdM (3) Equations 2 and 3 were discretized i
32、n time and solved using successive replacement. The adsorption rate coefficient was assumed to equal 1.6 ft/hr (0.5 m/hr), a reasonable value for transport-limited deposition velocity in indoor environments (Cano-Ruiz et al., 1993). It can be shown that for a linear sorption model the equilibrium pa
33、rtition coefficient is simply the ratio of the adsorption to desorption rate coefficients. As such, the desorption rate coefficient was estimated by: eqiaidKkk (4) Won et al. (2001) reported Keqfor o-dichlorobenzene (o-DCB) for carpet Keq = 22 1.6 ft (Keq = 6.6 0.5 m) and painted gypsum board Keq =
34、3.3 1.3 ft (Keq = 1 0.4 m). The mean values reported by Won et al. (2001) for o-DCB were used in this study for p-DCB. Values for naphthalene were extrapolated from the linear relationship between the negative natural log (-LN) of naphthalenes vapor pressure. Concentration curves for the p-DCB produ
35、cts and naphthalene products were calculated based on the model and parameters listed above. In each case the source was removed after a four day period. Concentration profiles for p-DCB and naphthalene are presented in Figures 2 and 3, respectively. Figure 2 Concentration of p-DCB in the air with t
36、he presence and absence of sorption in a home for a seven day period. After four days the sources are removed and the HVAC system cycles every 6 hours. 422 ASHRAE TransactionsFigure 3 Concentration of naphthalene in the air with the presence and absence of sorption in a home for a seven day period.
37、After four days the sources are removed and the HVAC system cycles every 6 hours. RESULTS The median air exchange rate in US homes is approximately 0.5 hr-1(Murray and Burmaster, 1995). At this air exchange rate the predicted concentrations for p-DCB are 164 ppb and 13 ppb (1,000 g/m3 and 67 g/m3)fo
38、r the closet air freshener and moth box respectively as shown in Figure 1. The odor threshold for p-DCB is 180 ppb (1,100 g/m3), in the absence of sorption this concentration is exceeded for the single closet air freshener at air exchange rates of less than 0.5 hr-1. The odor threshold for naphthale
39、ne is 84 ppb (447 g/m3),interestingly naphthalene concentrations, shown in Figure 1, for the moth box are never achieved. Implying that in this hypothetical home, such products will not be easily detected. However, if these products are stored in smaller volumes, i.e., garment bags or storage closet
40、s, concentrations will exceed the odor threshold. For energy efficient newer homes or weatherized older homes, it is not uncommon for air exchange rates to be 0.3 hr-1or lower. At an air exchange rate of 0.2 hr-1, the p-DCB concentrations when using a closet air freshener is predicted to exceed 410
41、ppb (2,500 g/m3). Concentrations of p-DCB and naphthalene increased rapidly to 95% of steady state within six hours in the absence of sorption as shown in Figures 2 and 3. However, in the presence of sorption and HVAC cycling, the time to reach steady state changes. The HVAC cycling definitely affec
42、ted the concentrations in the home, specifically when the HVAC systems cycles off. Naphthalene and p-DCB concentrations increase to 7 and 33 ppb (36 and 200 g/m3), respectively when the HVAC system is off. Therefore, when the HVAC system cycles on, the concentrations are less than 2 ppb and 8 ppb (1
43、0 and 50 g/m3)for naphthalene and p-DCB, respectively, decreasing indoor pollutant concentration by 75%. Adsorption to indoor materials was higher when naphthalene is used as opposed to p-DCB. A similar observation in the experiments conducted by 2011 ASHRAE 423Colombo et al. (1993) and Van der Wal
44、et al. (1998) were reported, demonstrating that adsorption increases with increasing boiling points and decreasing vapor pressure. In this model after a four day period the sources are removed, thus with the absence of sorption, concentrations decrease rapidly to 0 ppb (g/m3) within a couple minutes
45、. However, in the presence of sorption, achieving 0 ppb (g/m3)indoors for each compound considerably increases from minutes to days. In the case of naphthalene, concentrations never reach 0 ppb ( g/m3), especially since the HVAC cycles on and off after the four days. Thus, increasing indoor concentr
46、ation when the HVAC cycles off. When the source is removed after the four days of use, results show that there is a continued p-DCB and naphthalene concentration increase after the HVAC cycles off. This is known as chemical desorption, which can occur after the source is removed due to off-gassing f
47、rom the contaminated materials, i.e., painted gypsum board and carpet. Figures 2 and 3 both show the off-gassing of p-DCB and naphthalene even though sources of each chemical are no longer present. Resulting in continued elevated exposures to p-DCB and naphthalene for a relatively prolonged period a
48、fter the source is removed. This period of persistence will be longer for homes with lower air exchange rates. CONCLUSION Adsorption to indoor surfaces can have a significant effect on p-DCB and naphthalene concentrations for the first day or two after the source begins to emit, and can allow p-DCB
49、and naphthalene to persist at elevated concentrations in homes days after the source is removed. HVAC cycling definitely decreased indoor concentrations while the system remained on, however when the system cycled off, concentrations steadily increased in both compounds. Concentrations of p-DCB and naphthalene can be reduced inside the home if there are adequate materials present that adsorb both compounds. Additional knowledge related to adsorption and desorption of p-DCB and naphthalene
