Ventilation effectiveness criteria and measurement methods applicable to animal buildings--a review.
The effectiveness of an animal building's ventilation system is usually characterized in terms of air exchange capacity (by measuring the air speed) and air distribution (by measuring the inlet air speed and airflow pattern). These parameters are more apt for troubleshooting and not sufficient to quantify the system's performance in relation to contaminant removal. Limited studies have been done on the evaluation of the animal building's performance in terms of contaminant removal effectiveness. Standard quantitative criteria and measurement methods applicable to animal buildings are also lacking. This paper reviews the existing ventilation effectiveness criteria and measurement methods that have been found useful for application to other mechanically ventilated buildings. Based on the reviewed criteria and measurement methods, the paper also discusses the most applicable criteria and methods for evaluation of an animal building's ventilation system. This work is a major component of ASHRAE Research Project 1301, "Quantification of Ventilation Effectiveness for Air Quality Control in Plant and Animal Environments."
Ventilation is an air exchange and distribution process that brings clean outdoor air by either natural or mechanical means to occupied spaces, distributes it within the space, and exhausts hot, humid, and contaminated air from the building. It is the primary method for maintaining acceptable air quality in animal buildings. During summer, the major concern in animal buildings is heat buildup, and ventilation must be able to remove excess heat; during winter, the major purpose for ventilation is to remove excess moisture and carbon dioxide (C[O.sub.2]) while at the same time conserving heat produced by the animals. Although animal buildings' ventilation rates are modulated primarily to remove excess moisture, heat, and C[O.sub.2], it has been found that the ventilation rates designed to remove these components are in general sufficient to maintain the levels of other contaminants (e.g., ammonia [N[H.sub.3]], hydrogen sulfide [[H.sub.2]S], particulate matter [PM]) below the threshold values. In certain situations (i.e., low ventilation rate during colder months, too much activity in the barns during feeding and animal stocking), however, the concentration of these contaminants exceeded threshold values (Zhu et al. 2000; Predicala et al. 2001; Wathes et al. 2003; Jerez et al. 2005). When one or more of these pollutants rise to a level that is too high, one of the control measures is to adjust the management practices, i.e., removing the manure inside more often, storing manure outside the buildings, and adding additives and chemicals to feed and to manure storage, respectively. Ventilation, however, is still seen as the primary method and frequently the only means available to remove airborne pollutants, especially gases.
As in the case of other ventilated buildings, ventilation rates and air distribution are often varied to provide thermal comfort to the occupants and reduce the levels of the contaminants inside the building to a minimum. The desire for thermal comfort and acceptable indoor air quality usually provides conflicting constraints to the operation of the ventilation system. While higher ventilation rate may increase the removal of gaseous contaminants, albeit not necessarily PM, it also may result in cold draft, which affects the animals' performance and growth and may even be fatal for small animals. On the other hand, a lower ventilation rate may not be sufficient to evacuate indoor contaminants. Even if the ventilation rate is enough to achieve optimum thermal and IAQ conditions inside the building, this doesn't guarantee that air will be well mixed inside the ventilated space. In some cases, however, well mixed air is not desired; i.e., if the interest is in reducing the air contaminants in areas occupied by the animals and workers, it is desirable to have more fresh air going into these areas than in unoccupied spaces. What makes a ventilation system effective as a control method not only depends on the level of ventilation rate but also on the resulting air distribution as affected by the location of the air inlets and outlets within the building.
The effectiveness of mechanical ventilation systems in animal buildings is usually characterized in terms of the air exchange capacity (by measuring the fan air speed) and air distribution (by measuring the inlet air speed and airflow pattern); these parameters are more apt for troubleshooting and are not sufficient to provide a quantitative value of the system's performance in relation to contaminant removal. Although researchers have made great strides in improving animal building ventilation system design and control, limited studies have been done on the evaluation of these systems in terms of contaminant removal effectiveness. Standard quantitative criteria and measurement methods that researchers can use to compare the ventilation effectiveness of different types of animal building ventilation systems are also lacking.
The objectives of this research were to (1) conduct a detailed literature review of the ventilation effectiveness criteria and measurement methods that have been applied in mechanically ventilated airspaces and (2) determine the ventilation effectiveness criteria and method applicable to animal buildings. This paper summarizes the current knowledge on ventilation effectiveness criteria and methods of measurements. No attempts were made to cover the derivation of the parameters presented herein since it is covered by other authors (Sandberg 1981, 1983, 1992; Sandberg and Sjoberg 1983; Skaaret and Mathisen 1983; Skaaret 1984, 1986). Reviews of parameters used for ventilation effectiveness quantifications have also been written by Brouns and Waters (1991) and Liddament (1987, 1993). Roos (1999) wrote a thesis on ventilation effectiveness, while Roulet and Vandaele (1991) had extensive discussion on the ventilation effectiveness parameters and their measurement methods. Instead, this paper considers the ventilation effectiveness parameters that have been developed in order to come up with the most applicable and practical ventilation effectiveness measure for animal buildings. It should be noted that in the literature, the definition of ventilation effectiveness is usually limited to the quantification parameters for contaminant removal, while in other cases, ventilation efficiency is used instead of ventilation effectiveness. In this paper, ventilation effectiveness refers to the series of parameters that quantify the performance of a ventilation system in terms of both providing fresh air to occupants and diluting or removing internally generated contaminants.
