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Download Factsheet: and more Schemes and Mind Maps Law in PDF only on Docsity! NBSIR 88-3708 Tracer Gas Techniques for Studying Building Air Exchange Andrew K. Persily U.S. DEPARTMENT OF COMMERCE National Bureau of Standards Center for Building Technology Gaithersburg, MD 20899 February 1988 1913-1988 U.S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS Abstract A variety of procedures have been developed that employ tracer gases to study the air exchange characteristics of buildings. These procedures enable the examination of several features of building air exchange including ventilation rates, air movement within buildings, and building envelope airtightness. This paper reviews tracer gas measurement techniques that have been used to study air exchange in buildings. Background information is discussed such as the instrumentation used in these tests, building features that influence their application, and the fundamental theory of tracer gas measurement. Several specific applications are discussed including air exchange rate measurement in large buildings, low-cost procedures for measuring air exchange rates in large numbers of buildings, techniques for evaluating the performance of air distribution systems, and pressurization testing of envelope airtightness in large buildings. A detailed bibliography is also included to facilitate a more thorough examination of the topics discussed. Key Words: airflow measurement; building performance; infiltration; measurement; tracer gas; ventilation. TABLE OF CONTENTS 1 . INTRODUCTION 2 . IMPORTANT FACTORS IN AIR EXCHANGE EVALUATION. 2 . 1 Building Characteristics ............................................... 2 . 2 Instrumentation 3 . FUNDAMENTALS OF TRACER GAS MEASUREMENTS ..................................... 3 . 1 Decay 3 . 2 Constant Concentration 3.3 Constant Injection 3 . 4 Multi-Chamber Techniques 3.4.1 Decay 3.4.2 Constant Concentration.......... 3.4.3 Constant Injection 4 . AIR EXCHANGE RATE MEASUREMENT APPLICATIONS 4 . 1 Tracer Gas Measurement in Large Buildings .............................. 4 . Low Cost Measurement Procedures 4.2.1 Decay 4.2.2 Constant Injection 5 . OTHER TRACER GAS APPLICATIONS 5 . 1 Air Distribution Evaluation ............................................ 5 . Ventilation System Airflow Rates ....................................... 5 . 3 Pressurization Testing of Large Buildings 5 . 4 Qualitative Evaluation Techniques ...................................... 6 . REFERENCES ........ n 1. INTRODUCTION This paper reviews tracer gas measurement techniques for studying air exchange in buildings. These techniques are used to examine three basic aspects of building air exchange including air exchange rates, interior air movement, and building envelope airtightness. There are numerous motivations for investigating these three aspects of building air exchange. The measurement of building air exchange rates is important as these rates relate to energy consumption and indoor air quality. Their measurement is useful for determining whether the outdoor air intake rate is compatible with design specifications, and whether relevant ventilation standards are being complied with. Tracer gases can also be used to examine interior air movement patterns, which are important as they relate to thermal comfort within the occupied space. These interior air movement measurement procedures can also be used to evaluate the performance of a building's air distribution system in providing ventilation air to the occupants and removing internally generated contaminants. Finally, these tracer gas procedures can be used to verify the isolation of special-use spaces, where hazardous substances are in use, from the rest of a building's interior. The third area of building air exchange appropriate to tracer gas measurement is the determination of the airtightness of a building's envelope. Envelope airtightness is important as it relates to energy consumption due to uncontrolled air infiltration through the building shell. The existence of such infiltration influences interior thermal comfort due to drafts, and may lead to material, and possibly structural, damage to the building envelope due to moisture condensation. Air infiltration due to envelope leakage is often a significant fraction of the total building air exchange rate and therefore is important in understanding the indoor air quality of a building. Finally, the airtightness of the building envelope interacts with a building's mechanical ventilation system, affecting its ability to control air exchange rates and air distribution. While there have been previous reviews of tracer gas measurement techniques (Alexander et al 1980; Harrje et al 1981 and 1982; Hitchin and Wilson 1967; Hunt 1980; Lagus 1980; Sherman et al 1980), this paper concentrates on more recent developments such as the study of large buildings, evaluations of air distribution effectiveness, and multi-tracer and multi-chamber techniques. The first section of this review discusses several of the basic issues related to evaluating building air exchange using tracer gases such as relevant building characteristics and tracer gas instrumentation. The second section provides a discussion of the fundamentals of tracer gas measurement including the mass balance theory on which the measurements are based and the three basic approaches, i.e. decay, constant concentration and constant injection. Multi- chamber and multi-tracer theory and applications are also discussed. The third section presents two applications of air exchange rate measurement procedures including measurements in large buildings and low-cost procedures that are useful for air exchange measurements in large numbers of buildings. The fourth and final section discusses other tracer gas applications including the evaluation of air distribution effectiveness, measurement of airflows within ventilation systems, pressurization testing of large buildings, and qualitative tracer gas evaluation techniques. 