United States Office of Air Quality EPA-450/4-80-023R
Environmental Protection Planning and Standards June 1985
Agency Research Triangle Park NC 27711
__
f/EFA Guideline for
Determination of
Good Engineering
Practice Stack
Height (Technical
Support Document
For the Stack
Height Regulations)
(Revised)
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EPA-450/4-80-023R
Guideline for Determination of Good
Engineering Practice Stack Height
(Technical Support Document for the
Stack Height Regulations)
(Revised)
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Roar
Chicago, !L 60604-3590
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
June 1985
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DISCLAIMER
This report has been reviewed by The Office of Air Quality Planning and Standards, U.S. Environmental
Protection Agency, and has been approved for publication. Mention of trade names or commercial products
is not intended to constitute endorsement or recommendation for use.
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Preface
Section 123 of the Clean Air Act, as amended, requires EPA to
promulgate regulations to assure that the degree of emission limitation
required for the control of any air pollutant under an applicable State
implementation Plan (SIP) is not affected by that portion of any stack
height which exceeds good engineering practice (GEP) or by any other
dispersion technique. The stack height regulations are somewhat complex.
They define a number of statutory terms, such as "nearby" and "excessive
concentrations," provide methods for determining GEP height and specify
when each may be used, and implement a statutory bar on credit for use of
"dispersion techniques" other than stack height. The fundamental principles
used in these demonstrations are well-established, but where decisions must
be made concerning a particular study, the fundamental principles frequently
do not provide specific guidance. There is a need for additional basic and
systematic modeling studies that can be used to provide more specific
guidance. This guideline will be periodically revised as experience is
gained, new techniques are developed, and old ones refined.
This document is a revision of the "Guideline for Determination of
Good Engineering Practice Stack Height (Technical Support Document for the
Stack Height Regulations)," EPA-450/4-80-023, July 1981. The text contains
basically the same structure as the original guide, but includes changes
and additions throughout. A demonstration refers to fluid modeling and
wind tunnel simulation studies and the terms are used interchangeably in
this document.
111
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TABLE OF CONTENTS
Page
PREFACE in
1.0 OVERVIEW AND RECOMMENDATIONS 1
1.1 Overview 1
1.2 Recommendations 5
2.0 TECHNICAL BASIS FOR GEP STACK HEIGHT 9
2.1 Description of Aerodynamic Effects 9
2.2 Building Effects 11
2.3 Quantitative Rationale for GEP Equation 19
2.4 Terrain Influences 28
2.5 Minimum Stack Height 31
2.6 Porous, Rounded, or Sloping Structures 32
3.0 DETERMINATION OF GEP STACK HEIGHT 35
3.1 Initial Assumptions 35
3.2 Simple Structures 36
3.2.1 Low Structures 38
3.2.2 Tall Structures 38
3.3 Complex Structures 41
3.3.1 Tiered Structures 41
3.3.2 Groups of Structures 43
3.4 Framework for Demonstrating GEP Stack Height 45
3.5 Determining GEP Stack Height 48
3.6 Modeling Terrain Effects 52
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4.0 AIR QUALITY ESTIMATES 57
4.1 Determining Emission Limits 57
4.2 Treatment of Terrain 60
4.3 Multiple Source Impacts 60
4.4 Special Situations 61
REFERENCES 63
Appendix A. Annotated Bibliography A-l
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1.0 OVERVIEW AND RECOMMENDATIONS
1.1 Overview
As required by Section 123 of the Clean Air Act, the Administrator
has promulgated regulations (40 CFR 51) to assure that the degree of emission
limitation required for the control of any air pollutant under an applicable
State implementation Plan (SIP) is not affected by (1) that portion of any
stack height which exceeds good engineering practice (GEP) or by (2) any
other dispersion technique. Section 123 defines GEP, with respect to stack
heights, as "the height necessary to insure that emissions from the stack do
not result in excessive concentrations of any air pollutant in the immediate
vicinity of the source as a result of atmospheric downwash, eddies or wakes
which may be created by the source itself, nearby structures or nearby terrain
obstacles."
Section 123 further provides that GEP stack height shall not exceed
two and one-half times the height of the source unless a demonstration is
performed justifying a higher stack. In addition, Section 123 provides
that the Administrator regulates only stack height credit, rather than actual
stack height. The statute delegates to the Administrator the responsibility
for defining key phrases: "excessive concentrations," "nearby," with respect
to both structures and terrain obstacles, "other dispersion techniques," and
what constitutes an adequate demonstration justifying stack height credits
in excess of two and one-half times the height of a source.
According to 40 CFR 51.1(ii), "Good Engineering Practice (GEP) Stack
Height" means the greater of:
"1. 65 meters, measured from the ground-level elevation at the
base of the stack;
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2. (a) for stacks in existence on January 12, 1979, and for which
the owner or operator had obtained all applicable permits or
approvals required under 40 CFR Parts 51 and 52,
Hg = 2.5H
provided the owner or operator produces evidence that this
equation was actually relied on in designing the stack or
establishing an emission limitation to ensure protection
against downwash;
(b) for all other stacks,
Hg = H + 1.5L
where: Hg = good engineering practice stack height, measured from
the ground-level elevation at the base of the stack,
H = height of nearby structure(s) measured from the ground-
level elevation at the base of the stack,
L = lesser dimension, height or projected width, of nearby
structure(s),
provided that the EPA, State or local control agency may
require the use of a field study or fluid model to verify
GEP stack height for the source; or
3. The height demonstrated by a fluid model or a field study
approved by the EPA, State or local control agency, which
sures that the emissions from a stack do not result in excess-
ive concentrations of any air pollutant as a result of atmos-
pheric downwash, wakes, or eddy effects created by the source
itself, nearby structures or nearby terrain features."
The term "excessive concentration" is defined in 40 CFR 51.1(kk) for
the purpose of determining good engineering practice stack height and means
"(i) for sources seeking credit for stack height exceeding that
established under §51.l(ii)(2), a maximum ground-level concentra-
tion due to emissions from a stack due in whole or part to down-
wash, wakes or eddy effects produced by nearby structures or
nearby terrain features which individually is at least 40 percent
in excess of the maximum concentration experienced in the absence
of such downwash, wakes, or eddy effects and which contributes to
a total concentration due to emissions from all sources that is
greater than an ambient air quality standard. For sources subject
to the prevention of significant deterioration program (40 CFR
51.24 and 52.21), an excessive concentration alternatively means a
maximum ground-level concentration due to emissions from a stack
due in whole or part to downwash, wakes, or eddy effects produced by
nearby structures or nearby terrain features which individually is
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at.least 40 percent in excess of the maximum concentration
experienced in the absence of such downwash, wakes, or eddy
effects and greater than a prevention of significant deterio-
ration increment. The allowable emission rate to be used in
making demonstrations under this part shall be prescribed by
the new source performance standard that is applicable to the
source category unless the owner or operator demonstrates this
emission rate to be infeasible. Where such demonstrations are
approved by the authority administering the State implementation
plan, an alternative emission rate shall be established in con-
sultation with the source owner or operator;
(ii) for sources seeking credit after October 1, 1983, for
increases in existing stack heights up to heights established
under §51.l(ii)(2), either (A) a maximum ground-level concentra-
tion due in whole or part to downwash, wakes, or eddy effects
as provided in subparagraph (i), except that the emission rate
specified by any applicable State implementation plan (or, in
the absence of such a limit, the actual emission rate) shall
be used, or (B) the actual presence of a local nuisance caused
by the existing stack, as determined by the authority admin-
istering the State implementation plan; and
(iii) for sources seeking credit after January 12, 1979, for
a stack height determined under §51.1(ii)(2) where the authority
administering the State implementation plan requires the use of
a field study or fluid model to verity GEP stack height, for
sources seeking stack height credit after November 9, 1984, based
on the aerodynamic influence of cooling towers, and for sources
seeking stack height credit after December 31, 1970, based on the
aerodynamic influence of structures not adequately represented
by the equations in §51.l(ii)(2), a maximum ground-level concen-
tration due in whole or part to downwash, wakes or eddy effects
that is at least 40 percent in excess of the maximum concentration
experienced in the absence of such down-wash, wakes or eddy effects.
According to 40 CFR 51.1(jj), the term "nearby" as used in 51.1(ii)
is defined for a specific structure or terrain feature and
"(1) for purposes of applying the formulae provided in
§51.1(ii) (2) means that distance up to five times the
lesser of the height or the width dimension of a structure,
but not greater than 0.8 km (0.5 mile), and
(2) for conducting demonstrations under §51.1(ii)(3) means
not greater than 0.8 km (0.5 mile), except that the portion
of a terrain feature may be considered to be nearby which
falls within a distance of up to 10 times the maximum height
of the feature, not to exceed 3.2 km (2 miles) if such
feature achieves a height 0.8 km (0.5 mile) from the stack
that is greater than or equal to 40 percent of the GEP stack
height determined by the formulae provided in §51.1(ii)(2)(b)
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of this part or 26 meters, whichever is greater, as measured
from the ground-level elevation at the base of the stack."
The term "dispersion technique" as defined in 40 CFR 51.1(hh) means
"(1) any technique which attempts to affect the concentration
of a pollutant in the ambient air by.
A. using that portion of a stack which exceeds good engineering
practice stack height;
B. varying the rate of emission of a pollutant according to atmos-
pheric conditions or ambient concentrations of that pollutant; or
C. increasing final exhaust gas plume rise by manipulating
source process parameters, exhaust gas parameters, stack para-
meters, or combining exhaust gases from several existing stacks
into one stack; or other selective handling of exhaust gas
streams so as to increase the exhaust gas plume rise.
(2) The preceding sentence does not include:
A. the reheating of a gas stream, following use of a pollution
control system, for the purpose of returning the gas to the tem-
perature at which it was originally discharged from the facility
generating the gas stream;
B. the merging of exhaust gas streams where:
(i) the source owner or operator demonstrates that the
facility was originally designed and constructed with such
merged gas streams;
(ii) after [date of publication], such merging is part of a
change in operation at the facility that includes the instal-
lation of pollution controls and is accompanied by a net
reduction in the allowable emissions of a pollutant. This
exclusion from the definition of "dispersion techniques"
shall apply only to the emission limitation for the pollutant
affected by such change in operation; or
(iii) before [date of publication], such merging was part of
a change in operation at the facility that included the instal-
lation of emissions control equipment or was carried out for
sound economic or engineering reasons. Where there was an
increase in the emission limitation or, in the event that no
emission limitation was in existence prior to the merging, an
increase in the quantity of pollutants actually emitted prior
to the merging; the reviewing agency shall presume that, merging
was significantly motivated by an intent to gain emissions
credit for greater dispersion. Absent a demonstration by the
source owner or operator that merging was not significantly
motivated by such intent, the reviewing agency shall deny
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credit for the effects of such merging in calculating the
allowable emissions for the source;
C. smoke management in agricultural or silvicultural prescribed
burning programs;
D. episodic restrictions on residential woodburning and open
burning; or
E. techniques under 51.l(hh)(1)(C) which increase final exhaust
gas plume rise where the resulting allowable emissions of sulfur
dioxide from the facility do not exceed 5,000 tons per year."
This guideline provides technical support for the definitions and
specifications of GEP stack height as found in the stack height regulation.
The technical basis for the GEP definition is provided in Sections 2 and 3.
The technical basis for Part 1 of the "excessive concentration" definition,
which is the engineering requirement, is given in Sections 2 and 3 of this
guideline. The basis for Part 2 of the definition, which is the "ambient
requirement" is given in the preamble to the regulation. The technical
information on which the definition of "nearby" is based is summarized in
Sections 2 and 3 of this guideline, while the method for treatment of
entire terrain features in a demonstration is also given in Section 3. All
emission limitations must ensure that ambient air quality standards and PSD
increments will not be exceeded. Guidance for making air quality estimates
within the GEP framework is contained in Section 4.
An annotated bibliography is included that provides a representative
selection of statements found in the scientific literature concerning the stack
height for which adverse aerodynamic effects may be a problem.
1.2 Recommendations
The scientific literature in general indicates that a case
specific review is integral to assuring the prevention of adverse aerodynamic
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effects near a given source. However, the literature also identifies genera-
lized formulations which are designed to establish the minimum stack height
to prevent this phenomenon. The following recommendations are based on
these generalized findings:
(1) It appears from a scientific and technical standpoint that
the most appropriate procedure to follow in determining GEP stack height is
to conduct fluid modeling or a field study. A framework for a fluid modeling
demonstration of GEP stack height is presented in Section 3.4.
(2) The scientific literature in general indicates that a case
specific review is integral to assuring the prevention of adverse aerodynamic
effects near a given source. However, the literature also identifies general-
ized formulations which are designed to establish the minimum stack height to
prevent this phenomenon. The guidance provided in Equation 1 is based on
these generalized findings
Hg = H + 1.5L (Equation 1)
where: Hg = good engineering practice stack height, measured from
the ground-level elevation at the base of the stack,
H = height of nearby structure(s) measured from the ground-
level elevation at the base of the stack,
L = lesser dimension, height or projected width, of nearby
structure(s).
In Equation 1, both the height and width of the structure
are determined from the frontal area of the structure, projected onto a
plane perpendicular to the direction of the wind. If the structure is
asymmetrical, the GEP stack height should be based on the plane projection
lying upwind from the source (stack) which results in the greatest justifiable
height. The plane projection may have a multitude of heights or widths, par-
ticularly for a multilayered structure. Each combination of the height, H,
and lesser dimension (height or width), L, should be evaluated for each
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segment of the structure to determine which one results in the greatest GEP
stack height as defined by Equation 1. Adjacent and nearby structures
whose plane projections lying upwind from the source are overlaying should
be considered as one structure. Likewise, structures aside of each other
should be considered as one structure if their distance of separation is
less than their smallest dimension (height or width).
The downwind area in which a nearby structure is presumed to
have a significant influence on a source should be limited to five times the
height or width of the structure, whichever is less. Thus, application of
Equation 1 should be limited to emission points within 5L of the building
structure. The area of influence becomes diminishingly small as the height
to width ratio of a structure increases. Thus structures such as stacks and
radio or TV transmission towers should not be considered in GEP stack height
determinations. Assumptions associated with the determination of GEP stack
height and appropriate examples are presented in Section 3. Complex struc-
tures with a multitude of heights and widths are discussed in Section 3.3.
Where concern exists for possible significant effects on sources from a
distance greater than 5L but less than 0.8 km (0.5 mi), a wind tunnel or
field study should be conducted unless an analogy to a similar study is
available.
(3) The GEP stack height required to minimize the adverse effects
of elevated terrain should be determined on a case-by-case basis. A demon-
stration of the application of the fluid modeling approach to the determi-
nation of GEP stack height for a plant in complex terrain is shown in "Fluid
Modeling Demonstration of Good Engineering Stack Height in Complex Terrain,"
(Snyder and Lawson, 1985). Field studies designed to evaluate the specific
situations under the variety of adverse meteorological conditions are the
7
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best source of information. Where field studies are not possible, comparable
fluid model studies are acceptable. A framework for demonstrating GEP stack
height by fluid modeling is presented in Section 3.4.
(4) To avoid natural atmospheric effects which cause excessive
concentrations around very low level sources, a stack height of 65 meters is
defined as good engineering practice, without demonstration of necessity for
any source (see Section 2.5).
(5) There are certain types of structures that are more aerody-
namical^ smooth or more streamlined than block-shaped structures. These
include porous structures such as unenclosed metal supporting framework, and
rounded and sloping structures such as hyperbolic cooling towers. GEP stack
height calculated from Equation 1 is not applicable to these types of struc-
tures and must be determined on the basis of a field study or fluid modeling
demonstration.
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2.0 TECHNICAL BASIS FOR GEP STACK HEIGHT
2.1 Description of Aerodynamic Effects
Atmospheric flow is disrupted by aerodynamic forces in the immediate
vicinity of structures or terrain obstacles. The aerodynamic forces evolve
from interacting frictional forces and pressure gradients induced by the
local obstruction. The surface friction and pressure gradients combine to
retard the atmospheric surface layer flow enough to produce regions where the
flow is locally distorted, causing an area of stagnation (cavity) to develop.
The flow within the stagnant region is highly turbulent and conceptually
perceived as circulating eddies. The outer boundary of the eddy or cavity
region extends from the point of separation to reattachment downwind, as
shown in Figure 1. The wake is defined as the entire region of the flow
field that is disturbed by the obstacle. The upper boundary of the wake is
called the "envelope", as shown in Figure 1. The reattachment point is taken
as the ground-level position where the flow is no longer drawn back towards
the backside of the building. Downwind, beyond the reattachment, the flow
readjusts itself to a boundary layer appropriate to local surface roughness.
For sharp-edged obstacles the flow distinctly separates at the leading edges.
For rounded obstacles the point of separation can vary greatly. The disrupted
flow near either building structures or terrain obstacles can both enhance
the vertical dispersion of emissions from the source and reduce the effective
height of emissions from the source. For elevated sources, these aerodynamic
effects tend to cause an increase in the maximum ground-level concentrations.
Additional discussions of the aerodynamically induced disruption around
obstacles can be found, for example, in Hunt, £t al_. (1978); Cermak (1976);
Halitsky (1969); and Batchelor (1967). The complex pattern of flow around
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a rectangular block is depicted by Figure 2. A review of the literature
clearly indicates that the aerodynamic influences and the extent of the wake
are highly dependent on the particular shape and design of the obstruction.
