&EFK
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-340/1-84-012
July 1984
Air
Performance
Evaluation Guide
For Large Flow
Ventilation Systems
M>R
f EPA LIBRARY SERVICES RTP NC
Eft 340/1- £4--0
TECHNICAL DOCUMENT COLLECTION
-------
EPA-340/1-84-012
Performance Evaluation Guide
For Large Flow Ventilation Systems
by
William Kemner
Richard Gerstle
Yatendra Shah
Contract No. 68-01 -6310
Work Assignment No 119
EPA Project Officer: John Busik
EPA Project Manager: Dwight Hlustick
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards *«>»•'
Stationary Source Compliance Division { Pt"r,-T-!?C» ?
Washington, DC 20460
May 1984
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DISCLAIMER
The Information in this document has been funded wholly or in part by
the United States Environmental Protection Agency under Contract No. 68-01-
6310 to PEDCo Environmental, Inc. It has been subject to the Agency's peer
and administrative review, but it does not necessarily reflect the views of
the Agency and no official endorsement should be inferred.
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CONTENTS
Page
Figures v
Tables viii
Acknowledgment ix
1. Introduction 1
2. General Ventilation and Hooding Principles 3
Design basis 5
3. Hood Design Considerations 11
Determining air flow requirements 16
4. Duct Design and Considerations 33
Transport velocities 34
Energy losses 36
Branched systems 43
5. Fan Systems 46
Fan types and operating characteristics 46
Forced versus induced draft 55
Fan requirements for emission control system applications 57
Fan arrangements 60
Fan drives 60
Fan controls 61
Fan sizing 66
6. Ventilation System Inspection 71
Preparing for inspection 71
Safety considerations 72
Onsite company-inspector interaction 73
Inspection procedures 73
Operation and maintenance (O&M) considerations 79
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CONTENTS (continued)
Page
7. Total Furnace Enclosures 85
Electric arc furnaces 85
Basic oxygen furnaces 95
8. Special Applications 103
Coke oven sheds 103
Electric arc furnace ventilation 107
Blast furnace casthouse control 115
Control systems on basic oxygen furnaces (BOF's) 117
Building evacuation 123
Copper converters 124
References 128
Appendix A - Bibliography A-l
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FIGURES
Number Page
1 Matrix of Hooding Applications for Participate Control 4
2 Example of Open and Closed Hooding on the Discharge End
of a Sinter Strand 6
3 Direct Shell Evacuation on an Electric Arc Furnace 8
4 Variation in Gas Flow Rate From a BOF During the Course
of a Heat 10
5 Conveyor Transfer Point Hooding Using Total Enclosure 12
6 Exterior EAF Canopy Hood 14
7 Effect of Excessive Plume Velocity 17
8 Velocity Contours (Expressed in Percentage of Opening
Velocity) and Streamlines for Circular Openings 19
9 Velocity Contours and Streamlines for Flanged Hood 19
10 Formulas for Estimating Hood Air Flows 20
11 Dimensions Used to Design High-Canopy Hoods for Hot
Sources 21
12 Schematic Arrangement of Ladle Hood for Reladling
Emission Control 25
13 Controlled Airflow From a Heated Source 28
14 Uncontrolled Airflow From a Heated Source 28
15 General Principle of the Push-Pull (Air-Curtain) Type
System 29
16 Comparison of Face Velocity Decay for Blowing Versus
Exhausting 31
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FIGURES (continued)
Number
17 Air Curtain Control System on a Capped Converter 32
18 Dew-Point of Air Containing Various S03 Concentrations 35
19 Pressure Measurements in Ducts 38
20 Simple Pressure Diagram 42
21 Taper Duct System 44
22 Centrifugal Fan Components and Layout of a Typical
Industrial Fan System 47
23 Centrifugal Fan Blade Configurations and Impeller
Arrangements 48
24 Fan Testing Procedure and Typical Characteristic Curves 50
25 Typical Characteristic Curves for a Backward-Curved-Blade
Centrifugal Fan 51
26 Typical Characteristic Curves for a Forward-Curved-Blade
Fan 53
27 Typical Characteristic Curves for a Straight-Blade Fan 54
28 Basic Principle of Induced Versus Forced Draft 56
29 Louvre Damper: (a) Parallel Blade Multilouvre; (b) Opposed
Blade Multilouvre; (c) View of Parallel Blade, Multi-
louvre Damper View Showing Linkage 63
30 Guillotine Damper: (a) Simplified Cross-Sectional View of
a Guillotine Damper; (b) Guillotine Isolation Damper
Using Seal Air; (c) Top-Entry Type Guillotine Damper,
Showing Operation 64
31 Butterfly Damper: (a) Simplified Cross-Sectional View of
a Butterfly Damper; (b) Butterfly Damper Showing Hand
Operator 65
32 Typical Furnace Enclosure 87
33 Furnace Enclosure at North Star Steel Company 91
34 Furnace Enclosure at Birdsboro Corporation 93
vi
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FIGURES (continued)
Number Page
35 Typical BOF Furnace Enclosure 97
36 Schematic of Basic Oxygen Secondary Emission Control
System of Kaiser Steel-Fontana 98
37 Schematic of Q-BOP Secondary (Charging) Emissions
Control System of Republic Steel, Chicago 100
38 Various Shed Configurations 104
39 Thermal Expansion of Hot Gases From a Push 105
40 Electric Arc Furnace Utilizing Partial Enclosure 108
41 Ventilation Systems for Electric Arc Furnaces 110
42 Combined Direct Shell Evacuation With Canopy Hood 111
43 Multiple Pickup Points Vented to Common Control Device 112
44 Flow Rate Required for Electric Arc Furnace Control 114
45 BOF Hood Arrangements 119
46 Canopy Hood Concept for BOF Charging Emissions 121
47 Gaw Damper (Closure Plate) Use in BOF Control 122
48 Pierce-Smith Converter 125
49 Secondary Converter Hood Configuration 126
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TABLES
Number Page
1 Comparison of Principal Data for 10, 30, and 100 Percent
Combustion in BOF Hooding 9
2 Range of Capture Velocities 15
3 Range of Design Velocities 37
4 Conversion Table for Duct Velocity to Velocity Pressure 39
5 Relative Advantages of Using Duct Dampers 45
6 Basic Fan Laws 58
7 Basic Damper Types 62
8 Air Density Correction Factor 68
9 Density of Common Gases 69
10 Data on EAF Plants Designed for Total Furnace Enclosure 94
11 Flow Balancing of a Typical Furnace Enclosure With
Additional Pickup Hoods, Connected to a Common Baghouse 95
12 BOF/Q-BOP Shops Utilizing Furnace Enclosures 101
13 Example of Flow Balancing of Multiple Evacuation System
on Electric Arc Furnace 113
14 Blast Furnace Casthouse Typical Volume Requirements 116
vm
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ACKNOWLEDGMENT
This report was prepared for the U.S. Environmental Protection Agency by
PEDCo Environmental, Inc., Cincinnati, Ohio. Mr. Dwight Hlustick was the EPA
Project Officer. Mr. William Kemner served as the PEDCo Project Manager.
The principal authors were Mr. Kemner and Messrs. Richard Gerstle and Yatendra
Shah. Messrs. Gopal Annamraju, Gary Saunders, and Lario Yerino prepared
specific sections of the report. The authors wish to thank Mr. Hlustick for
his overall guidance and direction on this task.
