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Andrew K. Persily
National (^) Bureau of Standards Center for Building Technology
February 1988
1913-
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.
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.
In conducting a tracer gas evaluation of air (^) exchange in any building there are several issues that must be considered* These include the (^) characteristics of the building being studied and the instrumentation (^) that will (^) be used in the investigation* This section discusses these issues* In addition, there are some questions of terminology that need to be addressed at this stage. In (^) this paper the term infiltration refers to uncontrolled air (^) leakage into a building through unintentional openings in the building envelope, i.e., leaks, due to pressure differences across these openings. These pressure (^) differences are caused by weather effects, wind and temperature differences between indoors and outdoors, as well as by mechanical equipment operation, both fans and (^) combustion appliances. Exfiltration^ refers^ to airflow^ out of^ the building through these same leaks. The term ventilation refers to intentional outdoor air intake through a ventilation system, with mechanical ventilation being that ventilation induced by a mechanical system. Natural ventilation is unpowered airflow through intentional openings such as open windows and vents. The term air exchange rate refers (^) to the total airflow into (^) a building (^) due to all of these processes, and in general they are all important in determining the net air exchange rate of (^) a building. In many buildings, particularly residential, (^) there is no mechanical ventilation except for bath and kitchen exhaust fans, and all the air exchange is due to infiltration and natural ventilation.
2.1 Building Characteristics
In order to apply any tracer gas evaluation procedure to a building one must consider several features of that particular structure in order to determine exactly what (^) questions to ask about the building and how to conduct the measurements. It is very important to examine the building plans, the mechanical equipment specifications, and the building itself before making these decisions. The (^) basic physical (^) characteristics of the building, including (^) floor area, volume, height, and number of stories, are obviously important. Other important aspects of a building' (^) s physical layout include the manner in which the building is divided into zones, and the existence, size and layout of special-use spaces such as lobbies, atria, kitchens, cafeterias, conference rooms, and laboratories. Such special-use spaces often have dedicated mechanical ventilation systems (^) and unique ventilation needs. The existence and design of ventilation and air distribution equipment is another important building feature that must (^) be considered. These equipment issues include the number of fans, their role in ventilation and distribution (e.g. supply,^ return, exhaust, mixing), the amount of air they are intended to move, the manner in which they are controlled, and the particular zone within^ the^ building^ that^ they serve. In addition, the location of outdoor air intakes and exhaust vents, and their proximity to one another, are important factors. All^ of^ these^ features^ of a building must be studied and understood before the measurements are planned and conducted. This process of developing an^ understanding^ a^ building^ and^ its ventilation system requires a significant amount of^ effort^ but^ is^ absolutely essential in order (^) to conduct a worthwhile investigation.
In addition to understanding the above features of a particular^ building, it is also important to understand^ and^ recognize^ the^ complex^ interactions between the ventilation equipment, building envelope, and interior^ configuration in determining (^) a building's air exchange characteristics.' It is impossible^ to consider these building systems in isolation because they^ interact^ so^ strongly. Measurements and modelling in mechanically ventilated office^ buildings^ have
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
(h A ) The^ solution^ to^ equation^ (1)^ is^ given by
C(t) =^ C (^0) -It
where C 0 is the concentration at time t=0. For those cases in which F is constant, the solution integrates further to
C(t) -^ C 0 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) =^ C 0 e~ Ito (5)
Solving Eq (^) (5) for I yields
I = (^) d/t) In (^) [ (C 0 /C (t) (^) ] ( 6 )
where C 0 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^ C^0 , one^ can^ measure^ C^ periodically^ during^ the^ decay^ and fit the data^ to^ an^ equation^ of^ the^ form
lnC(t) =^ lnC 0 - 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
from chamber (^) j to the chamber i, (^) qj_ 0 is the airflow rate from chamber i to outdoors, and (^) q-j_j_ =^ 0.^ Note^ that^ as^ in^ the^ case^ of^ Eq^ (1), we^ are^ assuming^ that the outdoor tracer^ gas^ concentration^ is^ zero.^ For^ each^ tracer^ gas^ that^ is^ used, there will be one set of these equations,^ one equation for each zone.^ In designing and applying a multi-chamber experiment one must conceptually divide the building into^ separate^ zones,^ and^ it^ is^ not^ always^ obvious^ how^ to^ make^ these divisions
3.4.1 Decay