The extent of the wake also depends on the characteristics of the approaching
atmospheric flow. Presently, theoretical and quantitative understanding of
the extent of obstacle influences are limited. Further examinations of the
extent of influence for a wide range of structures and terrain obstacles are
needed.
2.2 Building Effects
The scientific literature in general indicates that a case specific
review is integral to assuring the prevention of adverse aerodynamic effects
in the immediate vicinity of a given source. However, the literature also
identifies generalized formulations which are designed to establish minimum
stack heights to prevent this phenomenon. One such formulation is the "2.5
times rule," which specifies that stacks designed to discharge their effluent
at least 2.5 times the height of the highest nearby structures would escape
building influences. This rule arose during the early part of this century
as a practical formula. Hawkins and Nonhebel (1955) reported that the rule
had been successfully used by the British electricity generating industry
during the previous 20 years. A British government report (Beaver, 1954),
which summarizes the informed opinion at that time, presents the 2.5 times
rule as successfully used in practice. According to Sutton (1960) the rule
was probably originally deduced by Sir David Brunt from W. R. Morgan's study
of the height of disturbances over a ridge in connection with an investigation
into the disaster of an airship. Marks' Standard Handbook for Mechanical
Engineers (Baumeister, £t _aj[. 1978) states that "a ratio of stack height to
11
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building height of 2-1/2 to 1 or more is commonly used to avoid entrapment
of the plume in the vortex of adjacent buildings and the associated high
ratios of ground-level concentration."
No matter what its origins, the rule can be generally supported by
scientific literature. In some instances where application of the 2.5 times
rule was considered impractical, individual evaluations of the specific case
have been made. Most of these studies were conducted as scale model studies
in a wind tunnel where the design parameters could be easily adjusted to
determine the necessary stack placement and height. Unfortunately, field
studies have been limited to a few case-specific problems. The following are
among the most significant findings from studies of building wake effects.
Evans (1957) estimated the smoke visualized shape and size of the
cavity region for nearly two hundred variations of basic building shapes in
a wind tunnel study. He found that regardless of the height of the building
the pattern of the air going over the top of the buildings appeared the same.
Examination of the published sketches shows the cavity to extend from the
ground vertically to about 1.5 times the height of the building. In case of
pitched roofs the height scale should be taken as the height of its apex.
When the width of the building was increased from 1 to 8 times its height,
the downwind extent of the cavity increased from 2 to 5 times the building
height. As the width of the building was further increased to 28 times its
height, the downwind extent was found to increase at a somewhat smaller rate
to 9 times the building height.
Wind tunnel tests defining the influence of block-type structures
on smoke emissions from roof-mounted chimneys were conducted by Lord, ejt al.
(1964). An examination of their results shows that the height of the cavity
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is nearly equal to the building height plus one-half times the building
height or width, whichever is less. However, the maximum vertical extent
of the disturbed flow above the cavity was found to be equal to the building
height plus up to 3 times the building height or width, whichever is less.
Halitsky (1968) reviewed several wind tunnel studies of flow near
structures. One of the studies (Halitsky, et_ jil_. 1963) demonstrated that the
wake in the lee of a rounded building is not as great as that found in a
study of sharp-edged buildings (Halitsky, 1963). Meroney and Yang (1971)
found that for a stack less than 1.5 times the building height the plume was
downwashed into the lee side of the building. When the stack height was
increased to 2.0 times the height of the building the influences were greatly
diminished.
A formulation that prescribes the stack height sufficient to avoid
significant building influences has been presented by Lucas (1972) and Briggs
(1973). They state that a stack should equal the height of the building plus
1.5 times the height or width, whichever is less. Snyder and Lawson (1976)
in a series of wind tunnel tests showed that this formulation is adequate for
a stack close to a building whose height is three times its width, and for a
building whose width is twice its height.
Peterka and Cermak (1975) present an evaluation of mean velocity
and turbulence characteristics in the wake of buildings based on wind tunnel
studies. They found for wider buildings, that the mean velocity defect and
turbulence excess did not begin until 3 to 5 building heights downstream
while the decay began almost immediately downstream of tall, narrow buildings.
Differences were found in the flow behind a rectangular shaped building
(height to width ratio of 2.44) when oriented perpendicular to the approach
flow compared to that when oriented at 47 degrees to the approach flow.
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The mean velocity defect decayed fairly rapidly over the first 20 building
heights in both cases. However, for the 47 degree case, an excess of 3 to
4 percent of the freestream velocity remained constant to 80 building heights
downwind. No evidence of a turbulence excess or defect was found at such a
great distance. The existence of a mean velocity defect to 80 building
heights is believed evidence of a vortex pair with axes parallel to the flow
direction which are a remnant of the corner vortices formed at the leading
roof corner. The vertical profiles of mean velocity defect and turbulence
intensity excess which are reported for the perpendicular case, show values
less than 5 percent at a-11 heights greater than 2.5 times the building.
Hansen and Cermak (1975) and Woo, Peterka, and Cermak (1976) present
additional wind tunnel measurements of mean velocity and turbulence character-
istics in the wakes of structures. The results are similar to those discussed
above. The downstream extent of the recirculation (i.e., cavity) region
determined from mean velocity and turbulence measurements is identified in
Woo, Peterka, and Cermak (1976) for a range of model sizes and test conditions.
In most cases, the downstream extent was found equal to 3 to 5 building heights
except for tall, narrow structures whose downstream extent was much less.
Robins and Castro (1977) examined the wind tunnel flow field in the
vicinity of a model cube. The flow around the cubes was found to be highly
dependent on orientation. Strong vortices generated by the top leading edges
were found for an approach flow at 45 degrees to the building edge. They
found the cavity region to extend to 1.5 times the building height for an
approach flow perpendicular to the building edge and to 2 times the building
height downwind for an approach flow at 45 degrees to the building edge. The
downwind zone, where the flow was significantly affected, extended, however,
15
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to 5 building heights for both cases. The effluent from a stack 2.5 times
the building height having a stack exit velocity 3 times the wind speed, was
found to be insignificantly affected by the building for an approach flow
perpendicular to the building edge and to result in a 20 percent increase in
maximum ground-level concentrations for an approach flow at 45 degrees to the
building edge.
Huber and Snyder (1976) evaluated a series of wind tunnel studies
designed to examine building wake effects near a building whose width was
twice its height. The size of the cavity was found to be approximately 1.5
building heights above ground level in the vertical and 2.5 building heights
downwind. In evaluating the building influence on dispersion, aerodynamically
generated turbulent flow was found to rapidly decay in the region 3 to 10
building heights downwind. The most significant disturbed flow occurred with-
in 5 buildings heights downwind. A significant building influence on ground-
level concentrations was found for cases with the stack less than 2 times the
building height. The building influences were found to be significantly
reduced for a stack 2.5 times the height of the building.
In the vicinity of building structures where mechanically generated
turbulence dominates the undisturbed atmospheric flow, wind tunnel modeling
has been found to be very reliable. However, near the outer boundaries of
the wake, differences can be significant. In the above early studies of
Evans (1957) and Lord, et afL (1964) no attempt was made to simulate an
atmospheric boundary layer. Thus, preference should be given to the results
in the most recent studies.
A review and evaluation of the current literature as reflected
above and in the annotated bibliography reveals a consensus that the height
of the cavity downwind of structures extends to the height of the structure
16
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plus 0.5 times the height or width, whichever dimension is less. However,
significant influences on plume behavior are found to extend farther. The
well established 2.5 times rule is found to be the consensus as the stack
height necessary to avoid significant effects for buildings whose projected
width is greater than its height, although individual studies show some
deviation. For tall buildings, where the width is less than the height,
the stack height need only be equal to the height of the building plus 1.5
times it width. Thus, the good engineering practice stack height has been
determined to be equal to the height of the structure plus 1.5 times the
height or width, whichever is less. This determination is most applicable
to sharp-edged structures. The extent of significant effects for rounded
structures are likely not as great as those for sharp-edged structures,
although there is very little information available.
The downwind extent of the highly turbulent region where there
are significant effects is, unfortunately, not as well defined. Based on
the current literature, it is recommended that, for the purposes of deter-
mining GEP stack height, the downwind extent of the highly turbulent region
be taken downwind of the lee side as 5 times the height or width of the
structure, whichever is less, i.e., 5L from Equation 1. This choice is
most applicable to a structure whose width is less than 10 times its height.
In situations where the structure is wider than 10 times its height, there
may be significant adverse effects extending farther downwind. The distance,
5L, generally corresponds to the cavity length. Most sources that are so
close to a structure will likely be greatly affected if their height is less
than GEP stack height as determined above. Sources at increasingly greater
distances would need a decreasingly lower stack height in order not to be
17
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significantly affected. This would be especially true for highly buoyant
sources whose emissions would rapidly rise to heights well above the disturbed
flow. General rules for defining a GEP stack height for sources at distances
greater than 5L are not presently feasible. Where concern exists for possible
significant effects on sources greater than a distance 5L, a wind tunnel or
field study should be conducted unless some reference to a similar study is
available.
Evaluation of the wind tunnel results of Evans (1957) indicates
that for extremely wide buildings the maximum extent of adverse effects likely
do not extend beyond 10 times their height. In the wind tunnel studies and
literature review reported by Huber, et al. (1976) on flow over two-dimensional
obstacles a maximum extent of 10 times the obstacle height was found, for the
cavity region, except in the case of very thin obstacles where the extent was
found to be much greater. Hosker (1979) has made an extensive review of the
literature and has developed an empirical estimation procedure for cavity
length behind two and three dimensional shape-edged rectangular buildings.
This presentation could be used as a guide to indicate where a demonstration
study for credit beyond 5L is likely justified. However, additional factors
which are not presently understood may effect the downwind extent of significant
influence. Again, very little information was found for rounded structures
which are unlikely to have as great a downwind influence as do sharp-edged
structures. However, hemisphere shaped obstacles and sharp-edged obstacles
placed with a 45 degree orientation to the approach wind have been both shown
by Peterka and Cermak (1975) to have weak vortex patterns which may persist
far downstream. It is not known, however, what effect the weak vortex pattern
could have on emissions from stacks.
18
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2.3 Quantitative Rationale for GEP Equation
Little of the literature on building effects presented above and in
the annotated bibliography contains specific data that can be used in evaluating
building influences. Design stack height near buildings has been based mostly
on theory and experience with minimal supporting data. Also, some of the data
available cannot be used because no measurements of concentrations in the
absence of buildings were taken for comparison. Specific data available from
the literature concerning cavity and wake height are summarized in Figures 3
and 4.
In Figure 3, cavity height (hc) is found to be well represented by
hc = H + 0.5L (Equation 2)
The scatter of data appears evenly distributed about the line with slope
equal to 1.0. The three sets of data included in Figure 3 were taken from
wind tunnel studies where smoke was used to visualize the region where flow
was circulating.
Figure 4 presents data from studies defining the necessary stack
height in the absence of any plume rise to avoid some wake effect. These
data are only qualitatively useful since no measure of the significance of
the effect on air quality problems can be inferred. The wake height (hw)
estimate has been used above to define GEP stack height as formulated by
hw = H + 1.5L (Equation 3)
The data from Meroney and Yang (1971) and Lord, £t al_. (1964) came from
observations of the plume center!ine visualized through smoke. The wake
height estimate was defined as the minimum plume centerline height found to
be unaffected by the building. The other data are from an examination of
vertical concentration profiles. For these data, the wake heights were
defined as the plume centerline height where profiles both with and without
19
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CAVITY HEIGHT ESTIMATE
2.6
2.4
2.2
2.0
a
i 1.8
e
1.6
CO
1.4
1.2
1.0
0.8
O LORD, ETAL (1964)
— OHUBER&SNYDER(1976)
»--A-H EVANS (1957)
CAVITY HEIGHT ESTIMATED FOR
— 90 BLDG. TYPES. AVG = 1.5
STDDEV = 0.36
hc = H+0.5L (EQ 2) Q
WHERE: hc = CAVITY HEIGHT
H =BLDG.HEIGHT
L = LESSER DIMENSION
(BLDG. HEIGHT OR WIDTH)
0.8 1.0 1.2 1.4 1.5 1.6
(EQUATION 2)
Figure 3. Cavity height estimate.
20
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WAKE HEIGHT ESTIMATE
4.6
4.4
4.2
4.0
3.8
3.6
3.4
< 3.2
3.0
CC 2.8
u.
2 2-6
S 2.4
uj 2.2
X 2.0
.C
1.8
1.6
1.4
1.2
1.0
D UKEGUCHI, ET AL. (1967)
A HUBER&SNYDER (1976)
O SNYDER & LAWSON (1976)
• MERONEY& YANG (1971)
• JENSEN & FRANK (1965)
A SHERLOCK & STALKER (1940)
O LORD, ETAL. (1964)
hw = H-H.5L (EQ 3)
WHERE: hw = WAKE HEIGHT
H =BLDG. HEIGHT
L = LESSER DIMENSION
(BLDG.HEIGHT OR WIDTH)
8-
8"
1.1
1.3
1.5
1.7
1.9 2.1
(EQUATION 3)
2.3
2.5
Figure 4. Wake height estimate.
21
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the building were judged to be essentially the same. One must be very careful
in interpreting the data in Figure 4. The visualized studies can be strongly
biased by the observer's eye and are extremely sensitive to the density of
the smoke. The information from concentration profiles is influenced strongly
by where the traverse through the plume is made and the judgment in determining
what constitutes a significant concentration difference. In all these studies
a higher stack would have been required if the objective were to determine
the height at which there was no building wake effect on the emissions. Most
of the data presented in Figure 4 came from studies which did not fully
consider proper simulation of atmospheric flow. Influences due to building
effects would be diminished in highly unstable and/or turbulent atmospheric
conditions.
The data presented in Figure 4 show Equation 3 to approximate the
lower bound of these measurements. Although the consensus opinion in the
scientific literature strongly supports using Equation 3 to determine GEP
stack height, actual studies could show the need for a much taller or lower
stack depending on one's interpretation of what is a significant influence
and on the effect of possible plume rise. To more precisely define that
height for a specific stack, ground-level measurement both in the wake of and
in the absence of the building are needed to assess the increase in maximum
concentrations. The ground-level measurements must be sufficient to determine
the location of the maximum concentration which may occur at a different
position in the wake of the building than found in absence of the building.
The increase in maximum concentration is simply the difference between the
maximum concentration found in the wake of the building and that found in
absence of the building. This concentration increase can be assessed to
determine whether the increase is at least 40% in excess of that which is
22
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projected to occur in the absence of such structures. In practice, successive
runs varying the physical stack height would be conducted until the concentra-
tion increase due to building influence meets the "40% criterion."
There are only a few data sets having ground-level measurements that
included increased maximum concentrations in the literature which can be used
to determine the effect of increasing stack heights on ground-level concentra-
tions. Snyder (1979) reported on additional EPA data at the May 30, 1979,
public hearing on the stack height regulation. All data are presented in
Figures 5, 6 and 7. A theoretical estimate (Britter, et al. 1976) of in-
creased maximum is also presented. The theoretical estimate assumes the
building is much wider than it is high, and should be considered as providing
an upper estimate. For all data, the plume rise was very small and thus
plume centerline height is nearly equal to stack height. In all cases, the
simulated atmospheric flow is likely typical of that which occurs for high
wind, neutrally stable situations. Thus differences among the data are due
to change in stack height, building size, or building orientation.
The maximum ground-level concentrations downwind of the building
and in absence of the building were used to form the concentration ratios in
Figures 5, 6 and 7. The maximum ground-level concentrations occur naturally
at different positions. The data in Figures 5 and 6, as presented by Snyder
(1979), show that higher concentrations downwind of buildings depend quite
strongly on building width. Ground-level maximum concentrations associated
with a stack 2.5 times the cubical and the wide buildings oriented perpendi-
cular to the approach wind (i.e., zero degrees building angle) are found to
be increased by roughly 20 to 40 percent by the building wake. The theoretical
estimate suggests an 80 percent increase as an upper limit. The data for the
same buildings oriented 45 degrees to the approach flow are found to have
23
-------
3.5
o
a
o
5 2.5
d
Q
CD
x 2
1.5
T
T
T
W/H
-BRITTER, ET AL. (1976)
THEORY: W»H
D HUBER AND SNYDER (1976): W/H = 2
O UKEGUCHIETAL. (1967): W/H = 2
A ROBINS AND CASTRO (1977): W/H = 1
SNYDER (1979):
A W/H = 0.33
• W/H = 1
• W/H = 3
WHERE: Hs = STACK HEIGHT
H = BLDG. HEIGHT
W = BLDG. HEIGHT
1 1.25 1.5 1.75
2 2.25 2.5 2.75
HS/H
3.25 3.5
Figure 5. Comparison of increased maximum ground-level concentrations for 0° building
angle cases.
24
-------
i
i
3.5
CO
o
2.5
C3
Q
CO
X
1.5
--BRITTER, ET AL. (1976)
THEORY: W»H
A ROBINS AND CASTRO: W/H =
SNYDER (1979):
A W/H = 0.33
• W/H = 1
• W/H = 3
WHERE: Hs = STACK HEIGHT
H = BLDG. HEIGHT
W = BLDG. WIDTH
1.25
1.5
1.75
2.25
HS/H
2.5
2.75
3.25
3.5
Figure 6. Comparison of increased maximum ground-level concentrations
for 45° building angle cases.