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SECTION 1
INTRODUCTION
The purpose of this manual is to familiarize Agency inspectors with the
design principles and O&M considerations for large-scale (i.e., generally
>50,000 cfm) ventilation systems commonly found in the metallurgical indus-
try. The emphasis is on the steel industry because most large, complex
systems are found in the many individual processes used in this industry.
Applications in copper smelting and other industries are also discussed.
Inasmuch as ventilation systems are highly complex from a design standpoint
and experience plays a major role in most designs, this manual should be
considered an introductory primer rather than a detailed design manual.
Several standard publications discuss ventilation principles and fan
1-4
engineering. In general, however, these publications emphasize smaller,
more traditional applications. Furthermore, their emphasis is on ventilation
in the general sense, as opposed to air pollution control. The complicating
factors found in ventilation of large metallurgical systems are either
treated in the abstract or not at all. Literature on operation and mainte-
nance practices and inspection procedures is very limited.
Air pollution control systems in the primary metals industry, particu-
larly the steel and copper segments, rely on large capture and ventilation
systems with flow rates commonly in the range of 50,000 to 1,000,000 acfm and
greater. These systems are used primarily to control process fugitive emis-
sions from various furnaces and for building evacuation.
Because these systems are an integral feature of the compliance programs
of the industries involved, this manual was initiated to accomplish the
following:
0 To provide inspection and operation and maintenance guidance to
state and local agency personnel who evaluate the performance of
these systems.
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0 To provide a comprehensive treatment of the existing literature
with regard to technical and specific aspects of typical designs.
0 To provide an easy-to-read technical manual on design and operation
for the use of inspectors.
Although not treated in detail in this manual, other technologies are
available for control of process fugitive emissions in the metallurgical
industries. These technologies, which do not involve hooding and ventilation
systems, include roof-mounted electrostatic precipitators ' and fume sup-
pression utilizing inert gases to suppress the oxidation of molten metal.
Both of these technologies offer promise of lower cost than conventional
ventilation approaches. They have been applied for control of charging and
tapping emissions in steel making plants and control of blast furnace casting
emissions. The former have not yet found application in the United States.
Sections 2 through 5 present technical factors of design for hooding,
ducting, and fans. Section 6 describes inspection procedures for use in
assessing the effectiveness and maintenance of ventilation systems. Section
7 deals with total furnace enclosures. In Section 8, the foregoing informa-
tion is supplemented in the context of special problems that are found in
several specific applications. The appendix contains a bibliography for
those interested in pursuing the subject matter further.
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SECTION 2
GENERAL VENTILATION AND HOODING PRINCIPLES
Process hooding and ventilation systems are required to capture and
transport emissions to some control device or vent. These hooding and venti-
lation systems sometimes also eliminate potential industrial hygiene problems
by reducing employees' exposure to an air contaminant and by removing heat
from the process area. Figure 1 summarizes the various types of hooding used
in processes in the metallurgical industries.
The three basic parts of a ventilation system are the hood or air in-
take, for initial capture of the emissions; the ductwork, for transport of
the gas stream to the vent or control device; and a fan, to move the gas
stream. Whereas the design of the basic hood and ventilation system is well
understood for small and medium-sized systems, the application of the same
principles to large processes often results in marginal or inadequate sys-
tems, especially when high-temperature processes are involved. This inabil-
ity to apply the same principles results primarily from the large size of the
equipment, the high heat loads, the variability of conditions in batch proc-
esses, the need for access to the process, and greater maintenance require-
ments. For larger systems, much of the design is left to the ingenuity and
experience of the designer, who must fit the hood around the process and lay
out the ductwork with minimal interferences.
Inadequate design of a ventilation system can compromise overall per-
formance. In all cases, the hood must be sized and oriented to capture the
maximum quantity of emissions without requiring excessive gas volumes (a
trade-off between performance and energy consumption). It makes little sense
to install a high-efficiency control device if a major portion of the emis-
sions are not captured initially. The hood should be as close as possible to
the point of generation without interfering with equipment movement and
process operation. It should be oriented to minimize cross-drafts and to
take advantage of thermal drafts.
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Application
§
U-
errous
*-
i
L.
V
O
Coke oven pushing
Basic oxygen furnace blowing
Basic oxygen furnace fugitives
Electric arc furnace refining
Electric arc furnace fugitives
Open hearth taoptng
Blast furnace casting
Blast furnace slag granulation
Steel scarfing
Grinding and shotblastlng
Hot rolling operations
Sinter strand discharge
Hot metal transfer
Raw metal conveyors
Gray Iron cupola
Primary copper converters
Primary copper reverberatory
furnaces
Primary lead blast furnace
Primary aluminum electrolytic
furnace
Casting shakeout system
Minerals handling
Minerals crushing
Type of hooding used
Fixed
hoods
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Movable
hoods
X
X
X
X
X
X
X
X
Air
curtain/
push-pull
systems
X
X
X
X
Enclosures
X
X
X
X
X
X
X
X
X
X
Curtain
walls
X
X
X
X
Runner
covers
X
X
X
X
Roof
canopies
X
X
Ladle
hoods
X
X
X
X
Close-
fitting
hoods
X
X
X
X
X
X
X
X
S1de-
draft
hoods
X
Telescoping
hoods
X
X
X
Sheds
X
Figure 1. Matrix of hooding applications for participate control.
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The ductwork leading from the hood (or pickup point) to the control
device must be sized to provide the needed transport velocity—generally
between 15 and 25 m/s (2700 and 4500 ft/min) --depending on particulate
loading and size distribution. Layout of ductwork should minimize energy
losses caused by bends, transitions, branches, etc., and should also minimize
air inleakage. If the source is hot, refractory lining or water-cooled hoods
and ducts may be required.
The three basic types of process hooding and venting systems are close-
fitting hoods, canopy hoods, and so-called building evacuation. (The latter
term is used loosely because in very few cases is an entire building actually
evacuated.) More than one of these three systems may be utilized on a single
process.
2.1 DESIGN BASIS
The most important of the three ventilation system components (i.e., the
hood, the duct, and the fan) is the hood. The ventilation system will not
perform well unless the hood effectively captures the emissions. The hood
design and open face area determine the amount of air that is drawn into the
system to capture the emissions. The volume of air and the process emissions
then determine the size of the ductwork, and these factors and the pressure
drop required by the control device in turn determine the size of the fan.
To minimize capital cost and fan power requirements, the designer tries to
minimize the amount of outside air drawn into the system. A face velocity of
200 to 500 ft/min is usually required through the hood's open area. Thus, to
minimize total ventilation air requirements, the hood must fit closely to the
process and have a small open area. The inability to achieve these goals is
the major problem in the applications discussed in this manual. Figure 2
illustrates the enclosure principle applied to a hood on the discharge end of
a sinter strand. In this application, nearly total enclosure is possible.
Under some conditions, the hood may be fitted directly to the process
[(e.g., direct shell evacuation (DSE) on an electric arc furnace (EAF) in
which the furnace roof serves as the hood)]; this arrangement allows only the
process gases (and minimal air infiltration) to pass through the vent system.
Even in these cases, however, high temperatures and explosive gases must be
considered. The qases must be transported at concentrations that are less
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INDRAFT AIR
COOLER
0 V*
LOSED
ENCLOSING
HOOD
SINTER f ,~
STRAND V v
NDRAFT AIR
Figure 2. Example of open and closed hooding on the
discharge end of a sinter strand.
Courtesy: Industrial Ventilation. 16th Ed. Published by American
Conference of Governmental Industrial Hygienists. 1980.
