DEVELOPMENT AND TESTING OF AIR EXCHANGE RATE METHODS TO IMPROVE COST EFFECTIVENESS WHILE MAINTAINING ACCURACY Prepared by: Joachim Eberharter (Joachim.Eberharter@tetratech.com) and James Elliot (Tetra Tech, Inc., Santa Barbara, CA) Bill Mills and Katya Sayenko (Tetra Tech, Inc., Lafayette, CA) Introduction Tetra Tech, Inc. (Tetra Tech) was contracted by the United States Air Force (USAF), Air Force Center for Engineering and the Environment (AFCEE) to conduct experiments designed to validate an innovative and cost-effective methodology for estimating the air exchange rate (AER) of a building. The objective of the work was to quantitatively validate the methodology against a standard American Society for Testing and Materials (ASTM) method (ASTM E741-00). Research for this study was conducted at (1) Cape Canaveral Air Force Station (CCAFS), Florida; (2) former Kelly Air Force Base (AFB), Texas; and (3) Travis AFB, California. The AER of a building is critical in determining whether vapor intrusion (VI) may result in unacceptable concentrations of volatiles in indoor air. A building with relatively high rates of VI may never develop high indoor air concentrations, as vapors will be continually flushed out of the building and replaced with ambient air. Conversely, a building with very low AER could be subject to significant buildup of indoor air concentrations, even with a relatively low concentration source or low rate of intrusion. Commonly used methods of measuring AERs tend to be somewhat cumbersome and costly; however, during previous vapor intrusion investigations conducted at CCAFS, Travis AFB, former Kelly AFB, and Vandenberg AFB, a relatively simple and cost-effective method for determining AERs was developed and used (Tetra Tech 2009a, b, c, d). The method consists of instantaneously releasing a finite quantity of helium (He) inside a building and then measuring the helium concentration in the air over time as it dissipates. A formula is then applied to the results to calculate the building AER. The purpose of the recent experiments conducted at CCAFS, Kelly AFB, and Travis AFB, and discussed in this paper, was to conduct side by side comparisons of the He release methodology with an existing published method, with the objective of validating the He method as a tool for use at other sites. A modified version of ASTM Method E741-00 was used to obtain an independent estimate of the AERs at three buildings while simultaneously deploying the helium methodology. The ASTM method used involves the release of a tracer gas into a building at a constant, known rate, and then measuring concentrations in the building air. A formula is then applied to calculate the AER. An additional method using a single data point from a SUMMA canister sample was also developed during this study to allow for further testing comparisons. 1
Air Exchange Rate Study: Former Kelly AFB Of the three site investigations, Building 1416, Former Kelly Air Force Base is chosen as an example. A summary of results at all installations investigated is also provided subsequently. Building Description and Indoor Volume Estimates Building 1416 is an approximately 2,400-square-foot, single story slab-on-grade structure. The building consists of five areas: an entry way room, class room, shop, restroom, and paint room (Figure 1). The paint room has a vaulted ceiling, while the remaining rooms have drop ceilings. The paint room and the shop have an uncovered concrete floor, while the entry, classroom, and restroom have linoleum floors. As shown on Figure 1, the front door and the double doors leading from the shop area into the paint room were closed during the experiment. The paint room was excluded from the experiment. Figure 1 Floor Plan of Building 1416 illustrating components of the AER experiment. Experimental Design The following sections provide details of the former Kelly AFB field experiments and Table 1 summarizes the field activities conducted during the period of the experiment. 2
Table 1 Summary of Field Activities at Building 1416, Former Kelly AFB 22 March 2011 23 March 2011 Arrive at Building 1416 and set up components of AER experiments Upon arrival, HVAC system was on but not operating due to the moderate outside temperatures. The thermostat was adjusted to cause the HVAC system to start up and the blower fan to operate. Steady flow of SF 6 tracer gas into building initiated at 1526 Continuous logging of SF 6 concentrations in the Shop initiated at 1526 Helium from six 111-cf cylinders released into building at 0625 Indoor air samples for on-site field meter and off-site laboratory analysis of He collected over course of eight hours Discrete indoor air samples collected for off-site analysis