25
-------
O 5
O
\
d
o
_j
DO
100% LOAD
75%LOAD
37.5% LOAD-
_ 100% LOAD
58% LOAL
A
8
I I I 1 |
OENVIROPLAN AND CSU (1981a): W/H=1.2
DENVIROPLAN AND CSU (1981b): W/H=3.7
APETERSEN (1982): W/H=0.9
OHOYDYSH et. al. (1980): W/Hs1.2
• PETERSEN AND CERMAK (1979): W/Hs2.8
• ESSCO (1981): W/Hs2.1
^,ALAWSON AND SNYDER (1983): W/Hs4.4
WHERE: Hs = STACK HEIGHT
H = BLDG. HEIGHT
W = BLDG. WIDTH
1.4
1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50
HS/H
Figure 7. Comparison of increased maximum ground-level concentrations based on recent fluid
model demonstrations for actual power plants.
26
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concentrations increased by roughly 40 to 80 percent. The differences are due
to the presence of longitudinal vortices in the wake of buildings having a 45
degree orientation to the approach wind as discussed in Section 2.2. The 80
percent increase found for the building with W/H = 3 and having a 45 degree
building angle very likely represents the maximum effect of changing building
orientation since, for wider buildings, the longitudinal vortices generated
at the sides of the building would be less likely to interact. Also the data
are for a source centered on the building and having no plume rise. These
conditions should result in the greatest potential effect.
Thus, it is anticipated for most situations, that maximum ground-
level concentrations downwind of building structures should not be increased
by more than 40 to 80 percent if the stack is equal to 2.5 times the building
height. Data for the tall thin building (W/H = 0.33) shows that a stack much
less than 2.5 times the building height is needed to avoid increases. GEP
stack height as given by Equation 1 is equal to 1.5 times the building height
for the 0 degree building angle case and equal to 1.7 times the building
height for the 45 degree building angle case. The increase in maximum ground-
level concentration from such stack heights was found by Snyder (1979) to be
increased by less than 40 percent.
Figure 7 presents the results of recent wind tunnel studies for six
separate power plants (one plant was modeled twice) which considered EPA
guidance and conducted fluid modeling demonstrations. The GEP height for
five of these plants, based on Equation 1, is HS/H = 2.5 (since the plants
are wider than they are tall). The results for the Eastlake Power Plant
(Enviroplan 1981a) demonstrated a GEP height, based on the 40% excess concen-
tration and MAAQS exceedance criteria, less than the GEP formula height given
by Equation 1. Also, it should be noted that the results shown by Lawson and
27
-------
Snyder (1983) are based on data collected at EPA's wind tunnel facility for a
building orientation fixed at 0 degrees. Use of oblique angles may have
resulted in a greater plume downwash effect, i.e. a larger concentration ratio.
Also shown in Figure 7 are data based on 100% plant load factor, and
where available, several other plant load conditions. For the three studies
that utilized several load factors, the resulting demonstrated GEP height is
shown to be only slightly influenced by this variable.
Thus, recent fluid modeling studies for separate power plants indicate
that application of the GEP formula (Equation 1) generally yields lower stack
heights than were justified using fluid modeling. The severity of building
effects on plume downwash is naturally affected by the building design, orien-
tation to the wind, and the stack-building separation distance. Additional
experience and research may lead to refinements of the GEP formula.
2.4 Terrain Influences
Elevated terrain can be much larger than most building structures.
Atmospheric phenomena on these scales can have a great influence on the develop-
ment of aerodynamic forces, beyond those found in the wake of low-lying struc-
tures. Very few definitive evaluations of the extent of significant adverse
effects in the wake of terrain obstacles are found in current literature.
The review of published field studies presented by Huber, et al.
(1976) strongly supports the assertion that, on the leeward side of a
mountain ridge, a circulating eddy with strong downwash and dispersion
characteristics can exist. Many of these studies are contained in the
annotated bibliography. However, information that could define the point
where the flow separates and the size and extent of the cavity was not found.
The point of separation appears to be a function of mean flow speed and
28
-------
direction, atmospheric stability, downslope and upslope angle of the ridge
sides, and the location of the ridge with respect to surrounding terrain.
For a particular situation, the greatest cavity occurs when flow
separation occurs at the ridge apex. Both field studies and fluid modeling
results confirm a natural expectation that the more obtrusive the ridge, the
larger the cavity region. Obstructions with salient features should exhibit
definite separation at their edges under all atmospheric conditions. The
size of the cavity region is greatest for isolated ridges with steep sloping
sides. Stable atmospheric conditions act to restrict the size and extent of
the cavity region. Under highly stable flows other phenomena, such as lee
waves and rotors, may be found. Terrain features that most adversely affect
flow are two-dimensional in nature. Lateral air motion around a hill under
neutral stability results in a smaller eddy size than would be observed for a
two-dimensional ridge.
Sporn and Frankenberg (1966) and Frankenberg (1968) recognized the
potential for adverse terrain influences in the late 1960's when their pioneer-
ing experience with tall stacks began. A wind tunnel study was conducted for
the Clifty Creek plant since preliminary evaluations indicated that there
would be unusual difficulties from an aerodynamic standpoint. An abrupt rise
of the terrain to a plateau approximately 100 m above plant level was found
in the prevailing downwind direction. The authors indicate that the results
of the wind tunnel study showed that stacks with a gas exit velocity of 36 m/s
and a height twice the plateau height (200 m) would be adequate to insure
that the plume would not intercept the boundary layer flow along the hillside
and be immediately brought to the ground. The Kyger Creek plant presented no
special terrain problems so the stack height was determined from diffusion
calculations only. The results of the analyses at Clifty Creek and Kyger
29
-------
Creek were used as a guide in determining the necessary stack design for
newer facilities. For example, the stacks at the Cardinal Power Plant were
constructed 251.8 m high; this makes them about 1.5 times the height of the
surrounding terrain, Frankenberg, ^t jjl_. (1970.)
Williams and Dowd (1969) report that wind tunnel studies of gaseous
diffusion have been used in many cases to help determine stack heights. It
has been observed, however, that for scaling ratios larger than 600:1, consis-
tent and repeatable results become difficult to obtain.
A study, "Plume Dispersion in Complex Terrain," by Johnson and Mage
(1978) was found to provide some specific cases applicable to assessment of
potential terrain effects for two American Electric Power generating plants.
The stack of the Mitchell Power Plant is more than 2.5 times higher than the
maximum terrain features in the vicinity of the plant, while the stacks at
the Kammer Power Plant are nearly equal to the elevations of the surrounding
terrain. The horizontal spread of the plume from the stacks of the Kammer
Power Plant were found on the average to be twice as large as the spread
found for the Mitchell Power Plant.
Recent results from wind tunnel research conducted by EPA (Lawson,
1984) show that for a three-dimensional axisymmetric hill with maximum slope of
16°, a region of 40% increase in concentration was found to extend a maximum
of 1.8 hill heights in the vertical, 14 hill heights upstream, and 10 hill
heights downstream. For a two-dimensional ridge with a maximum slope of 24°,
this region of 40% increase in concentration extended 2.2 hill heights in the
vertical, 8 hill heights upstream, and 15 hill heights downstream. A terrain
amplification factor was defined as the ratio of maximum ground level concen-
trations in the presence of hills to those in absence of hills (in flat
terrain). Maximum terrain amplification factors for both the axisymmetric
30
-------
hill and the two-dimensional ridge were found on the downstream side of the
hills and have values of approximately 5.6 and 6.8, respectively. These
initial results give a first indication of the extent to which terrain fea-
tures may significantly affect source emissions.
Because of the complex air flow over terrain and the general uniqueness
of each situation, no simple definition of GEP stack height is possible as
has been recommended for building and other structures. Until further studies
better define the extent of the region where significant terrain influences
can affect nearby sources, determination of GEP stack height in the vicinity
of terrain obstacles should be made on a case-by-case basis.
2.5 Minimum Stack Height
In the case of very low structures or where there is essentially no
structure to which a stack is attached, application of the 2.5 times rule may
yield answers which have little or no meaning. Isolated release points may
require some physical height for security, safety or other public health
reasons. Excessive ground-level concentrations may result from low level
releases, due to adverse meteorological phenomena in the lower few tens of
meters above the surface. The specific height of this layer often called
'the surface boundary layer', varies not only with certain meteorological
factors but also among the definitions used by micro-meteorologists such as
Sutton (ca. 50 m)(1953), Busch (30 m or so) (1973), and others. In this
layer, the vertical atmospheric structure is largely a function of thermal
and mechanical turbulence generated at the surface, i.e., surface heating by
the sun or cooling by terrestrial radiation, and the surface roughness caused
by obstacles to air flow.
31
-------
To minimize the influences of these natural atmospheric effects,
one alternative is to consider that good engineering practice should not
preclude the construction of stacks up to a reasonable height of 65 meters.
This will certainly minimize the deleterious effects of stable and/or
stagnant conditions, and allow reasonable dilution to take place in the
short travel time to nearby locations by permitting a wider spectrum of
atmospheric eddy sizes to act in the dispersion process significantly
without contributing to problems which arise from long range transport and
transformation of pollutants. It should be noted, however, that reasonable
stack heights will not eliminate instantaneously high concentration peaks
associated with looping plumes. Eigsti (1979) shows that emissions from
stacks whose heights are less than 65 m are not likely to contribute signi-
ficantly to the overall loading of sulfate in the atmosphere. However, for
taller stacks, the increase in height can contribute significantly to
additional sulfate formation and transport. This should also apply to
other chemical transformation mechanisms in the atmosphere.
Thus, it is recommended that Equation 1 be applied unless the
resulting height is less than 65 meters. If this is the case, the stack
height credit allowed is equal to the actual stack height, up to 65 meters.
2.6 Porous, Rounded or Sloping Structures
It is known that wind disturbance patterns around some structures
are not as great as in the case of simple idealized block structures used
in the development of the GEP formula. Moreover, the possibility exists
that the formula height may exceed actual GEP height for porous structures
such as the unenclosed metal supporting framework or "lattice" used in some
refineries and power plants, and domed, rounded or sloping structures, such
32
-------
as natural draft hyperbolic cooling towers, whose shapes are aerodynamically
smoother than the block structures used in the development of the formula.
Presently, sufficient data do not exist, nor is the state of the analytical
art sufficiently advanced to enable the establishment of a mathematical
formula to calculate GEP stack height for these categories of structures.
Sources seeking GEP stack height credit for the effects of downwash, wakes,
or eddy effects due to porous1, rounded, or sloping structures should
conduct field studies or fluid modeling demonstrations to determine GEP
stack height on a case-by-case basis.
^Sources that wish to base stack height credit on Equation 1 may do so by
using only the dimensions of the "solid" structure which is "enclosed"
by lattice work.
33
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3.0 DETERMINATION OF GEP STACK HEIGHT
3.1 Initial Assumptions
GEP stack height is designed to ensure that emissions from a stack
do not result in excessive concentrations as a result of aerodynamic effects
from nearby structures or terrain features. Determination of excessive con-
centration is dependent on the 40% criterion and the NAAQS and available PSD
increments as discussed in Section 1. Lower ground-level concentrations will
result when: (1) the emission point is well above the disturbed flow, (2)
the effluent rise is sufficiently great to keep a significant part of the
effluent plume above the disturbed flow or (3) the wind direction places the
stack outside the area of disturbed flow.
GEP stack height as determined by Equation 1 does not consider
plume rise. However, plume rise should not be significant in the determina-
tion of GEP stack height because under high wind speeds, plume rise near
the source is negligible. For most sources, even those with a relatively
high exit velocity, a wind speed of 15-20 m/s will result in significantly
reduced plume rise and thus increase the potential for adverse effects from
downwash. Therefore, the critical conditions for determining GEP stack
height for most sources are considered likely to be high winds associated
with neutral atmospheric stability with little plume rise near the sources.
Sources situated within 5 times the lesser of the height or the
width dimension of a structure but not greater than 0.8 km (0.5 mi) downwind
from the trailing edge of the structure are presumed nearby enough to the
building to be of concern in determining downwash potential. The height of
the structure is measured from the ground-level elevation at the base of
the stack. Procedures for calculating GEP stack height are contained in
Sections 3.2, 3.3, 3.5 and 3.6. A demonstration based on fluid modeling or
35
-------
field studies must be used to show the necessary stack height where Equation
1 is not applicable, e.g. porous, rounded or sloping structures. A framework
for demonstrating GEP stack heights in these cases is presented in Section
3.4. The "Guideline for Use of Fluid Modeling to Determine Good Engineering
Practice Stack Height" (EPA, 1980) provides specific guidance to be followed.
Field studies should be designed and evaluated on a case-by-case basis since
the complexities of field studies do not make it feasible to propose specific
c ri te ri a.
3.2 Simple Structures
GEP stack height has been defined to be equal to the height of
adjacent or nearby structures plus 1.5 times the structure height or width,
whichever is less. Both the height and width of the structure are determined
from the frontal area of the structure, projected on a plane perpendicular to
the direction of the wind. If the structure is asymmetrical, the GEP stack
height should be based on the plane projection lying upwind from the source
(stack) which results in the greatest justifiable height (refer to Section 1).
In some situations the projected area may be very irregular, thus
resulting in a multiplicity of scales. However, structural protuberances are
seldom a significant factor in determining GEP stack height. For the purpose
of determining GEP stack height, nearby is limited to 5 structure heights or
widths, whichever is less, downwind from the trailing edge of the structure.
Figure 8 illustrates applications to three types of buildings. A
GEP stack should have a height equal to the upper edge of the shaded regions
of the vertical cross-section if the stack lies within the associated shaded
region of the horizontal cross-section. Note for both the tall, thin structure
36
-------
DETERMINATION OF THE IMMEDIATE VICINITY, R,
FOR THREE TYPES OF STRUCTURES
W2=0.1 H2
W3=2 H2
TOP VIEW
GEP2=1.15 H2
GEP^Z.5 H,
GEP3=2.5 H3
W3
SIDE VIEW
Figure 8. Determination of the GEP stack height near three types of structures.
37
-------
and the short, long structure, the expected sphere of influence is less than
that found for the moderately tall cubical structure.
3.2.1 Low Structures
The nearby region of adverse influence downwind, R, for a
uniform low structure (one whose width all around is greater than its height)
is easy to determine. It is 5 times the structure height, downwind in all
directions from the trailing edge of the building. The vertical extent of
disturbed flow is 2.5 times the structure height throughout the entire vicin-
ity of the structure. Thus GEP stack height is defined as 2.5 times the
structure height. This determination for a low structure is presented in
Figure 9 where the sphere of influence is outlined. Figure 9 also depicts
the maximum projected structural width, W, affecting each of the four given
sources. Note that these projected widths are only valid for a wind which
is perpendicular to the actual or the cross sectional surfaces. Since the
projected width for all directions is greater than the height, the width
scale is not a factor in determining GEP stack height.
3.2.2 Tall Structures
The width scale becomes the significant factor in determining
GEP stack height whenever the structure is taller than it is wide. In Figure
10, the structure is tall and thin (one whose lateral dimensions are less
than its height). The determination of the structural width and resulting
presumed aerodynamically effected nearby region for four wind directions is
presented in Figure 10. The nearby region, R, is 5 times the projected
width, downwind from the trailing edge of the structure. Note that the
extent is highly dependent on the wind direction. The GEP stack height for
a tall structure is determined to be equal to the structure height plus 1.5
times the projected structure width. Thus, GEP based only on the side view
38
-------
TOP VIEW
i n 111111 i i i i i i
-1111 11 11 i 11 11 11 11
I I LI I I I I I I I I I I I I I I I
•f-Z.SH-GEP
STACK HEIGHT
4-H
SIDE VIEW
Figure 9. Determination of the maximum projected structure width and
associated region of adverse influence for four stacks placed near a low
structure.
39
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DETERMINATION OF THE STRUCTURAL WIDTH
AND DOWNWIND EXTENT OF THE IMMEDIATE VICINITY
FOR FOUR STACKS PLACED NEARBY A TALL,
THIN STRUCTURE
SIDE VIEW
0.5H-
1.75H-GEP STACK HEIGHT
" ~ BASED ON WIND DIRECTION 3
Figure ^.Determination of the projected structure width and associated region of
adverse influence for four possible wind directions near a tall, thin structure.
40
-------
in Figure 10 would be equal to 1.75 times the height of the structure.
Since the projected width of the structure is dependent on the wind direction,
all directions projecting downwind towards the source need to be assessed.
The maximum allowable GEP for sources near a tall structure is then equal
to the structure height plus 1.5 times the maximum projected structure width.
3.3 Complex Structures
3.3.1 Tiered Structures
Figure 11 presents a more complex, tiered structure. For
this situation, tier 1 by itself has a nearby region, R, extending downwind
for five heights. The addition of tier 2, which is equal in height to tier
1, causes both the vertical and downwind extent of the region of significant
influence to double since the height scale is the overall height which still
is less than the width. The projected area downwind of tier 3 which is placed
above tier 2 has a height 4 times greater than its width, as can be seen from
examination of Figure 11. However, the downwind region of influence of tier 3
extends downwind less than the influence of tier 2. Should a source be located
directly downwind of tier 3, although out it its influence, GEP is then based
on the influence of tier 2. Note that the vertical and downwind extent of
influence of tier 2 totally engulfs the influence of tier 1. However, the
across flow extent is, of course, greater.
For the situation presented in Figure 11, GEP stack height
is equal to GE?3 (1.4H3) for all sources downwind of tier 3 and placed within
R3- GEP for sources farther downwind, but not beyond Rg, is equal to GEP2
(2.5H2). For sources outside of the projected width of tier 2, however
within the projected width of tier 1 and downwind distance R]_, the GEP stack
height is equal to GEPi (2.5Hi). Other orientations of the building to the
41
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VARIATION IN THE DETERMINATION OF THE
IMMEDIATE VICINITY FOR ADDITIONS
TO A TIERED STRUCTURE
W2=1.