-------
than the lower explosive limit (LEL). In the electric arc furnace arrange-
ment just mentioned, for example, a gap is provided to allow sufficient
infiltration of air for combustion of carbon monoxide and dilution of its
concentration (CO). This gap also permits the furnace to be tilted as illus-
trated in Figure 3.
In other arrangements, the hood is separated from the process for access
purposes or to allow outside air to mix with and cool, dilute, or combust the
process exhaust. The much larger open area of these arrangements requires
much greater total exhaust flow. Two basic designs are used in the control
of emissions from basic oxygen furnaces (BOF's), the so-called open-hood and
the more-energy-efficient closed-hood approach. Application of a ventilation
system to a BOF is more complex than most applications because the exhaust
gas is primarily carbon monoxide, which, in open-hood systems, is combusted
in the hood by the indraft air. This combustion leads to temperature and
volume variations that must be accounted for in the design of the exhaust
system. With closed-hood systems, the CO is not burned in the hood. The
CO-rich gas is cleaned and then flared or used as a fuel. Thus, for closed-
hood systems, air infiltration must be minimized to avoid explosions in the
gas-cleaning system; its major advantage is the greatly reduced size of the
gas cleaning equipment. Table 1 illustrates the difference in flow rate
associated with these BOF systems. The difference in flow rate also dictates
the type of control device used. A scrubber, for example, would be very
expensive to operate on the high flows resulting from the open-hood approach.
Another common problem in many systems is the variability of conditions.
Conditions may vary from one season to another (i.e., ambient temperature
and/or ventilation requirements), from one heat to another, or even from one
moment to the next as process conditions change. For example, Figure 4
illustrates the variation in gas flow during a heat in the BOF process. This
variation occurs over a period of 15 to 20 minutes.
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INFILTRATED
AIR,
EXHAUST POSITION
TILT POSITION
Figure 3. Direct shell evacuation on an electric arc furnace.
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TABLE 1. COMPARISON OF PRINCIPAL DATA FOR 10. 30, AND 100 PERCENT
COMBUSTION IN BOF HOODING3
Varying parameters
Total gas volume, scfm
Theoretical gas temperature
inlet hood, °F
Heat to be removed in hood,
106 Btu/h
Fan horsepower of high-energy
scrubber, kW
Type of hooding and combustion rate
Semi -open
100%
158,800
4352
889
4100
Semi -closed
30%
87,000
3992
325
2200
Closed
10%
66,700
3272
167
1640
Open
200%
318,600
b
b
8200
Reference 8.
NA = Not available.
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SHOP A
BLOWING TIME
Figure 4. Variation in gas flow rate from a BOF
during the course of a heat.
Courtesy: JAPCA, 18(2):98-101, February 1968. Article by D. H. Wheeler
entitled "Fume Control in L-D Plants."
10
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SECTION 3
HOOD DESIGN CONSIDERATIONS
The three principles of optimum hood design are:
0 Enclosure of the process or source insofar as possible.
0 Location of exterior hood in path of exhaust.
0 When exterior hood is used, minimization of interference from
cross-drafts.
The goal of good hood design is high capture efficiency. Ideally, a
process should be entirely enclosed, which would permit almost 100 percent
capture efficiency. Simple conveyor transfer hoods (Figure 5) provide an
example of total enclosure. Because frequent access to a process (to charge
materials, remove products, or perform maintenance) is usually required, most
hoods have open areas to provide this access. These open areas must be
maintained under a negative pressure by drawing air into the system, which
prevents fumes from escaping. Although this concept is simple in principle,
its application is complicated by variations in process emissions, thermal
currents from hot processes, and cross-drafts that interfere with the inflow
of air into the hood.
Hoods can be classified into three broad groups: enclosures, receiving
hoods, and exterior hoods. Enclosures usually surround the point of emis-
sion, but sometimes one face is partially or even completely open. Examples
of enclosures are paint spray booths, abrasive blasting cabinets, totally
enclosed bucket elevators, and enclosures for conveyor belt transfer points,
screens, crushers, etc. The sides of the enclosure effectively reduce cross-
drafts and also direct the plume toward the capture hood.
Receiving hoods are those in which the air contaminants are injected
into the hoods and inertial forces carry these emissions into the hood.
These hoods are generally applied to smaller processes that impart a velocity
11
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ENCLOSE TO PROVIDE 150-200 fpm
INDRAFT AT ALL OPENINGS
MIN. Q = 350 efm/ft BELT WIDTH FOR BELT
SPEEDS <200 fpm
= 500 cfm/ft BELT WIDTH FOR BELT
SPEEDS >200 fpm
FOR FALLS GREATER THAN 3 FT WITH DUSTY
MATERIAL, PROVIDE ADDITIONAL EXHAUST QA
BELT WIDTH 12 in. to 36 in. QA = 700 cfm
ABOVE 36 in. QA = 1000 cfm
FLEX STRIPS
RUBBER SKIRT
Figure 5. Conveyor transfer point hooding using total enclosure,
12
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to the emissions, such as grinders and paint sprayers. They are not appli-
cable to the large systems discussed in this manual.
Exterior hoods must capture air contaminants that are being generated
from a point outside the hood itself. These hoods are generally used for
large systems that generate heat and require frequent access. Figure 6 is a
simplified diagram of an exterior or canopy-type hood. In this example, the
hood design is augmented with baffles and dampers to direct suction to one of
three sections, depending on the source of the emissions. This enhances
capture efficiency by deer-easing the effective face area.
The total air flow into the hood system is determined by Equation 1:
Q = A V (Eq. 1)
where Q = Total air flow, cfm ~
A = Cross-sectional area, ft
V = Air velocity perpendicular to open face area, ft/min
This simple equation is the root of inadequate system design. Because cost
is directly proportional to flow, Q, the user is continually tempted to
decrease either hood area, A, or face velocity, V. As described later, a
decrease in either of these causes rapid deterioration in hood capture per-
formance.
The desired air velocity, or capture velocity, designed into the hood
must be based on experience, but the guidelines in Table 2 may be helpful.
In many larger industrial processes, the third category—active generation
into a zone of rapid air motion—is encountered, and face velocities in the
range of 200 to 500 ft/min are required. Air motion or currents in the room
may be caused by thermal drafts from hot processes, building drafts, movement
of machinery or material, movement of the process, or rapid discharge of
gaseous emissions.
Because all of these factors cannot be accurately evaluated, a high-effi-
k>
ciency hood must have high face velocities in which a large safety factor is
incorporated. In addition, reducing cross drafts by using partial enclosures
(both fixed and movable) will greatly enhance capture efficiency.
In the design of a hood system, it is useful to consider the concept of
a null point. This point is defined as the point where the inertia! energy
(mass times velocity) of the emission has decreased to zero or been nul-
lified. Because the mass of most emissions (gases and/or particles) is
13
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TAPPING
DAMPER
PERIMETER FLANGE
INTERNAL BAFFLES
FURNACE ROOF
IN OPEN POSITION
SIDE DRAFT HOOD
TAPPING PIT
FURNACE
Figure 6. Exterior EAF canopy hood.
14
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TABLE 2,
RANGE OF CAPTURE VELOCITIES
Condition of dispersion of contaminant
Examples
Capture velocity, ft/min
Released with practically no initial
velocity into quiet air.
Released at low velocity into
moderately still air.
Active generation into zone of rapid
air motion.
Released at high initial velocity into
zone of very rapid air motion.