of SF 6 at 0655 and 1025 SF 6 analyzer carried through building periodically to collect readings from each room 2 collocated SUMMA canisters deployed in Shop next to SF 6 analyzer at 0655 to collect 3.5-hour and 9-hour time-integrated samples for He and SF 6 analyses Final He sample collected at 1425 (8 hours after He release) SF 6 gas turned off at 1655 Site cleaned up and restored to condition as found SF 6 Tracer Gas Release The release of SF 6 tracer gas into Building 1416 for the ASTM method started prior to sampling and continued for the duration of the experiment through 23 March 2011. The SF 6 tracer gas was released as a 4.0 percent mixture of SF 6. Throughout the experiments, the SF 6 gas analyzer logged the SF 6 concentration at 10-minute intervals in the center of the Shop. The instrument was set to continually monitor the SF 6 concentration and to log the minimum, maximum, and average concentrations during each 10-minute interval. Helium Gas Release The He source for the former Kelly AFB experiment was six 111 cubic-foot cylinders of at least 99 percent pure He, which were placed at the locations shown on Figure 1. The valves were fully opened simultaneously by the field crew and the tanks allowed to empty into the building. The cylinders emptied in approximately 90 seconds. The cylinders were weighed before and after release to estimate the mass of helium released. Kelly AFB Implementation Composite indoor air samples of He were collected in Tedlar bags for He analysis at 15, 30, 60, 90, 120, 180, 240, 360, and 480 minutes (8 hours) after the release. Concentrations of SF 6 were continuously logged with the portable SF 6 gas analyzer and discrete measurements were collected in each room approximately once per hour. In addition, two time-integrated indoor air samples for SF 6 and He analysis were collected from the Shop in two collocated 6-liter SUMMA canisters. The SUMMA tests were experimental and are not part of the protocol developed from this investigation, but air exchange rates were calculated using this method and compared to predictions generated using the methods described above. 3
Experimental Results The He concentrations in the composite Tedlar bag samples and the time integrated SUMMA canister samples collected during the experiments at Building 1416 are presented in Table 2. A plot of the logged SF 6 concentrations is presented in Figure 2. Sample ID HVAC On (23 March 2011) Table 2 Helium Concentrations in Building 1416 at Former Kelly AFB Elapsed Time¹ (minutes) Collection Time Field Result² (ppmv) Laboratory Result³ (ppmv) 1416-HON-15 15 06:40 36,000 21,000 1416-HON-30 30 06:55 26,000 15,000 1416-HON-60 60 07:25 16,300 9,500 1416-HON-90 90 07:55 10,975 7,300 1416-HON-120 120 08:25 8,425 5,900 1416-HON-180 180 09:25 4,625 3,400 1416-HON-240 240 10:25 2,300 1,800 1416-HON-360 360 12:25 700 ND 1416-HON-480 480 14:25 75 ND 1416-SUMMA-3.5HR 30 240 06:55 10:25 NA 7,900 1416-SUMMA-9HR 30 570 06:55 15:55 NA 3,500 Definitions: HVAC heating, ventilation and air conditioning system NA not applicable ND not detected ppmv part per million by volume Notes: 1 Elapsed time since release of He. 2 Instrument detection level for He was 25 ppmv 3 Laboratory detection level for He was 500 ppmv 4
Figure 2 Plot of logged SF 6 concentrations measured on-site at former Kelly AFB Building 1416 and laboratory sample results. The MIRAN gas analyzer was set to continuously monitor SF 6 concentrations and to log the measurements every 10 minutes. Figure 2 shows the maximum, minimum, and average SF 6 concentrations logged for each 10-minute period. The sudden drop in SF 6 concentrations immediately following the release of He was also noted after the He releases during the CCAFS experiments, suggesting that the release of He into the building space may displace a measurable amount of SF 6 gas. Six cylinders of He represent approximately 650 cubic feet of pure He, which represents approximately 4.6 percent of the total indoor air volume of Building 1416; however, this volume is not sufficient to account for the total drop in SF 6 concentration. The SF 6 concentration is likely also affected by ingress and egress through the front door beginning immediately prior to He release and continuing throughout the experiment. Air Exchange Rate Analysis Analyses of the AER data for each of the three buildings investigated for this study include the following: Analysis of He release methods: For these methods, He was released instantaneously and AERs were calculated based on the decay in He concentrations and using three leastsquares (LS) methods. 5