1 important to measure and record environmental conditions during the measurements, including wind speed and direction and indoor and outdoor air temperatures. In an automated system, devices for measuring environmental conditions can be installed at the building site and their output recorded by the data acquisition and control system. In manual measurements, hand held devices can be used as well as reports from nearby weather stations. Other building conditions, such as fan operation and damper position, are also important to air exchange and need to be examined and recorded. 3 o FUNDAMENTALS OF TRACER GAS MEASUREMENTS There are three basic tracer gas techniques for measuring air exchange rates, decay, constant concentration, and constant injection. To understand these methods, one employs a mass balance of the tracer gas released within the building. Assuming that the tracer gas mixes thoroughly and instantaneously within the structure, this mass balance equation is VC(t) = F (t ) - q C(t) (1) where V is the building volume, C ( t ) is the tracer gas concentration at time t, C is the time derivative of concentration, q is the airflow rate out of the building, and F is the tracer gas flow rate. The outdoor tracer gas concentration is assumed to equal zero. The air exchange rate I is given by I = q/V ( 2 ) where I is in air changes per hour -1 (h A ) The solution to equation (1) is given by C(t) = C0 -It e F(u) / V ) du (3) where C0 is the concentration at time t=0. For those cases in which F is constant, the solution integrates further to C(t) - C0 e' :t + (F/q) (1 - e _It ) . (4) 3.1 Decay The simplest tracer gas technique is the tracer gas decay method, which has been discussed previously by Lagus (1980), and is the subject of a standardized measurement procedure (ASTM 1983) . In the decay method, one injects a small amount of a tracer gas into the structure and allows the tracer to mix with the interior air. After the mixing period one monitors the rate of decay of the tracer gas concentration within the building. During the decay there is no source of tracer gas, hence F(t)=0 and the solution to Eq (1) is C(t) = C0 e~ Ito (5) Solving Eq (5) for I yields I = d/t) In [ (C0 /C (t) ] ( 6 ) where C0 is the initial tracer gas concentration at t=0, when the decay begins. To minimize errors in the determination of I due to errors in the measurement of C(t) and C0 , one can measure C periodically during the decay and fit the data to an equation of the form lnC(t) = lnC0 - It. (7) The concentration measuring equipment can be located within the structure, or building air samples may be collected in suitable containers and analyzed off- site (Grot 1980; Harrje et al 1982) . As with all tracer gas techniques, there are advantages and disadvantages associated with the tracer gas decay method. The advantages include the fact that the equation used to determine the air exchange rate is an exact solution to the tracer gas mass balance equation. Also, because one takes logarithms of concentration, only relative concentrations are needed, which may simplify the calibration of one's concentration measuring device. Finally, one need not measure the tracer gas injection rate, although it must be controlled such that the tracer gas concentrations are within the range of one's concentration measuring device. The most serious problem with the tracer gas decay technique is imperfect mixing of the tracer gas with the interior air, both at initial injection and during the decay. Equations (1) and (5) employ the assumption that the tracer gas concentration within the building is uniform and can be characterized by a single value. If the tracer gas is not well mixed, either within zones (e.g. floors or rooms) or between zones of a building, this assumption is not appropriate and the use of Eq (6) or (7) to determine I will lead to errors. It is extremely difficult to estimate the magnitude of the errors due to poor mixing, and there has only been very limited analysis of this problem (Hunt 1980) . The only way to determine if there is or there is not good mixing is to monitor the tracer gas concentration at several locations within the building. It has been suggested (Dick 1949) that if one obtains different tracer gas decay rates in different rooms, due to poor mixing, one may obtain an estimate of the whole building air exchange rate using a volume-weighted average of the individual room decay rates. 3.2 Constant Concentration In the constant concentration technique one injects appropriate quantities of tracer gas in order to maintain a constant concentration within the building. If the tracer gas concentration is truly constant, Eq (1) reduces to q (t ) = F(t)/C (8) This technique is less well developed than the tracer gas decay technique, but examples of its application do exist (Collet 1981; Bohac et al 1986) . An advantage of the constant concentration technique is that it can avoid some measurement problems that occur in the tracer gas decay procedure due to nonuniform mixing of the initial tracer gas injection. Because the tracer gas injection is continuous, there is no initial mixing period to be concerned with once the test is underway. There are, however, other serious mixing concerns as discussed below. Another advantage of the constant concentration technique is 5 that one can separately control the tracer concentration in each zone of a building by separately injecting tracer into each zone, and thereby determine the amount of outdoor air flowing into each zone (Honma 1975; Sinden 1978) . Tracer gas procedures appropriate to multi-chamber analysis are discussed further below. This procedure has the disadvantages of requiring absolute concentration measurement and precise measurement of the tracer gas injection rates. Also, imperfect mixing of the tracer gas and the interior air causes a delay in the response of the tracer gas concentration to changes in the tracer gas injection rate. This delay in the concentration response, due to imperfect mixing, makes it essentially impossible to keep the concentration constant and therefore Eq (8) is actually an approximation. The errors induced by these mixing problems have not been well examined. 