EXISTING VENTILATION EFFECTIVENESS CRITERIA
Ventilation effectiveness is defined as the ability of a ventilation system to achieve its design goals (Persily 1994), the main goals being (1) to provide fresh air to the occupants and (2) to dilute and remove the internally generated contaminants (Liddament 1993). The definition of ventilation effectiveness in ASHRAE Standard 62-2001 addresses both objectives through prescription of required ventilation rates, and it is defined as the fraction of the outdoor air delivered to the space that reaches the occupied zone (ASHRAE 2001). But the same standard does not have practical information on how to evaluate and measure the ventilation effectiveness of the system; the information that is provided in Appendix E was derived from a two-zone model and defined ventilation effectiveness ([E.sub.v]) as a function of mixing factor S and the recirculation factor R (Equation 1) without stating the procedures on how to measure these values.
[E.sub.v] + (1 - S)/(1 - RS) (1)
There are many other ways to interpret the ventilation effectiveness of a system, including how effective it is in providing acceptable velocities in the occupied zone and delivering comfortable temperature level for the occupants. The Air Diffusion Performance Index (ADPI) has been developed to quantify ventilation performance based on air velocity and effective draft temperature (a combination of local temperature variations from the room average) (ASHRAE 2005). The ADPI is only an indicator of the thermal comfort of the occupants. In terms of indoor air quality, quantitative analysis of a ventilation system's performance focuses on the abilities of the ventilation system to provide fresh air to the occupants and remove internally generated contaminants. The parameters that define these two objectives depend on the spatial application scale, i.e., local vs. global, and time scale of interest, i.e., steady state vs. transient. The local scale relates the ventilation effectiveness to specific zones of interest in the room, while the global scale yields the performance of the whole ventilation system. Although a majority of the work on ventilation effectiveness measurements was based on a steady-state condition, Sandberg (1981) emphasizes the importance of characterizing the effectiveness of the ventilation system in both transient and steady-state conditions. In office and residential buildings, the steady-state phase is more applicable since the concern is with the longer-term pollution level; the transient phase is more important in industrial facilities, especially when there is an accidental release of toxic gas, to be able to know how rapidly the concentration can be brought back to a safe level. In animal buildings, contaminants are being generated all the time; thus, the longer-term pollution level is the primary concern. Discussion in this paper focuses primarily on steady-state parameters that may be relevant to animal building applications. The parameters were divided into two categories: (1) indices for room air renewal, which are based on age of air and residence times, and (2) indices for contaminant removal, which are based on the relation between the concentration of contaminants at the exhaust ports and at various locations in the room.
Indices for Room Air Renewal
Nominal Air Exchange Rate. Since the ventilation air not only dilutes but also acts as carrier of gaseous and particulate contaminants, its freshness can then be used, to some extent, as an indicator of the effectiveness of the ventilation system. In this regard, a number of indices that describe the freshness of the air, usually in "time" parameters, have been developed and introduced. The most commonly cited of these indices is the specific airflow rate, or nominal air exchange rate ([eta]) (air exchange frequency in Skaaret 1984). The nominal air exchange rate is the ratio of the supplied fresh airflow rate (Q) to the volume of the ventilated room (V); it is usually expressed in terms of air changes per hour (ach), i.e., the higher [eta] is, the fresher is the air in the room.
[eta] = Q/V (2)
Although this definition may mean that the room air is completely replaced with fresh air at a given ach, this isn't the case since the displaced air is always a mixture of fresh and old air. The nominal air exchange rate has been used in ventilation measurements in offices, residential housing, and other mechanically ventilated spaces (Howard 1966; Dols and Persily 1992; Chow and Wong 1999). Although the nominal air exchange rate provides information regarding the total amount of outside air entering the building, it provides no information on the distribution of the outside air in the building.
When room air is not perfectly mixed, the local exchange rate [[eta].sub.p] can be used to measure the amount of ventilation that occurs at specific locations in the room. This concept was first introduced by Sandberg (1981) and is defined as the flow rate of air [Q.sub.p] entering a certain volume [V.sub.p] irrespective of the time that air has been inside the building.
[[eta].sub.p] = [Q.sub.p]/[V.sub.p] (3)
The definition of local exchange rate in Equation 3 by Sandberg (1981) is difficult to measure physically. Offermanm et al. (1983) introduced a local ventilation exchange rate based on the mass balance equation for a small, perfectly mixed volume element p in an imperfectly mixed indoor space and is defined as the change in pollutant concentration C divided by the area under the concentration curve C(t) integrated over a period of one hour. In Equation 4, [C.sub.p](0) and [C.sub.p] (t) are the pollutant or tracer concentrations observed at point p at time t = 0 and at t, respectively. This latter definition of local exchange rate is easier to measure using the tracer-gas decay technique.
[[eta].sub.p] = [[C.sub.p](t).[-[integral]dC].[[C.sub.p](0)]]/[t.[integral].0][C.sub.p]dt (4)
Age of Air. The age of air, also called internal age, is the time that has passed since a molecule of air entered the ventilated space; the younger the air, the better is its dilution capability and the more efficient is the ventilation system. Since not all molecules of air arrive at a point at the same time, the age of air is characterized by statistical age distribution, which is defined by a whole range of parameters such as the mean value, variance, maximum value, skewness, etc. The most important and widely used parameter is the mean value, i.e., local mean age of air and room mean age of air. The ventilation effectiveness parameters that use the age of air concept were first introduced by Sandberg and Sjoeberg (1983) and were applied later by others (Skareet 1984, 1986; Sherman and Wilson 1986; Breum 1988, 1992; Haghighat et al. 1990; Olufsen 1991; Sateri et al. 1991; Persily and Dols 1991; Mundt 1994; Heiselberg 1996; Xing et al. 2001; Novoselac and Srebric 2003). The indices were derived by Sandberg and Sjoeberg (1983) by considering an ideal unidirectional parallel plug or piston flow of air. The commonly used indices are the nominal time constant, local mean age, and room mean age.