Evaporation from tanks; degreasing,
etc.
Spray booths; intermittent container
filling; low speed conveyor transfers;
welding; plating; pickling
Spray painting in shallow booths;
barrel filling; conveyor loading;
crushers; melting and refining
Grinding; abrasive blasting; tumbling
50-100
100-200
200-500
500-2000
NOTE: In each category above, a range of capture velocity is shown. The proper choice of values depends
on several factors:
Lower end of range Upper end of range
1. Room air currents minimal or favorable to capture. 1. Disturbing room air and thermal currents.
2. Contaminants of low toxicity or of nuisance value only. 2. Contaminants of high toxicity.
3. Intermittent, low production. 3. High production, heavy use.
4. Large hood, large air mass in motion. 4. Small hood, local control only.
Courtesy: Brandt, A. D. Industrial Health Engineering, John Wiley and Sons, New York. 1947.
Kane, J. M. Design of Exhaust Systems. Heating and Ventilatino, 42, 68. November 1945.
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small, their momentum is soon dissipated by air resistance. Hot process
exhausts often have significant momentum due to thermal updraft. Examples
are electric arc furnace emissions and coke pushing emissions. Thermal
momentum can be misinterpreted in the sense, that one might think the gases
would be easily captured because they are headed directly into the capture
hood. If the upward velocity is greater than the hood face capture velocity,
however, the gas stream will be deflected to the side as if it had struck a
barrier. This is illustrated in Figure 7.
At the null point the emissions have no momentum of their own, and if an
adequate draft or air velocity toward the hood is provided at the null point,
the contaminants will be captured. What constitutes an adequate velocity
toward the hood depends on the drafts in the area, and therefore cannot be
determined precisely.
Establishing the null point in advance for a new process is not always
possible. For existing equipment, however, direct observation will usually
establish a locus of null points. In the absence of external disturbances,
any positive velocity toward the hood at the null point will give complete
capture. In practice, however, complete capture is difficult to achieve
because of drafts and thermal currents that disturb the air flow and prevent
the formation of the null point. Sufficient velocity must be induced to
overcome the disturbance caused by drafts and thermal currents. Because
these drafts and thermal currents vary with the activity near the process, an
exact entrainment velocity cannot be calculated; therefore, a safety factor
must be incorporated to ensure good capture.
3.1 DETERMINING AIR FLOW REQUIREMENTS
3.1.1 Cold Processes
As shown in Figure 8, air moves from all directions toward openings
under suction. By definition, flow contours are lines of constant velocity
in front of a hood. Similarly, streamlines are lines perpendicular to veloc-
ity contours.
The equation for air flow around free-hanging round hoods and rectangu-
lar hoods that are approximately square is :
16
-------
/ £
\CAPTURE
AFACE VELOCITY!
1300 ^gn J ^—
HOOD
DEFLECTION
PLUME VELOCITY, 500 fpm
Figure 7. Effect of excessive plume velocity.
17
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v - . 2)
v 10X2 + A
where V = Center!ine velocity at X distance from hood, ft/min
X = Distance outward along axis, ft (Equation is accurate only for
limited distance of X, where X is within 1.5D.)
Q = Air flow, cfm
A = Area of hood opening, in ft2
D = Diameter of round hoods or side of essentially square hoods
As shown in Equation 2 and Figure 8, velocity decreases rapidly with
increasing distances from the hood and varies almost inversely with the
square of the distance. The velocity decreases less rapidly with a flanged
hood, as shown in Figure 9.
Where distances of X are greater than 1.5D (as is the case in most
applications), the velocity decreases less rapidly with distance than Equa-
tion 2 indicates. Figure 10 illustrates other hood types and gives the air
volume formulae that apply.
In addition to canopy-type hooding systems, many other configurations of
hood systems are applied to spray booths, grinding, end open tanks. Few of
these systems have exhaust flows greater than 100,000 cfm.
3.1.2 Hot Processes—High Canopy Hoods
In hot processes, significant quantities of heat are transferred to the
surrounding air by conduction end convection, and a thermal draft is created
that causes a rising air current. The design of the hood and the ventilation
rate provided must take this thermal draft into consideration.
As the heated air stream that rises from a hot surface moves upward, it
mixes turbulently with the surrounding air. The higher the air column rises,
the larger it becomes and the more it is diluted with ambient air. As illus-
trated in Figure 11, the rising air column expands approximately according to
the following empirical formula:
Dc - 0.5 xf°'88 (Eq. 3)
where D = The diameter of the hot column of air at the level of
the hood face, ft
18-
-------
vo
-STREAMLINE
FLOW
CONTOURS
AIR FLOW
DIRECTION
(TANGENT TO
STREAMLINE)
0 50 100
X OF OPENING DIAMETER
- STREAMLINE
FLOW CONTOURS
50 100
X OF DIAMETER
Figure 8. Velocity contours (expressed in
percentage of opening velocity) and
streamlines for circular openings.
Courtesy: Silverman, L. Velocity Characteristics
of Narrow Exhaust Slots. Journal of In-
dustrial Hygiene and Toxicology, 24,
267. November 1942.
Figure 9. Velocity contours and stream-
lines for flanged hood.
Courtesy: Silverman, L. Centerline Velocity
Characteristics of Round Openings Under
Suction. Journal of Industrial Hygiene
and Toxicology, 24, 259. November 1942.
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ro
o
HOOD TYPE
w
A*WL(sq.ft)
DESGRPTION
SLOT
FLANGED SLOT
PLAIN OPENING
FLANGED OPENING
CANOPY
ASPECT RATIO,"
0.2 or less
0.2 or less
0.2 or greater
and round
0.2 or greater
ond round
To suit work
AIR VOLUME
0-37LVX
Q*2.8LVX
0' VflOX +A)
Qs0.75V(K)X*+A)
Qst.4POV
P -perfneter of wont
Ds height above work
Figure 10. Formulas for estimating hood air flows.
Courtesy: DallaValle, J. M. Exhaust Hoods. Industrial Press, New York. 1946.
Silverman, L. Velocity Characteristics of Narrow Exhaust Slots.
Journal of Industrial Hygiene and Toxicology, 24, 267. November 1942.
-------
o
, HOT SOURCE
HYPOTHETICAL
POINT SOURCE X
Figure 11. Dimensions used to design high-canopy hoods for hot sources.
12
21
-------
xf = The distance from the hypothetical point source to the
hood face, ft (equal to y + z)
1 3R
where z = (2D )1'JO and D = diameter of source, ft
y = Distance from top of source to hood, ft
To allow for drift in the rising column of emissions caused by cross-
drafts and air disturbances, the designer must increase the overall hood
diameter by adding 80 percent of the distance between the hood and the proc-
ess, as shown in Equation 4.
Dh = Dc + 0.8y (Eq. 4)
where D. = Overall hood diameter, ft
Where cross drafts occur, the hood diameter may be increased still
farther and the distance between the hood and source may be decreased. When
possible, side shields in the form of steel sheets (curtain walls) or chains
suspended from the hood should be utilized to decrease cross drafts. Asbes-
tos and tarpaulin curtains have been tried, but they are rarely successful
because their light weight makes them tear easily and they do not hang
straight.
The total flow through the hood system may be estimated by the use of
Equation 5:
Vf - "TO? <<£ As "I"'"
-------
In addition to the volume of the hot gases rising through the hood area
defined by diameter D (Figure 3-8), room air is also drawn into the hood
\f
through the balance of the hood area. Estimates of the desired velocity of
the air through this portion of the hood depend entirely on engineering
judgment and are based on expected cross-drafts, air disturbances, and the
toxicity of the emissions.