W3=0.25H3
TOP VIEW
WIND DIRECTION
GEPs=1.4H3
GEP2=2.5H2
GEPi=2.5H!--
SIDE VIEW
Figure 11. Variation in the determination of the region of adverse
influence for additions to a tiered structure.
42
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wind can result in different determinations of GEP stack height where the
projected width is less than its height. For the building design in Figure Tl,
only the influences of tier 3 change the GEP determination since only its
projected width is less than its height.
The influence of tiers has been assumed to be complementary.
Very little information relative to such situations is found in the present
scientific literature. The influence of tiers may not be exactly complemen-
tary since additional tall tiers, similar to tier 3 in the above example,
may result in some streamlining of the flow around the lower tiers and thus
some reduction in their effects. Since such effects are likely minimal, it
is recommended that, until further evaluations are reported, the effects of
tiers should be considered totally additive as presented here. A demonstra-
tion should be provided if a noncomplementary assumption is used or in
situations where there is concern for additional complications.
3.3.2 Group of Structures
Figure 12 presents an evaluation of the region of adverse
influence downwind of a group of structures. The top view shows the projected
downwind extent for three wind directions. The effects of adding building 3
is shown as the added region of influence beyond that shown for building 2.
The downwind extent of the region of adverse influence is equal to five times
the height or projected width of the building, whichever is less. The influ-
ence of nearby buildings is assumed to be exactly complementary, similar to
that shown for tiered structures. Where the projected widths of adjacent
buildings do not overlay but whose lesser projected dimension (height or pro-
jected width) of either building is greater than the projected distance of
separation, treat the gap as if it were filled by a structure equal in height
to the lesser projected height. This is demonstrated in the front view. The
43
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distance of separation between building 1 and building 2 is too large while
building 2 and building 3 are assumed to be sufficiently close to be treated
as a single building for purposes of determining GEP stack height. The side
views show all three buildings to be simply complementary. GEP stack height
based only on the front view and the side views is presented in Figure 12.
As for single structures, the maximum allowable GEP stack height is equal to
that resulting from an evaluation of all wind directions.
The influence of groups of buildings has been assumed to be
complementary. Very little information relative to such situations is found
in the scientific literature. The above general procedure is recommended
until further evaluations are reported from which more specific guidance may
evolve. A demonstration should be provided for special situations where
support for the above assumption is desirable or in situations where there is
concern for additional complications.
3.4 Framework for Demonstrating GEP Stack Height
As outlined in 40 CFR 51.1(ii), a demonstration may be required to
determine GEP stack height for a source (refer to Section 1). A demonstra-
tion can be performed through fluid modeling (wind tunnel or water channel)
or a comparable field study subject to the conditions discussed below. In
field studies and fluid modeling simulations, a quantitative evaluation of
the building and/or terrain influence on GEP stack height is a necessary part
of demonstrating GEP stack height. Upon acceptance of such a demonstration,
sources may base GEP stack height on the field study or fluid modeling results
as described in Section 3.5. Comparable fluid modeling studies require certain
similarity criteria to be considered. Discussion of similarity criteria can
45
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be found, for example, in Snyder (1981); Snyder (1972); Sundaram, et al.
(1971); Cermak (1980); and Halitsky (1968).
Modeling simulations rely on the continuing development and
refinement of state-of-the-art techniques. The specific criteria and proce-
dures for an adequate GEP modeling demonstration are presented in separate
guidance documents. Specifications for such fluid modeling demonstrations
are found in the "Guideline for Use of Fluid Modeling to Determine Good
Engineering Practice Stack Height" (EPA, 1980). This guideline is based on
a separate guideline entitled, "Guideline for Fluid Modeling of Atmospheric
Diffusion" (Snyder, 1981), which reviews the fundamental principles and
practical applications of fluid modeling, establishes the capabilities and
limitations of fluid modeling, and also establishes EPA standards for the
conduct of fluid modeling studies. EPA published a fluid modeling demonstra-
tion study for a power plant (Lawson and Snyder, 1983) that illustrates how
the 1980 fluid modeling guideline should be applied. In addition, EPA has
published a report entitled "Fluid Modeling Demonstration of Good-Engineer-
ing Practice Stack Height in Complex Terrain" (Snyder and Lawson, 1985) which
modelers may use as guidance.
As the state-of-the-art improves, future guidance may require
additional data and/or specific critical assessments. For this reason,
reviewing agencies should establish a requirement that a study plan be
submitted prior to the conduct of the demonstration study so that the
latest EPA quality assurance procedures and guidance will be considered.
In some situations field studies may be desired in conjunction
with or as support for a fluid model study. Proposed field studies should be
designed and evaluated on a case-by-case basis since the complexities of field
46
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field studies do not make it feasible to propose specific criteria. The
following discussion presents, generally, the essential components for
demonstrating a GEP stack height by a field study.
The cause(s) and magnitude of the disturbed flow used to justify
the GEP stack height should be clearly identified. In the case of an inso-
lated building, this can be easily accomplished by documenting the release
of visible smoke at ground level and on top of the building to demonstrate
the general region of influence. Effects caused by atmospheric phenomena
such as oscillations in the flow and inversion breakup are not creditable
toward determining a GEP stack height.
A field demonstration of GEP stack height requires experiments to
determine the concentration patterns from two release points—one with the
structure(s) and/or terrain; the other in the absence of structure(s) and/or
terrain. This means there must be a location near the site of the source
where the atmospheric flow is similar except for differences caused by struc-
tures and/or terrain near the source. A monitoring array must be arranged
to clearly identify the maximum concentrations downwind of similar releases
at both sites. Meteorological instrumentation must be placed upwind of both
sites to show that the approaching atmospheric flow is similar. In areas
where the upwind fetch at both sites is similarly homogeneous with no nearby
obstructions such as buildings or elevated terrain, one may expect similar
approach flows. A light wind, stable atmospheric flow is very sensitive to
external influences, often resulting in great differences between even close
sites. Generally, moderate to high wind speeds with near neutral stability
conditions can be expected to result in the more severe downwash, wakes, or
eddy effects.
47
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3.5 Determining GEP Stack Height
Determining the GEP stack height under these regulations is
required in order to set the correct emission limitations for the source.
The regulation has exempted certain sources and stacks from these determina-
tions while requiring that others perform a more rigorous evaluation includ-
ing a fluid modeling demonstration. The steps to be taken in this evaluation
are shown in Table 3.1; however, the discussion of how to determine the
emission limitations is deferred to Section 4.
Stacks less than 65 m in height are considered de minimi's. Sources
with such stacks should use actual stack height in calculating emission
1 imitations.
For stacks based on the 2.5H formula and in existence on January
12, 1979, (but after December 31, 1979), and for which all applicable permits
or approvals have been obtained, reliance on the 2.5H formula must be shown.
This showing includes the use of reconstructed evidence, or affidavits (as
described in the regulation) that the 2.5H formula was actually relied on
in designing the stack or establishing an emission limitation to ensure pro-
tection against downwash. If this showing is unsuccessful, Equation 1 must
be used to determine stack height.
Sources and stacks in existence on December 31, 1970 are
grandfathered and not subject to this regulation. The actual stack height
is the GEP height.
Sources that sought credit for stack height before November 9,
1984 based on the aerodynamic influence of cooling towers must show actual
reliance on Equation 1 as prescribed in the Guideline for Determination of
Good Engineering Practice Stack Height, July 1981. The requirements for a
48
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showing are similar to those for reliance on the 2.5H rule described above.
If this showing is unsuccessful, a fluid model demonstration of GEP stack
height is required.
Although sources may generally use Equation 1 to determine stack
height, a demonstration may be required by the regulatory agency (i.e., EPA,
the State or local air pollution control agency) for stacks greater than 65m
but less than the formula height, if the agency believes that the formula is
not applicable and a demonstration of the GEP stack height is necessary.
Also, a demonstration may be justified where there is concern for additional
complications near buildings, or to support the noncompl ementary building
influence assumptions discussed in Sections 3.3.1 and 3.3.2, or in connect-
ion with porous, sloping or rounded structures (discussed in Section 2.6).
In these situations, it is only necessary to demonstrate equivalence to
formula height by determining the stack height needed to avoid a 40% increase
in concentrations.
Sources with stack height greater than 65 meters but less than
the GEP height given by Equation 1, and wishing to raise the stack to that
height given by Equation 1, must provide evidence that additional height is
necessary to avoid downwash-rel ated concentrations raising health and welfare
concerns. This can be accomplished by either one of two methods: (1) demon-
strate by fluid modeling or a comparable field study, using the existing stack
and emission rate (before the stack is raised) and adding in the background
air quality, that both "excessive concentration" criteria are met; or (2)
show by site-specific information that the existing short stack(s) have in
fact caused a local nuisance. A nuisance caused by air pollution from the
stack could include widespread citizen or employee complaints (i.e. choking,
49
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Table 3.1
Determining GEP Stack Height For Modeling Emission
Limitations For Sources in Flat or Elevated Terrain
A. Stack height _< 65 m
Use actual stack height in calculating emission limitations.
B. For stacks = 2.5H and in existence prior to January 12, 1979
but after December 31, 1970.
1. Show reliance on the 2.5H formula (use reconstructed evidence
or other conditions specified in the rulemaking).
2. If successful, use 2.5H height to set emission limitations.
3. Otherwise, use Equation 1 to determine GEP stack height and set
emission limitations.
C. For sources and stacks in existence prior to December 31, 1970 use
the actual stack height to set emission limitations.
D. For sources that sought credit for stack height before November 9,
1984, based on the aerodynamic influence of cooling towers.
1. Show reliance on Equation 1 as prescribed in EPA guidance (use
reconstructed evidence or other conditions specified in the
rul emaking).
2. If successful, use Equation 1 height to set emission limitations.
3. Otherwise, demonstrate by fluid modeling the stack height needed
only to avoid a 40% increase in concentrations and set emission
1 imitations.
E. Regulatory Agency discretion to require fluid modeling when Equation 1
is not acceptable (e.g. in connection with porous, sloping or rounded
structures).
1. Demonstrate equivalence to formula height by fluid modeling the
stack height needed only to avoid a 40% increase in concentration.
2. Use the demonstrated height or Equation 1, whichever is less, to
set emission limitations.
F. Stack height > 65 m but < Equation 1
1. Use actual stack height to set emission limitations.
50
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Table 3.1 (Cont.)
2. If a stack height increase to Equation 1 is requested, then:
2a. Demonstrate by fluid modeling or a field study! that both excessive
concentration criteria^ are met, using existing stack and existing
emission rate, and adding in background air quality, or
2b. Show, by site-specific information, that the stack is causing
a local nuisance.
2c. Determine GEP height based on Equation 1. May increase physical
stack height up to this height.3
2d. Otherwise, use actual stack height to set emission limitations.
G. Stack height > Equation 1, wish to determine correct GEP height
1. Determine stack height based on Equation 1; or
2a. Demonstrate by fluid modeling or a field studyl tne stack height
that satisfies both excessive concentration criteria^, using
applicable emission rate^ and adding in background air quality.
2b. Select the lowest height necessary to meet the more restrictive
of the "excessive concentration" criteria^.
3. Use this physical stack height (from step 1 or 2b) to set emission
limitations^.
H. All other sources
1. Should use Equation 1 to define their GEP stack height. The emission
limitations should be based on this height.
^Proposal to conduct a field study shall be reviewed on a case-by-case basis
as discussed in Section 3.4.
^"Excessive Concentration" criteria include both an exceedance of a NAAQS or
available PSD increment and 40% excess concentration, as defined in Section 1
3where some other meteorological condition i s more controlling than downwash,
adjust the emission rate to avoid a violation of a NAAQS or available PSD
increment.
applicable emission rate is defined as that equivalent to NSPS for that
source category.
51
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stinging eyes) or property damage (i.e., soiling). If a successful demonstra-
tion is made, the stack height can be increased up to Equation 1 height and
the emission limitations established at this new height. Otherwise, the
existing stack height is used to set the emission limitations.
Sources, with stack height greater than the GEP height given by
Equation 1, who wish to determine the correct GEP height can either determine
the stack height based on Equation 1 or demonstrate by fluid modeling or a
field study the correct GEP height. In conducting a demonstration, a source
should use the modeled stack height, input the applicable emission rate that
is equivalent to NSPS for that source category1, and add in the background air
quality as determined by procedures contained in two EPA guidance documents
(EPA, 1978, 1981). After demonstrating that both "excessive concentration"
criteria are met as defined in Section 1, the source must determine the
lowest stack height necessary to meet the more restrictive of the two
excesive concentration criteria. This lower height is the new GEP height.
All other sources, e.g. those sources not excepted or included
above, can use Equation 1 to define their GEP stack height.
3.6 Modeling Terrain Effects
As discussed earlier, a GEP stack based on Equation 1 is theoretically
high enough to avoid downwash, wakes, or eddy effects caused by nearby struc-
tures. However, even though the stack is tall enough, there is still the
possibility of pi une downwash caused by nearby elevated terrain.2 Criteria
However sources may on a case-by-case basis demonstrate that such an emission
is not feasible for their situations and determine their emission limitations
based on Best Available Retrofit Technology.
evated terrain is defined as a setting with significant topographical
complexities, e.g., topographic features exceed the GEP stack height of the
source being modeled.
52
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for determining GEP stack height for sources in elevated terrain are shown
in Table 3.1 and the justification for the need to make a demonstration are
the same as those for sources in flat terrain. The implementations of the
model demonstration techniques are, however, different. In conducting a
fluid modeling demonstration, considerations of downwash, wakes, or eddy
effects of terrain features are limited to those features that can be classi-
fied as being "nearby" as that term is defined in Section 1 (i.e., not further
than 0.8 km (0.5 mi) from the stack). However, that portion of a terrain
feature may be considered to be nearby which falls within a distance of up to
10 times the maximum height (Hj) of the feature, not to exceed 3.2 km (2 mi)
i f such feature achieves a height (Ht), at or within 0.8 km (0.5 mi) from the
stack, that is at least 40% of the GEP stack height (Hg) determined by Equation
1 or 40% of the 65 m ^e minimi's heigh t (26 m), whichever i s greater. The
height of the terrain feature is measured from the ground-level elevation at
the base of the stack. This is illustrated in Figure 13. The nearby and
distance limitations apply with respect to the terrain feature inserted and
removed while fluid modeling.
The specific steps undertaken to simulate the effects of nearby
terrain are provided in the docunent "Fluid Modeling Demonstration of Good-
Engineering-Practice Stack Height in Complex Terrain," (Snyder and Lawson,
1985). A model baseline should be established by initially representing in
the model all relevant terrain features beyond a distance of 3.2 km (2 mi) or
10 times terrain height (HT), whichever is less, but excluding the nearby
features, i.e., smoothing and sloping those features falling within the
appropriate distance limitation to minimize their effects. To evaluate the
effects of nearby terrain, these latter features are then inserted into the
53
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model, and the resulting concentrations compared to the baseline as described
in Section 3.5. Refer to Figure 14.
In summary, stack height may be increased to eliminate excessive
concentrations caused by down wash due to nearby terrain. However, sources
having excessive concentrations due to downwash, wakes or eddy effects caused
by terrain features not classified "nearby," as defined in the regulation,
may not receive credit for increasing the height of their stacks to eliminate
such effects.
54
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(a)
Ht >0.4HG@0.5 mi
Ht>0.4HG@0.5 mi (b>
H = H+1.5L
END OF NEARBY TERRAIN IS > 2 MILES, CASE (a), OR 10 HT, CASE (b).
THE LESSER DISTANCE CF 2 MILES OR 10HT IS SELECTED
Ht<0.4HG@0.5mi
(c)
= H+1.5L
NO NEARBYTERRAIN FOR GEP FLUID MODELING
2.0
1.5 1.0
UPWIND DISTANCE, mi
0.5
Figure 13. Examples of determining the extent of nearby upwind terrain for
fluid modeling of all sources. In all cases H. must be at least 85 ft (26 m)
for any upwind terrain to be considered nearby.
55
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(NEAR TERRAIN DELETED FROM BASE CASE)
UPWIND EXTENT OF NEARBY TERRAIN
(NEAR TERRAIN ADDED INTO BASE MODEL)
ACTUAL TERRAIN
Figure 14, Simulating terrain effects in a wind tunnel.
56
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4.0 AIR QUALITY ESTIMATES
4.1 Determining Emission Limits
Air quality dispersion modeling is used for determining if emissions
from a stack contribute to exceedance of a NAAQS or applicable PSD increments.
It is the intent of the stack height regulation to set a limit on the maximum
stack height credit to be used in air quality modeling for the purpose of
determining an emission limitation. In the event that air quality modeling
shows violations of the NAAQS or applicable PSD increments, the emission rate
must be reduced accordingly. No stack height credit, i.e. an increase in
emission limitations, is given for that portion of an actual physical stack
height greater than the GEP height. Nor can credit be given for any GEP stack
unless such stack is actually constructed and put in operation. A GEP stack
height based on the physical configuration of the source and any nearby struc-
tures and terrain features should be determined by the procedures in the
preceding sections.
Sources with stacks less than or equal to 65 meters, should use the
actual stack height to calculate the emission limitations. Refer to Table 3.1,
item A. Sources with stacks equal to 2.5H and in existence prior to January
12, 1979, but after December 31, 1970, that can show reliance on the 2.5H
formula may use this 2.5H height to set their emission limitations. However,
if the showing is unsuccessful, sources shall use Equation 1 stack height to
set the emission limitations. Refer to Table 3.1, item B.