The multi-chamber tracer gas decay method^ involves^ releasing^ the^ tracer^ gas as a pulse and^ then^ monitoring^ the^ concentration^ response^ in^ all^ of^ the^ chambers (Sinden 1978). The^ decay^ technique^ can^ be^ applied^ with^ a^ single^ tracer^ or several different tracer gases, and can^ be used to obtain the values^ of^ all^ the interzonal airflows (^) q-j_j and (^) qj_ 0. In the single tracer, multi-chamber decay approach, one injects tracer into a single zone and monitors the concentration response in^ all^ the^ zones.^ The^ measured^ concentrations^ are^ used^ in^ conjunction with Eq (^) (11) to solve for the airflows. Depending on the characteristics of the airflow between the various zones, one such test may be sufficient to determine all of the airflows of^ interest.^ In^ general,^ however,^ several^ such^ tests^ will be required with the tracer^ being^ injected^ into^ a different^ zone^ each^ time.^ For this series of measurements to provide meaningful results, the airflows can not change significantly over the test period, generally an unreasonable expectation. To minimize the requirement for constant conditions, one can employ a multi-tracer decay technique in which a different tracer gas is injected into each^ zone^ and^ the^ concentration^ response^ of^ all^ the^ gases in^ all the zones is monitored.^ Both^ the^ single-^ and^ multi-tracer^ approaches^ make^ the assumption that each individual zone is prefectly mixed and that the test begins with a uniform tracer gas concentration in each zone. These assumptions are approximations at best and the errors associated with^ the actual conditions of imperfect mixing within zones and nonuniform initial conditions are not well known
Several measurement systems employing the multi-tracer decay method have been developed, including those of Irwin (^) et al (^) (1984) and Prior (^) et al (^) (1983) Both systems employ gas chromatograph electron capture detectors, to measure refrigerants in the first system and perfluorocarbon tracers in the latter. I' Anson et al (1982) also employs a decay system involving refrigerant tracers. In figure (^) 2, we show representative data from their paper in which two tracers are used (^) to study the airflow between the upper (^) and lower (^) levels of a building. The refrigerant C 2 CI 2 F 4 is injected downstairs and CCI 2 F 2 upstairs. The movement of tracer between the zones and the subsequent (^) decay is evident in the data. Analysis of these data yields the airflow rates between the two zones and from each zone to the outdoors.
3.4.2 Constant Concentration
The (^) constant concentration method can also be applied to study buildings in which a single zone approximation is inappropriate. As mentioned earlier one can (^) separately control the tracer gas concentration in each zone of a building such that all zones are at the same target concentration (^) C'. This technique involves (^) sampling the tracer gas concentration from, and injecting tracer into, each zone. The procedure can be used to determine the outdoor airflow rates into each zone (^) qj_ 0 , but not the interzonal airflow rates (^) qij. Using Eq (^) (11) and setting (^) Ci(t) =^0 and (^) Cj_(t) =^ C' (^) , one obtains,
F (^) i ~ (^) 3io. (^) ( (^12) )
From this equation one obtains values of (^) q-[ 0 for each zone. The total (^) air exchange rate of the building is determined by adding together all (^) of the (^) qi 0 's. Of course this procedure involves the same (^) approximations as the single zone constant concentration procedure, i.e., that the concentration is maintained constant despite lags between the tracer gas injection and (^) the concentration response (^) due to imperfect mixing within (^) a zone. (^) Again, the errors associated with this mixing problem are not well known. As mentioned earlier, (^) there have been several systems developed that employ the (^) constant concentration procedure for multi-chamber studies (Collet et al 1981; Bohac et al 1986)
3.4.3 Constant Injection
The constant injection procedure can also be (^) used to study multi-chamber (^) air exchange by injecting^ a different^ tracer^ into^ each^ zone^ of a building at a constant rate and measuring the equilibrium concentrations of each tracer in each zone. By employing the appropriate form of Eq (^) (11), one can solve for all of the interzonal airflow rates. This procedure is most appropriate for situations in which these airflows are fairly constant since one is assuming steady-state conditions^ in^ solving^ for^ the^ airflow^ rates.^ The multi-chamber, constant injection procedure is less well developed than some of the other multi-chamber techniques but some examples of its application (^) exist.
Figure 3 shows an example of a constant injection measurement of ventilation and interzonal airflows in two adjacent, negative pressure laboratory rooms (Lagus 1984). The two laboratories are under negative pressure (as shown in the figure) (^) , and a different tracer gas is injected at a constant rate into each laboratory. The equilibrium (^) tracer concentrations in each zone are shown in the figure. The measured airflow rates agree well with the ventilation rates measured (^) by tracer (^) gas decay. The presence of the refrigerant CBrFg into (^) the lower laboratory serves as evidence of undesired airflow between the laboratories.
