Laboratories for the 21st Century:
 

Best Practices
 

Modeling exhaust dispersion for specifying 

acceptable exhaust/intake designs 

Introduction 

This guide provides general information on specifying acceptable exhaust and intake designs. It also offers 

various quantitative approaches (dispersion modeling) 

that can be used to determine expected concentration (or 

dilution) levels resulting from exhaust system emissions. 

In addition, the guide describes methodologies that can 

be employed to operate laboratory exhaust systems in a 

safe and energy efficient manner by using variable air 

volume (VAV) technology. The guide, one in a series on 

best practices for laboratories, was produced by 

Laboratories for the 21st Century (Labs21), a joint program of the U.S. Environmental Protection Agency (EPA) 

and the U.S. Department of Energy (DOE). Geared 

toward architects, engineers, and facility managers, the 

guides contain information about technologies and practices to use in designing, constructing, and operating 

safe, sustainable, high-performance laboratories. 

Studies show a direct relationship between indoor 

air quality and the health and productivity of building 

occupants1,2,3. Historically, the study and protection of 

indoor air quality focused on emission sources emanating from within the building. For example, to ensure that 

the worker is not exposed to toxic chemicals, “as manufactured” and “as installed” containment specifications 

are required for fume hoods. However, emissions from 

external sources, which may be re-ingested into the 

building through closed circuiting between the building’s 

exhaust stacks and air intakes, are an often overlooked 

Figure 1. Photographs of wind tunnel simulations showing fumes 

exiting fume hood exhaust stacks. In looking at the photograph, one 

should ask: Are the air intakes safer than a worker at the fume hood? 

Only a detailed dispersion modeling analysis will provide the answer. 

Photo from CPP, Inc., NREL/PIX 13813 

aspect of indoor air quality. 

United States 

Environmental 

Protection Agency 
2 LABS FOR THE 21ST CENTURY

If the exhaust sources and air intakes are not properly 

designed, higher concentrations of emitted chemicals may 

be present at the air intakes than at the front of the fume 

hood, where the chemical was initially released. 

Furthermore, if a toxin is spilled within the fume hood, the 

worker can take corrective action by closing the sash and 

leaving the immediate area; thus, reducing his or her 

exposure to the released chemical vapors. Conversely, the 

presence of the toxic fumes at the air intake, which can distribute the chemical vapors throughout the building, typically cannot be easily mitigated. The only option may be to 

evacuate the entire building, which results in an immediate loss of productivity and a long-term reduction in occupant satisfaction with the working conditions.. 

Dispersion modeling predicts the amount of fume 

reentry, or the concentration levels expected at critical 

receptor locations, with the goal of defining a “good” 

exhaust and intake design that limits concentrations below 

an established design criterion. Receptors considered in 

the assessment may include mechanically driven air 

intakes, naturally ventilated intakes like operable windows and entrances, leakage through porous walls, and 

outdoor areas with significant pedestrian traffic like plazas 

and major walkways. 

Petersen et al. gives a technical description of various 

aspects of exhaust and intake design4. Some of the challenges of specifying a good stack design mentioned in that 

article include the existing building environment, aesthetics, building design issues, chemical utilization, source 

types, and local meteorology and topography. For example, if a new laboratory building is being designed that is 

shorter than the neighboring buildings, it will be difficult 

to design a stack so that the exhaust does not affect those 

buildings. Figure 1 illustrates the effect of a taller downwind or upwind building. The figure shows how the 

plume hits the face of the taller building when it is downwind and how, when it is upwind, the wake cavity region 

of the taller building traps the exhaust from the shorter 

building. In either case, the plume has an impact on the 

face of the taller building. 

Typically, laboratory stack design must strike a balance between working within various constraints and 

obtaining adequate air quality at surrounding sensitive 

locations (such as air intakes, plazas, and operable windows). The lowest possible stack height is often desired for 

aesthetics, while exit momentum (exit velocity and volume flow rate) is limited by capital and energy costs, 

noise, and vibration. 

General Design Guidelines or 

Standards 

1.	 Maintain a minimum stack height of 10 ft (3 m) to protect 

rooftop workers.(5) 

2.	 Locate intakes away from 

sources of outdoor contamination such as fume hood 

exhaust, automobile traffic, 

kitchen exhaust, streets, cooling 

towers, emergency generators, Photo from CPP, Inc., NREL/PIX 13814 and plumbing vents.(6) 

3.	 Do not locate air intakes within the same architectural screen 

enclosure as contaminated exhaust outlets.(6) 

4.	 Avoid locating intakes near 

vehicle loading zones. Canopies 

over loading docks do not 

prevent hot vehicle exhaust 

from rising to intakes above the 

canopy.(6) 

5.	 Combine several exhaust Photo from CPP, Inc., NREL/PIX 13815 

streams internally to dilute 

intermittent bursts of contamination from a single source and 

to produce an exhaust with greater plume rise. Additional air 

volume may be added to the 

exhaust at the fan to achieve 

the same end.(6) 

6.	 Group separate stacks together 

(where separate exhaust 

systems are mandated) in a 

tight cluster to take advantage 

of the increased plume rise 

from the resulting combined 

vertical momentum.(6) Note 

that all the exhausts must operate continuously to take full 

advantage of the combined momentum. If not all of the exhausts 

are operating at the same 

time, however, such as in an 

n+1 redundant system, the 

tight placement of stacks 

may be detrimental to their 

performance. 