For sources and stacks in existence prior to December 31, 1970^,
actual stack height should be used to set the emission limitations. Refer
to Table 3.1, item C.
1 According to the stack height regulation, stacks in existence prior to
December 31, 1970 are grandfathered and not subject to the regulation.
57
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Emission limitations for a source that sought stack height credit
before November 9, 1984 based on cooling tower influences are set using the
stack height resulting from following Table 3.1, item D. If there is a
successful showing that the source has built a stack based on EPA's guidance
then in effect for applying Equation 1 to hyperbolic cooling towers, then that
stack height should be used to set the emission limitations. If the showing
is unsuccessful, then that stack height resulting from a fluid modeling demon-
stration where the 40% increase in concentration criterion has been satisfied
should be used to set the emission limitations.
Sources that are required by the Regulatory Agency to demonstrate
through fluid modeling the equivalence of the stack height to Equation 1,
must use the smaller of the demonstrated height or Equation 1 height to set
the emission limitations. Refer to Table 3.1, item E.
Sources with a physical stack height greater than 65m but less
than that determined by Equation 1 may use their actual stack height to set
the emission limitations. Sources that are successful in demonstrating the
need for a GEP stack height up to Equation 1 height may then use this GEP
stack height to determine the emission limitations. A source may receive
stack height credit up to formula height only if it actually raises the
physical stack, not simply claim more credit for a short stack already in
existence. If a source cannot demonstrate that the reason for raising the
physical stack height is in fact the desire to avoid a problem caused by
downwash, then the inference is a desire for more dispersion credit, which is
prohibited. Refer to Table 3.1, item F.
Sources, with a physical stack height greater than the GEP height
based on Equation 1, that wish to establish the correct emission limit should
56
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input the GEP height (given by Equation 1, fluid model or field study) into an
air quality model to set the emission limitations. Refer to Table 3.1, item 6.
All other sources should use Equation 1 to define the GEP stack
height in setting the emission limitations. Refer to Table 3.1, item H.
For all sources, specific modeling techniques have been recommended
for estimating the air quality impact of these sources and determining the
emission limitations (EPA, 1978, 1981). A simple screening analysis should
first be conducted to eliminate from further consideration those new sources
that clearly will not cause an air quality problem. Screening procedures
(Budney, 1977) provide a conservative estimate of maximum concentrations, i.e.,
a margin of safety is incorporated to insure that maximum concentrations will
not be underestimated. If a more refined analysis is necessary the analysis
should be consistent with techniques recommended in the "Guideline on Air
Quality Models" (EPA, 1978). The Guideline makes specific recommendations
concerning air quality models, data bases and general requirements for concen-
tration estimates.
Sources in elevated terrain should use a complex terrain screening
model to determine source impact (EPA, 1978, 1981). When the results of the
screening analysis demonstrate a possible violation of a NAAQS or applicable
PSD increment, a more refined analysis should be conducted. Since there are
no refined techniques currently recommended for complex terrain applications,
a refined model should only be applied after discussion with the EPA Regional
Office. In the absence of an appropriate refined model, screening results
may need to be used to determine air quality impact and/or emission limits.
59
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4.2 Treatment of Terrain
The effect of terrain elevation must be considered in routine model
calculations. If the terrain is less than the GEP height, the GEP stack
height should be the model input (See schematic below).
-WIND-
ACTUAL HEIGHT
GEP HEIGHT
I I
1 I
l i
PLUME CENTERLINE
FOR MODELING ASSESSMENT
If the terrain is greater than the GEP height and is not within
the definition of "nearby", there is a possibility that plume interaction
with this elevated terrain will be modeled, resulting in high concentrations
and violation of the NAAQS or applicable PSD increment. Stack height
credit cannot be increased to avoid this terrain impaction and the emission
rates must be reduced to eliminate the violation.!
4.3 Multiple Source Impacts
In situations where there is a significant contribution to ambient
concentrations due to sources other than the one in question, first calculate
the contribution from other sources (background). GEP-based emission rates
should be used in conjunction with GEP or allowable stack heights as input to
the model assessment. However, if emission limits have been set for a source
iRefer to court decision on plume impaction discussed in the preamble to the
stack height regulation.
60
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operating with less than a 6EP stack height, the associated emission rate and
stack height should be input to the model. Second, estimate the air quality
impact of the source in question, as discussed in Section 4.1 and 4.2. Fin-
ally, add the background to the air quality impact of the source in question
to estimate the total air quality impact. The emission limitation for the
source in question should be determined such that the NAAQS and applicable
PSD increment will be met even after natural background and the additive
impact of other sources are considered. Guidance is available for estimating
contributions from other sources (Budney, 1977 and EPA, 1978, 1981).
4.4 Special Situations
The term "dispersion technique" includes any practice carried out
to increase final plume rise to avoid control requirements and is thus not
allowable (refer to Section 1). Increasing final plume rise raises the
effective release height of pollutants into the atmosphere which could
result in less stringent emission limitations. Examples of practices that
are not considered dispersion techniques and are thus allowable for use are
given in the preamble to the stack height regulation. Sources with allowable
S02 emissions below 5,000 tons per year are exempt from the prohibition on
manipulating plume rise.
Reconstructing a facility to vent multiple flues from the same
stack solely for the purpose of increasing final plume rise is considered
a prohibited dispersion technique and no credit for the merged plume is
allowed in setting emission limitations. Therefore, if multiple flues are
to be vented from the same GEP stack using multiple liners, each flue/liner
must be modeled as a separate source and their combined impact determined.
This is accomplished by separately putting the temperature and volume flow
61
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rate of each flue/liner in the air quality model along with GEP stack
height and calculating the total concentration, including background, at
each receptor in order to determine the emission limitations.
Credit for merging gas streams is allowed under three scenarios:
(1) the source owner or operator demonstrates that the facility was origin-
ally designed and constructed with such merged gas streams; (2) after date
of promulgation, demonstrate that such merging is associated with a change
in operation at the facility that includes the installation of pollution
controls and results in a net reduction in the allowable emissions of the pol-
lutant for which credit is sought; or (3) before date of promulgation, demon-
strate that such merging did not result in any increase in the allowable
emissions and was associated with a change in operation at the facility
that included the installation of emissions control equipment or was carried
out for sound economic or engineering reasons, as demonstrated to EPA.
Any exclusion from the definition of "dispersion techniques"
applies only to the emission limitation for the pollutant affected by such
change in operation. For example, a source tears down two stacks and builds
one GEP stack with an electrostatic precipitator that results in net reduction
in particulate matter emissions. This source could model using stack gas
characteristics resulting from merging the two gas streams in setting the TSP
emission limit, but may not so model when setting the S02 emission limit.
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REFERENCES
Batchelor, G. K., 1967: An Introduction to Fluid Dynamics. Cambridge
University Press, Cambridge, (Great Britain), 325-331.
Baumeister, T., E. A. Avollone, and T. Baumeister, III, (editors), 1978:
Marks' Standard Handbook For Mechanical Engineers. McGraw-Hill, New
York, Chapter 18, page 16.
Beaver, S. H. (Chairman), 1954: Report of Government Committee on Air
Pollution. Her Majesty's Stationary Office, CMD. 9322.
Bowers, J. F., J. R. Bjorklund, and C. S. Cheney, 1979: Industrial Source
Complex (ISC) Dispersion Model User's Guide. (EPA-450/4-79-031),
Environmental Protection Agency, Research Triangle Park, NC.
Briggs, G. A., 1973: Diffusion Estimation for Small Emissions. Atmospheric
Turbulence and Diffusion Laboratory, NOAA, Oak Ridge, TN, (Draft)
ATDL No. 75/15.
Britter, R. E., J. C. R. Hunt and J. S. Puttock, 1976: Predicting Pollution
Concentrations Near Buildings and Hills. Symposium on Systems and
Models in Air and Water Pollution, September 22-24, London.
Budney, L. J.} 1977: Procedures for Evaluating Air Quality Impact of New
Stationary Sources. Guidelines for Air Quality Maintenance Planning
and Analysis: Volume 10 (EPA 4bU/2-/7-001, OAQPS Guideline Number
1.2-029R), Environmental Protection Agency, Research Triangle Park, NC.
Burt, E. W., 1977: Valley Model User's Guide. (EPA-450/2-77-018), Envion-
mental Protection Agency, Research Triangle Park, NC.
Busch, N. E., 1973: On the Mechanics of Atmospheric Turbulence. In
Workshop on Micrgmeteorology, by American Meteorological Society, Science
Press, Ephrata, PA.Chapter 1.
Cermak, J. E., 1970: Laboratory Simulation of the Atmospheric Boundary Layer.
American Institute of Aeronautics and Astronautics, 3rd Fluid and Plasma
Dynamics Conference, Los Angeles, CA, June 29 - July 1, No. 70-751.
Cermak, J. E., 1976: Aerodynamics of Buildings. In Annual Review of Fluid
Mechanics. Vol. 8, Van Dyke, M. And W. G. Vincenti (Co. Ed.), Annual
Reviews, Inc., Palo Alto, CA, pp. 75-106.
Cramer, H. E., and J. F. Bowers, Jr., 1976: West Virginia Power Plant
Evaluation. Prepared for U. S. EPA Region III, Philadelphia, PA.
Counihan, J., 1969: An Improved Method of Simulating an Atmospheric
Boundary Layer in a Wind Tunnel. Atmospheric Environment, 3, 197-214.
63
-------
Egan, B. A. 1975: Turbulent Diffusion in Complex Terrain. Lectures on
Air Pollution and Environmental Impact Analysis, American Meteorological
Society, Boston, MA.
Eigsti, S. L., 1979: An Assessment of the Potential Effects of Stack
Height on Sulfate Formation and Sulfur Deposition. U. S. Environmental
Protection Agency, Office of Air Quality Planning and Standards,
Research Triangle Park, NC, 18 pp.
Environmental Protection Agency, 1978: Guideline on Air Quality Models,
(EPA-450/2-78-027) Office of Air Quality Planning and Standards,
Research Triangle Park, NC.
Environmental Protection Agency, 1980: Guideline for the Use of Fluid
Modeling to Determine Good Engineering Practice Stack Height. (EPA-450/
4-80-015) Office of Air Quality Planning and Standards, Research Triangle
Park, NC.
Environmental Protection Agency, 1981. Regional Workshops on Air Quality
Modeling: A Summary Report (amended) EPA Publication No. EPA 450/4-
82-015. U. S. Environmental Protection Agency, Research Triangle
Park, NC.
Environmental Science & Service Corp., 1981. Niles Generating Plant. Ohio
Edison Company, Wind Tunnel, Good Engineering Practice Stack Height
Study.
Enviroplan, Inc. and Colorado State University, 1981. Eastlake Power Plant
Units 1-4 Good Engineering Practice Stack Height Physical Modeling
Study. Submitted to Cleveland Electric Illuminating Company.
Enviroplan, Inc. and Colorado State University, 1981. Avon Lake Power
Plant Units 6 and 7 Good Engineering Practice Stack Height Physical
Modeling Study. Submitted to Cleveland Electric Illuminating Company.
Evans, B. H., 1957: Natural Air Flow Around Buildings, Research Report
No. 59, Texas Engineering Experiment Station, Texas A&M College
System.
Frankenberg, T. T., 1968: High Stacks for the Diffusion of Sulfur Dioxide
and other Gases Emitted by Electric Power Plants, Am. Ind. Hyd. Assoc.
J_., 29, pp. 181-185.
Frankenberg, T. T., I. Singer, and M. E. Smith, 1970: Sulfur Dioxide in the
Vicinity of the Cardinal Plant of the American Electric Power System.
Proc. 2nd Int. Clean Air Cong. Washington, DC.
Halitsky, J., 1963: Gas Diffusion Near Buildings. ASHRAE (Trans.), 69,
pp. 464-484. ~~
64
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Halitsky, J., 1963: Gas Diffusion Near Building. Meteorology and Atomic
Energy - 1968. D. H. Slade (Ed.), Chapter 5-5.
Hansen, A. C. and J. E. Cermak, 1975: Vortex-Continuing Wakes of Surface
Obstacles. Project Themis Technical Report, No. 29,
CER75-76 ACH-JEC16.
Hawkins, J. E. and G. Nonhebel, 1955: Chimneys and the Dispersal of Smoke.
J. of the Institute of Fuel, 28_, pp. 530-545.
Hosker, R. P., Jr., 1979: Empirical Estimation of Wake Cavity Size Behind
Block-Type Structures. American Meteorological Society 4th Symposium
on Turbulence, Diffusion, and Ai r Pollution, Reno, Nevada,January
15-18, pp. 603-609.
House of Representatives, 1977. Report No. 294, 95th Congress, First Session.
Washington, DC, p. 93.
Hoydysh, W. G., et al., 1980. Eastlake Plant Units 1-4 Wind Tunnel Model GEP
Stack Height Study. Submitted to The Cleveland Electric Illuminating
Company.
Huber, A. H. and W. H. Snyder, 1976: Building Wake Effects on Short Stack
Effluents. Third Symposium on Atmospheric Turbulence Diffusion and Air
Quality, Raleigh, NC. October 19-22, pp. 235-241.
Huber, A. H., W. H. Snyder, R. S. Thompson, and R. E. Lawson, Jr., 1976:
Stack Placement in the Lee of a Mountain Ridge. U. S. Environmental
Protection Agency, EPA-600/4-76-047, Research Triangle Park, NC.
Huber, A. H., 1977: Incorporating Building/Terrain Wake Effects on Stack
Effluents. AMS-APCA Joint Conference on Applications of Air Pollution
Meteorology, Salt Lake City, Utah, November 29 - December 2, pp. 353-356.
Hunt, J. C..C.J. Abell, J. A. Peterka, and H. Woo, 1978: Kinematical Studies
of the Flows Around Free or Surface Mounted Obstacles; Applying Topology
to Flow Visualization. J. Fluid Mech., 3, Part 1, 179-200.
Johnson, F. G., and D. T. Mage, 1978: Plume Dispersion in Complex Terrain.
Presented at the Annual Meeting of the APCA, Houston, Texas, paper No.
78-73.10.
Lawson, R. E. Jr., and W. H. Snyder, 1983. Determination of Good-Engineering-
Practice Stack Height: A Fluid Model Demonstration Study For A Power
Plant, EPA 600/3-83-024. U S. Environmental Protection Agency, Research
Triangle Park, NC.
Lawson, R. E. Jr., 1984. Effect of Terrain Induced Downwash on Determination
of Good-Engineering-Practice Stack Height. U.S. Environmental Protection
Agency, Research Triangle Park, NC.
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Lord, G. R., W. D. Baines, and H. J. Leutheusser, 1964: On the Minimum
Height of Roof-Mounted Chimneys, Results of an Exploratory Wind-Tunnel
Study. Report TP-6049, Technical Publication Series, Dept. of Mechanical
Engineering, University of Toronto.
Lucas, D. H., 1972: Choosing Chimney Heights in the Presence of Buildings.
Proceedings of the International Clean Air Conference, Melbourne,
Australia, May 15-18, pp. 47-52.
Meroney, R. N. and 8. T. Yang, 1971: Wind-Tunnel Study on Gaseous Mixing
Due to Various Stack Heights and Injection Rates Above an Isolated
Structure. USAEC Report No. COO-2053-6.
Peterka, J. A. and J. E. Cermak, 1975: Turbulence in Building Wakes.
Fourth International Cnference on Wind Effects on Buildings and
Structures, London, United Kingdom. Colorado State University Report
No. CPE 74-755, AP-JEC 34.
Peterson, R. L., 1982. Building Downwash and Optimum Stack Height Assessment
for Dallman Units 31 and 32. Vol. 1. Submitted to Springfield City Water,
Light and Power Department.
Peterson, R. L., and J. E. Cermak, 1979. Wind Tunnel Study of Downwash at the
Bay Shore Power Station. Submitted to the Toledo Edison Company.
Robins, A. G. and I. P. Castro, 1977: A Wind Tunnel Investigation of Plume
Dispersion in the Vicinity of a Surface Mounted Cube-1. The Flow Field,
II. The Concentration Field. Atmospheric Environment, 17, pp. 291-311.
Scorer, R. S., 1968: Air Pollution. Pergamon Press, Oxford, England, pp.
107-123.
Snyder, W. H., 1972: Similarity Criteria for the Application of Fluid Models
to the Study of Air Pollution Meteorology. Boundary-Layer Meteorology,
3, 113-134.
Snyder, W. H. and R. E. Lawson, Jr., 1976: Determination of a Necessary
Height for a Stack Close to a Building--a Wind Tunnel Study.
Atmospheric Environment, 10, 683-691.
Snyder, W. H., 1979: Testimony on Behalf of the U. S. Environmental
Protection Agency at the Public Hearing on Proposed Stack Heights
Regulations, May 31, 1979.
Snyder, W. H., 1981: Guideline for Fluid Modeling of Atmospheric Diffusion
(EPA-600/8-81-009). U. S. Environmental Protection Agency, Environmental
Sciences Research Laboratory, Research Triangle Park, NC.
Snyder, W. H., and R. E. Lawson, Jr., 1985. Fluid Modeling Demonstration of
Good-Engineering-Practice Stack Height in Complex Terrain, EPA 600/3-85-022,
U.S. Environmental Protection Agency, Research Triangle Park, NC.