The nominal time constant, [[tau].sub.n], (also called transit time) is the average period of time in which air, once entering the enclosure, will remain. Thus, it gives the average age of the air leaving the room and is the inverse of nominal air exchange rate, [eta], i.e.,
[[tau].sub.n] = V/Q = (1/[eta]). (5)
The nominal time constant can be measured by the tracer decay method, which will be discussed in the following section. It is not dependent on the flow pattern, and it has been shown that even during poor mixing, [[tau].sub.n] is equal to the mean age of air at the exhaust, [[tau].sub.e], i.e., [[tau].sub.n] = [[tau].sub.e] (Sandberg 1983; Skaaret 1984).
The local mean age of air at an arbitrary point p, [[tau].sub.p], is the average time that it takes for an air molecule, once entering the enclosure, to reach point p. When measured using the tracer decay technique, it is the area under the decay curve divided by the initial concentration and is given by Equation 6 (ASHRAE 2002):
[[tau].sub.p] = [[infinity].[integral].0][C.sub.p](t)dt/[C.sub.p](0) (6)
where [C.sub.p](t) is the concentration of tracer at time t and [C.sub.p](0) is the initial concentration at time 0.
The room mean age of air, <[tau]>, is the average of the local ages of all air particles in the room. For a piston flow, it is equal to [[tau].sub.n]/2. During complete mixing, the room mean age of air can be calculated from the contaminant or tracer concentration in the exhaust duct [C.sub.e] (Seppanen 1986):
<[tau]> = [[infinity].[integral].0][C.sub.e](t)tdt/[[infinity].[integral].0][C.sub.e](t)dt (7)
Air Exchange Effectiveness. The air exchange effectiveness, <[[epsilon].sub.a]>, describes the replacement of room air with fresh air compared to an ideal plug (piston) flow pattern. It is the ratio of the room mean age for the piston flow to the room mean age for the real flow through the room and is given by Equation 8 (Etheridge and Sandberg 1996):
<[[epsilon].sub.a]> = [[tau].sub.n]/2<[tau]> (8)
In ASHRAE Standard 129 (ASHRAE 2002), the air-change effectiveness, designated with E, is defined and is twice the air exchange effectiveness, i.e., E = 2<[[epsilon].sub.a]>. The air exchange efficiency provides a measure of how quickly air in an enclosure is replaced under different conditions of mixing. The theoretical upper limit for <[[epsilon].sub.a]> is 1, which happens when the flow is piston-type, whereas for complete mixing, it is equal to 0.5. While the air-exchange efficiency may indicate a mixing problem within the space, it doesn't indicate where the problem exists (Liddament 1992). Only by monitoring the mean age of air at specific locations ([[tau].sub.p]) within a zone can the location of poor mixing be identified. The local air exchange effectiveness [epsilon]p, defined by Equation 9 (Heiselberg 1996), accomplishes this purpose.
[[epsilon].sub.p] = [[tau].sub.n]/[[tau].sub.p] (9)
The local air exchange effectiveness indicates the rate of ventilation supplied at different locations in the room and, thus, the air distribution. At complete mixing, values of [[tau].sub.p] are the same throughout the space and are equal to the inverse of [[tau].sub.n], thus, [[epsilon].sub.a] is 1. If there is nonuniform air distribution within a space, locations with poor ventilation will have local ages of air that are higher than the average. When there is a short-circuiting airflow pattern, locations in the stagnant regions will have values of [[tau].sub.p] that are relatively large and [[epsilon].sub.p] will be smaller than 1.
Indices for Contaminant Removal
Average Contaminant Removal Effectiveness. The air exchange efficiency and the air exchange rate are not adequate to quantify the ability of the ventilation system to remove contaminants from the space because they are only related to the ventilation air. When contaminants are introduced in the room, they are not well mixed and their spatial distribution may not follow the room air since their release points are different from that of the air, which is released at the supply or inlet terminals of the building. In addition, contaminants may have different densities than the air due to temperature differences that allow them to set up their own motion, and their movement is also affected by the diffusion process; particles, in particular, possess inertia that affects their own movement. Thus, ventilation effectiveness parameters that are based on the ratios between concentrations of contaminants have been developed, the reference point being the exhaust concentration, and are based on the premise that in order not to increase the contaminant concentration indoors, the exhausted concentration should be at least equal to the amount created in or brought to the room; these parameters only apply to steady-state conditions. The definition that is frequently encountered in literature is the average contaminant removal effectiveness, <[[epsilon].sub.c]>, which is defined by Equation 10 (Malmstrom and Ahlgren 1982).