Photo from CPP, Inc., NREL/PIX 13816 

Photo from CPP, Inc., NREL/PIX 13817 
 

 

 

 

LABS FOR THE 21ST CENTURY 3 

7.	 Maintain an adequate exit velocity to avoid stack-tip downwash. 

The American National 

Standards Institute (ANSI)/ 

American Industrial Hygiene 

Association (AIHA) standard 

for laboratory ventilation, 

Z9.5-2011,(9) suggests that 

the minimum exit velocity 

from an exhaust stack should Photo from CPP, Inc., NREL/PIX 13818 

be at least 3,000 fpm. The 

American Society of Heating, Refrigerating, and Air-Conditioning 

Engineers (ASHRAE)(6) recommends a minimum exit velocity of 2,000 

to 3,000 fpm. 

8.	 Apply emission controls where viable. This may include installing 

restrictive flow orifices on compressed gas cylinders, scrubber 

systems for chemical specific releases, low-NOx (oxides of 

nitrogen) units for boilers and emergency generators, and oxidizing 

filters or catalytic converters 

for emergency generators. 

9.	 Avoid rain caps or other 

devices that limit plume rise 

on exhaust stacks. Although 

widely used, conical rain 

caps are not necessarily 

effective at preventing 

Photo from CPP, Inc., NREL/PIX 13819 rain from infiltrating the 

exhaust system because rain 

does not typically fall straight down. Alternate design options are 

presented in Chapter 44 of 

the ASHRAE Handbook–HVAC 

Applications.(6) 

10. Consider the effect of 

architectural screens. An 

ASHRAE-funded research 

study found that screens Photo from CPP, Inc., NREL/PIX 13820 

can significantly increase 

concentrations on the roof and reduce the effective stack height as 

a result11. A solid screen can decrease the effective stack height 

by as much as 80%. Alternatively, the effect of the screen can be 

minimized by installing a highly porous screen greater than 70% 

open). 

11. Avoid a direct line of 

sight between exhaust 

stacks and air intakes. An 

ASHRAE research project 

demonstrated a distinct 

reduction in air intake 

Photo from CPP, Inc., NREL/PIX 13821 concentrations from rooftop 

exhaust stacks when air intake 

louvers are “hidden” on sidewalls rather than placed on the roof12. 

Depending on the specific configuration, concentrations along the 

sidewall may be half to a full order of magnitude less than those 

present on the roof. 

Exhaust and Intake Design Issues 

Qualitative Information on Acceptable Exhaust 

Designs 

Several organizations have published standards for or 

recommendations on laboratory exhaust stack design, as 

summarized in the sidebar. 

Exhaust Design Criteria 

Laboratory design often considers fume hood stack 

emissions, but other pollutant sources may also be associated with the building. These could include emissions 

from emergency generators, kitchens, vivariums, loading 

docks, traffic, cooling towers, and boilers. Each source 

needs its own air quality design criteria. An air quality 

“acceptability question” can be written: 

Cmax < Chealth/odor ? 	 (1) 

In this equation, Cmax is the maximum concentration 

expected at a sensitive location (air intakes, operable 

windows, pedestrian areas), Chealth is the health limit concentration, and Codor is the odor threshold concentration 

of any emitted chemical. When a source has the potential 

to emit a large number of pollutants, a variety 

of mass emission rates, health limits, and odor thresholds 

need to be examined. It then becomes operationally 

simpler to recast the acceptability question by normalizing 

(dividing) the equation above by the mass emission rate of 

each constituent of the exhaust, m: 

(2) — — ? ( m

C )max < ( m

C )health/odor 

The left side of the equation, (C/m)max, is dependent 

only on external factors, such as stack design, receptor 

location, and atmospheric conditions. The right side of the 

equation is related to the emissions and is defined as the 

ratio of the health limit, or odor threshold, to the emission 

rate. Therefore, a highly toxic chemical with a low emission rate may be of less concern than a less toxic chemical 

emitted at a very high emission rate of each emitted 

chemical. Three types of information are needed to 

develop normalized health limits and odor thresholds: 

1. A list of the toxic or odorous substances that may be 

emitted; 

2. The health limits and odor thresholds for each emitted 

substance; and 

3. The maximum potential emission rate for each substance. 

Recommended health limits, Chealth, are based on the 

ANSI/AIHA standard Z9.5-20119, which specifies that air 

intake concentrations should be no greater than 20% of the 

acceptable indoor concentrations for routine emissions 
4 LABS FOR THE 21ST CENTURY

Table 1. Typical Design Criteria
 

Source Type Design Criteria Basis for Design Criteria 

Type (μg/m3) / (g/s) 