3.3 Constant Injection The third technique for measuring air infiltration and ventilation is referred to as the constant injection or constant flow technique. In this procedure, one injects tracer at a constant rate and, setting C0 = 0, Eq (4) reduces to C(t) = (F/q) (1 - e _It ) (9) After a sufficient period of time, the transient term reduces to zero, the concentration attains equilibrium, and one obtains the simple constant flow equation q = F/C. (10) This relation is valid only for cases in which the air exchange rate is constant, thus this technique is appropriate only for systems at or near equilibrium. This technique is particularly useful in areas with mechanical ventilation or in locations with high air exchange rates. The constant flow technique avoids the concentration control problems of the constant concentration procedure. The constant injection procedure requires the measurement of absolute concentrations and the tracer gas injection rate. 3.4 Multi-Chamber Techniques The previous theory applies to structures that are modelled as a single zone, i.e., the tracer gas concentration within the building can be characterized by a single value. In many cases this assumption is inappropriate and a multi- chamber approach must be used (Sinden 1978) . A variety of multi-chamber measurement techniques exist, involving the decay, constant concentration, and constant injection techniques, and the use of one or several tracer gases. The equations describing the multi-chamber case are similar to Eq (1), except for the addition of airflow between chambers. A mass balance for each chamber yields, ViCi = Qio^i + X< cIijCj “^^ji^i j 3 where Vj_ is the volume of chamber i, Cj_ is the tracer gas concentration in that same chamber, Cj. is its derivative with respect to time, qj_j is the airflow rate 6 4. AIR EXCHANGE RATE MEASUREMENT APPLICATIONS In this section we discuss two applications of air exchange rate measurement. The first is tracer gas measurements of air exchange in large buildings, while the second application concerns low cost procedures for studying air exchange in large numbers of buildings. 4.1 Tracer Gas Measurements in Large Buildings This section reviews techniques which employ a single tracer gas to measure infiltration and ventilation in large buildings, arbitrarily defined as structures with floor areas greater than about 5000 ft (500 in ) . These buildings may have mechanical ventilation systems or they may be naturally ventilated. The types of large buildings that have been studied include office buildings, industrial buildings such as warehouses and airplane hangers, stores, shopping centers, and institutional facilities such as schools and hospitals. Most of the measurements in large buildings have employed the tracer gas decay method and this particular application is discussed below, along with other procedures applied to large buildings. The basic approach of tracer gas decay measurements in large buildings is the same as that described earlier. The tracer gas is released into the building and allowed to mix with the interior air. The decay in the tracer gas concentration is monitored over time and the air exchange rate is determined from the decay rate. Several characteristics of large buildings influence the manner in which the decay method is applied in these structures. First, because of the large building volumes, the quantity of tracer gas required foj a test and its cost become important. The expense depends on the cost per m° of tracer gas, the building volume, and the magnitude of measurable tracer gas concentrations. Table 2 shows the range in the maximum building volume that can be measured for one dollar's worth of tracer gas (leased on approximate 1986 prices) . These volumes range from 4 g00 |t° (jjlOO^iTi ) for helium in the 300 parts per million (ppm) range, to about 10° ft J (10° in ) for carbon dioxide and nitrous oxide. The ability to measure SFg, CBrFg and PDCH in the parts per q trillion (ppt) range yields measurable volumes of 10° to 10 1 ft' (10 to 10 m ) per dollar's worth of tracer gas. From this table it is apparent that tracer gases such as SFg, refrigerants, and perfluorocarbons analyzed at ppt, or even parts per billion, levels are most appropriate for large buildings. However, measurements have been performed in large buildings using infrared adsorption (Potter et al 1983; Zuercher and Feustel 1983) and flame ionization gas chromatography (Prior et al 1983) . The mixing of the tracer gas injection in these large building volumes is an important issue. Mixing by diffusion alone is a slow process; however, even in naturally ventilated buildings there are significant convective mixing mechanisms. In mechanically ventilated buildings the air distribution system can be used to mix the tracer gas, but mixing can still require fifteen minutes to one hour. In naturally ventilated buildings, tracer gas mixing is a more difficult problem. If the building interior is open with few internal partitions, then the gas will mix with the air, although it can take a long time. Fans can be used to mix the tracer, but they will alter the interior air movement, which may or may not affect the measurement results. The fans may be used to obtain an initially uniform concentration, and then be turned off during the decay. The only way to determine if good mixing has been achieved is to measure the tracer gas concentration at several locations within the building. 