<[[epsilon].sub.c]> = ([C.sub.e] - [C.sub.s])/(<C> - [C.sub.s]) (10)
Using the steady-state concentrations, [C.sub.e] and [C.sub.s] are the concentrations in the exhaust and supply air, respectively, and is the mean concentration in the room. Equation 10 describes the overall performance of the ventilation system in removing indoor contaminants. For complete mixing, the contaminant concentration is uniformly distributed and <[[epsilon].sub.c]> is unity. When there is short-circuiting, the room average concentration will be greater than that at the exhaust point and, thus, <[[epsilon].sub.c]> will be less than unity. The steady-state concentration of the contaminant at the exhaust [C.sub.e] is equal to the ratio of the contaminant injection rate q to the supplied fresh airflow rate Q, whereas the steady-state average contaminant concentration is the ratio of the volume of contaminant in the room [V.sub.c] to the total volume of the room V. Thus, substituting the values for [C.sub.e] and in Equation 10 gives the expression for <[[epsilon].sub.c]> in terms of the ratio between the nominal time constant for the ventilation air [[tau].sub.n] and the nominal time constant for the contaminant [[tau].sub.n.sup.c] (Skaaret 1986), i.e.,
<[[epsilon].sub.c]> = ([[tau].sub.n]/[[tau].sub.n.sup.c]), (11)
where [[tau].sub.n.sup.c] is equal to the ratio of equivalent volume of contaminant in the room [V.sub.c] to the contaminant injection rate q; [[tau].sub.n] is defined by Equation 5. Evidently, the shorter the contaminant time constant is, the higher is <[[epsilon].sub.c]>, which will happen only when there is short-circuiting of contaminants.
A variation of the average contaminant removal effectiveness is defined in Equation 12 (Zhang et al. 2001), i.e.,
[[epsilon].sub.f] = V([C.sub.e] - [C.sub.s])/[N.summation over (p=1)] [V.sub.p][C.sub.p] - V[C.sub.s], (12)
where [V.sub.p] is the representative volume at location p and N is the number of measured locations and should be greater than 1. In Zhang et al. (2001), Equation 12 was called the ventilation effectiveness factor and was derived by introducing the ratio [V.sub.p]/V as a weighting factor for the mean concentration described in Equation 10. As in other contaminant removal effectiveness parameters described previously, higher values of [[epsilon].sub.f] are desired. When [[epsilon].sub.f] is equal to unity, the ventilation system is as effective as a complete mixing system. Using complete mixing as the reference, the ventilation system is effective in removing internally generated contaminant if [[epsilon].sub.f] is greater than unity, while a value less than unity suggests otherwise. The ventilation effectiveness factor has been applied in field measurements in a livestock building, and it was found to be primarily affected by the ventilation system and less affected by the ventilation rate.
Local Contaminant Removal Effectiveness. The average concentration of contaminants in Equation 10 can be replaced with the contaminant concentration at point p ([C.sub.p]) resulting in the local contaminant removal effectiveness [[epsilon].sub.p.sup.c], i.e.,
[[epsilon].sub.p.sup.c] = ([C.sub.e] - [C.sub.s])/([C.sub.p] - [C.sub.s]). (13)
Equation 13 is the definition of relative ventilation effectiveness given by Sandberg (1981) and it expresses how the ventilation capability of the system varies among different locations in the room. It is a measure of dispersion, and its value is always positive and can be greater than unity. It is equal to unity when there is perfect mixing, i.e., the concentration throughout the room is uniform and equal to the exhaust concentration. In the case of displacement ventilation, the value of [[epsilon].sub.p.sup.c] depends on the location of the source. If the source is generated downstream from measurement point p then [C.sub.p] may equal [C.sub.s] and the value of [[epsilon].sub.p.sup.c] is infinite; whereas when the pollutant source is upstream of the measurement point, [C.sub.p] may be higher than [C.sub.e] and [[epsilon].sub.p.sup.c] will be lower than unity. It can also be higher than unity when there is local exhaust system within the room.
Absolute Contaminant Removal Effectiveness. If the maximum value of contaminant concentration [C.sub.m] in the room is used as the reference instead of <C>, then Equation 10 becomes the absolute contaminant removal effectiveness [[epsilon].sub.m.sup.c] (absolute ventilation efficiency in Sandberg ), i.e.,
[[epsilon].sub.m.sup.c] = ([C.sub.e] - [C.sub.s])/([C.sub.m] - [C.sub.s]). (14)
The ventilation effectiveness defined above expresses the ability of the system to reduce contaminant concentration relative to the maximum concentration, but it doesn't tell the time it takes to achieve the condition. It is always equal to or less than unity no matter what type of ventilation system (e.g., piston flow, displacement flow) is used.
The contaminant removal effectiveness indices defined in Equations 10 to 14 have been selectively used in test rooms (Sandberg et al. 1986; Sandberg and Blomqvist 1989; Breum 1992; Nielsen 1992; Mundt 1994; Heiselberg 1996; Jerez and Maghirang 2003; Novoselac and Srebric 2003; Chao and Wan 2004) and in real buildings (Persily and Dols 1991; Sateri et al. 1991) to test the performance of different types of ventilation systems and determine the effect of the ventilation parameters on those indices. It has been found that these indices were strongly dependent on the type of ventilation system, contaminant source location, and the presence or absence of disturbance. In addition, the performance of the ventilation system depends on which indicator was used, i.e., for displacement ventilation, calculated contaminant removal effectiveness for occupied zones is sometimes higher than that of the whole room and vice versa; while for mixing ventilation, the local indices are usually the same or worse than that for the whole space (Novoselac and Srebric 2003).