Laboratory fume hood Health 

Odor 

400* 

400* 

ASHRAE (2003) example criterion for a spill in a fume hood 

ASHRAE (2003) example criterion for a spill in a fume hood 

30,000-cfm vivarium Health 

Odor 

N/A 

706† 

Not applicable 

1:100 recommended dilution for a vivarium 

5,000-cfm kitchen hood exhaust Health 

Odor 

N/A 

1,412† 

Not applicable 

1:300 recommended dilution for kitchen exhaust 

400-hp diesel truck Health 

Odor 

156,522 

5,293† 

Health limit associated with NOx emissions 

1:2,000 odor dilution threshold for diesel exhaust 

250-kW diesel generator Health 

Odor 

2,367 

492† 

Health limit associated with NOx emissions 

1:2,000 odor dilution threshold for diesel exhaust 

2,000-kW diesel generator Health 

Odor 

296 

66† 

Health limit associated with NOx emissions 

1:2,000 odor dilution threshold for diesel exhaust 

100-hp boiler (4.5 MMBtu) — oil-fired Health 

Odor 

21,531 

23,576 

Health limit associated with NOx emissions 

Odor threshold associated with NO 

— gas-fired (20 ppm NOx) Health 

Odor 

132,278 

192,122 

Health limit associated with NOx emissions 

Odor threshold associated with NO 

500-hp boiler (21.0 MMBtu) — oil-fired Health 

Odor 

4,613 

5,052 

Health limit associated with NOx emissions 

Odor threshold associated with NO 

— gas-fired (20 ppm NOx) Health 

Odor 

28,345 

41,169 

Health limit associated with NOx emissions 

Odor threshold associated with NO 

*	 This criterion is more restrictive than the 0.05 ppm criterion stated in Z9.5-2011(9) for the maximum concentration present at the face of the fume hood, 

which corresponds to a normalized concentration of approximately 750 μg/m3 per gram per second. Less restrictive criteria may be applicable for exhausts 

with light chemical usage such as biological-safety cabinets. 

†	 Normalized concentration design criteria based on dilution standards depend on the volume flow rate through the exhaust stack. 

and 100% of acceptable indoor concentrations for 

accidental releases. Acceptable indoor concentrations 

are frequently taken to be the short-term exposure limits 

(STEL), which can be obtained from the American 

Conference of Governmental Industrial Hygienists 

(ACGIH), the Occupational Safety and Health 

Administration (OSHA), and the National Institute of 

Occupational Safety and Health (NIOSH), as listed in 

ACGIH13,14. ACGIH also furnishes odor thresholds, 

Codor15. 

For laboratories, emission rates are typically based on 

small-scale accidental releases, either from spilling a liquid 

or evacuating a lecture bottle of compressed gas. For other 

sources, such as emergency generators, boilers, and vehicles, chemical emissions rates are often available from the 

manufacturer. Table 1 outlines typical design criteria for 

various sources. 

Dispersion Modeling Methods 

Concentration predictions (C/m) at sensitive locations 

can be accomplished with varying degrees of accuracy 

using three different types of studies: 

1. A full-scale field program; 

2. A reduced scale wind-tunnel study; or 

3. A mathematical modeling study. 

A full-scale field program, although it may yield the 

most accurate predictions of exhaust behavior, may be 

expensive and time consuming. If the nature of the study 

is to estimate maximum concentrations for several stacks 

at several locations, many years of data collection may be 

required before the maximum concentrations associated 

with the worst-case meteorological conditions are measured. In addition, it is not possible to obtain data for 

future building configurations. 

Wind-tunnel modeling is often the preferred method 

for predicting maximum concentrations for stack designs 

and locations of interest, and is recommended because it 

gives the most accurate estimates of concentration levels 

in complex building environments16. A wind-tunnel modeling study is like a full-scale field study, except it is conducted before a project is built. Typically, a scale model of 

the building under evaluation, along with the surrounding buildings and terrain within a 1,000-foot radius, is 

placed in an atmospheric boundary layer wind tunnel. A 
LABS FOR THE 21ST CENTURY 5 

tracer gas is released from the exhaust sources of interest, 

and concentration levels of this gas are then measured at 

receptor locations of interest and converted to full-scale 

concentration values. Next, these values are compared 

against the appropriate design criteria to evaluate the 

acceptability of the exhaust design. ASHRAE10 and the 

EPA16 provide more information on scale-model simulation and testing methods. 

Wind-tunnel studies are highly technical, so care 

should be taken when selecting a dispersion modeling 

consultant. Factors such as past experience and staff 

technical qualifications are extremely important. 

Mathematical models can be divided into three categories: geometric, analytical, and computational fluid 

dynamic (CFD) models. The geometric method defines an 

appropriate stack height based on the string distance 

between the exhaust stack and a nearby receptor location10. This method is entirely inadequate for exhaust 

streams that contain toxic or odorous material because it 

does not yield estimated concentration values at air 

intakes or other sensitive locations. Hence, no information 

is provided for stack designs to avoid concentrations in 

excess of health or odor limits. 

Analytical models assume a simplified building configuration and yield concentration estimates based on 

assumed concentration distributions (i.e., Gaussian). 

These models do not consider site-specific geometries that 

may substantially alter plume behavior; thus, concentration predictions are not as reliable. When properly 

applied, the analytical equations provided in the ASHRAE 

Handbook on HVAC Applications tend to give conservative results for an isolated building or one that is the same 

height or taller than the surrounding buildings and has air 

intakes on the roof 10. As such, the analytical model can be 

useful for screening out sources that are unlikely to be 

problematic, thus reducing the scope of more sophisticated modeling. Neither the geometric nor the analytical 

models are appropriate for complex building shapes or in 

locations where taller buildings are nearby. 