9 The attainment of a uniform concentration is also assisted by injecting the gas at several locations. In mechanically ventilated buildings fan operation and damper position are important issues. Most of these buildings have automatic control systems that turn fans on and off, modulate airflow rates and adjust exhaust, recirculation and supply damper positions. Thus, to conduct useful tracer gas measurements and interpret the results, one must be aware of the fan operating schedules and ventilation control strategies. One may make long term measurements in a building or conduct only a small number, of tests. Long term studies are useful for examining the dependence of air leakage on weather and the performance of the ventilation control system. When making long term measurements, one's equipment, sensors and air sampling lines must be unobtrusive with regard to the building occupants and the automatic operation of the building equipment. In this section we present an example of tracer gas decay measurements in mechanically ventilated office buildings. We also present other examples of air exchange measurements in large buildings. The example of measurements in a mechanically ventilated office building involves the use of automated equipment to conduct long term measurements (Grot et al 1980; Grot 1982) . The tracer gas equipment is generally located in the building's mechanical equipment room where the main air handlers are located. Figure 5 is a schematic of such a building with the mechanical equipment room located in a penthouse. Most office buildings have separate air handlers, for spaces such as lobbies, which may be located some distance from the main mechanical equipment room. Such an air handler is shown in the figure. In order to obtain a uniform tracer gas concentration throughout the building, one must inject tracer into all the supply fans. This requires the installation of injection tubing that runs from the measurement equipment to each supply fan. As seen in figure 5, the tracer gas concentration is measured at several locations within the building in order to verify that the tracer is indeed well mixed. For example, one may sample in the building's main return duct, on individual floors and in the return ducts of any other air handlers. One must install air sample tubing connecting the measurement equipment to each of the sampling locations, and use pumps to bring the air to the measurement equipment. The individual floor sampling locations can be in the return air plenums immediately upstream of the return air shafts. In order for these sampling locations to give meaningful concentration measurements, the air handlers must be operating. In some buildings it may be possible to run the air sampling tubes into the occupied space in which case the air handlers need not be running during the tracer gas decay. Since building geometry and air handler arrangements vary greatly among buildings, tracer injection and air sampling locations are different for each building. In office buildings, there are two types of measurements that are of interest, referred to here as ventilation and infiltration. Ventilation rates refer to measurements made when the building HVAC system is operating normally under occupied conditions. In this case the various spill, recirculation and intake dampers open or close as the control system dictates in response to indoor and outdoor temperature and humidity, and time of day. Infiltration rates refer to the measurements obtained when the spill and intake dampers are closed (including any minimum outdoor air dampers) . These test results give an indication of the airtightness of the building envelope. The operation of the fans during these measurements may be necessary for mixing, and their operation 10 may affect the test results. Short term measurements can also be made in such a building using injection and sampling by hand. The tracer is injected into the supply fans and air is collected in containers at locations throughout the building. The tracer gas concentrations in these containers are determined at some later time. This "manual" technique has a shorter set-up time than the automated procedure described above, but each infiltration measurement must be made by hand. Long term automated infiltration and ventilation measurements have been made in several mechanically ventilated office buildings (Grot 1982; Grot and Persily 1983) . In these tracer gas decay tests, hourly average infiltration and ventilation rates were measured for hundreds of hours in each building and the results were related to indoor-outdoor temperature difference and wind speed. The infiltration rates of the different buildings were found to exhibit varying degrees of weather dependence, and a range in the leakage of the building envelopes was observed. The ventilation rates reflected the strategies used to control the outdoor air intake rates and also revealed that uncontrolled envelope infiltration is generally an important portion of these building's total air exchange rates, even with significant amounts of outdoor air intake. The tracer gas decay technique has been applied in an eleven-story office building, employing hand injection of the tracer gas and sampling of the interior air with polyethylene bottles (Harrje et al 1982) . The tracer gas concentration in the bottles was later determined at a central location. The air was sampled on four different floors and in the main return duct of the air handler. Only a small number of measurements were made in the building, but the results demonstrated the utility of this "air sample container" technique in large buildings. A small number of industrial buildings have also been studied with the tracer gas decay technique. These buildings are often characterized by large open volumes, such as warehouses, where tracer gas mixing can take a long time or require the use of fans. In a study of three large naturally ventilated single-zone structures in England, a fan was used to mix the tracer gas (Waters and Simons 1984) . A uniform tracer gas concentration throughout the space was generally obtained within about twenty or thirty minutes after injection, although some spatial variation did remain. A series of tracer gas decay measurements in airplane hangers has also been conducted (Ashley and Lagus 1984). There are other examples of the use of tracer gas in large buildings (Potter et al 1983; Zuercher and Feustel 1983) . In one particular application, a constant injection scheme with gas bag sampling of the interior air was applied to a laboratory building (Freeman et al 1983) . In these measurements, tracer gas was injected at a constant rate at twelve locations and the equilibrium tracer gas concentration was determined from air sample bags filled at twelve other locations. This "bag sample equilibrium" method was compared to measurements based on tracer gas decay and the agreement was good when mixing was thorough. 