A velocity between 100 and 200 ft/mi n is recommended, and the velocity
should increase with greater air disturbances. In extreme cases in which
more violent reactions occur with sudden heat release (such as in the charg-
ing and tapping of steelmeking furnaces), even greater velocities are re-
quired:
Q = VfAc + Vr (Ah - Ac) (Eq. 6)
where Q = Total hood flow, cfm
»
V, = Velocity of hot air, ft/mir
A = Area of hood face through which hot gases enter
C (= *Dc2/4), ft2
V = Desired velocity of air entering balance of hood
(100 to 200 ft/min)
A. = Area of total hood, ft2
n /
The control of emissions from sources that are other than circular in
shape is best handled by hoods of the appropriate shape. Thus, a rectangular
source would require a rectangular hood to minimize the ventilation require-
ments. The equations used for circular hoods are appropriate for rectangular
hoods, but increases of 0.8 times the distance to the source (y) should be
made in both length and width.
Total hood volume determines retention time in the hood. For inter-
mittent processes of short duration, such as charging, a large hood has the
advantage of containing the exhaust gas for several minutes until the ven-
tilation system can withdraw the fumes. Partitions can be added that will
not only minimize cross-drafts, but also essentially increase hood size and
retention time. In any event, it is difficult to predict performance based
solely on theoretical design. Scale model studies can be helpful during the
23
-------
design stage, but final modifications in the field may be necessary based on
observation of system performance.
3.1.3 Hot Processes--Low Canopy Hoods
An exact distinction cannot be made between a low and high canopy hood,
but a low hood is usually defined as one in which the distance between the
hood and the source does not exceed the diameter of the source, or 3 feet,
whichever is smaller. The primary difference in the design of low hoods is
that the hood diameter and source diameter are essentially the same. A
safety factor is usually included, and for practical purposes, the hood
diameter should exceed the source diameter by at least one foot. The dimen-
sions of larger rectangular hoods should exceed the source's dimensions by at
least one foot in all directions.
For circular hoods, the total exhaust flow may be determined by the
12
following equation :
Q = 4.7(Dh)2'33(At)°-417 (Eq. 7)
where D. = Hood diameter in feet and is equal to the source
diameter plus 1 or 2 feet
For rectangular hoods, the exhaust flow may be determined by:
Q = (6.2JW1-33 (At)°'417L (Eq. 8)
where VI = Hood width, which is 1 to 2 feet larger than the source
width
L = Length, which is 1 to 2 feet longer than the source
length
Lowest flow rates are achieved with close-fitting hoods. Figure 12
illustrates the latest design for hooding a hot metal transfer operation.
Note that the open area is essentially limited to the hood slot through which
the metal is poured.
3.1.4 Building Evacuation
A building acts as a large process enclosure. By drawing air through
the building and out the roof, the building essentially serves as a hood.
24
-------
TORPEDO CAR
LADLE MOOD
tn
HOT METAL LADLE
Figure 12. Schematic arrangement of ladle hood for reladling emission control
Courtesy: Pennsylvania Engineering Corporation, Pittsburgh.
-------
The large size of a building compared with a more closely fitted hood re-
quires utilization of a much larger exhaust flow. Many large buildings, and
especially those that contain processes that release heat, are ventilated by
natural draft. Heat released by the processes warms the air, which rises and
is pushed out through openings (roof monitors) in the roof. Cooler ambient
air is drawn into the building through openings near ground level. Wind
action at the building openings may either increase or decrease the natural
ventilation rate, depending on the location and size of the openings end the
wind speed and direction.
A forced-ventilation system that is applied to a building must be sized
to include the air flow resulting from natural draft and also maintain suffi-
cient draft into the building to prevent any emissions from escaping through
building openings.
For buildings containing hot processes, the natural ventilation rate can
be estimated by the ASHRAE equation :
Q = 9.4(AL)°-5(Atavg)°-5 (Eq. 9)
where Q = Air flow, cfm
A = Total inlet or outlet air flow area, whichever is
smaller, ft2
L = Building height from air inlet to outlet, ft
At = Difference between average temperature in building
y and air entering building, °F
When the heat released from sources within the building can be quan-
tified, Equation 10 can be utilized:
Q = 20(0.67L)°-33(0.67H)0-33A0'67 (Eq. 10)
where Q = Air flow, cfm
L = Building height from air inlet to outlet, ft
H = Heat released within building, Btu/min
A = Total inlet or outlet area (whichever is smaller), ft2
26
-------
Building ventilation requirements may also be defined in terms of air
changes per unit of time. Again, a great deal of judgment enters into the
selection of the number of changes required, which depends on the heat gen-
erated in the building and the industrial hygiene considerations regarding
fumes and gases within the building. On the order of at least 20 air changes
per hour are required for metallurgical processes.
The distribution of ventilation is also very important. Uncontrolled
air flowing into a building as a result of negative pressure in the building
or because of poorly designed air-supply distributors not only may cause
recirculation of contaminants, but also may upset the local ventilation
systems. Therefore, the amount of air, the location of its entry into the
building, and its direction must be controlled. Figure 13 shows a controlled
air supply that results in a convective flow from a heat source (such as a
ladle of molten metal) rising to be exhausted through a roof ventilator.
Figure 14 shows an uncontrolled air supply, which results in a disrupted
plume and recirculation of the contaminant throughout the building. The
latter could cause a buildup of contaminants in the building and possible
leakage to the outside air.
Because of the huge air volumes it would require (on the order of 5 to
10 million cfm), true building evacuation is rare. Care must be taken in
closing the roof monitors or ventilators in a building and replacing this
natural ventilation with induced-draft ventilation. If the induced draft is
inadequate, both ambient dust and heat levels in the building can rise
rapidly and create health and safety hazards. This is particularly true in
hot climates.
3.1.5 Push-Pull Systems
Hood capture efficiency can sometimes be improved by the use of a push-
pull or air curtain approach. This approach involves a source of compressed
air (push) to direct the emission plume toward the exhaust hood (pull).
These applications are used to control blast furnace casthouses, copper
converters, and electric arc furnace enclosures. Figure 15 illustrates the
general principle of the push-pull system. The effective face velocity of a
27
-------
Figure 13. Controlled airflow from a heated source.^
Figure 14. Uncontrolled airflow from a heated source.
13
28
-------
NO MAJOR OBSTRUCTIONS
IN PATH OF JET
PRESSURE SLOT
EXHAUST HOOD
PUSH PULL HOODS
Figure 15. General principle of the push-pull (air-curtain) type system.
29
-------
blowing source is sustained at much larger distances than that of a suction
source (as illustrated in Figure 16).
In one system used to control tapping emissions from a blast furnace, an
air curtain in front of the taphole directs the emissions into the capture
hood above the taphole in an arrangement very similar to that in Figure 15.
Figure 17 illustrates the air curtain application on a copper converter
furnace. The total enclosure uses an air curtain to prevent emissions from
escaping the enclosure when the doors ere opened for access. Each appli-
cation is unique in its design and must be evaluated by actual observation.
It is difficult to base suitability on design data alone.
30
-------
FAN
30 diameters
BLOWING
400 f pm
r^
)
4000 fpm Alfc
VELOCITY AT
FACE OF BOTH
I I
400 fpm
APPROXIMATELY 10% OF FACE VELOCITY
AT 30 DIA. AWAY FROM PRESSURE JET
OPENING.