Analysis of ASTM methods: For these methods SF 6 was released continuously at a known rate, and AERs were calculated. These tests were conducted simultaneously with the He tests. A modification was made to the ASTM method for this project to allow the ASTM methods to be excecuted under conditions that were not at steady-state (not at equilibrium). This was useful because data collected during the project clearly showed the SF 6 tracer concentrations were not at steady-state during the tests. Comparison of He-release results and SF 6 -release results. The three LS methods are all similar to each other, and differ primarily in that for LS method #1 (LS-1), the initial well-mixed He concentration (C0) is used in the equations to calculate AER, while for method #2 (LS-2) and method #3 (LS-3), C0 is not required to calculate the AER, although it can be estimated from the sample data. For LS-1, C0 is calculated in one of two ways: either by directly calculating the difference in weights of the He cylinders before and after He release, or using the vendor provided volume of the cylinders. These analyses are presented for the site chosen as an example, former Kelly AFB. Figure 3 illustrates the temporal relationships between the AER tests conducted at Building 1416. Also shown is when the instantaneous He release occurred, and the beginning and end of the SF 6 release. 6
Figure 3 Relationship between air exchange rate tests for former Kelly AFB. By examining Table 3 and Table 4 and cross-comparing results from the 13 different tests, the results are shown to be generally consistent between methods. Tests #3 and #4, using lab data, show that methods LS-1 produces higher air exchange rates (~22.2/day) than LS-2 or LS-3 (14.7/day to 16.5/day). One hypothesis for this is that He is being lost from the Tedlar bags between the time of sample collection and the time of sample analysis. Since method LS-1 uses a well-mixed initial He concentration (C0) and if He were lost from the first sample, the LS-1 method would tend to predict a higher air exchange rate. Methods LS-2 and LS-3 are less affected because C0 is not imposed as for LS-1. 7
8 Table 3 Air Exchange Rates, Day -1, Over Interval Shown
Table 4 Summary of Air Exchange Rate Analysis for Former Kelly AFB Tracer Range (day -1 ) Average (day -1 ) Results for Short Duration Tests (3 4 hours), March 23, 2011 He SF 6 LS 14.7 22.6 17.9 SUMMA 24.6 24.6 SUMMA 13.7 14.4 14.1 Finite Difference 16.6 18.6 17.6 Results of Long Duration Tests (8 9 hours) He SF 6 Definitions: He helium LS least squares method SF6 sulfur hexafluoride LS 17.7 18.8 18.1 SUMMA 22.9 22.9 SUMMA 18.0 18.0 Finite Difference 17.6 17.7 17.6 Discussion Including Summary From all Three Sites Cape Canaveral Air Force Station From the comparative AER tests completed at CCAFS, the following conclusions can be reached regarding the two types of tests (He and SF 6 ): The tests generate comparable results. Less field equipment is needed for the He tests, reducing costs and reducing the possibility of equipment failure. The weight of the He released can be accurately estimated, and this is a useful feature of the He tests. This information is directly used in the AER tests to eliminate a source of uncertainty. It is straightforward to implement an uncertainty analysis of results for the He methods, so that confidence intervals of results can be estimated. For the SF 6 methods, such an analysis is more difficult to implement, and was not attempted during this work. Safety issues are of less concern for He. SF 6 concentration limits to protect human health exist, although it is likely not to be an issue for well-designed tests. SF 6 is a greenhouse gas with a high global warming potential, and is being phased out as a tracer gas. These conclusions tend to favor the use of He tests over SF 6 tests. However, SF 6 continuous release tests may be superior to the He instantaneous tests in the duration of the tests. Typically He tests last only a fraction of the day, while SF 6 tests can continue for a day or more. It may be desirable to design He tests that last longer in order to get a more accurate estimate of daily AERs. 9