66
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Sporn, P. and T. T. Frankenberg, 1966: Pioneering Experience with High
Stacks on the OVEC and American Electric Power Systems, Presented at
the International Clean Air Congress, London, Paper No. IV/9.
Sundaram, T. R., G. R. Ludwig, and G. T. Skinner, 1979: Modeling of the
Turbulence Structure of the Atmospheric Surface Layer. American Institute
of Aeronautics and Astronautics, 9th Aerospace Sciences Meeting, New York,
NY, No. 71-136.
Sutton, 0. G., 1953: Micrometeorology, McGraw-Hill, NY.
Sutton, 0. G., 1960: Discussion before the Institute, in London, 23.d,
May 1960. J. Institute of Fuel, 33_, 495.
Ukeguchi, N., H. Sakato, H. Okamoto, and Y. Ide, 1967. Study on Stack
Gas Diffusion. Mitsubishi Technical Bulletin No. 52.
Williams, D. H., Jr., and J. T. Dowd, 1969: Design and Construction
Features of the 1600 MW Mitchell Plant, Combustion, August, 19-23.
Woo, H. B., J. A. Peterka, and J. E. Cermak, 1976: Wind-tunnel
Measurements in the Wakes of Structures, Technical Report for NASA
Marshall Space Flight Center, NASA CR-2806, Colorado State University
Report No. CER75-76HGCW-JAP-JEC40.
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APPENDIX A
ANNOTATED BIBLIOGRAPHY
Sherlock, R. H. and E. A. Stalker, 1940: The Control of Gases in the
Wake of Smokestacks. ASHE Journal, 6£, 455-458.
A wind-tunnel- investigation was used to determine whether an addi-
tion of the height of the existing stacks would prevent downflow of
stack gases into the area surrounding the Crawford Station of the
Commonwealth Edison Company, Chicago. An additional study of the
nature and cause of the behavior of the gas in the wake of smokestacks
is reported. The turbulent region immediatly adjacent to the downstream
surface of the stack was found to cause plume downwash. If the gases
thus brought down come within the influence of the turbulence flow over
the roof of the building, they were then quickly brought to the ground
behind the building.
Zero downwash into the wake of the smokestack was observed when the
stack gas exit velocity was greater than twice the wind velocity.
Downwash was approximately one stack diameter below the top of the stack
when the stack gas exit velocity was only twice the wind velocity. The
model study of Crawford Station demonstrated the need for a stack increase
of 50 feet to prevent downwash from any direction, provided that the gas
velocity is high enough to prevent the first step of downwash. This
additional increase results in the stack being approximatley 2.5 times
the highest part of the building structure.
Davidson, W. F., 1959: Studies of Stack Discharge Under Varying Con-
ditions. Combustion, 23(4), 49-51.
The problem encountered in designing stacks for the new Astoria
Station in New York City is reviewed. Design of the stack to have a
height greater than 2.5 times the height of the power station is stated
as a long time recognized "rule of thumb". However, the author believes
that, despite the importance of this factor, except for stacks of limited
height and the number of investigations made, it is still impossible to
give any rules or criteria that can be used with reasonable assurance
to predict the stack performance of a new station. Thus, carefully
planned wind-tunnel tests seem to be required. In the case of Astoria
Station, increase in stack height was originally limited by nearby
airport runways. A wind-tunnel model was tested to determine the necessary
exit gas velocity to provide a sufficient plume height to minimize
adverse building effects. A special stack nozzle was designed to keep
velocity of the exit gas equal to the full load parameter regardless of
the actual load.
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Strom, G. H., 1952: Wind-Tunnel Techniques Used to Study Influence of
Building Configuration on Stack Gas Dispersal. Industrial Hygiene
Quarterly, 1_3, 76-80.
Wind-tunnel experimentation is presented as a research tool that
has yielded answers difficult, if not impossible, to obtain by other
means. Stack gas dispersal in the presence of buildings and other
nearby structures is given as the most frequently investigated problem
in the wind tunnel. Wind-tunnel modeling is suggested when use of
empirical rules for stack height such as requiring a stack to be 2.5
times the building may lead to unnecessarily high and costly structures.
Discussion of wind-tunnel modeling methods and criteria then follow.
Beaver, S. H. (.Chairman), 1954: Report of Government Committee on Air
Pollution. Cdm. 9322. Her Majesty's Stationery Office.
A committee was appointed in July, 1953, with the following terms
of reference:
"To examine the nature, courses and effects of air
pollution and the efficacy of present preventive
measures; to consider what further preventive measures
are practicable; and to make recommendations."
Discussion of desirable stack height is taken from Appendix VI.
APPENDIX VI
The Influence of Chimney Design and Height on the
Dispersion of Flue Gases From Industrial Chimneys
Memorandum by the Industrial Sub-Committee
INTRODUCTION
The original function of high chimneys was to create draught for
the furnaces. With the introduction of mechanically created draughts
early in the century, many factories were equipped with only short
chimneys and as a consequence smoke dispersal was not good. More
recently, however, there has been a trend towards use of high chimneys
in order to improve dispersion by discharge into the higher levels of
the air.
We have found that the information on a chimney design and height
and the effect of chimney height on probable conditions on the ground to
the lee of the chimney is widely scattered and in general inaccessible
to industrial engineers. We have therefore felt it necessary to go into
the subject in some detail in this appendix. The following is a summary
of the best informed opinion at present, but further investigation may
cause these opinions to be revised.
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1. Down-draught
When a wind blows across a building or a hill a down-draught is
created on the lee side. (1) It is important that chimneys should
discharge their smoke high enough for it to escape these down-draughts
if possible.
A rule used successfully for about 20 years by the Electricity
Industry is that the height of a chimney shall be at least 2-1/2 times
the height of the highest adjacent building. When the chimney is sited
in hilly country or among buildings which make it impracticable to apply
the "2-1/2 times" rule, wind tunnel tests on models may be necessary to
determine where to site the chimney and how high to make it to avoid
down-draughts. Pending further research on the subject, a good working
rule for low buildings is to make the chimney not less than 120 feet
high — though discretion must of course be exercised for small install-
ations.
2. Down-wash
Down-wash is the drawing downward of chimney smoke by the system of
stationary vortices or eddies that form in the lee of a chimney when a
wind is blowing. If the velocity of emission of the smoke is not great
enough to overcome down-wash some of the smoke will be drawn by these
eddies down into the down-draughts of the buildings beneath.
The down-draught will then carry the smoke to the ground. Ex-
periments have shown that down-wash will not occur if the velocity of
emission is sufficiently high. It is clear to us that further research
on the design of chimney mouths is required.
Reference C21 gives a graph showing for a given wind speed the
minimum velocity of emission for avoiding down-wash.
3. Chimney height and dispersal of smoke and gases
At whatever height smoke is discharged, gravity will eventually
bring the larger particles of dust and soot to the ground. Moreover,
because of the natural turbulence and mixing of the atmosphere, a propor-
tion of the finer particles and gases in the smoke will reach the ground,
although their motion is unaffected by gravity. The higher the point of
discharge the greater will be the dilution of the gases and dust by the
time they reach the ground.
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Corby, G. A., 1954: Airflow Over Mountains: A Review of the State of
Current Literature. Quart. J. Roy. Met. Soc., 8CU
The work of J. Forchtgott, who gathered about 35 different sets of
observations involving five different mountain ridges located in Bohemia
is reviewed. Mountain airflow is classified into four main types:
01 undisturbed streaming, C21 standing eddy streaming, C31 wave streaming,
and C4I rotor streaming. The case of standing eddy streaming corresponded
to the situation of boundary layer separation at the ridge apex with
cavity formation in the lee. This type of flow is reported to have been
observed frequently. Forchtgott implied that this situation was predominant
under moderate wind speed and wind shear conditions. Even for the cases
with smooth waves above, some form of turbulent wake was found in the lee
of the ridge. No discussion of the extent of the region of modified
airflow is presented.
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Hawkins, J. E. and G. Nonhebel, 1955: Chimneys and the Dispersal of
Smoke. J. of the Institute of Fuel, 28, 530-545.
To avoid parts of a smoke plume being blown rapidly to the ground
by local disturbances of the wind, the authors report that it is necessary
to choose minimum heights of chimney and exit velocities of flue gases
which are related to the height of surrounding buildings, diameter at
chimney and local ground contour. Disturbances of the atmospere set up
by the wind flowing past the chimney and over buildings can, under
certain circumstances, draw the smoke rapidly to the ground so that the
efficiency of the chimney as a smoke disperser is much impaired. The
region of so-called "down-draughts" is stated to stretch from the top of
the windward face of the building, rise to about twice the building
height and stretch for about six times the height downwind of the building.
These dimensions are stated to.be approximate and to increase with
cross-wind width of the building. Also similar effects occur in the lee
of hills.
It is reported that a committee appointed by the Electricity Commissioners
CGreat Britain] proposed the rule that, to discharge flue gas clear of
down-draughts, chimneys should be 2.5 times the heights of the highest
adjacent building. "This rule has been used successfully by the electricity
generating industry during the last 20 years, although there is some
evidence that at high wind speeds cool gas plumes can be brought down by
down-draught even though the chimney height satisfies the 2.5 times
rule." The usefulness of the wind-tunnel tests as an indication of how
high the chimney height should be to avoid down-draught, in difficult
cases is stated. For large plants in complicated locations, advice is
given to obtain confirmatory data by observation of the spread of smoke
from smoke generations and observations of the trajectories of "zero-
buoyancy" balloons. It is noted that when a chimney is discharging into
a region of down-draughts and turbulence behind a building, changes in
the velocity of emission or temperature of the flue gas as it emerges
from the chimney will make little or no difference to conditons on the
ground. The work of Sherlock and Stalker (1950} is referenced in determining
the necessary exit velocity to avoid the drawing-down of the smoke plume
by the chimney wake. Also stated is the likelihood that a more intense
wake-region will occur for a square-shaped chimney in comparison to the
circular chimney.
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Scorer, R. S., 1955: Theory of Airflow Over Mountains: IV-Separation
of Flow from the Mountain Surfaces. Quart. J. Roy. Met. Soc., 81, 340-
350. ~~
According to the author, the flow separation point is stationary
when there is a salient edge at the top of a hill or ridge. Numerous,
but limited, field studies relating to the zone of recirculation and
instances of intense mixing and general down-draughting in the leeward
regions of ridges are cited. Details are insufficient to draw firm
conclusions relating to formation of separated flows. No specification
of the size of the region modified is given. Three types of flow separation
in mountainous areas are discussed. These are (1) air-mass (.i.e.,
valley flow independent of the flow aloft), (2) two-dimensional aerodynamic
type (.i.e., flow over a ridge}, and C3} three-dimensional aerodynamic
type [i.e., flow around an isolated peak). In general, the influences
of a three-dimensional hill are reported to be less than that of a two-
dimensional ridge. Also, katabatic winds tend to reduce the likelihood
and size of the region of separated flow, whereas anabatic winds should
enhance the size of any region.
Evans, B. H., 1957: Natural Air Flow Around Buildings. Research Report
No. 59, Texas Engineering Experiment Station, Texas A&M College System.
The shape and sjze of the downwind eddy caused by the model building
was determined in a wind tunnel study for nearly two-hundred variations
of the basic building shape. The downwind eddy was defined as the area
between the building and the point downwind of the building where some
particles of the air close to the ground are found to flow upwind toward
the building. Smoke patterns were used to determine the observed dimensions
of the eddy. The shape of the building, the roof type, the position of
openings, and the orientation with repsect to the wind, were all found
to have an effect on the air flow over the building. Several significant
findings are reported. It was found that, regardless of the height of
the building, the pattern of the air going over the top of a tall building
appeared the same. For pitched roofs, the depth of the downwind eddy
increased due to the increase in the height of the building. When the
building was extended in the downwind direction, the depth of this
downwind eddy decreased. When the width of the building (perpendicular
to the wind direction! was increased from one times its height to eight
times its height, the downwind depth of the eddy increased from 2 to 5.25
times its height. As the width of the building was further increased to
28 times its height, the downwind depth of the eddy increased at a
somewhat smaller rate to 8.75 times its height.
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Scorer, R. S., 1959: The Behavior of Chimney Plumes. Int. J. of Air
Pollution. 1, 198-220.
The 2.5 times rule concerning chimney heights is presented as being
a well-known commendable rule because it is comprehensible as a practical
working rule: it has no precise theoretical justification, and if
experience proved it to be inadequate it could be changed by Act of
Parliment! It is also argued that architects should accept the chimney
heights necessary for the proper dispersal of pollution as a requirement
and design buildings with the chimney as an integral part instead of as
an- undesirable appendage. Also in the lee of a cliff there may be
eddies into which, if a chimney is sited in the downdraught of the eddy,
the plume may be carried down to the ground bodily. This is more serious
than being diffused down by ambient turbulence. A case at Hope Cement
Works near Sheffield is discussed. A problem of downdraught was solved
by installing a 150 meter chimney which reaches above the eddies downwind
of the nearby hill.
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Nonhebel, G., 1960: Recommendations on Heights for New Industrial
Chimneys. J. Institute of Fuel. 3_3_, 479-495.
A review of the present state of knowledge and experience, and
recommendations are put forward as the basis of discussion between
industrialists and those responsible for the administration of the Clean
Air Act of 1956. This technical review was felt necessary since no
detailed technical advice had so far been issued by any governmental
department to assist those frequently faced with difficulty in deciding
the height of chimney required under the provisions of this Act.
Appendix VI of the Beaver Report 0954) is referenced as providing
guidance on technical considerations governing the height of chimneys.
Where a chimney rises from or is adjacent to a high, large building, the
recommended height is stated to be at least 2.5 times the height of the
building. For small plants (.reference to very low buildings appears to
be intended) the Beaver Report 0954) makes the recommendation that
chimney heights be not less than 120 feet high. The author goes on to
point out that, where there is a choice in the orientation of a long
building to which is attached a chimney, the longitudinal axis should be
at right angles to the prevailing wind. It is suggested that when a
chimney of a large plant is to be built among a group of high buildings
which makes it costly to apply the "2.5 times rule," the only satisfactory
solution is to make tests with models in a wind tunnel to determine its
minimum height and its position with respect to the buildings. For
small installations where the chimney plume is not expected to be
seriously affected by downdraughts exerted by a neighboring building, a
sliding scale of minimum stack height from 50 feet to 120 feet for
plants with steam output up to 33,000 Ib/hr is given. This minimum.
height is suggested to insure adequate dispersal of flue gases and is
based on specific estimates of maximum desirable ground-level concentrations,
A stack gas discharge velocity of 1.5 times the wind velocity is referenced
as sufficient to keep the centerline of the plume from being drawn below
the chimney top. The impracticalility of achieving the necessary discharge
velocity in relatation to very high wind velocities is noted as not too
important since dispersion of the gases is increased under such conditions.
It is suggested that consideration be given to increasing the velocity
of the exit gas by addition of aerodynamically designed nozzles to the
chimney top.
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Sutton, 0. G., 1960: Discussion before the Institute, in London, 23.d,
May 1960. J. Institute of Fuel, 33_, 495 (comment).
It is pointed out that the 2.5 times rule be strictly applied only
to a building which is very long across wind, and only near the central
point. Sutton believes the origin of the rule was deduced by Sir David
Brunt from W. R. Morgan's study of the height of disturbances over a
long ridge, in an investigation into the disaster of the airship R. 101;
if a wind were blowing perpendicular to the longside of a building, the
disturbances should extend upwards to about 2.5 times the height of the
roof. Another significant point raised by Sutton was that, since it is
impossible to take every factor into account in the mathematics of
atmospheric turbulence, the only thing to do is look at a situation with
the aid of scaled-down models.
Scorer, R. S. and C. F. Barrett, 1962: Gaseous Pollution from Chimneys.
Int. J. of Air and Water Pollution. 6_, 49-63.
The wake region of the building is given by a vertical circular
cylinder centered on the building of height 2.5 times the height of the
building and of horizontal radius equal to 3.5 times the width of the
building. For a building whose width is less than its heights, the wake
region is of height 2.5 times the maximum width.
Skinner, A. I., 1962: Model Tests on Flow from a Building Ventilation
Stack. Atomic Energy Establishment, Winfrith, Report AEEW-W 227.
Wind tunnel tests were conducted on a model of a building to assess
the minimum requirements for a stack which would effectively disperse
the ventilation air clear of the building wind eddies and also avoid
recirculation into the inlet grille, A stack 2.25 times the average
roof height was found to be just sufficient.
Davidson, B., 1963: Some Turbulence and Wind Variability Observations
in the Lee of Mountain Ridges. J. Appl. Meteor., 2C4). 463-472.
The results of a number of balloon releases made in two valleys in
Vermont are reported. Balloon releases were made at several positions
along the sides of ridges that had approximately 20 degree slopes.
Balloon paths were determined using theodolites. The limited results
could not be used to confirm a point of separation of the extent of a
leeward cavity region. The extreme turbulence generated in the lee of
the ridges, however, appeared to be dissipated at most elevations at a
distance of 4 to 6 ridge heights downind.
A-9
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Thomas, F. W., S. B. Carpenter, and F. E. Gartnell, 1963: Stacks—How
High? JAPCA, 13C5), 198-204.
TVA experience has demonstrated that when stacks are less than
twice the height of the main powerhouse structure, the plume may, during
high velocity wind, be caught in the turbulent vortex sheath and brought
to the ground level in relatively high concentrations very near the
plant and sometimes re-enter the building air supply. Also, extensive
wind-tunnel tests are stated to have demonstrated that downwash does not
pose a problem where the stack height is at least 2.5 times the height
of the powerhouse or other nearby structures and appropriate efflux
velocities are provided.