EXHAUSTING
APPROXIMATELY 10% OF FACE VELOCITY
AT ONE DIA. AUAY FROM EXHAUST
OPENING.
Figure 16. Comparison of face velocity decay for blowing versus exhausting.
Courtesy: Industrial Ventilation, 16th Ed.
31
-------
JET SIDE
EXHAUST SIDF
AIR
CURTAIN
JET
CONVERTER
(FUME SOURCE)
LADLE
BAFFLE
WALL
TO SUCTION FAN AND
HOOD SAMPLE LOCATION
Figure 17. Air curtain control system on a copper converter.
32
-------
SECTION 4
DUCT DESIGN AND CONSIDERATIONS
The three design principles for ducting are:
0 Minimization of changes in flow direction.
0 Maintenance of smooth duct surfaces.
0 Avoidance of abrupt expansions.
Basically, a duct is a pipe or channel that conveys a gas and contained
emissions from a collection point to a more convenient point for rehandling,
cleaning, or blending. In some cases a duct also acts as a cooler.
Duct configurations range from small-diameter ducting (6 to 12 in.) to
ducting having cross-sectional areas of 600 square feet or more. Ductwork
can be a combination of circular, square, or rectangular, based on location,
space limitations, equipment or building design, and length of run to the
control point. Flows should be continuous and smooth in direction, with no
abrupt expansions or contractions. Larger duct sizes usually are fabricated
in the field in square or rectangular cross sections. Circular ducts are
limited in size by the availability of plate widths. The cost of wider
plates must be balanced against forming and welding costs. Theoretically, a
circular cross section is preferable because it minimizes friction losses and
nonstreamline flow. Duct configuration is generally dictated by economics,
however. Duct thickness in large metallurgical systems usually varies from
1/4 to 3/8 in. Strengthening ribs and expansion joints are generally re-
quired for larger sections (over about 6 feet in diameter or square). Ex-
pansion joints are required for ductwork carrying hot gases (above about
300°F). If temperature fluctuations occur regularly, cracking of the ex-
pansion joints and the resultant leakage can be a major source of maintenance
problems.
33
-------
Ducting must be designed to deal with the following gas conditions:
temperature, abrasiveness, acidity, dust concentration, and moisture. For
temperature control, the ductwork may be lined with refractory materials or
water-cooled in its entirety. An example of this is the BOF shop, where the
hood and duct above the furnace are cooled by a variety of designs that are
discussed later.
Damage from abrasion, acidity, and moisture may be controlled by special
types of refractory or, depending on the temperature, by the use of various
coatings or special alloyed steels. Carbon steel, for example, generally is
considered suitable for gas temperatures up to 1000°F. Above this tempera-
ture, stainless steel or refractory-lined carbon steel is required. Acid
condensation (primarily from sulfates) is a problem if gas temperature falls
below the acid dew point. In copper converters, sulfur trioxide (SOg) typi-
cally constitutes about 1 percent of the sulfur dioxide (SO^) present, or
about 0.02 to 0.1 percent of the total gas stream. This can condense and
form sulfuric acid. Figure 18 indicates the dewpoint of air containing
various SO, concentrations.
4.1 TRANSPORT VELOCITIES
The minimum design duct velocity is that required to prevent buildup in
a duct. At elbows and other sections where the gas stream slows down, pro-
vision should be made for inspection for dust dropout and for cleaning. If
the substances in the gas tend to be sticky, or if moisture condensation is
possible, cleanout ports or flanged sections should be provided to gain
access. Buildup decreases the effective duct cross-sectional area and in-
creases transport velocity; the latter counterbalances the former and thus
prevents further buildup. If buildup is due to stickiness or moisture,
however, it can proceed to the point of total pluggage, especially in smaller
branch ducts. A further concern caused by buildup is the possibility of its
breaking loose during startups or vibration. Serious buildup can also create
an excessive mechanical load on the duct structure and supports.
Normally, the minimum transport velocity required is greater than that
required merely to prevent settling or buildup. Buildup or pluggage can
cause system upset in the main duct or in any of the branches, and this in
34
-------
CO
en
Figure 18. Dew-point of air containing various $03 concentrations.
Courtesy: "Combating Fuel Oil Heating Problems", Plant Engineering, January 7, 1974.
-------
turn affects the remainder of the system because of the increase in resist-
ance and decrease in flow in the blocked section. Anything that decreases
the duct size (such as damage from outside forces) affects overall collection
performance.
A change in the temperature of the gas at any point can also affect the
overall collection efficiency of the system.
The transport velocity must account for the velocity needed for gas and
particulate removal, resistance due to friction along the duct surface, end
dynamic losses due to air turbulence.
Table 3 gives a recommended range of design velocities. Metallurgical
process control systems with heavy dust loadings operate in the transport
velocity range of 3500 to 4500 fpm. When the dust consists primarily of
small particles at low concentration, lower transport velocities ere pos-
sible. Higher velocities are still preferable, however, to decreese duct
diameter. Duct cost savings normally outweigh the energy penalty of the
higher velocity.
4.2 ENERGY LOSSES
Figure 19 summarizes the three pressure measurements of concern in a
duct. The sum of the energy losses that must be considered in the duct
system are:
0 Inertia - The energy required to accelerate the gas from zero to
duct velocity is equal to (V /4005)2, where V is the gas velocity
in feet per minute. This vafue is referred to8 as velocity pressure
or as the velocity head (h ). Table 4 can be used to convert
velocity (V or V) in feet per minute to velocity pressure (h or
VP) in inches of water.2 v
0 Straight duct friction - Friction loss (or pressure drop) in
streight duct runs is usuelly negligible relative to the pressure
drop required for elbows, branch entries, and the control device.
As shown in the following equation, the total loss in 500 feet of
10-foot diameter duct (essuming V = 4000 fpm) is about 0.5 in. H20.
The equation used for clean round ducts is:
f - 2.74 ,_ ...
-- TT2 — (Eq. 11
D
36
-------
TABLE 3. RANGE OF DESIGN VELOCITIES'
Nature of
contaminant
Examples
Design velocity
Vapors, gases,
smoke
Fumes
Very fine light
dust
Dry dusts and
powders
Average indus-
trial dust
Heavy dusts
Heavy or moist
All vapors, gases, and smokes
Zinc and aluminum oxide fumes
Cotton lint, wood flour, litho
powder
Fine rubber dust, Bakelite
molding powder dust, jute
lint, cotton dust, shavings
(light), soap dust, leather
shavings
Sawdust (heavy and wet),
grinding dust, buffing
lint (dry), wool jute dust
(shaker waste), coffee beans,
shoe dust, granite dust,
silica flour, general mate-
rial handling, brick cutting,
clay dust, foundry (general),
limestone dust, packaging and
weighing, asbestos dust in
textile industries
Metal turnings, foundry tumbling
barrels and shakeout, sand
blast dust, wood blocks, hog
waste, brass turnings, cast
iron boring dust, lead dust
Lead dust with small chips,
moist cement dust, asbestos
chunks from transit pipe
cutting machines, buffing
lint (sticky), quick-lime
dust
Any desired velocity
(economic optimum
velocity usually
1000-1200 fpm)
1400-2000
2000-2500
2500-3500
3500-4000
4000-4500
4500 and up
A rule of thumb for items or contaminants not tested is to operate in the
3500-4500 fpm transport velocity range.
a From Reference 14.