The most common type of computational fluid 

dynamics models resolve fluid transport problems by 

solving a subset of traditional Navier-Stokes equations at 

finite grid locations. CFD models are used successfully to 

model internal flow paths within areas, such as vivariums 

and atriums, as well as in external aerodynamics for the 

aerospace industry. The aerospace CFD turbulence models, however, are ill suited for modeling the atmospheric 

turbulence in complex full-scale building environments 

because of the differing geometric scales. Background 

information on the use of CFD for dispersion modeling 

can be found in ASHRAE Handbook on HVAC 

Applications10. The chapter includes discussions on the 

various solutions methods that can be used. The general 

conclusion is that RANS (Reynolds-Averaged Navier 

Stokes), which is the most commonly used and most cost 

and time effective, can lead to “large, and sometimes very 

large, discrepancies in comparison with wind tunnel and 

full-scale measurements.” Furthermore, LES (Large Eddy 

Simulations) have a greater potential to provide accurate 

results. However, it requires significantly greater expertise 

and the computational time and cost can be prohibitive. 

Whether using RANS or LES, CFD models should be 

used with extreme caution when modeling exhaust 

plumes resulting from laboratory pollutant sources. If a 

CFD study is conducted for such an application, supporting full-scale or wind-tunnel validation studies should be 

carried out. 

Effective Stack Height and Induced-Air Fans 

Induced-air fan manufacturers often quote an “effective stack height” for exhaust fan systems. Many designers 

incorrectly interpret this value to be a physical stack 

height and compare it to the height requirement defined 

from a dispersion modeling study. The manufacturer’s 

specified effective stack height is actually a prediction of 

the exhaust plume centerline’s final height, based on a 

mathematical plume rise equation from an outdated version of the ASHRAE Applications Handbook18. This final 

height typically occurs far downwind of the exhaust stack 

(approximately 100 to 200 feet) as predicted using the 

updated plume rise equations presented in the most recent 

ASHRAE Applications Handbook10. The “new” equaDownwind Distance (m) 

0 10 20 30 40 50 60 70 

0 50 100 150 200 250 

35 

30 

25 

20 

15 

10 

5 

0 

9.2 

6.1 

3.1 

0.0 

Downwind Distance (ft) 

Plume Centerline Height (ft)

Plume Centerline Height (m)

Conventional 

Induced 

Difference 

Exhaust Parameters 

Conventional Induced-Air 

Stack height (ft, m) 

Stack diameter (in., m) 

Discharge flow rate (cfm, m/s) 

Exit velocity (fpm, m/s) 

Wind speed (mph, m/s) 

Fan power (bhp, bkW) 

10.2 

30.3 

15,000 

3,000 

20 

14.5 

3.10 

0.77 

7.08 

15.24 

8.94 

10.8 

10.2 

45.0 

32,466 

2,940 

20 

17.86 

3.10 

1.14 

15.32 

14.94 

8.94 

13.3 

Figure 2. Plume centerline height for conventional and induced-air 

exhaust systems 
6 LABS FOR THE 21ST CENTURY

tions, which are actually a more precise version of the 

original Briggs plume rise equations, predict the height 

of the plume centerline as a function of downwind 

distance19. 

A better method of comparing two different exhaust 

systems is to specify the effective increase in the plume 

height versus downwind distance. The increase may not 

be as great as one might expect as the following analysis in 

Figure 2 points out. 

Figure 2 shows the predicted plume centerline height 

versus downwind distance for an induced-air exhaust 

stack and a conventional exhaust fan system at a 20 mph 

stack height wind speed. The curves indicate that the difference in plume height between the two exhaust systems 

is only 1 to 2 feet at 20 feet downwind with a maximum 

difference of 6 feet after both plumes have reached their 

final rise. Therefore, using an induced-air fan may reduce 

the necessary stack height by only a few feet depending on 

the location of the nearby air intake locations. This analysis shows why the effective stack height specification is 

misleading. 

Plume Rise and Exit Velocity 

Adequate plume rise is important to ensure exhaust 

escapes the high turbulence and recirculation zones 

induced by a building’s roof. Plume rise increases with 

increased exit momentum and decreases with increased 

wind speed10. Reducing the diameter to increase exit 

velocity increases the exit momentum and thus the plume 

rise. There are limitations on how much the exit velocity 

can be increased before noise, vibration, and energy problems develop. Therefore, it is often preferable to increase 

the plume rise by augmenting the volume flow rate, possibly by bringing in additional air via a bypass damper at 

the base of the stack. Plume rise is adversely affected by 

atmospheric turbulence because the vertical momentum of 

the exhaust jet is more quickly diminished. In areas of high 

turbulence, the only method for obtaining an adequate 

plume centerline may be to increase the physical height of 

the stack. 

If the ratio of exit velocity to approach wind speed is 

too low, the plume can be pulled downward into the wake 

of the stack structure to create negative plume rise, a condition called stack-tip downwash. This downwash defeats 

some of the effect of a taller stack and can lead to high concentrations. A rule of thumb for avoiding stack-tip downwash is to make the exit velocity at least 1.5 times the wind 

speed at the top of the stack17. This stack top wind speed 

is commonly taken to be 1% wind speed, which can be 

obtained from ASHRAE for various worldwide metropolitan areas21. Note that ASHRAE-provided wind speed 

must be adjusted from the anemometer location to the 

stack top21. 