Buettner, K. J. K., 1964: Orographic Deformation of Wind Flow. Uni-
versity of Washington, Seattle, Washington. Prepared for U.S. Army
Electronics Research and Development Laboratory, Fort Monmouth, New
Jersey, under Project No. 1AO-11001-B-021-01, Contract No. DA 36-039-SC-
89118, 70 p.
The general features of flow over a ridge are treated theoretically
and experimentally. A ridge station was constructed on the lee side of
the Ipsut Pass area of Mount Rainier National Park in Washington as part
of a study of the effect of terrain obstacles on the fallout of particulate
matter through the atmosphere. Tracer particles of zinc sulfide were
released and collected. Data were collected for 5 days during which the
airflow approach was perpendicular to the ridge. During the period of
experimental set-up, only light to moderate winds were observed. The
most common wind field occurrence is reported as a "Vortex sheet flow"
with the airstream separating from the ridge top and forming a wake zone
in the lee of the ridge. For this flow, the wind field was constant
above and zero below a plane representing the wake zone. Only a small
amount of particulate penetrated down through the horizontal vortex
sheet. A contaminant released in the calm zone is reported to meander
in an unpredictable manner. Previously a lee eddy with the main airstream
moving first horizontally away from the ridge, then down, and then up
again close to the valley bottom was visually observed. At this site,
such a flow pattern was believed to exist only for strong winds. Laminar
flow complicated by thermal winds is reported to occur when stable
settled conditions prevail and the gradient wind at ridge level was less
than 6 knots C3.1 meters per second).
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Eimern, J., R. Karschon, L. A. Razumova, and G. W. Robertson, 1964:
Windbreaks and Shelterbreaks. Word Meteorological Organization Technical
Note No. 59.
Part of this report summarizes the literature on the influence of
shelterbelts on air flow. The region leeward of shelterbelts is reported
to have reduced winds, and a degree of turbulence and eddying of the
flow in the lee. According to one reference, the air flow is affected
up to even three or four times the height of the belt.
Most of the literature is concerned only.with defining the downwind
extent of the region of reduced winds. The literature offers a wind
range of distances. It is reported that, according to West European,
North American, and Russian experiences, the rule of thumb applies that
the shelter zone extends to 30 times the obstruction height. However,
for a wind reduction of 20 percent and more, the effect is noted to only
20 times the height. Moreover, the extent is very dependent on the
permeability, shape and width of the belt, roughness of the ground
surface, thermal stratification of the air. No discussion defining the
vertical extent that shelterbelts can effect stack effluents is given.
Lord, G. R., W. D. Baines, and H. 0. Leutheusser, 1964: On the Minimum
Height of Roof-Mounted Chimneys, Results of an Exploratory Wind-Tunnel
Study. Report TP-6409, Technical Publication Series, Dept. of Mechanical
Engineering, University of Toronto.
Wind-tunnel tests of smoke emission from roof-mounted chimneys on
both block-type and pyramidal structures are described. The tests were
performed in a constant velocity low turbulence wind field. The wind
velocity was equal to the stack emission speed. Four conditions defining
a minimum stack height are given, each corresponding to a different
degree of plume distortion by the structures. For a given stack location,
building configuration, and wind direction, the height of the stack
necessary to meet each of the four conditions is reported.
A discussion of building wake effects is included. The point is
made that, even if the source is above the wake, the effluent may later
enter the region of influence. At several building heights downstream,
the turbulent region is stated to be about twice the building cross-
section. For the tests, the stack was placed over the center of the
building. The vertical extent of building influences was found to scale
with the building width for tests where the building height is greater
than its width. The height above the building of the stack at which
smoke began to be entrained to the stagnant wake of the building was 0.5
times the building width. For the tests when the building width was
greater than the building height, the vertical extent of the building
Influences were similar to above definitions, however, with the height
scale replacing the width scale.
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Moses, H., G. H. Strom, and J. E. Carson, 1964: Effects of Meteorological
Engineering Factors on Stack Plume Rise. Nuclear Safety. 6_, 1-19.
This paper contains a review and disucssion of several reports
concerning desirable stack height near buildings and terrain. Movies of
smoke flow patterns over buildings with small stacks at Argonne National
Laboratory v/ere said to illustrate "cart wheels" forming on the lee side
with a diameter several times the height of the building and thus providing
high concentrations of contaminant. The wind-tunnel studies of air flow
around buildings by Evans 0957} Halitsky (1962) and Strom (1962), are
discussed. The likely origins of the 2.5 times rule of thumb, which has
been used by the British Electricity Industry since the 1930's is presented
in light of comments of Sutton (1960). It is reported that the Dutch
require that a stack must only be 1.5 times the height of the highest
building in the neighborhood. It is concluded that no elementary rule,
such as a 1.5 or 2.5 times rule, can be applied to all situations. The
air flow in mountainous areas is stated to be quite complicated with
terrain irregularities located many stack heights upwind and downwind
influencing plume motions. It is suggested that, whenever a potential
pollution problem results from an effluent emitted by a stack located in
all but perfectly uniform terrain, wind-tunnel studies should be considered.
Gloyne, R. W., 1965: Some Characteristics of the Natural Wind and Their
Modification by Natural and Artificial Obstructions. Scientific Horticul-
ture, XVII, 7-19.
Some characteristics of wind field modification by natural obstructions
are reported. An eddy flow 2 barrier heights in vertical extent and 10
to 15 barrier heights in horizontal extent to the leeward side of a
"near solid" barrier was diagrammed. At ground level, the region of
distubed flow extended to about 30 barrier heights. Downwind of a
steeply sloped, wooded hill with a wind blowing at right angles to its
length, the disturbed flow is reported to also extend downwind to about
30 times its height. Additional discussions relevant to wind modifications
were also presented, and the point is made that each case must be assessed
separately. Slope angle and thermal stability and wind speed were
influential factors in determining the extent of terrain induced dis-
turbances.
Jensen, M., and N. Frank, 1965: Model-Scale Tests in Turbulent Wind.
Danish Technical Press, Copenhagen.
A large number of systematic wind-tunnel studies of concentration
downwind from an isolated chimney and a chimney on a house are reported.
An evaluation of the data indicates some building influence even for a
stack height three times the house height.
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Halitsky, J., G. A. Magony, and P. Halpern, 1966: Turbulence Due to
Topographical Effects. New York University, New York, Geophysical
Laboratory Report No. TR-66-5, 75 p.
Comparisons between the author's wind tunnel model results and
Davidson's (1963) field observations in the lee of Green Peak, Vermont
are reported. Best agreement resulted for the higher model wind speeds
suggesting that tests of this type be run with a minimum ridge height
Reynolds number of 1 x 10s. The field observations of a cavity and wake
flow generally fitted the model test results. The boundary layer and
upstream turbulence conditions were not simulated in the wind tunneV
tests.
Ukeguchi, N., H. Sakata, H. Okamoto, and Y. Ide, 1967: Study on Stack
Gas Diffusion. Mitsubishi Technical Bulletin No. 52.
The authors reported that downdraughts occur where the structures
and/or buildings stand near the stack, but these can be prevented on the
whole with the increase of stack height to 2.5 times greater than the
structures and/or buildings surrounding the stack. They stressed that
downdraughts produce very high ground level concentrations, depend on
the layout of structures and/or buildings, and must be avoided. A wind-
tunnel study examined the influence of a nearby building complex on
plume diffusion and found only a small effect when the stack was 2.5
times the building height and a negligible effect when the stack was
over 3 times the building height. No general rules are given as being
applicable to the effects of topography; thus wind-tunnel models are
used to assess air quality impact.
World Meteorological Organization, 1967: The Airflow over Mountains.
WMO, Geneva, Switzerland. Report No. 98, 43 p.
The World Meteorological Organization technical note concludes
that, over rugged terrain, whether the flow aloft is smooth or other-
wise, it usually rests on a turbulent wake. Although little descriptive
detail of such regions is presented in the report, many photographs
showed the wave structures above the wakes, as revealed by cloud formations,
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Berlyand, M. E., 1968: Meteorological Factors in the Dispersion of Air
Pollutants in Tovm Conditions. Symposium of Urban Climates and Building
Climatology, Brussels.
The author mentions that the character of air motion changes considerably
near hilly relief and can substantially influence pollutant dispersion.
The increase of concentration was reported to sometimes occur even if
the pollutant sources are located on elevated places, but near leeward
slopes where wtnd velocity decreases sharply and downward currents
arise. He states that at present numerical solution of the equations of
motions and wind tunnel experiments are carried out for each case.
Experiments on models of separate plants and buildings have permitted
determination of zones in which downward currents and pollutant stagnations
are possible. The "2.5 times rule" is referenced as the recommended
stack height in order to avoid considerable increases of concentration.
Halitsky, T., 1968: Gas Diffusion Near Buildings. Meteorology and
Atomic Energy - 1968, D. H. Slade (Ed.), Chapter 5-5.
A detailed discussion of flow separation and wake formation near
buildings is presented. The introduction of a building into a backgound
flow is stated to cause changes in the velocity and pressure fields.
The new fields are called aerodynamically distorted, with the amount of
distortion measured by the difference between the distorted and the
Background properties. The author presents a literature review of flow
near characteristic structures. It appears that the flow downwind of
sharp-edged buildings is disrupted to a greated extent than for rounded
buildings. No definition of the vertical or horizontal extent of the
building wake which could be used to determine the height of a stack
sufficient to avoid adverse influence is presented.
Scorer, R. S., 1968: Air Pollution. Pergamon Press, Oxford, England,
pp 107-108.
The author discusses the consequences of a separated flow in the
wake of obstacles. Several examples- of adverse influences on chimney
effluents in the wake of buildings and steep hills are presented. The
examples are quite descriptive of the problem; however, no specific
definitions to the size and extent of wake effects are given. It is
suggested that chimney tops be cleanly shaped without elaborate decoration
or increase in exterior size and that the efflux velocity should be
enough only to prevent downwash into the wake and cold inflow. It is
noted that devices have been employed to prevent chimney downwash. The
author also states that, if chimneys need to be short, there are many
devices which can be employed to prevent separation at salient edges.
One such device is shown.
A-U
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Strom, G. H., 1968: Atmospheric Dispersion of Stack Effluents. In:
Air Pollution. Vol. I, Stern, A. C. (Ed.), Academic Press, New York,
A brief discussion of the effects on plume dispersion induced by
terrain and Buildings 15 presented. The results of several wind tunnel
experiments are presented. The need for experimental procedures is
stated stnce there are no accurate analytical procedures. The adverse
effects were seen to be greater when the wind was normal to the long
dimension of the Building. The desirability of designing stacks high
enough to have the plume remain clear of the highly turbulent regions is
s.tated. No specific definitions of the extent of the highly turbulent
regions is. presented. Evans 0957 1 is referenced as providing guidance
when experimental data is not available for specific cases.
Forsdyke, A. G., 1970: Meteorological Factors in Air Pollution.
Technical Note No. 114, World Meteorological Organization, Geneva,
Switzerland.
The following sentence is the only mention of stack height in
relation to the effect of building eddies which, if the chimney is not
high enough, wi-ll bring high concentrations of the pollutant down to
ground level in puffs. "To overcome this effect it is required in some
countries that the chimney height shall be at least two and one half the
freight of the but! ding from which it rises."
Pooler, F., Jr., and L. E. Niemeyer, 1970: Dispersion from Tall Stacks:
An Evaluation. Presented at the Second International Clean Air Congress,
Washington, D. C. December 6-11, 1970, Paper No. ME-14D. 31 p.
The authors present, as part of a study evaluating dispersion from
tall stacks, several situations in which unexpectedly high ground level
concentrations could be associated with mountain lee effects. On days
with neutral flow, the plume from a stack located 13 ridge heights
downwind from a 450. m ridge was carried down to ground level within a
very short distance. This phenomenon could well be a result of the
strong downwash that occurs near the leeward edge of a standing eddy.
A-15
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World Meteorological Organization, 1970: Urban Climates and Building
Climatology. Proceeding of the Symposium on Urban Climates and Building
Climatology, Jointly organized by the World Health Organization and WMO,
Brussels, October 1968, WMO Technical Note No. 108, 109.
Concern for potential adverse building effects upon plume dispersion
was mentioned in several of the symposium presentations. Only one of
the authors alluded to the "2.5 times rule" as referenced by Hawkins and
Nonhebel (1955). One of the general conclusions as reported by T. J.
Chandler was that "there is an urgent need to define much more vigorously
the physics of the urban surface—particularly its thermal and aerodynamic
properties." He also concluded that wind measurements within the cubic
of the city are clearly dependent upon very local conditions which
"makes it very difficult to use such field observations to construct any
general theory although simple models of airflow around single structures
may still prove of practical use. Wind tunnel and similar laboratory
techniques have a very real contribution to make in these enquiries."
Meroney, R. N. and B. T. Yang, 1971: Wind-Tunnel Study on Gaseous
Mixing Due to Various Stack Heights and Injection Rates Above an Isolated
Structure. USAEC REport No. COO-2053-6.
This wind-tunnel study examines the influence of a simple cubical
structure on the dispersion of a tracer gas released from short stacks
at varying heights and exhaust velocities. Both smoke visualization and
quantitative concentration measurements were made. The conclusions of
this study include;
(11 For a stack less than 1.5 times the building height, high
exhaust velocities cannot prevent some immediate downwash.
(21 As the stack height increases, the effect of building en-
trainment decreases. Exhaust velocities, for stack heights greater than
twice the building height, apparently need only be high enough to avoid
downwash behind the stack itself.
C3) Building orientation apparently aggravates entrainment even
for a simple cubical structure, however, the effect is not a major
consideration here. (For more complicated building complexes, the
influences may be more significant.)
A-16
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Orgill, M. M., 0. E. Cermak, and L. 0. Grant, 1971: Laboratory Simu-
lation and Field Estimates of Atmospheric Transport - Dispersion Over
Mountainous Terrain. Colorado State University, Fort Collins, Colorado.
Technical Report Mo. CER70-71MM-JEC-LOG40.
An extensive literature review relating to both field and fluid
modeling studies and a discussion as to how mountainous terrain can
alter atmospheric airflow is presented. The authors report that, for
neutral airflow over a mountain, a large semipermanent eddy occurs on
the lee side. An area in the central Rocky Mountains of Colorado was
chosen for a field and laboratory study of transport and dispersion over
irregular terrain. Two different atmospheric conditions were simulated:
the thermal stability used in the wind tunnel model was near-neutral in
the lower levels and stable in the upper levels for one case and totally
neutral throughout for the other case. Field data yielded information
on the mean velocity and dispersion characteristics over the local
terrain. Totally neutral atmospheric stability conditions were observed
on only one day. No specific information as to where and when boundary
layer separation occurs or the size or shape of the cavity region in the
lee of ridges is reported in either the field or laboratory study
results. The purpose of the report is to generalize on flow patterns in
complex terrain on a much larger scale.
Yasuo, I., 1971: Atmospheric Diffusion Theory of Factory Exhaust Smoke
and Its Applications. Water Engineering Series, published fay the Japan
Society of Civil Engineers, Hydraulics Committee.
The author presents equations for providing air quality estimates
that are intended for flat land. When the stack height is less than
2.5 times the height of buildings [or the mountains near the stack), it
is suggested that the exhaust gas will be swept down into the turbulence
area caused by the buildings. When this phenomena occurs, simulation
methods using wind tunnels and other special techniques are used.
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Lucas, D. H., 1972: Choosing Chimney Heights in the Presence of Buildings.
Proceedings of the Interantional Clean Air Conference, Melbourne Australia,
May 15-18, 1972, 27-52.
A chimney 2.5 times the height of any adjacent building is reported
to follow the widely accepted rule of thumb to avoid effects by building
turbulence. The fact that the building width must also be relevant in
deciding the effect of the building is discussed. The essential dif-
ference for a tall thin building is that flow around the building reduces
the effect of flow over the building. It is generalized for all buildings
that a building wake has a height above the building of 1.5 times the
height of width of the building, whichever is less. The extent of the
turbulent wake is reported to be pronounced for a distance downwind of
approximately five building heights or half-widths, whichever is less.
While there is no abrupt cut-off in fact, it is considered convenient to
take the effect as declining progressively to zero from 5 to 10 building
heights or half-widths, whichever is less.
Schultz, J. G., 1972: Self Pollution of Buildings. The ASME Proceedings
of the 1972 National Incinerator Conference, New York, NY. June 4-7,
1972, 201-210.
It is suggested that good design for a chimney or exhaust system is
to locate them above the eddy area. Otherwise, there will be recycling
of exhaust products in to the air intake to contaminate the entire
building, the vertical extent of the eddy over a cubical building is
given according to Evans (1957) as 1.5 times the building width.
Shingi, K., 1972: Wind Tunnel Experiment on Ascent Height of Exhaust
Gas. Central Research Institute of Electric Power Industry Report,
71053 (Translated from Japanese)., 26 p.
The results of wind tunnel exepriments on the ascent height of
exhaust gas from thermal and nuclear power plants are reported, and
studies are made of the ascent height with relation to down-washing,
down-draught, and the "stack type. The laws of wind tunnel similarity
are also discussed. It was found that stack down-washing does not occur
if the ratio between the exhaust gas speed and wind speed is more than
two. For the power plants studied, down-draught in the wake of the
building did not occur even when the stacks were much lower than 2.5
times the building hieght, if the exhuast gas rate was large enough.