37
-------
AIR FLOW
TOTAL
PRESSURE
STATIC
PRESSURE
VELOCITY
PRESSURE
Figure 19. Pressure measurements in ducts.
14
38
-------
TABLE 4. CONVERSION TABLE FOR DUCT VELOCITY TO VELOCITY PRESSURE15
v , fpm
d
400
500
600
700
800
900
1,000
1,100
1,200
1,300
1,400
1,500
1,600
1,700
1,800
1,900
2,000
2,100
2,200
2,300
2,400
2,500
2,600
2,700
2,800
2,900
3,000
3,100
3,200
3,300
hv. in.H20
0.010
0.016
0.022
0.031
0.040
0.051
0.062
0.075
0.090
0.105
0.122
0.140
0.160
0.180
0.202
0.225
0.249
0.275
0.301
0.329
0.359
0.389
0.421
0.454
0.489
0.524
0.561
0.599
0.638
0.678
va, fpm
3,400
3,500
3,600
3,700
3,800
3,900
4,000
4,100
4,200
4,300
4,400
4,500
4,600
4,700
4,800
4,900
5,000
5,100
5,200
5,300
5,400
5,500
5,600
5,700
5,800
5,900
6,000
6,100
6,200
hy, in.H20
0.720
0.764
0.808
0.853
0.900
0.948
0.998
1.049
1.100
1.152
1.208
1.262
1.319
1.377
1.435
1.496
1.558
1.621
1.685
1.751
1.817
1.886
1.955
2.026
2.098
2.170
2.244
2.320
2.397
39
-------
where
f = friction loss in inches of water per 100 feet
V = velocity in fpm in duct
D = inside diameter of duct in inches
Actual values can be twice as high because duct internal surfaces
are not ideally clean and smooth. Note that as duct diameter
decreases and length of ductwork increases (as in a system with
multiple miscellaneous pickup points), pressure drop can become
significant. If the system fan is not designed to provide this
pressure drop (for example, where miscellaneous pickup points have
been added letter), the duct pressure drop will result in less
suction at the hoods.
A rectangular duct can be converted to the circular equivalent in
the following manner:
1. A = the duct cross-sectional area in square feet
2. P = the perimeter in feet
A
3. R = -p the hydraulic radius in feet
4. 12R = Conversion of R to inches, r
5. D = 4r = equivalent diameter in inches
Elbows - Losses for 90-degree elbows are determined as equivalent
resistance in feet of straight duct. For other elbow angles, use:
60-degree elbow = 0.67 x loss for 90-degree elbow
45-degree elbow = 0.5 x loss for 90-degree elbow
30-degree elbow = 0.33 x loss for 90-degree elbow
For radius of 1.5D:
D 1.175
Equivalent feet = 130 (^-) (Eq. 12)
For a radius of 2D:
D
Equivalent feet = 89 (—-} (Eq. 13)
40
-------
For radius of 2.5D:
D <
Equivalent feet = 73 (--) (Eq. 14)
Branch Entry - Losses due to branch entry are best expressed as the
equivalent feet of straight duct of the same diameter. The equiva-
lent length is added to the actual length of straight duct and the
loss is computed from the earlier friction loss equation.
For an entry angle of 30 degrees:
D 1>214
Z = 20 (£.) (Eq. 15)
where
Z = equivalent feet
D = diameter in inches
For an entry angle of 45 degrees:
D
Z = 32 (^g-) (Eq. 16)
All branches should enter the main duct at the large end of the
transition, at an angle not to exceed 45 degrees, preferably 30
degrees or less. Branches should be connected only to the top or
sides of the main duct, never to the bottom. Two branches should
never enter a main at diametrically opposite points.
0 Contraction and Expansion - When the cross-sectional area of a duct
contracts, a pressure loss occurs. This loss is a function of the
abruptness of the contraction. When the cross-sectional area
expands, a portion of the decrease in velocity pressure becomes
static pressure. The increases and decreases in pressure from
expansion and contraction are calculated from equations in Refer-
ence 15:
The summation of the transport velocity requirements from all of these
losses plus the hood or entry losses plus the control device and stack dis-
charge loss determines the size and power of the fan. A pressure diagram can
be useful in characterizing and understanding a given system. Figure 20
presents a hypothetical pressure diagram for a simple system.
41
-------
DUCT. ELBOW, DAMPER. AND BRANCH ENTRY LOSSES =1.1
>
N
CONT
LO
(FAB
v ,
\
ROL DEVICE
SS = 10
RIC FILTER)
CANOPY HOOD\
-0.2
/CANOPY HOOD\
-0.2
DISCHARGE
PRESSURE = 1.5
ro
2
0
o
i~ -2
5 .4
•
tt -6
=>
2 -8
o:
°- -10
-12
TOTAL PRESSURE REQUIRED TO SIZE FAN
A-12.8
Figure 20. Simple pressure diagram.
-------
4.3 BRANCHED SYSTEMS
The addition of new or additional pickup points that are in turn tied
into a main duct system has to be carefully designed to achieve the desired
flow balance throughout the system. If duct sizes are not compatible with
the static pressure throughout the system, the desired air flows will not be
achieved. Thus, it is necessary to provide a means of distributing air flow
between branches, either by balanced design or by the use of dampers. Figure
21 illustrates the widely used approach of tapered cross section for branch
entry systems. This design maintains constant duct velocity in the main
duct.
The two approaches for designing duct systems are 1) to balance duct
diameters to match desired flows in each branch, and 2) to use dampers or
blast gates to control flow. Many systems use a combination of both ap-
proaches.
Balanced design is theoretically preferable to minimize the tampering
with dampers and the dependence on operating personnel. The design calcula-
tions begin at the branch of greatest resistance and proceed as follows:
branch to main, section of main to section of main, main to control equipment
on to the fan. At each junction point, the static pressure necessary to
achieve the desired flow in one stream must match the static pressure in the
other.
Damper adjustments permit the desired flow to be varied through each
portion of the system after initial design. This permits adjustments to be
made by trial and error to accommodate variation in actual conditions. The
design calculations begin at the branch of greatest resistance, and pressure
drops are calculated through the branches and duct sections to the fan. At
each point where two gas streams meet, the combined flow is then used, and
when it reaches the main duct, this combined flow is added to the main duct
flow with no attempt to balance the static pressure in the joining gas
streams. Branches are sized for the desired minimum duct or transport veloc-
ity at the desired flow rate.
Advantages and shortcomings of these two methods are further outlined in
Table 5.
43
-------
SIZE FOR BALANCE
AND TRANSPORT
VELOCITY
TO FAN
BRANCH
DUCTS
Figure 21. Taper duct system.
Courtesy: Industrial Ventilation.
-------
TABLE 5. RELATIVE ADVANTAGES OF USING DUCT DAMPERS
14
Balance without dampers
Balance with dampers
Air volumes cannot be easily
changed by the operator.
Small degree of flexibility for
future equipment changes or addi-
tions; the ductwork is "tailor-
made" for the job.
Choice of exhaust volumes for a
new unknown operation may be in-
correct; in such cases some duct-
work revision is necessary.
No unusual erosion or accumulation
problems.
Ductwork will not plug if veloci-
ties are chosen wisely.
Total air volumes are slightly
greater than design air volumes
because of the additional air
handled to achieve balance.
Poor choice of "branch of greatest
resistance" will show up in design
calculations.
Layout of system must be in com-
plete detail, with all obstructions
cleared and length of runs accu-
rately determined. Installations
must follow layout exactly.