Variable volume exhaust systems should be designed 

to maintain adequate exit velocity during turndown periods. The exit velocity should be sufficient to avoid stack-tip 

downwash at all times. A high exit velocity can be maintained by having adjustable makeup air at the exhaust stack 

via a bypass damper or by employing several stacks that 

can be brought on/offline in stages as flow requirements 

change. Products are also available that can change the 

geometry of the stack exit in an attempt to maintain a high 

exit velocity with variable volume flow rates. Many of these 

devices do not properly condition the flow as it exits the 

stack, which reduces the vertical momentum and ultimately the plume rise out of the stack. As an alternative, smart 

control systems can be used to set minimum exit velocity 

requirements based on the current wind conditions measured at a nearby anemometer. 

Energy Issues 

Several factors affect exhaust system energy 

consumption, including the design and operation of 

the laboratory, specifically the exhaust volume flow rates 

and exit velocities and the chemical utilization within the 

fume hoods; the environment surrounding the laboratory, 

including the presence of nearby structures, air intakes, 

and other critical receptor locations; and the local 

meteorology. 

Chemical utilization is the basic criterion used to 

judge whether a specific exhaust/intake design is acceptable. An overly conservative judgment about the potential 

toxicity of an exhaust stream may result in a high-energyuse exhaust system as volume flow or exit velocity is 

increased unnecessarily. A more accurate assessment of 

the intended chemical use, with some consideration of the 

future program, results in an exhaust system that yields 

acceptable air quality while consuming a minimum 

amount of energy. 

Local wind speeds may be used to set exit velocity 

targets, as discussed previously. However, exhaust 

momentum is the true parameter governing exhaust 

plume rise and dispersion. In cases of high-volume flowrate exhausts (e.g., 30,000 cfm or greater), studies show 

that exit velocities as low as 1,000 fpm can produce acceptable plume rise and dispersion. Specific designs should be 

evaluated on a case-by-case basis, regardless of exhaust 

design parameters, to ensure that adequate air quality is 

maintained at all sensitive locations. 

Figure 3 was developed using the laboratory fume 

hood criteria and the analytical models for dispersion 

described previously. The figure shows that shorter 

exhaust stacks can be used to meet the design criteria as 

volume flow rate increases. The shorter stacks, however, 

are obtained at the cost of increased exhaust fan power. 
	 	 	 	 	

	 	 	 	 	 	 	 	

	 	 	 	 	 	 	 	 	 	

	 	 	 	 	 	 	 	 	 	 	

	 	 	 	 	 	 	 	 	 	

	 	 	 	 	

	 	 	 	 	 	 	 	

	 	 	 	 	 	 	 	 	 	

LABS FOR THE 21ST CENTURY 7 

Figure 3. Stack height above top of intake required to meet a 

specified design criterion for various exhaust volume flow rates at 

a range of downwind distances 

Fan Power Requirements — Design Q . 

(at 4 in. W.C.) 

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 

140 

120 

100 

80 

60 

40 

20 

0 

Power (bhp) 

Conventional Exhaust at 1,000 fpm 

Conventional Exhaust at 2,500 fpm 

Conventional Exhaust at 4,000 fpm 

Induced-Air Exhaust 1 

Induced-Air Exhaust 2 

Design Q (cfm) 

Figure 4. Required fan power versus design exhaust volume flow 

rate, Q 

Key Questions for
 

Exhaust/Intake Design
 

Questions for the project team 

•	Can an exhaust manifold be utilized?

•	Are induced-air systems required or will conventional, lower

energy systems suffice? 

•	Is the site sufficiently complex to warrant a detailed wind-tunnel

modeling evaluation? 

•	Do the laboratory exhausts have a high enough volume flow and

exit velocity to escape the building envelope? 

Questions to ask when selecting a dispersion modeling consultant 

•	Does the method you are using predict concentrations or dilution

at building air intakes? 

•	Is your technique validated or conservative?

•	Do you utilize chemical emission rates in the analysis?

•	Does your method account for all wind conditions expected at

the site? 

The figure also demonstrates the advantage of manifolding exhaust systems. For example, a single stack operating 

at 5,000 cfm should be approximately 22 feet tall to 

achieve the design criterion at a receptor 180 feet downwind. Conversely, five stacks operating at 1,000 cfm 

would need to be nearly 30 feet tall to provide the same air 

quality at the same receptor location. 

Figure 4 shows how fan power may increase with 

exhaust flow rate for various system designs. The figure 

illustrates the relationships between the design volume 

flow rate, Q, and the fan power requirements for two typical induced-air systems and for a conventional system at 

three different exit velocities. For the conventional exhaust 

systems, the figure shows the benefit of decreasing the exit 

velocity for a given design flow rate, always assuming 

that the specified system meets the design goals. 

Consider the following example to better understand 

data presented in Figure 4: A building exhaust system 

requires 30,000 cfm at a static pressure of 4-inch water column (W.C.) to adequately ventilate the building. An assessment of the exhaust plume shows that a 10-feet tall, 30,000 

cfm exhaust fan with a 2,500 fpm exit velocity would meet 

the design criterion established for the exhaust stack. Figure 

4 shows that a conventional exhaust system meeting these 

parameters requires fan power of approximately 27 brake 

horsepower (bhp). An equivalent induced-air system 

requires between 32 and 42 bhp to exhaust the same 30,000 

cfm from the building, an increase of 19% to 55%. 

This discussion illustrates the importance of using 

dispersion modeling to evaluate exhaust performance, 

taking fan energy costs into consideration, to ensure that 

acceptable air quality is achieved. 