Except for the ventilation effectiveness factor [[epsilon].sub.f], which was verified in the laboratory by measuring the airborne dust concentrations, the rest of the ventilation effectiveness indices presented in the previous section were all derived and applied using tracer gases. Roulet and Vandaele (1991) listed the ideal properties of gases: (1) neither flammable nor explosive, (2) nontoxic, (3) density should be close to air density to ensure easy mixing, (4) should not be absorbed by furnishings or decompose or react with air or building components, (5) should be easily detectable at low concentrations, (6) low background concentration in the air, (7) the flow of natural sources within the test space should be lower than the flow of the source used for measurements, and (8) cheap in the quantity required for measurements. Although no tracer satisfies all these requirements, nitrous oxide ([N.sub.2]O) and C[O.sub.2] are the most commonly used tracer gases both for laboratory and field measurements. Other works utilized sulfur hexafluoride (S[F.sub.6]) and very few used Refrigerant 12 (CC[l.sub.2][F.sub.2]) and methane (C[H.sub.4]) as tracer gases. [N.sub.2]O and C[O.sub.2] are often used in small buildings because of their density being close to air--1.5 times that of air--while S[F.sub.6] and CC[l.sub.2][F.sub.2] are at least 5 times denser than air; for large industrial buildings, S[F.sub.6] is preferred since it is cheaper and can be easily detected. If the tracer has a density higher than that of the air, the tracer is either diluted in compressed air (1% concentration) or mixed with helium to adjust the density close to that of air to achieve better mixing. However, if the simulated contaminants are particles, higher density of the tracer is required, and test particles are probably better tracers than gases for contaminant removal effectiveness measurements. In some studies, smoke particles are used to evaluate the contaminant removal effectiveness of ventilation systems.
Use of tracer gas techniques proposed by Sandberg and Sjoberg (1983) to measure the ventilation effectiveness of the system has expanded its application from laboratory evaluations to measurements in real buildings, although laboratory studies still dominate the literature. The three most commonly used techniques are step-down, step-up, and pulse method. However, for applications in real buildings, only step-down and step-up are commonly used as shown in Table 1; they have been applied in exchange rate, age of air, and air exchange effectiveness measurements. Field measurements of contaminant removal effectiveness are less frequently done compared to the indices for room air renewal. Although ASHRAE Standard 129 (ASHRAE 2002) limits the standard methods to only tracer step-up and step-down, the pulse method is also briefly described here.
Tracer Step-Down (Decay, Washout) Method
The decay method is applicable for any space, mechanically ventilated or not and with either single or multiple supply and exhaust ducts, as long as uniform distribution of tracer can be achieved by whatever means. In this method, the tracer gas is injected into and dispersed throughout the space with the aid of mixing fans or portable oscillating fans to achieve uniform distribution--uniform distribution can also be achieved by injecting tracer at a constant rate. The injection point depends on the desired measurements, i.e., the tracer is injected into the supply airstream for the age of air measurements while for age of contaminants, tracer gas is injected at representative locations in the room. After uniform condition is achieved, tracer addition is stopped at t = 0 and concentration is recorded as it falls at all measurement locations, either to zero (or background) or at least 95% of the uniform initial concentration.
The recorded decay of concentration with time gives the cumulative age distribution of the air. The nominal time constant [[tau].sub.n] is equal to the mean age of air at the exhaust while [[tau].sub.p] and <[tau]> are calculated by integrating Equations 6 and 7, with concentration values corrected for the background concentration. The nominal exchange rate [eta] and local air exchange rate [[eta].sub.p] are the reciprocals of [[tau].sub.n] and [[tau].sub.p], respectively. Air exchange effectiveness can be directly calculated using Equations 8 and 9. The contaminant removal effectiveness defined in Equations 10 to 14 can also be obtained by measuring the steady-state concentrations of the contaminants at the specified locations, with the contaminant released within the room.
Since the tracer decay method requires uniform distribution of contaminants in the space, applying this method in large building measurements may pose difficulty. Similarly, when mixing fans are used to achieve uniform distribution of tracer, the added mixing disturbs the natural airflow pattern. In small buildings, where uniform tracer concentration can be easily achieved, it has these advantages: it is simple to implement, saves tracer gas, doesn't require initial knowledge on how the air is distributed within the space, and it is already known at the outset that the desired equilibrium concentration after an infinitely long time is equal to the background concentration or zero (Olufsen 1991).
Tracer Step-Up (Source) Method
The step-up method is the inverse of tracer gas decay. It is based on the assumption that at time zero (t=0), there is a step change in the supply concentration from the background (or zero) to some value [C.sub.x] and this concentration must be maintained throughout the measurement period. This method is only applicable to buildings with a single air inlet duct, and it requires complete mixing of the tracer gas with the supply air. To ensure good mixing of the tracer with the incoming air, the tracer should be injected upstream of the inlet fans, if present, or within the supply duct several duct diameters away from the inlet or grilles. If the building has more than one inlet, each must be injected with the same tracer concentration within 15%, which is generally impractical, especially if the air inlets have different airflow rates.
At time [t.sub.0], tracer gas is injected into the supply air at a constant rate, which depends on the ventilation rate Q and the concentration range of the measurement device (ASHRAE 2002), i.e.,
q = Q x [C.sub.mid], (15)
where [C.sub.mid] is tracer concentration near the middle of the measurement range. Constant injection of tracer should continue until a steady-state concentration C([infinity]) is observed at all measurement points; the growth of tracer gas concentration is then recorded at different points. The equations for calculating <[tau]> and [[tau].sub.p] are shown in Equations 16 and 17:
[[tau].sub.p] = [[infinity].[integral].0][1 - [[C.sub.p](t)]/[[C.sub.p]([infinity])]]dt (16)
<[tau]> = [[infinity].[integral].0]t[[C.sub.e]([integral]) - [C.sub.e](t)]dt/[[C.sub.e]([integral]) - [C.sub.e](t)]dt (17)
where [C.sub.e]([integral]) is the steady-state concentration at the exhaust. As in the step-down method, [[tau].sub.n] is the age of the air at the exhaust; [eta] and [[eta].sub.p] are the reciprocals of [[tau].sub.n] and [[tau].sub.p], respectively. Equations 8 and 9 are used to calculate <[[epsilon].sub.a]> and [[epsilon].sub.p], respectively; the contaminant removal effectiveness indices can also be calculated using the steady-state concentrations at the locations specified in Equations 10 to 14 and, as in the step-down method, the contaminant must be released within the room.