4.2 Low Cost Measurement Procedures The tracer gas measurement techniques described above involve bringing sophisticated equipment for measuring tracer gas concentration to the building 11 occupants and to remove internally generated pollutants. Many different definitions of ventilation effectiveness exist (Persily 1985) and can be divided into those that quantify the distribution of supply air and those that quantify pollutant removal effectiveness. A great deal of valuable ventilation effectiveness research has been conducted involving experiments in test rooms (Sandberg 1981 and 1983; Sandberg and Sjoberg 1983; Sandberg et al. 1982; Malmstrom and Ahlgren 1982; Skaret and Mathisen 1982 and 1985) . These experiments have employed test rooms with reconfigurable intake and exhaust openings, and controllable supply air temperatures and ventilation rates, to « study the dependence of ventilation effectiveness on these variables. The procedures employed in these laboratory measurements can also be used to measure ventilation effectiveness in actual buildings. Several definitions and theoretical frameworks have been used to discuss ventilation effectiveness, but there are essentially two basic approaches. The first type of ventilation effectiveness measures can be referred to as "concentration efficiencies" and are based on relations between gas concentrations in the supply air, the exhaust air, and the air at various locations in the space. Efficiencies based on age distributions and residence times, using approaches of chemical reactor engineering, constitute the second approach to ventilation effectiveness. For a detailed review of the various definitions of ventilation effectiveness and the associated measurement techniques see Persily (1985), as well as the original articles on the material (Malmstrom and Ahlgren 1982; Sandberg 1981 and 1983; Sandberg and Sjoberg 1983; Sklret and Mathisen 1982) . The techniques for quantifying air distribution effectiveness in actual buildings are still being studied. No standard procedures exist yet for typical North American buildings, but the procedures based on age distribution theory appear to have potential for being useful and are therefore discussed below with reference to mechanically ventilated office buildings. Ventilation effectiveness definitions based on age distributions involve average and local ages of the interior air, and tracer gases can be used to determine these ages. The measured values of these ages are compared to each other or to their values for idealized reference cases (i.e., perfect mixing or pure plug flow through the space) to determine various ventilation effectivenesses. In age distribution theory applied to ventilation effectiveness, one considers three populations of air parcels for a given ventilated space of volume V and volumetric air exchange rate q: the air at some specific location within the space, all of the air contained in the space, and the air leaving the space. One defines the average age of the air at a specific location by considering all the air molecules at that location and determining the average amount of time that has elapsed since these air molecules entered the space or building. The average age of the air at a point i is denoted as tj_. One defines the average age of all the air in a given space, denoted by [t], as the average value of tj_ for all locations in the space under consideration. One may also consider the age of the air leaving the room, denoted by tn , which is equal to the inverse of the air exchange rate, i.e. (V/q) , regardless of the airflow patterns within the space (Sandberg and Sjoberg 1983) . It is revealing to compare the values of the local air age tj_, the average age of the air in the space [t] and the age of the air leaving the space tn , for three reference cases. First, if the air within the space under consideration is perfectly mixed, then all the local ages have the same value throughout the 14 space, equal to [t]=tn . In addition, contaminant concentrations are identical throughout the space. For the second case, pure piston flow from the supply to the exhaust, the value of t-j_ depends on the particular location within the space, with its value increasing from 0 to tn as i moves from the supply to the exhaust. The average age of the air in the space [t] is equal to t n /2. For pure piston flow, the effectiveness of pollutant removal and outdoor air distribution are maximized, but thermal comfort may be compromised due to excessive air velocitites or low air temperatures near the supply vents. In addition, occupants located immediately downstream of a pollutant source will be subjected to higher contaminant concentrations than in the case of perfect mixing. The third case is often referred to as "short circuiting," in which a portion of the supply air flows directly into the exhaust vent without mixing with the rest of the space air. In this situation, the value of ti again depends on the location of i. In stagnant zones that are bypassed by the supply air, tj_ is greater than [t] and t n . In the regions through which the short- circuiting flow passes, ti is less than [t] and tn . The average age of the air within the space [t] is greater than t n . Such short-circuiting flow has serious negative implications for indoor air quality because the ventilation air is only partially effective in providing outdoor air to the occupants and removing internally generated pollutants. Depending on the degree of short-circuiting, the concentrations of internally generated contaminants in the stagnant zones can be much higher than for the cases of perfect mixing and piston flow. Comparisons