Variable Air Volume Exhaust 

Designing a laboratory to utilize a VAV exhaust system allows the exhaust ventilation system to match, or 

nearly match, the supply ventilation airflow requirement 

of the building. This allows the designer to take full 

advantage of energy-saving opportunities associated with 

employing various strategies to minimize airflow requirements for the laboratory. However, just as arbitrarily 

reducing the supply airflow may adversely affect air quality within the laboratory environment, blindly converting 

an exhaust system to VAV without a clear understanding 

of how the system will perform can compromise air quality at nearby sensitive receptor locations (e.g., air intakes, 

operable windows, plazas, etc.). Therefore, before employing a VAV system, the potential range of operating conditions should be carefully evaluated through a detailed 

dispersion modeling study as described earlier in this 

guide. Since the nature of these assessments is to accurate­
8 LABS FOR THE 21ST CENTURY

ly determine the minimum volume flow requirements for 

the exhaust system, the preferred method is the use of 

physical modeling in a boundary-layer wind tunnel. 

Numerical methods can be used, but these will more often 

than not result in higher minimum volume flow rates 

when properly conducted and the resulting energy 

savings potential will be reduced. 

Three different strategies that can be used for operating VAV laboratory exhaust systems are described below. 

Strategy 1: Passive VAV 

In a passive VAV system, the exhaust flow is based on 

the greater of two values: the minimum air quality set 

point and the building’s ventilation demand. The minimum air quality set point is defined as the minimum 

flow/exit velocity/stack height needed to provide acceptable air quality at all sensitive receptor locations as 

defined in the dispersion modeling assessment. During 

the assessment, when a passive VAV system is to be 

employed, the stack design often focuses on the minimum 

potential volume flow rate for the laboratory building 

rather than the maximum value as evaluated for a constant volume exhaust system. In many cases, this minimum flow rate will be roughly half of the maximum value 

and is associated with nighttime turndown or minimum 

fume hood utilization. For a system to operate safely at 

50% of full load, taller stack heights and/or the optimum 

placement of air intakes to minimize re-entrainment of the 

exhaust at these reduced flow rates are often required. 

Typically, 5 feet or 10 feet increases in stack height have 

been effective. From a controls standpoint, this is likely the 

simplest system to employ, particularly for retrofit of existing laboratories. 

Strategy 2: Active VAV with Anemometer 

If the passive VAV system does not lower the air quality exhaust set point equal to, or lower than, the building 

ventilation demand, further optimization is available 

through knowledge of the current wind conditions at the 

stack through use of an onsite anemometer. Recall that the 

passive VAV set point assumed the worst-case wind condition — a relatively low-frequency event. 

In this strategy, a local anemometer is connected to the 

building automation system (BAS) and the minimum 

required exhaust flow rate is varied based on current wind 

conditions (direction and speed). When the wind conditions are anything but worst-case, the exhaust system may 

be turned down to more closely match the building 

demand. Essentially, the air quality minimum set point is 

specified for each wind direction/speed combination. This 

usually results in air quality set points well below building 

demand for many wind conditions, allowing the entire 

ventilation system to operate at optimum efficiency. 

This strategy requires physical exhaust dispersion 

modeling in a wind tunnel as most numerical models do 

not provide off-axis concentration predictions. Minimum 

air quality set points as a function of wind direction (WD) 

and wind speed (WS) require concentration predictions at 

all sensitive locations (receptors) for all wind directions, 

wind speeds, stack heights, and exhaust flow parameters. 

Typically, initial testing is conducted to identify an acceptable stack height. Subsequent testing is conducted for all 

wind directions and speeds using a fixed stack diameter to 

produce concentrations for each stack/receptor combination for all combinations of wind direction, wind speed, 

and volume flow rate (Figure 6). 

Figure 6. Active VAV with Anemometer Figure 5. Passive VAV 
LABS FOR THE 21ST CENTURY 9 

Table 2. Example of Minimum Fan Load Percentages versus Anemometer Reading (BAS Lookup Table) 

Wind Direction 

(Deg) 

Anemometer Wind Speed (m/s) 

Min Max <1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 

350 10 57 57 78 81 76 68 61 58 57 57 56 49 33 6 0 

10 30 57 41 62 75 81 82 79 75 71 66 62 60 58 57 57 

30 50 57 31 38 40 40 38 36 33 31 30 29 29 30 30 30 

50 70 57 29 27 24 21 18 15 12 10 7 6 4 3 2 2 

70 90 57 36 47 48 43 37 31 27 25 25 26 27 27 27 24 

90 110 57 39 50 48 42 34 29 26 25 26 27 26 25 22 22 

110 130 57 33 47 51 48 42 36 31 27 26 25 26 26 27 27 

130 150 57 22 22 21 19 17 15 13 11 11 10 10 11 11 12 

150 170 57 34 45 46 42 36 31 26 24 24 24 26 26 26 25 

170 190 57 35 46 46 41 34 29 25 24 24 25 26 27 26 23 

190 210 57 21 20 19 17 16 15 15 15 15 16 16 15 15 14 

210 230 57 5 2 0 0 0 0 0 0 0 0 0 0 0 0 

230 250 57 5 2 1 0 0 0 0 0 0 0 0 0 0 0 

250 270 57 10 8 7 6 6 6 6 6 6 7 6 5 1 0 

270 290 57 13 11 9 7 7 7 8 8 8 7 7 7 10 19 

290 310 57 13 12 10 7 8 7 7 8 8 8 8 7 6 7 

310 330 57 13 12 10 7 8 7 7 8 8 8 7 7 6 8 

330 350 57 32 40 39 32 36 29 28 29 30 31 31 29 27 26 

Similar data for all receptors is then compiled into 

either a single lookup table or a series of wind-directionspecific polynomial equations for the BAS. Table 2 

presents a lookup table of the air quality set point as a 

percentage of design flow. Note that the air quality set 

point for most directions is essentially 0 (no minimum set 

point so the exhaust flow can be set to match the building 

demand without the need for any by-pass air), although a 

few conditions require 80% of the design flow. 