The step-up method should only be used when it is possible to achieve uniform and identical tracer gas concentrations in all supply inlets. In large buildings that have multiple supply inlets, it may be difficult to achieve uniform mixing, especially if the inlets are not ducted through a single outdoor air source, which is the case in animal buildings. In addition, this method can give rise to large errors due to uncertainties in the values of the steady-state concentrations.
The pulse method is a steady-state variant of the step-down method. In this method, a small amount of tracer gas is injected for a short duration (one or two minutes) into the supply duct or at a point within the room. The time variation in tracer concentration at the exhaust and other locations of interest are then measured. Unlike the step-down technique, the measurement period begins when the tracer is injected and then waits until the concentration decays down to zero or background concentration. This method is relatively new compared to the other two methods, and applications of this approach in both laboratory and field settings are still limited.
The measured time variation of the tracer concentration gives the statistical density function of the age distribution. The local and room ages of air are calculated using Equations 18 and 19, respectively.
[[tau].sub.p] = [[infinity].[integral].0]t x [C.sub.p](t)dt/[[infinity].[integral].0][C.sub.p](t)dt (18)
<[tau]> = [[infinity].[integral].0][t.sup.2] x [C.sub.e](t)dt/[[infinity].[integral].0]t x [C.sub.e](t)dt (19)
As in the other two methods presented previously, the other indices can be derived from the calculated local and mean ages of air.
Particle Concentration Measurements
Although the tracer methods are widely used, tracer gases may not best represent the particle contaminants that are also present indoors, especially the large and dense particles in animal buildings. Thus, test dust with properties close to the particles encountered in the space is used. Very few researchers (Breum et al. 1990; Zhang et al. 2001; Maghirang et al. 2001; Fisk et al. 1991) have done measurements of ventilation effectiveness using dust as contaminants, and only the work of Breum et al (1990) was done in a real swine building.
The contaminant removal effectiveness defined in Equations 10 and 12 through 14 can be obtained using particles injected within the room by measuring these three dust concentrations: [C.sub.e] in the exhaust air, [C.sub.s] in the supply air, and [C.sub.p]'s at the locations of interest. Normally, to be able to get in Equation 10, spatial distribution of contaminant is measured (Zhang et al. 2001; Maghirang et al. 2001). The spatial distribution of particles also provides useful information on whether the ventilation in an area close to the source of contaminant is sufficient or not.
APPLICATION TO MECHANICALLY VENTILATED ANIMAL BUILDINGS
In any type of building, the choice of the ventilation effectiveness index that must be used to evaluate its ventilation performance depends on the objective of measurements. As in other occupied airspaces, animal buildings are ventilated to provide fresh air and remove excess heat, moisture, and contaminants. The ventilation rates of animal buildings are designed to remove excess heat during warm weather and remove excess moisture and carbon dioxide while conserving heat during cold weather. There is, however, a lack of quantitative understanding of how effective these ventilation systems are in achieving their objectives, primarily due to the lack of ventilation effectiveness measurement methods.
Evaluation of the effectiveness of the ventilation system in an animal building should include measurements of contaminant concentrations in as few measurement points as possible to make it practical for field applications. Hence, in the choice of parameters, the following issues must first be resolved: (1) number and definition of zones and (2) number and location of sampling points.
A zone is more often defined as a space where air is well mixed, i.e., the thermal properties and chemical properties of the air within the space are uniform. If this definition of zone is strictly applied to animal buildings, no zone can be identified, as air within the building is never uniformly mixed. Therefore, either a less stringent or a different definition of zone should be considered. When the concern in occupied spaces is the exposure of the occupants to contaminants, two kinds of zones exist in animal buildings: animal breathing zone (ABZ) and worker's breathing zone (WBZ). In swine buildings, ABZ is usually located from about 0.40 to 0.80 m from the floor or at pig level while WBZ is about 1.5 m from the floor or at the worker's level. However, if the purpose of measurement is to study the contaminant transport, more zones should be identified and monitored.
Selection of Sampling Locations and Number of Samples
There are always contaminant concentration gradients in a ventilated room, confirmed by recent publications (Wang et al. 2000, 2004; Wang 2000). Even if ABZ and WBZ are the only locations that matter for occupant exposure-related studies, the concentrations at these locations throughout the building still vary. The challenge in selecting the appropriate sampling locations is illustrated in the results of preliminary measurements of dust and ammonia spatial distribution conducted in a swine tunnel ventilated building (Figures 1 and 2). The building was 64.6 m (212.0 ft) long, 12.2 m (40.0 ft) wide, and 4.6 m (15.1 ft) high. The building was monitored in winter; thus, the inlets located opposite the exhaust fans were closed and air entered only through the attic. The exhaust fans were also off and contaminated air was exhausted only through the pit fans located underneath the floor. Concentrations of total suspended particulate (TSP) matter and N[H.sub.3] were measured at 50 and 30 sampling points (SPs), respectively.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Figures 3 to 6 present the trends in the measured TSP and N[H.sub.3] concentrations. The measured dust concentrations over the walk alley (Figure 3) in general are lower than those over the pens (Figure 4) since dust sources (feed, dried manure) originate from the pens. In addition, the dust concentrations in SPs 1 and 2 are also generally higher than the other SPs; these locations are farthest from the exhaust fans, but for these measurements, exhaust fans were not in operation so their effect on the spatial distribution is nil. In the case of N[H.sub.3], the concentrations at SPs 3 and 5 are, generally, higher than at SP1. It should be noted that the concentrations presented for both TSP and N[H.sub.3] were averages of measurements done over seven and five days, respectively; single-day measurements may have a different trend but may not be meaningful since they cannot be used to estimate the mean concentration; thus, the values will only be valid during that specific day of sampling.