of tj_, [t], and t n serve as the basis for definitions of ventilation effectiveness. Several such definitions exist, but only two are presented here, the mean air exchange effectiveness n and the local air exchange effectiveness ej_. The mean air exchange effectiveness quantifies the overall air distribution pattern for a space and is given by n = tn / [t] . (13) n achieves its maximum value under conditions of pure piston flow for which n=2. If there is perfect mixing, then n=l, and if there is short circuiting of the supply to the exhaust, then n is less than 1. The local air exchange effectiveness quantifies local conditions and is given by ei = [t]/ti. (14) ej_ can range from zero to infinity, and in the case of perfect mixing equals 1.0 throughout the space. For pure piston flow, the value of ej_ depends on the specific location within the space being considered. Near the supply, tj_ is close to zero and ej_ is much greater than one. In the middle of the room, tj_ = tn/2 and ej_ = 1.0. And as one approaches the exhaust, tj_ approaches tn , and e^ approaches 0.5. In the case of short circuiting, for a location within one of the so-called stagnant zones, tj_ > [t] and ej_ < 1, a generally undesirable situation. One may employ a tracer gas to measure these ages using several techniques that differ primarily in the tracer gas injection location and duration (Sandberg 1983) . One of the measuring techniques involves injecting tracer at a constant rate into the supply airstream and monitoring the build-up in tracer gas concentration in the exhaust vent and at various locations within the space until equilibrium is attained. In another technique, one begins with a uniform tracer gas concentration throughout the space and monitors the decay in concentration in the exhaust and within the space. The measuring procedures 15 have been used successfully in one- and two-room laboratory test facilities [Sandberg 1983; Sandberg and Sjoberg 1983; Sandberg et al. 1982; Skaret and Mathisen 1982, 1985], but the application of these procedures in actual buildings is much more complex. There are several features of real buildings, particularly large, mechanically ventilated office buildings, that complicate the application of age distribution measuring techniques (Persily 1987) . The theory of age distributions consider the ventilated space as a single zone with a small number of well-defined supply and exhaust (return) vents. The space is also assumed to have no airflow into or out of the space except through the mechanical ventilation system, a concern that was discussed earlier. In reality, the ventilation of an office space is much more complicated that this theory assumes. Office spaces communicate freely with adjoining spaces and with the outdoors through leaks in the building envelope. There are generally many supply vents serving a space and the number of return air vents is extremely variable among rooms, with some rooms having no return vents at all. This situation presents several problems for applying age distribution measuring techniques. These and other factors complicate the application of age distribution measuring techniques in modern North American office buildings and the interpretation of the test results, but research is currently in progress to develop measurement protocols that are appropriate to these field situations. 5.2 Ventilation System Airflow Rates As mentioned earlier, tracer gas measurements of air exchange rates in mechanically ventilated buildings determine the sum of the uncontrolled envelope leakage and the intentional outdoor air intake through the air handling system. Tracer gas procedures exist to measure these quantities simulataneously, as well as other airflow rates of interest in understanding the air exchange characteristics of mechanically ventilated buildings (Persily and Norford 1987) . These quantities can be measured using two different techniques, a steady-state, constant injection tracer gas procedure and a procedure combining tracer gas decay and airflow measuring stations. Both techniques are described below. The constant injection procedure is illustrated in the schematic in Figure 6. In this schematic Qqa is the rate of intentional outdoor air intake through the air handling system, Qgg is the recirculation airflow rate, and Qgy is their sum, the supply airflow rate. Qjn is the rate of uncontrolled air leakage into the building through the building envelope, and Qgx is the sum of the uncontrolled air leakage out through the building envelope and the airflow out of intentional openings such as bathroom exhausts. Qgg is the return airflow rate and Qgp is the airflow rate through the spill dampers. In the constant injection measurement procedure, one injects tracer gas at a constant rate F into the supply airstream as indicated in Figure 6. In this procedure, the value of the outdoor air tracer gas concentration must be constant, and for many tracer gases it will equal zero. The equations below are developed under the assumption that the outdoor concentration is zero, but an alternative set of equations can be easily developed for an outdoor concentration that is nonzero but constant. During the test, one measures the return air tracer gas concentration Cr, the supply air concentration Cg, and the mixed-air concentration Cjj. must be measured some distance downstream of the location where the recirculation air meets the new outdoor air in order to provide the two airstreams an opportunity to mix. Similarly, Cg must be measured downstream of the tracer gas injection location, after the tracer mixes with the supply airstream. The tracer gas mixing can be enhanced by releasing 16 separately determine the envelope infiltration rate into that zone. In the more general case of some mixing between zones, using the tracer gas decay/airflow rate measuring procedure, one can only determine the outdoor air intake rate into each zone and the tracer gas decay rate for each zone. If there is sufficient mixing between zones, then the tracer gas decay rate will be