A long-term averaging, constant injection technique has also been employed using perfluorocarbon tracers and passive samplers (Dietz and Cote 1982; Dietz et al 1984a). Long-term averaging, passive sampling techniques are discussed in more detail below. In these measurements, passive tracer sources are used to release a different tracer into each zone at a constant rate. The average concentration over the measurement period (which can range from^ hours^ to months) is determined in each zone with a passive sampler that collects tracer gas over the entire sampling period. From the injection rates^ and^ the^ average concentrations, one determines the airflows of interest. The determination of the airflow rates from Eq (11) involves an assumption^ that^ the^ concentration^ is at equilibrium, which can lead to measurement errors of unknown magnitude. Also, in solving these equations one uses average values of the terms (^) qijC£. Since one measures only the average of Cj_, in order to solve for^ qj_j one^ is making the assumption that the average of (^) qijCj_ equals the average of (^) q^j multiplied by the average of Cj_. This is not a mathematicailly valid^ assumption except in very special cases, and therefore the calculations of (^) qj_j are subject to errors of unknown magnituide. Figure 4 is^ an^ example^ of^ measured^ airflows for a two zone case using this technique (Dietz^ et al^ 1984)
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
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
tracer gas injection^ and^ air^ sampling,^ while^ the^ tracer^ gas^ concentration^ of^ the air sample is determined with^ equipment^ at a central^ facility.^ All^ long-term average, constant injection^ measurement^ techniques^ have^ the^ problem^ that^ they actually determine^ the^ average^ of^ the^ inverse^ air^ exchange^ rate^ and^ not^ the average air^ exchange^ rate.^ The^ difference^ between^ the^ average^ of^ the^ inverse and the actual average^ depends^ on^ the^ distribution^ of^ air^ exchange^ rates^ during the measurement period.^ The^ magnitude^ of^ these^ differences^ are^ just^ beginning to be^ studied,^ but^ can^ be^ on^ the^ order^ of^ 20%^ for^ one-month^ averaging^ periods (Sherman and Wilson 1987). Longer averaging periods will^ generally lead to larger errors.
One version of the constant injection technique is the average infiltration monitor (AIM) developed at the Lawrence Berkeley Laboratory (Harrje et al 1981) This system employs suitcase-sized^ injectors^ and^ samplers^ to enable^ unattended measurement of long-term^ average^ infiltration^ rates.^ The^ injector^ contains^ a pump which slowly releases the tracer gas at a constant rate into the building. The sampler slowly^ fills^ a^ sample^ bag^ with^ a^ pump^ to^ obtain^ the^ average^ tracer gas concentration during the measurement period. The concentration in the sample bag is later determined at a central location. Another technique employs small passive injectors and samplers^ in^ a similar^ procedure. The Brookhaven National Laboratory Air Infiltration Monitoring^ System (BNL/AIMS)^ employs a perfluorocarbon tracer gas (PFT) which diffuses out at a known constant rate from a fluoroelastomer plug impregnated with the tracer (Dietz and Cote 1982; Dietz et al 1984 and 1986). The passive sampler is a small capillary adsorption tube which collects the PFT from the building interior during the test. The sampler is later analyzed^ to determine^ the^ average^ tracer^ gas concentration^ in the building, and hence the average (inverse)^ air exchange^ rate.
The application of these constant injection, long-term averaging techniques to large buildings requires that several tracer sources be used and that they are well-distributed throughout the building. Several samplers are also required. In mechanically ventilated office buildings the intermittent operation of fans, changing damper positions, and variations in outdoor air intake rates must be considered when using these techniques.
In this section tracer gas measurement procedures are discussed that characterize aspects of building air exchange and air movement other (^) than building (^) air exchange rates. These procedures include recently developed techniques to evaluate the performance of air distribution systems and to measure airflow rates associated with (^) a ventilation (^) system. In addition, procedures exist to pressure test the airtightness of large building envelopes and to evaluate building air movement in a qualitative manner.
5.1 Air Distribution Evaluation
Building (^) ventilation systems are designed to satisfy minimum outdoor air intake levels (ASHRAE (^) 1981) in order (^) to provide (^) a safe (^) and comfortable environment for the (^) building occupants. Even if the ventilation system is bringing in a sufficient amount of outdoor air, the air may not be well distributed within the interior space. In (^) this case, not all of the air will be effective in maintaining (^) acceptable indoor air quality within the space. The concept of ventilation effectiveness has been developed in order to quantify the air distribution system's ability to provide freshly conditioned air (^) to the
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 t (^) n , 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^ t^ n , 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