If local ventilation effectiveness will be measured, it is then more appropriate to select the sampling locations indoors that will yield the maximum contaminant concentration or the longest age of air to ensure that the highest risk of exposure for occupants is taken into account. Thus, the preferred sampling locations are those away from the exhaust fans and inlet but close to the contaminant source. In the preliminary data presented, the points of interest for dust contaminant removal effectiveness measurement would be either SP1 or SP2 over pens for ABZ and over the walk alley for WBZ measurements. For N[H.sub.3] or other gases, the points of interest are either SP3 or SP5 over the alley for WBZ and over the pen for ABZ measurements. When the exhaust fans are on, the most appropriate location is probably SP3; this will be verified in future measurements.
[FIGURE 3 OMITTED]
When the contaminant removal effectiveness of the whole building is to be measured, a mean value of contaminant concentration is required and the number of SPs (N) necessary to be able to provide a valid estimate of the confidence interval around the mean can be estimated from the equation for standard error (S.E.) and accuracy (A):
N = (S.D./A x M)[.sup.2] (20)
where S.D. is the standard deviation, which can be obtained from previous measurements, A is usually 0.05,and M is the sample mean from previous measurements also. If Equation 20 is strictly applied, the resulting number of samples may be economically prohibitive. Therefore, the number of samples will depend on the budget and desired accuracy of measurements, i.e., using a higher value of A or lesser accuracy will result in a lesser number of samples.
Applicable Ventilation Effectiveness Index and Measurement Method
As in other real building measurements, assessment of the effectiveness of an animal building's ventilation system is difficult due in part to the following: (1) several gaseous contaminants are already present in significant quantities in and around the building (e.g., N[H.sub.3], C[O.sub.2], [H.sub.2]S, C[H.sub.4]); (2) the building is not airtight and air does not enter and leave the building only through the designed openings; (3) the area is usually large and uniform concentration of tracer is difficult if not impossible to achieve; (4) several inlets and exhaust fans are usually present in one building and sampling the air at all of these points can be impractical; and (5) ventilation rate varies throughout the day or the measurement period. Despite the aforementioned limiting factors, procedures can be employed to get around them to measure the ventilation system performance.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Contaminants, especially particles, are not usually spread in the same way as the supply air since they have different points of release. The air enters the building through the supply inlets, while contaminants are generated inside. Furthermore, contaminants may also have different densities than the air, which will allow it to generate its own motions; particles in particular possess inertia that dictates their movement with the air. Therefore, the age of air may not be sufficient as a parameter to quantify the ventilation system's performance; it will only suffice if it can be established that the movement of both the contaminants and the air are the same. Ventilation effectiveness indices that involve measurement of contaminant concentrations, such as those defined in Equations 10 through 14, therefore, are more appropriate indicators of system performance.
Measurement of the contaminant removal parameters mentioned above can be done either by using tracer gas (to measure the average contaminant removal effectiveness given in Equation 10) or by direct sampling of the contaminant concentrations. If a tracer method is employed, e.g., to calculate the average contaminant removal effectiveness defined in Equation 10, the pulse tracer method is the most applicable measurement technique since it does not require tracer gas concentration to be uniformly mixed nor a constant amount to be constantly injected--conditions required by step-up and step-down methods but are difficult, if not impossible, to achieve in animal buildings since these buildings are usually long and wide and the ventilation rate constantly varies even throughout the day. S[F.sub.6] is the ideal tracer gas for the tracer study since it is not typically found in the ambient air. However, the most limiting factor in its use is the cost of equipment to monitor the concentration in real time ($40,000 for a photoacoustic S[F.sub.6] analyzer, California Analytical Instrument). C[O.sub.2], on the other hand, can be easily measured with low-cost, accurate, and reliable photoacoustic infrared analyzers, but measurements can be contaminated since it is originally present in the ambient air (~350 ppm) and is generated by the animals. Background concentration of C[O.sub.2] both in the supply air and indoor air can be monitored, however, prior to the actual tracer measurements. N[H.sub.3] is more difficult to use as a tracer gas since its source in the building varies and is not stable, i.e., the concentration can be easily affected by the presence of urine on the floor and by even slight agitation of the manure in the pit.
The contaminant removal parameters can also be determined by measuring the concentration of the actual contaminants in the building instead of using any tracer. This may prove to be more practical, but the generation rate of contaminants inside may be unknown unless separate measurements are done. The contaminant generation rate is not really required unless mass balance of contaminants is desired; otherwise, measurement of contaminant concentrations is sufficient to be able to calculate the parameters in Equations 10 and 12 through 14. Contaminant removal effectiveness can be calculated for PM, C[O.sub.2], N[H.sub.3], and [H.sub.2]S.