identical for all the zones and equal to the total building air exchange rate. Even if the mixing is not perfect, the volume-weighted average tracer gas decay rate for all the zones can be used as an approximation of the total building air exchange rate. Based on the measured outdoor air intake rates for all the zones and the total building air exchange rate, one can determine the total building envelope infiltration rate. 5.2 Pressurization Testing of Large Building The constant injection technique has been used to measure airflow rates in the evaluation of the airtightness of large building envelopes using pressurization testing. In pressurization testing, a fan induces a large pressure difference across the building envelope and the airflow rate required to induce this pressure difference is measured. The airflow rate associated with a specific indoor-outdoor pressure difference is a measure of the airtightness of the building envelope. The airflow can be induced with a large fan which is brought to the building for the test or with the building's air handling equipment. Various means exist for measuring the airflow rate through the fan, but a constant injection tracer gas method has been used in some cases. This is a simple technique which does not require the duct lengths and flow straighteners associated with other flow measurement techniques. To measure the airflow rate with the constant injection tracer gas method, tracer gas is injected into the airstream at a constant and known rate. The tracer injection is generated using a compressed gas cylinder with a flowmeter such as a critical orifice, a float-type rotameter or an electronic flow controller. The tracer concentration is then measured as far downstream as possible from the injection point. Under conditions of perfect mixing of the tracer gas and the airflow, the airflow rate can be determined from the tracer injection rate and the measured concentration (see Eq (10)). Several large industrial buildings have been pressure tested using a fan that was brought to the buildings and employing the flow measurement technique described above (Lundin 1984) . Figure 7 shows a schematic of the flow measurement equipment used in these tests. The same flow measurement procedure was applied to seven modern office buildings in which the building supply fans were used to pressurize the structure (Persily and Grot 1984a) . Figure 8 shows a schematic of the test arrangement including the fan operating conditions, damper positions, tracer gas injection location and tracer gas concentration measurement point. 5.4 Qualtitative Evaluation Techniques It should be noted that one does not require the full solutions to Eq (1) to obtain useful information regarding a building's air exchange characteristics with tracer gases. A significant amount of qualitative information can be obtained using tracer gases in buildings. For example, the existence of the movement of air from one location to another can be verified by releasing tracer gas at the first location and measuring the concentration at the second. This simple procedure can be used to examine the occurence of reentrainment of 19 exhaust air into an outdoor air intake, movement of air from one location within a building to another, or to verify the isolation of a special-use space from the rest of a building. All such applications require extreme care regarding the tracer gas injection to insure that the tracer is released only at the intended location, such that any tracer detected elsewhere is due only to airflow and not to unintentional tracer release. 20 6. REFERENCES ASHRAE, "Ventilation for Acceptable Indoor Air Quality," Standard 62, American Society for Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1981. ASTM, "Standard Practice for Measuring Air Leakage Rates by the Tracer Dilution Method", E 741-83, American Society for Testing and Materials, 1983. Alexander, D.K., Etheridge, D.W., Gale, R. "Experimental Techniques for Ventilation Research, " in Air Infiltration Instrumentation and Measuring Techniques , proceedings of the First Air Infiltration Centre Conference, Berkshire, UK, 1980. Ashley, J.L., Lagus, P.L., "Air Infiltration Measurements in Large Military Aircraft Hangers," in Measured Air Leakage of Buildings , ASTM STP 904, H.R. Trechsel and P.L. Lagus, Eds., American Society for Testing and Materials, Philadelphia, 1986. Bohac, D., Harrje, D., Norford, L.K., "Constant Concentration Infiltration Measurement Technique: An Analysis of its Accuracy and Field Measurements,” Proceedings of the ASHRAE/DOE/BTECC Conference Thermal Performance of the Exterior Envelopes of Buildings III , ASHRAE SP 49, 1986. Collet, P.F., "Continuous Measurements of Air Infiltration in Occupied Dwellings," in Building Design for Minimum Air Infiltration proceedings of the Second Air Infiltration Centre Conference, Stockholm, 1981. Condon, P.E., Grimsrud, D.T., Sherman, M.H., Kammerud, R.C., "An Automated Controlled-Flow Air Infiltration Measurement System, " in Building Air Change Rate and Infiltration Measurement eds., Hunt, C.M., King, J.C., and Trechsel, H.R., ASTM STP 719, American Society for Testing and Materials, 1980. Dick,. J.B., "Experimental Studies in Natural Ventilation of Houses," Journal of the Institution of Heating and Ventilating Engineers , December 1949. Dietz, R.N., Cote, E.A., "Air Infiltration Measurements in a Home Using a Convenient Perfluorocarbon Tracer Technique, " Environment International, Vol.8, 1982. Dietz, R.N., Goodrich, R.W., Cote, E.A., Wieser, R.F., "Application of Perfluorocarbon Tracers to Multizone Air Flow Measurements in Mechanically and Naturally Ventilated Buildings," Brookhaven National Laboratory Report 35249, 1984. Dietz, R.N., Goodrich, R.W., Cote, E.A., Wieser, R.F., "Detailed Description and Performance of a Passive Perfluorocarbon Tracer System for Building Ventilation and Air Exchange Measurement, " in Measured Air Leakage of Buildings , ASTM STP 904, H.R. Trechsel and P.L. Lagus, Eds., American Society for Testing and Materials, Philadelphia, 1986. Freeman, J., Gale, R., Lilly, J.P., "Ventilation Measurements in Large Buildings," in Air Infiltration Reduction in Existing Buildings proceedings of the Fourth Air Infiltration Centre Conference, Elm, Switzerland, 1983. 