Strategy 3 Active VAV with Chemical Monitor 

An alternative to monitoring the local wind conditions is to monitor the contents of the exhaust stream22. 

When the monitor does not detect any adverse chemicals 

in the exhaust stream, the exhaust system is allowed to 

operate at a reduced volume flow rate. While there may be 

an increase in the plume concentrations at the nearby air 

intakes, air quality will not degrade since the exhaust 

plume is essentially “clean.” 

The usual assumption is that a contaminant is present 

in the exhaust stream, and the exhaust design is specified 

to achieve acceptable air quality through either mathematical or wind tunnel exhaust dispersion analysis. If a monitoring system were used, the “normal” mode would be to 

establish a minimum air quality exhaust set point that 

allows higher plume impact. Plume impact would still be 

limited, just to a less conservative criterion than otherwise 

allowed. If contaminants were detected in the exhaust 

stream, the exhaust flow would increase to achieve a more 

stringent criterion. 

Figures 8 and 9 show 1,500 μg/m3 per grams per second (g/s) as an example of the “normal” allowable impact 

and 400 μg/m3 per g/s as the criterion when a contaminant is detected. To put the 1,500 μg/m3 per g/s and 400 

μg/m3 per g/s into perspective, considerer the “as manufactured” and “as installed” chemical hood containment 

requirements outlined in Z9.5-20119 (i.e., a concentration 

at a manikin outside the chemical hood of 0.05 ppm or less 

for “as manufactured” and 0.10 ppm or less for “as 

installed” with a 4 liters per minute (L/m) accidental 

release in the hood as measured using the ANSI/ASHRAE 

110-1995 test method). The “as manufactured” requirement is equivalent to a design criterion of 750 μg/m3 per 

g/s and the “as installed” requirement is equivalent to a 

design criterion of 1,500 μg/m3 per g/s. Hence, the criterion for a manikin (i.e., worker outside the chemical hood) 

is 1.9 to 3.8 times less restrictive than that for the air intake 

or other outdoor locations when applying the 400 μg/m3 

per g/s design criterion when chemicals are detected 

within the exhaust hood. Applying the 1,500 μg/m3 per 

g/s when no contaminant is detected in the exhaust 

streams means that the exhaust system is still providing 

an equivalent level of protection to the nearby air intake 

that the fume hood is providing to the inhabitants of the 

laboratory. 
10 LABS FOR THE 21ST CENTURY

Data collected at operating research laboratories with typical system will be able to operate at the lower air quality set point more than 99% of the time, resulting in signifi­ air quality monitors in the exhaust manifold indicate that 

cant energy savings. emission events that would trigger the higher volume 

flow rate typically occur no more than one hour per month 

(12 hours per year; 0.1% of the time)22. This means that a 

Figure 8. Higher intake concentrations are allowable when the 

exhaust stream is essentially “clean.” 

Figure 9. When chemical concentrations are detected in the exhaust stream, the exhaust 

volume flow rate is increased, reducing downwind intake concentrations. 
LABS FOR THE 21ST CENTURY 11 

Summary and Conclusions 

An accurate assessment of exhaust dispersion can be 

used to produce exhaust/intake designs optimized for 

energy consumption. No matter what type of exhaust system is used, the important design parameters are physical 

stack height, volume flow rate, exit velocity, expected pollutant emission rates, and concentration levels at sensitive 

locations. Whether conventional or induced-air exhaust 

systems are used, the overall performance should be evaluated using the appropriate criterion that ensures acceptable concentrations at sensitive locations. When 

employing a VAV heating, ventilation, and air conditioning (HVAC) supply system for the laboratory, the design 

team should strongly consider opportunities to include 

VAV laboratory exhaust systems as well to fully realize the 

energy savings potential of VAV. However, blindly applying VAV can be detrimental to the air quality at air intakes 

and other locations of concern if a dispersion modeling 

study is not conducted to define acceptable minimum volume flow rates. Any implementation of a VAV exhaust 

system should include a building automation system 

designed to handle the appropriate control logic. In addition, commissioning of the system should include the full 

range of operating conditions. 

References 

1. Fisk, W.J., “Review of Health and Productivity Gains 

from Better IEQ,” Proceedings of Healthy Buildings, Vol. 4, 

pp. 23–34, 2000. 

2. Yates, A., “Quantifying the Business Benefits of 

Sustainable Buildings (Draft),” Building Research 

Establishment, Ltd., Project Report 203995, 2001. 

3. Kats, G., “The Costs and Financial Benefits of Green 

Buildings,” California’s Sustainable Building Task Force, 

Capital E, 2003. 

4. Petersen, R.L., B.C. Cochran, and J.J. Carter, 

“Specifying Exhaust and Intake Systems,” ASHRAE 

Journal, August 2002. 