This paper reviews the different ventilation effectiveness indices and the measurement methods that have been developed and applied both in controlled test spaces and in real buildings. When choosing an index that can be used to characterize the ventilation effectiveness of a real system, it should be measurable, applicable under different operating conditions, and practical in terms of the cost and effort that will be involved in the measurements. The following conclusions can be drawn from this work:
* The quantitative measures of the ventilation effectiveness of the system consist of parameters for air renewal and contaminant removal. A majority of the work done involved the use of the age of air concept in controlled airspaces to determine how other factors (obstructions, heat sources, types of ventilation system, etc.) affect the indices. A limited amount of research has been done on the applications of the age of air concept and the contaminant removal indices in real buildings.
* Tracer methods proved to be useful in the measurement of the ventilation effectiveness parameters. However, the application of these methods in real buildings is difficult due to factors such as large area of application, presence of unwanted openings in buildings, and presence of several supply inlets and exhaust locations.
* In real building applications, evaluation of local indices involves measurements from at least three up to more than ten sampling locations. In animal buildings, the number and location of sampling points can also be reduced to a minimum if prior data on the spatial distribution of gaseous and particulate contaminants are available.
* The contaminant removal indices are most applicable for animal building applications. The pulse tracer method and direct measurements of contaminants generated in the building can be applied to measure these indices.
This study was supported by ASHRAE (Grant No. TRP-1301). The technical advice provided by the PMS members, Rebecca T. Ellis, Henry Hays, Daniel Albert Ghidoni, James Reardon, and Jianshun Zhang, was greatly appreciated.
A = accuracy
<C> = mean contaminant concentration in a room
[C.sub.e] = contaminant concentration at exhaust air
[C.sub.m] = maximum contaminant concentration
[C.sub.mid] = contaminant or tracer concentration near the middle of the measurement range
[C.sub.p] = contaminant concentration at location p
[C.sub.s] = contaminant t concentration at supply air
[E.sub.v] = ventilation effectiveness
M = sample mean
N = total number of sampling locations
q = contaminant or tracer injection rate
Q = airflow rate
[Q.sub.p] = volume of air entering specific volume [V.sub.p]
R = recirculation factor
S = mixing factor
S.D. = standard deviation
t = time
V = volume of ventilated room
[V.sub.c] = volume of contaminant in the room
[V.sub.p] = volume of specific location p
<[[epsilon].sub.a]> = average air exchange effectiveness
[[epsilon].sub.c]> = average contaminant removal effectiveness
[[epsilon].sub.f] = ventilation effectiveness factor
[[epsilon].sub.p] = air exchange effectiveness at location p
[[epsilon].sub.m.sup.c] = absolute contaminant removal effectiveness
[[epsilon].sub.p.sup.c] = local contaminant removal effectiveness
[eta] = nominal air exchange rate
[[eta].sub.p] = local exchange rate
<[tau]> = room mean age of air
[[tau].sub.e] = age of air at exhaust location
[[tau].sub.n] = nominal time constant
[[tau].sub.n.sup.c] = nominal time constant for contaminant
[[tau].sub.p] = age of air at location p
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Sheryll B. Jerez
Student Member ASHRAE
Yuanhui Zhang, PhD, PE
Xinlei Wang, PhD
Sheryll Jerez is a PhD candidate, Yuanhui Zhang is a professor, and Xinlei Wang is an assistant professor in the Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL.
Table 1. Selected Field Measurement Techniques to Quantify Ventilation Effectiveness Index (*) Technique [eta], [[eta].sub.p] Step-up with S[F.sub.6] as tracer gas Step-down with C[H.sub.4], S[F.sub.6], CO, C[O.sub.2], [N.sub.2]O as tracer gases [tau], [[tau].sub.n], Step-up with [N.sub.2]O as tracer gas [[tau].sub.p], <[tau]>, <[[epsilon].sub.a]> Step-up with S[F.sub.6] as tracer gas Step-down with [N.sub.2]O as tracer gas <[[epsilon].sub.c]> Step-up with C[O.sub.2] as tracer gas (Equation 11) Step-down with S[F.sub.6] as tracer gas Index (*) Description of Building/Room and Measurements [eta], [[eta].sub.p] Office room, MV system, V = 128 [m.sup.3], 5 SPs at different heights and inlet, exhaust, return Tightly sealed room, fan-induced air-change rate, V = 66.5 [m.sup.3]; various SPs (unspecified [tau], [[tau].sub.n], Printing plant w/ displacement ventilation, V = [[tau].sub.p], 1100 [m.sup.3], five SPs at diff. heights and <[tau]>, inlet and exhaust <[[epsilon].sub.a]> Industrial hall, MV system, V = 38,000 [m.sup.3], three exhaust ducts were sampled Conference rooms and classrooms, MV system, V from 136 to 378 [m.sup.3], up to 5 SP at diff. heights and exhaust <[[epsilon].sub.c]> 49 residential houses, combination of MV and NV (Equation 11) systems, eleven SPs; measured tracer concentration variation with time Office/library bldg., MV system, V = 164,400 [m.sup.3]; nine SPs that include return ducts and outdoor location Index (*) Reference [eta], [[eta].sub.p] Offermann and Int-Hout (1989) Shaw (1984) [tau], [[tau].sub.n], Breum (1988) [[tau].sub.p], <[tau]>, <[[epsilon].sub.a]> Raatschen and Walker (1991) Olufsen (1991) Sateri et al. (1991) <[[epsilon].sub.c]> (Equation 11) Persily and Dols (1991) *At least one of the indices within each row was measured in each reference cited. Some authors measured other indices not cited in Table 1. SPs = sampling points; Bldg. = building; Rm. = room; MV = mechanical ventilation; NV= natural ventilation; A = floor area
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|Author:||Jerez, Sheryll B.; Zhang, Yuanhui; Wang, Xinlei|
|Date:||Jan 1, 2007|
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