21 Sandberg, M., Sjoberg, M., "The Use of Moments for Assessing Air Quality in Ventilated Rooms," Buildings and Environment , Vol.18, No. 4, 1983. Sandberg, M., Blomqvist, C., Sjoberg, M., "Warm Air Systems. Part 2. Tracer Gas Measurements and Ventilation Efficiencies," Bulletin M82:23, The National Swedish Institute for Building Research, 1982. Skaret, E., Mathisen, H.M., "Ventilation Efficiency - A Guide to Efficient Ventilation," ASHRAE Transactions , Vol.89, Part 2, 1983. Sklret, E., Mathisen, H.M., "Test Procedures for Ventilation Effectiveness Field Measurements," in Proceedings of the International Symposium on Recent Advances in the Control and Operation of Building HVAC Systems , Trondheim, Norway, 1985. Sherman, M.H., Grimsrud, D.T., Condon, P.E., Smith, B.V., "Air Infiltration Measurement Techniques," in Air Infiltration Instrumentation and Measuring Techniques proceedings of the First Air Infiltration Centre Conference, Berkshire, UK, 1980. Sherman, M.H., Wilson, D.J., "Relating Actual and Effective Ventilation in Determining Indoor Air Quality," Buildings and Environment, Vol.21, No. 3/4, 1986. Sinden, F.W., "Multi-Chamber Theory of Air Infiltration," Building and Environment , Vol. 13, 1978. Tamura, G.T., Evans, R.G., "Evaluation of Evacuated Glass Tubes for Sampling SFfi/Air Mixture for Air Exchange Measurements," ASHRAE Journal, October 1983. Waters, J.R., Simons, M.W., "The Measurement of Air Infiltration in Large Single Cell Industrial Buildings," in Measured Air Leakage of Buildings , ASTM STP 904, H.R. Trechsel and P.L. Lagus, Eds., American Society for Testing and Materials, Philadelphia, 1986. Zuercher, CH„, Feustel, H., "Air Infiltration in High-Rise Buildings," in Air Infiltration Reduction in Existing Buildings proceedings of the Fourth Air Infiltration Centre Conference, Elm, Switzerland, 1983. 24 TABLE 1 TRACER GASES AND MEASUREMENT TECHNIQUES Technique Gases Thermal Conductivity Detector Electron Capture Gas Chromatograph Flame Ionization Gas Chromatograph Infrared Absorption TABLE 2 RELATIVE TRACER GAS COSTS TAKING DETECTABILITY INTO ACCOUNT Gas Detectable Concentration Gas Per Volume Dollar Maximum Measureable Volume Per Dollar (ppm) ft 3 (m 3 ) ft .3 (m 3 ) He 300 1.4 (0.13) 4 X 10 3 (4 x 10 2 ) co2 1 7.0 (0.65) 6 X 10 4 (6 x 10 5 ) n2o 1 2.4 (0.22) 2 X 10 6 (2 x 10 5 ) sf 6 — fi 5 x 10 0.13 (1.2 x 10” 2 ) 2 X 10 10 (2 9 x 10 ) 5 x 10~ 3 2 X io 7 (2 x 10 6 ) CBrF 2 * 5 x 10~ 5 3.7 x 10" 2 (3.4 x 10~ 3 ) 7 X 10 8 (7 x 10 7 ) PDCH** 5 x 10“ 6 3.0 x 10" 3 (2.8 x 10" 4 ) 6 X 10 8 (6 x 10 7 ) * bromotrifluoromethane ** perfluorodimethylcyclohexane Hydrogen Helium Carbon Dioxide Sulfur Hexafluoride Refrigerants Perfluorocarbons Ethane Carbon Monoxide Carbon Dioxide Sulfur Hexafluoride Nitrous Oxide Ethane Methane 25 ' oo o CO o qd o •ST o C\J o 00 Io cr> o UO.i;PJ;U0DUO3 J9DPJ1 ^ -p o> TO 0J i/i Cl «3 UJ C o 0 r=C AJ 05 p-H °H AJ d a) > 05 c • r-4 V -o- 00 0) o-« AJ <u a US o 00 AJ CO AJ C QJ c/5 B 3 • i-4 AJ M CO QJ 05 CU X 3 w O c o • r— 1 • pa] AJ c o 0) QJ ) C c O i— i CQ l-i aj 3 c AJ to c AJ 1—) C/5 c -o o c o C0 co QJ l-i 3 oo 0 r- 1 WHOLE BLDG: 0.74 ACH 29 3864- 2nd Floor (2358 m3 ) 0.15 ACH 72 2605 * 345 1st Floor (2610 m3 ) 1.33 ACH 2554 34 681 232 ~r 177 Basement (872 m3 ) 0.58 ACH 507 LAKE OSWEGO LIBRARY (all flow rates in m3/h) F igure 4 Airflow Rates Measured in Multi-Tracer Tests (Dietz et al 1984a) Schematic of Tracer Gas Measurement in a Mechanically Ventilat \ ////i — I////H c o c Q) 03 Cd E 0 ^ c 0 0 o o cd cW oH O D C/3 cd 0 £ NBS-114A rev. 2 -aci U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication D ate BIBLIOGRAPHIC DATA SHEET (See instruction s) REPORT NO. NBSIR 88-3708 FEBRUARY 1988 4. TITLE AND SUBTITLE Tracer Gas Techniques for Studying Building Air Exchange 5. AUTHOR(S) Andrew Persily 6. PERFORMING ORGANIZATION (If joint or other than NBS. see in stru ction s) NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE WASHINGTON, D.C. 20234 7. Contract/Grant No. 8 . Type of Report & Period Covered 9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ("Street. City. State . ZIP) National Bureau of Standards 10. SUPPLEMENTARY NOTES Document describes a computer program; SF-185, FlPS Software Summary, is attached. 11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gn i fi cant bi bl iography or literature survey, mention it here) A variety of procedures have been developed that employ tracer gases to examine the air exchange characteristics of buildings. These procedures enable the examination of several features of building air exchange including ventilation rates, air movement within buildings, and building envelope airtightness. This paper reviews tracer gas measurement techniques that have been used to study air exchange in buildings. Back- ground information is discussed such as the instrumentation used in these tests, building features that influence their application, and the fundamental theory of tracer gas measurement. Several specific applications are discussed including air exchange rate measurement in large buildings, low-cost procedures for measuring air exchange rates in large numbers of buildings, techniques for evaluating the performance of air distribution systems, and pressurization testing of envelope airtightness in large buildings. A detailed bibliography is also included to facilitate a more thorough examination of the topics discussed. 12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) airflow measurement; building performance; infiltration; measurement; tracer gas; ventilation. 13. AVAILABILITY [X] Unlimited I i For Official Distribution, Do Not Release to NTIS JT Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. ly~ Order From National Technical Information Service (N„TIS), Springfield, VA. 22161 14. NO. OF PRINTED PAGES 38 15. Price $11.95 USCOMM-DC 6043-P 80 § I .. ' - , " : i , * ,, . -