Former Kelly Air Force Base The major conclusions from the field investigations at former Kelly AFB are: As at CCAFS, the AERs calculated from the SF 6 and He release methods were similar. All of the data analysis methods were successfully completed indicating that the methods are robust. Travis Air Force Base The AERs for Building 1130 at Travis AFB were the highest encountered at any of the buildings investigated. Using lab data the LS results ranged from 47.1/day to 86.7/day. Using field data the results were less: 36.6/day to 50.8/day. The difference in results based on field data vs. lab data appears to be related to loss of He from the sample containers during transport to the laboratory. Several SF 6 tests were completed and average AER values were within the range of LS results obtained from the He tests. Also, two SUMMA tests were completed, and AERs ranged from 27/day to 39/day. While the calculated AERs exhibited more variability than for previous sites, the values generally agreed by a factor of two. Overall, the results from Travis AFB are consistent with results from CCAFS and Former Kelly AFB, and indicate that the instantaneous He release method yields similar results to the ASTM method. Cost Comparison There are a variety of options for implementing the two methods that will impact the total cost; for example, field analysis only, laboratory analysis only, or both field and laboratory analysis. Due to the wide range in AERs at different buildings, it is advisable to use field meters when conducting either test so that the approach (e.g. number of samples collected or SF 6 flow rate) can be adjusted in response to site specific conditions. Therefore, for cost comparison purposes, two scenarios were compared: (1) implementing the methods using both field and laboratory analysis and (2) using field measurements only. For both scenarios, unit costs and labor rates are estimates based on experience implementing the methods during this study. Actual costs vary based on location and other factors. The cost of the He methodology is significantly lower than the SF 6 tests for both scenarios. The primary factors driving the higher expense of the SF 6 method are the cost of the SF 6 gas and the regulator and rotameter required to release it at a known, steady rate, and the cost of the Portable SF 6 analyzer. The difference in cost is less for scenario 1 vs. scenario 2 due to the cost of analyzing multiple samples for He. Protocol for Estimating Air Exchange Rate Using Concentration Decay of Instantaneously Released Helium Tracer Gas The protocol developed covers the methodology used for estimating the air exchange rate (AER) of a building, or portion (zone) of a building, and by evaluating the concentration decay rate of instantaneously released helium (He) tracer gas. Three mathematical methods to estimate AERs from field-collected data were developed from basic principles. These methods are presented in the protocol and are solvable using Excel spreadsheets. The major assumption that guides these 10
methods is that the He tracer gas is released instantaneously such that it becomes well-mixed in the building before sampling begins (typically 15 to 30 minutes after release). The results from this step-by-step protocol pertain only to those conditions of weather and building operation (e.g., heating, ventilation, and air conditioning [HVAC] system status) that prevail during the measurements. It is assumed that the He concentration within the test building can be characterized by a single value throughout the building. This test method has been evaluated and validated against an established method described in American Society for Testing and Materials (ASTM) Method E741-00 modified for this project. The method was validated using test buildings with volumes up to approximately 30,000 cubic-feet (cf). Overall Conclusions The primary objective of this project was to develop cost-effective, simple methods for determining AERs. Simplified methods to determine AERs were developed from a combination of field investigations and least-squares algorithms. The AERs calculated for the three test buildings (Facility 1381, Building 1416, and Building 1130) using instantaneous He release and the LS methods compared well with the AERs calculated using the modified ASTM method with SF 6 tracer gas. The resuls of this study demonstrate that the He release methodology provides a cost-effective, easy to implement method of measuring building specific AERs. References Tetra Tech, Inc. (2009a) Investigation and Validation of Multiple Lines of Evidence to Assess Vapor Intrusion at Cape Canaveral Air Force Station, FL for US Air Force School of Aerospace Medicine (USAFSAM/OE) at Brooks City-Base, TX. Tetra Tech, Inc. (2009b) Investigation and Validation of Multiple Lines of Evidence to Assess Vapor Intrusion at Kelly Air Force Base, TX for US Air Force School of Aerospace Medicine (USAFSAM/OE) at Brooks City-Base, TX. Tetra Tech, Inc. (2009c) Investigation and Validation of Multiple Lines of Evidence to Assess Vapor Intrusion at Travis Air Force Base, CA for US Air Force School of Aerospace Medicine (USAFSAM/OE) at Brooks City-Base, TX. Tetra Tech, Inc. (2009d) Investigation and Validation of Multiple Lines of Evidence to Assess Vapor Intrusion at Vandenberg Air Force Base, CA for US Air Force School of Aerospace Medicine (USAFSAM/OE) at Brooks City-Base, TX. 11