5. Bell, G.C., “Optimizing Laboratory Ventilation 

Rates,” Laboratories for the 21st Century, U.S. 

Environmental Protection Agency, Office of Administration 

and Resources Management, September, 2008. 

6. Weale, J., D. Sartor, L Eng, “Low-Pressure Drop 

HVAC Design for Laboratories,” Laboratories for the 21st 

Century, U.S. Environmental Protection Agency, Office of 

Administration and Resources Management, February, 

2005. 

7. Labs21 Benchmarking Tool, http://ww.labs21century.gov/toolkit/benchmarking.htm, Laboratories for the 

21st Century, U.S. Environmental Protection Agency, Office 

of Administration and Resources Management, online 

database. 

8. Kaushansky, J. and G. Maine, “Laboratories for the 

21st Century: Case Studies – Pharmacia Building Q, 

Skokie, Illinois,” Laboratories for the 21st Century, U.S. 

Environmental Protection Agency, Office of Administration 

and Resources Management, December, 2002. 

9. ANSI/AIHA, American National Standard for 

Laboratory Ventilation, Standard Z9.5-2011. 

10. ASHRAE, ASHRAE Handbook-HVAC 

Applications, Chapter 44, Building Intake and Exhaust 

Design, 2011. 

11. Petersen, R.L., J.J. Carter, and M.A. Ratcliff, 

“Influence of Architectural Screens on Roof-top 

Concentrations Due to Effluent from the Short Stacks,” 

ASHRAE Transactions, Vol. 105, Part 1, 1999. 

12. Petersen, R.L., J.J. Carter, and J.W. LeCompte, 

“Exhaust Contamination of Hidden vs. Visible Air 

Intakes,” ASHRAE Transactions, Vol. 110, Part 1, 2004. 

13. ACGIH, Guide to Occupational Exposure Values, 

2010. 

14. ACGIH, Threshold Limit Values for Chemical 

Substances and Physical Agents, 2010. 
 

12 LABS FOR THE 21ST CENTURY 

15. ACGIH, Odor Thresholds for Chemicals with 

Established Occupational Health Standards, 1989. 

16. EPA, Guideline for Use of Fluid Modeling of 

Atmospheric Dispersion, April 1981. 

17. ASHRAE, ASHRAE Handbook – Fundamentals, 

Chapter 24, Airflow Around Buildings, 2009. 

18. ASHRAE, ASHRAE Handbook-HVAC 

Applications, Chapter 44, Building Intake and Exhaust 

Design, 2003. 

19. Briggs, G.A., “Plume rise and buoyancy effects,” 

Atmospheric Science and Power Production, D. 

Randerson, ed. U.S. Department of Energy DOE/TIC27601, Washington, D.C., 1984. 

20. ASHRAE, ASHRAE Handbook-Fundamentals, 

Chapter 14, Climatic Design Information, 2009. 

21. ASHRAE, ASHRAE Handbook-Fundamentals, 

Chapter 24, Airflow Around Buildings, 2009. 

22. Cochran, B. and G. Sharp, “Combining Dynamic 

Air Change Rate Sensing with VAV Exhaust Fan Control 

to Minimize HVAC Energy Consumption in 

Laboratories,” Labs21 Conference Session VAV vs. CAV, 

San Jose, California, 2008. 

Laboratories for the 21st Century 

U.S. Environmental Protection Agency 

Office of Administration and Resources Management 

www.labs21century.gov 

In partnership with the 

U.S. Department of Energy 

Energy Efficiency and Renewable Energy 

www.eere.energy.gov 

Prepared at the 

National Renewable Energy Laboratory 

A DOE national laboratory 

Acknowledgments 

The following individuals contributed to the preparation

and publication of this guide to Modeling Exhaust Dispersion

for Specifying Acceptable Exhaust/Intake Designs: 

Authors: 

Ronald L. Petersen, Ph.D., CPP, Inc.
 

John J. Carter, M.S., CPP, Inc.
 

Brad C. Cochran, M.S., CPP, Inc.
 

Reviewers: 

Michael A. Ratcliff, Ph.D., Rowan Williams Davies and 

Irwin, Inc. (RWDI)
 

Thomas A. Scott, Ph.D., CPP, Inc.
 

Robert N. Meroney, Ph.D., Professor Emeritus, 

Colorado State University
 

Scott Reynolds, P.E., Computer Aided Engineering 

Solutions (CAES)
 

Otto Van Geet, P.E., NREL
 

For More Information 

On Modeling Exhaust Dispersion for Specifying 

Acceptable Exhaust/Intake Designs 

Brad Cochran, M.S., CPP, Inc.970-221-3371 

bcochran@cppwind.com 

On Laboratories for the 21st Century: 

Will Lintner, P.E. 

U.S. Department of Energy 

Federal Energy Management Program 

202-586-3120 

william.lintner@ee.doe.gov 

Best Practices Guides on the Web: 

www.labs21century.gov/toolkit/bp_guide.htm 

Laboratories for the 21st Century: 

U.S. Environmental Protection Agency Office of 

Administration and Resources Management. 

www.labs21century.gov 

In partnership with the U.S. Department of Energy Office 

of Energy Efficiency and Renewable Energy. 

www.eere.energy.gov 

DOE/GO-102011-3331 

September 2011 

Printed with a renewable-source ink on paper containing at least 

50% wastepaper, including 10% post consumer waste.