The author comments that the 2.5 times law does not have a theoretical
basis making it applicable to all cases.
A-18
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American Society of Mechanical Engineers, 1973: Recommended Guide for
the Prediction of Airborne Effluents. Smith, M. (Ed.), New York, AMSE,
_T_
One section of the book discusses the influence of buildings and
irregular terrain. It is reported that few quantitative diffusion
experiments have been made in irregular terrain; however, visual observations
of plume behavior in a variety of situations have been made. The plume
from a stack placed in the cavity leeward of a valley ridge is said to
become thoroughly diffused before passing downwind to the wake region
where the flow was in the direction of the upper wind. The air flow
disturbed locally by buildings is shown to influence that portion of the
plume which penetrates the disturbed flow region. Changes in building
shape and orientation to the wind are reported to affect the cavity
dimensions and flow to a marked degree, but the gross dimensions of the
displacement zone and wake for sharp-edged buildings appear to be a
function primarily of the frontal area of the building presented to the
wind. Also for rounded buildings, both the displacement zone and wake
are. smaller than for sharp-edged buildings since separation usually
occurs downwind of the center of the buiilding where the direction of
the surface flow just prior to separation is horizontally downwind
rather than normal to the wind. No quantitative definitions of th
vertical or downwind extent of the region of adverse influences near
buildings or terrain are given.
A-19
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Briggs, G. A., 1973: Diffusion Estimation for Small Emissions. Atmos-
pheric Turbulence and Diffusion Laboratory, "OAA, OAK Ridge, TN, (Draft)
ATDL No. 75/15.
A method for estimating air quality concentrations for emissions
influenced by buildings is presented. The plume is considered to be
within the region of building influence only when the estimated source
height is less than the building height plus 1.5 times the building
height or width, whichever is less. The "cavity" region where there is
circulation of the flow within the wake of the building is defined to
equal the building height plus 0.5 times the building height or width,
whichever is less.
Peterka, J. A. and 0. E. Cermak, 1975: Turbulence in Building Wakes.
Presented at 4th International Conference on Wind Effects on Buildings
and Structures, London, United Kingdom. Colorado State Univ. Report iio.
CEP74-75 JAP-JEC 34.
The mean velocity and turbulence characteristics in the wake of
simple rectangular-shaped builidngs were measured in a boundary layer
wind tunnel. The mean velocity deficit, turbulence excess, and longi-
tundinal vorticity relative to the undisturbed turbulent boundary layer
are presented and discussed. The conclusions of this study include;
(1) The turbulence wake effects of single building heights do not
extend beyond 15 to 20 building heights and can be much less for a tall,
narrow building.
(2) Mean velocity effects in the wake do not extend beyond 15 to 20
building heights except when the angle at flow is such that corner
vortices are formed over the building roof.
(3) Within the primary wake region, the wake can extend 4 to 5
building heights in the vertical direction and 4 to 5 building widths in
the lateral direction for a strongly three-dimensional building.
(4) The data show that the wake characteristics of tall, narrow
buildings and low, long buildings are different. Furthermore, neither
the characteristics for a building of complex shape nor for a group of
buildings has been investigated.
(5) When the flow relative to a builidng is such that corner vortices
(swirling motion) are formed over the building roof, a longitudinal
vorticity was observed as far as 30 building heights downwind.
A-2Q
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Smith, D. G., 1975: Influence of Meteorological Factors Upon Effluent
Concentrations On and Near Buildings with Short Stacks. Presented at
tHe. 618th\ Annua.1 Meeting of the Air Pollution Control Association,
Bjpston, flass., June 15-20, 1975, Paper No. 75-26.2.
Field data of concentrations from stack emissions near a scaled-
down model of an industrial butlding is presented. The tests were
conducted for selected conditions of atmospheric stafailility, aerodynamic
roughness of upwind fetch", and wind orientation angle of the building.
T5e extt velocity was greater than twice the wind speed for all tests to
eliminate stack downwash as available. The study was designed to measure
the. amount of effluent reaching the building and ground surfaces in the
downwind wake cavity of the building under a variety of stack heights.
Concentrations along the lee wall of the building were measurable, even
when tfte stack was 2 to 2.5 times the building height. However, much
frigHer concentrations were found when that stack was less than 1.5 times
the burldi.ng height.
Brttter, R. E., J. C. R.'Hunt, J. S. Puttock, 1976: Predicting Pollution
Concentrations Near Buildings and Hills. Presented at the Conference on
Systems and'Models in Air and Water Pollution, at the Institution of
Measurement, London, Sept. 22-24, 19.76.
Several simple mathematical representations of different parts of
the flow field near buildings and hills are presented. These models are
biased on theoretical arguments applicable to two-dimensional flow.
Reltable calculation methods for the mean turbulent flow around obstacles
.(three-dimensional ts implied! are stated to not exist. The effects of
the distorted flow, in the wake behind two-dimensional bluff surface
obstacles in a turbulent boundary layer, upon emissions of various
h.etght and downwind locations is evaluated. A source elevated to only
1.5 times the obstacle height is found to be greatly influenced unless
it ts placed farther than 10 obstacle heights downwind. The influence
upon a source elevated to 2.5 times the obstacle height is found to be
much less, however, the effect extends to sources as far downwind as 20
obstacle heights. No significant effect is found for source heights
that are greater than 3 times the obstacle height.
A-21
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Huber, A. H., W. H. Snyder, R..S. Thompson, and R. E. Lawson, Jr., 1976:
Stack Placement in tFie Lee of a Mountain Ridge. U. S. Environmental
Protection Asency, EPA^6J3Q/4-76^!I47, ftesearclTPtangU farfc NC.
A wind tunnel study was conducted to examine the effects the
highly turbulent region in the lee of a two-dimensional mountain ridge.
Smoke visualization and hot film anemometry measurements showed that the
cavity size and shape were minimally affected by the thickness and
turbulence intensity of the approach, boundary layer flow. The size of
the region of strong circulation in the lee of the model ridge was found
to be strongly dependent upon the upwind terrain and the gross topographic
features .(emglesL of the downslope. The largest cavity was found to
extend to two rtdge heights in the vertical and to ten ridge heights
downwind. A stack 2.5 times the height of the ridge is stated to avoid
the highly- turbulent region of the cavity proper. It is implied that a
taller stack, may- be necessary to avoid all wake effects since part of
the plume can, in only a short distance, spread downward into the wakes.
Tfi.e need for studies of the behavior of plumes from sources placed
downwind of the cavity region is stated since the turbulence intensity
downwind of the cavity was found to be still significantly greater than
i;n the undisturbed flow.
Ruber, A. H. and W. H. Snyder, 1976: Building Wake Effects on Short
Stack Effluents. American Meteorological Society, Third Symposium on
Atmospheric Turbulence Diffusion and Air Quality, Raleigh, NC Oct. 19-
22, 1976, 235-241.
A wind tunnel study was conducted to examine building wake effects
on effluents from stacks near a building whose width is twice its height.
Some discussion of the building influences on the plume dispersion is
presented, for those sources having an effective stack height less than
2.CL buvlding heights, very significant effects upon measured ground
level concentrations were found. Visual observations of smoke were also
made in order to assess the Building influence upon stack emissions.
There was significant reduction in building effect for the most elevated
stack wfnxh was 2.5 times the building.
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Snyder, W. H. and R. E. Lawson, Jr., 1976: Determination of a Necessary
Height for a Stack Close to a Building—a Wind Tunnel Study. Atmospheric
Environment, TJD, 683-691.
Wind tunnel tests shows a stack 2.5 times the building height is
adequate for a building whose width perpendicular to the wind direction
is greater than its height, but unnecessary for a tall, thin building.
Smoke was used for flow visualization and quantitative concentration
measurements of a tracer gas emitted with the stack effluent were made
downwind of the building. For a tall, thin building, application of an
alternative to the 2.5 times rule (Briggs, 1973) was shown to be ade-
quate. Thus, it is concluded that a sufficient stack height in order to
not have the plume entrained into the wake of the building is equal to
the building height plus 1.5 times the building height or width, whichever
is less.
Frost, W. and A. M. Shahabi, 1977: A Field Study of Wind Over a Simulated
Block Building. NASA CR-2804 prepared by the Univ. of Tenn. Space
Inst., Tullahoma, Tenn.
A field study of the wind over a building 2.4 m (deep) x 3.2 m
(.high) x 26.8 m (long) is reported. The study was designed to provide a
fundamental understanding of mean wind and turbulence structure of the
wind field. Eight instrumented towers were placed in the region both
upwind and downwind of the building. Horizontal and vertical wind
sensors were placed at the 3, 6, 12, and 20 meter levels. Approximately
100 experimental runs have been conducted. Hand held smoke candles and
anemomenters were used to define the extent of the region of recirculating
flow downwind from the building with its long side oriented perpendicular
to the flow. The downwind extent was about 12 ± 2 building heights.
This is compared to values of 13-16 building heights reported for similar
two-dimensional laboratory tests. The smoke patterns indicate that the
wake extends to a height of approximately 1.5-2 building heights. The
values of the velocity components at the 3 m level were strongly influenced
by the building, but at the 12 m (^ 3 building heights) level the influence
was not apparent.
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International Atomic Energy Agency, 1977: Guideline for Atmospheric
Dispersion Estimates. Vienna, Austria.
It is reported that the motion of effluents near bluff bodies, such
as buildings, is affected by-distortion of the windfield. Stacks at
least twice the hetght of the tallest adjacent building are usually
necessary except when the discharges are insignificant. Because of the
great variety-of possible terrain conditions, a generalized treatment of
the effects of features such as hills or valleys is stated as not feasible,
since tn.e exact flows will be extremely site-dependent. The use of
fluid flow modeling is suggested as providing some help in estimating
the plume trajectory near hilly terrain.
RoBins, A. G. and I. P Castro, 1977: A Wind Tunnel Investigation of
Plume Dispersion in the Vicinity of a Surface Mounted Cube-I. The Flow
field, It. Tfie Concentration Field. Atmospheric Environment,
17., 221-311.
Experiments investigating both, the flow field and plume behavior
downstream of an isolated surface mounted cube in the Marchwood Engi-
neering Laboratory wind tunnel are reported. The wake air flow was
found to be strongly affected by upstream turbulence. For both a 0° and
45P orientation of the building into the wind, the effective wake zone
in a turbulent boundary layer extended upwind to about five times the
height of the cube. The region of reversed flow extended downwind to
1.5 heights for wind angle, 9, of 0°, and 2 heights for e of 45°. The
mean velocity deficit was reported to extend to twice the building
height for both the Q° and 45° orientation. A tracer gas was emitted
from a stack over the roof center. The stack extended from building
height to 2.5 times the building height. The influence of the building
was found to be detectable for e = 0 degrees and a low stack emission
rate; however, for a ratio of emission velocity to wind speed of 3:1,
tfie influence was negligible for a stack height 2.5 times the building
Fietgftt. For 0 = 45° the influence of the cube was detectable for all
the stack heights and emission velocity ratios. It is concluded that
jnucft work remains to be done on the influence of nearby buildings on the
Behavior of chimney plumes. Also, it is especially important to model
correctly the approach flow when undertaking wind tunnel investigations
of diffusion in the vicinity of isolated buildings.
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Hosker, R. P. Jr., 1981: Methods for Estimating Wake Flow And Effluent
Dispersion Near Simple Block-Like Buildings. NOAA Technical Memorandum,
ERL ARL-108.
The report consolidates available data and methods for estimating
flow and effluent dispersion near isolated block-like structures. The
report is intended for those who routinely face air quality problems
associated with near-building exhaust stack placement and height and the
resulting concentration patterns.
Meroney, R. N., 1982: Turbulent Diffusion Near Buildings. Engineering
Meteorology. Elsevier Science Publishing Company, Amsterdam.
A review of information and methods for estimating flow and diffusion
near buildings is presented. Transportation associated diffusion and
buoyancy dominated dispersion problems are also reviewed.
Wilson, D. J. and Winkel, G., 1982. The Effect of Varying Exhaust Stack
Height on Containment Concentration at Roof Level. ASHRAE Transactions.
88:1.
The results of a wind tunnel model study of stack height on concentra-
tions at roof level is presented. The emphasis of this study is on the
design of industrial ventilation systems. Reference is made to Chapter
14 of the ASHRAE Handbook--1981 Fundamentals for Additional Design
Methods.
Wilson, D. J. and R. E. Britter, 1982. Estimates of Building Surface
Concentrations From Nearby Point Sources. Atmospheric Environment,
_16_, 2631-2646.
The results of a wind tunnel model study of concentrations at building
air intakes as a result of upwind sources, surface sources and downwind
sources within the near building wake recirculating region is presented.
A qualitative description of the relevant dispersion mechanisms and then
some theoretical and experimental results are given.
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Snyder, W. H., 1983. Fluid Modeling of Terrain Aerodynamics and Plume
Dispersion, A Perspective View. Preprints Volume, Sixth Symposium on
Turbulence and Diffusion, American Meteorological Society, Boston,
March 22-25, 317-320.
The results and conclusions of several recent fluid model studies
conducted at EPA's Fluid Modeling Facility are summarized. Including:
a. Arya, S.P.S. and Shipman, M.S., 1981: An Experimental Investigation
of Flow and Diffusion in the Disturbed Boundary Layer Over a
Ridge; Part I: Mean Flow and Turbulence Structure, Atmos. Envir.,
v. 15, no. 7, p. 1173-84.
b. Arya, S.P.S., Shipman, M.S. and Courtney, L. Y. 1981: An Experi-
mental Investigation of Flow and Diffusion in the Disturbed
Boundary Layer Over a Ridge; Part II: Diffusion from a Continuous
Point Source, Atmos. Envir., v. 15, no. 7, p. 1185-94.
c. Castro, I. P. and Snyder, W. H., 1982: A Wind Tunnel Study of
Dispersion from Sources Downwind of Three-Dimensional Hills,
Atmos. Envir., v. 16, no. 8, p. 1869-87.
d. Hunt, J.C.R. and Snyder, W.H., 1980: Experiments on Stably
and Neutrally Stratified Flow over a Model Three-Dimensional
Hill, J. Fluid Mech., v. 96, pt. 4, p. 671-704.
e. Khurshudyan, L.H., Snyder, W.H. and Nekrasov, I.V., 1981:
Flow and Dispersion of Pollutants over Two-Dimensional
Hills: Summary Report on Joint Soviet-American Study,
EPA-600/4-81-067, U. S. Environmental Protection Agency,
Research Triangle Park, NC.
Under neutral conditions, the maximum ground level concentrations
occurred with the source located just downwind of a two-dimensional ridge.
Terrain amplification factor's are found to range from 0.5 for sources on
top of two-dimensional ridges to 15 for sources downwind of two-dimensional
ridges. Terrain amplification factors for three-dimensional hills have
values in between the above for two-dimensional ridges.
Fackrell, J. E., 1984. Parameters Characterising Dispersion in the Near
Wake of Buildings. Journal of Wind Engineering and Industrial Aero-
dynamics, ]6_, 97-118.
The paper describes wind-tunnel measurements of near-wake parameters
for many different building shapes in a variety of boundary-layer flows.
A few cases were also examined with two buildings and variable downstream
spacing.
A-26
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Lawson, R. E. Jr., 1984. Effect of Terrain-Induced Downwash on Determination
of Good-Engineering-Practice Stack Height. U.S. Environmental Protection
Agency, Research Triangle Park, NC, July.
Terrain amplification factors were measured for a variety of source
positions (locations and heights) both upstream and downstream of two
model hills, an axisymmetric hill and a two-dimensional ridge. The
spatial variation of these terrain amplification factors was used to
delineate the vertical and longitudinal extent of the areas where excess
concentrations (terrain amplification factor >1.0) occurred. For the
axisymmetric hill, a region of 40% excess concentration was found to
extend a maximum of 1.8 hill heights in the vertical, 14 hill heights
upstream, and 10 hill heights downstream. For the two-dimensional ridge,
this region of 40% excess concentration extended 2.2 hill heights in the
vertical, 8 hill heights upstream, and 15 hill heights downstream.
Maximum terrain amplification factors for both the axisymmetric hill and
the two-dimensional ridge were found on the downstream side of the hills
and had values of approximately 5.6 and 6.8, respectively.
Hosker, R. P. Jr., 1984: Flow and Diffusion Near Obstacles. Atmospheric
Science and Power Production. D. Randerson, Editor, U.S. Department of
Energy Technical Information Center DOE/TIC-27601.
A comprehensive consolidation of information of flow and effluent
diffusion near obstacles is presented. Available data on the extent of
the upwind influence of a body, the characteristics and size of the
near body flow, and the behavior at the far wake is summarized.
A-27
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-450/4-80-023R
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
Guideline for Determination of Good Engineering
Practice Stack Height (Technical Support Document for
the Stack Height Regulations)(Revisedj
June 1985
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME. AND ADDRESS
Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This revised guideline provides background information used to develop a means of
computing good engineering practice (GEP) stack height according to the requirements
of Section 123 of the Clean Air Act, as amended. The report also summarizes the
application of the structure-based formula to determine GEP stack height under
different general building formations.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI field/Group
Air Pollution
Good Engineering Practice Stack Height
Air Pollution Control
18. DISTRIBUTION STATEMENT
Unlimited'
19. SECUR'TY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
102
20. SECURITY CLASS (This page)
Unclassified
!2. PRICE
EPA Form 22:0-1 (Rev. 4-77)
£ EDITION IS OBSOLETE
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