Air volumes may be changed rela-
tively easily.
Greater degree of flexibility for
future changes or additions.
Correction of improperly esti-
mated exhaust volumes is easy
within certain ranges.
Partially closed dampers or blast
gates may cause erosion and
thereby change the degree of
restriction or cause accumula-
tions of material.
Ductwork may plug if persons have
tampered with the blast gate
adjustment.
Balance may be achieved with
design air volume.
Poor choice of "branch of
greatest resistance" may remain
undiscovered. In such case the
branch or branches of greater
resistance will be "starved."
Leeway is allowed for moderate
variation in duct location to
miss obstructions or interfer-
ences not known at time of
layout.
45
-------
SECTION 5
FAN SYSTEMS
Centrifugal fans are normally used in conjunction with large emission
control systems because of the large flow rates and high pressure drops in
these systems. Although some axial fans can deliver large volumes of air at
high resistance, they are best suited for clean-air applications. The pres-
ence of dust causes rapid erosion of axial fan components because of the high
tip speed of the fan and the high air velocity through the fan housing.
Centrifugal fans can be designed for differing gas characteristics encountered
in various emission control applications. This section discusses the major
aspects of centrifugal fans with regard to their applicability for large
ventilation systems.
5.1 FAN TYPES AND OPERATING CHARACTERISTICS
A centrifugal fan is used to transfer energy to gases by centrifugal
action. Figure 22 shows the components of the centrifugal fan and layout of a
typical industrial fan system. It consists of a wheel or rotor that is ro-
tated by an electric motor in a scroll-shaped housing. The gases enter the
housing axielly, make a right-angle turn, and are forced through the blades of
the rotor end into the housing by centrifugal force. The centrifugal force
imparts velocity pressure to the air, and the diverging shape of the scroll
converts a portion of the velocity head into static head.
Centrifugal fans are classified according to the following blade configu-
rations:
0 Backward-curved blade
0 Forward-curved blade
Straight blade
Figure 23 shows a few basic centrifugal fan blade configurations and impeller
arrangements.
46
-------
COUPLING INBOARD END
DRIVE UNIT\!EAVRING
IMPELLER
OUTBOARD END
E F*OUNDAffoN^5^2^S:
BEARING
STEEL PEDESTAL
MAIN SHAFT
GRADE LINE
'CONCRETE PEDESTAL
Figure 22a.
Courtesy: Hydrocarbon Processing, June 1975. Article by J. W. Martz
and R. R. Pfahler entitled "How to Troubleshoot Large Indus-
trial Fans."
SCROLL SIDE
BLACKPLATE
BLADES
'OUTLET
OUTLET
AREA
\^
CUTOFF
INLET
SCROLL
SUPPORTS
INLET COLLAR
BEARING SUPPORT
Figure 22b.
Courtesy: "With permission of the American Society of Heating, Refrig-
erating & Air Conditioning Engineers, Inc., Atlanta, GA.
Figure 22. Centrifugal fan components and layout of a typical industrial
fan system.
47
-------
RADIAL BLADE FAN
RADIAL TIP FAN
SINGLE INLET
AIRFOIL FAN
DOUBLE INLET
AIRFOIL FAN
Figure 23a.
Courtesy: Hydrocarbon Processing, June 1975. Article by J. W.
Martz and R. R. Rfahler entitled "How to Troubleshoot
Large Industrial Fans,"
BACKWARD-
INCLINED
X
STRAIGHT
I
AIRFOIL
FORWARD-
CURVED
Figure 23b.
Courtesy:
Excerpted by special permission from CHEMICAL ENGINEER-
ING (date of issue) Copyright (c) (year), by McGraw-
Hill, Inc., New York, N.Y. 10020.
Figure 23. Centrifugal fan blade configurations and impeller arrangements.
48
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The size, shape, and number of blades affect the operating characteris-
tics of the fan. Fan performance is characterized by the volume of gas flow,
pressure, fan speed, power requirement, and operating efficiency. The rela-
tionship of these parameters is measured according to the testing methods
sponsored by the National Association of Fan Manufacturers or the American
Society of Mechanical Engineers. The fan is tested from shutoff conditions to
free-delivery conditions. At shutoff, the duct is completely blanked off; at
free delivery, the outlet resistance is zero. Between these two conditions,
various flow restrictions are placed at the end of the duct to simulate vari-
ous operating conditions. The operating parameters are measured at each test
point and plotted against volume on the abscissa. Figure 24 illustrates the
fan testing procedure and shows the typical fan characteristic curves.
Each fan type has a different performance characteristic. The fan per-
formance curves are used in the selection of a fan type. Generally, the
characteristics of geometrically similar fens are identical. The fan manu-
facturers can predict the performance of a large fan from the tests on a
smaller but geometrically similar fan.
5.1.1 Backward-Curved-Blade Fan
The blades in a backward-curved-blade centrifugal fan are inclined in a
direction opposite to the direction of rotation. The blades (usually 14 to
24) are supported by a solid steel backplate and shroud ring. The scroll-type
housing permits efficient conversion of velocity head into static head.
The characteristics of a backward-curved-blade fan are shown in Figure
25. The static pressure of this fan rises sharply from free delivery to about
50 percent volume point. Beyond this point and up to the no-delivery point
the pressure remains approximately constant. Maximum efficiency occurs at
maximum horsepower input. The horsepower requirement is self-limiting; it
rises to a maximum as the capacity increases and then decreases with addi-
tional capacity. This self-limiting horsepower characteristic of the back-
ward-curved-blade centrifugal fan prevents overloading of the motor when the
fan load exceeds its design capacity. The operating efficiency of the back-
ward-curved-blade fan is high, and this fan develops higher pressure than the
forward-curved-blade fan.
49
-------
Ji—BLANKED OFF
RESTRICTED
LESS RESTRICTED
FLOW STRAIGHTENERS
WIDE OPEN
VOLUME FLOW RATE, Q /
FREE DELIVERY
Figure 24a.
Courtesy: ASHRAE Handbook. Equipment Volume.
SP VP
MEASURING
STATION
i i i i I I i i
20 30 40 50 60 70 80 90 100
PERCENT OF WIDE OPEN VOLUME
Figure 24b.
Source: U.S. Environmental Protection Agency. Standards Support Documents:
An Investigation of the Best Systems of Emission Reduction for Elec-
tric Arc Furnaces in the Steel Industry. (Draft) June 1974.
Figure 24. Fan testing procedure and typical characteristic curves.
50
-------
VOLUME
Figure 25. Typical characteristic curves for a
backward-curved-blade centrifugal fan.
51
-------
Because the backward-curved blades are conducive to buildup of material,
they are not recommended for dirty streams. These fans are generally used in
ventilating applications where large volumes of clean air are to be handled on
e continuous basis. When used for emission control applications, the back-
ward-curved-blade fan must be installed on the clean-air side of the control
system.
5.1.2 Forward-Curved-Blade Fan
The forward-curved-blade fan generally has 20 to 64 blades. The blades
are shallow, and both the heel and tip are curved toward the direction of
rotation. The rotor of the forward-curved-blade fan is known as a "squirrel-
cage" rotor. A solid steel backplate holds one end of the blade, and a shroud
ring supports the other end. The scroll design is similar to that of the
backward-curved-blade fan.
As shown in Figure 26, the static pressure of this fan rises from a free
delivery to a point at approximate maximum efficiency, drops to about the 25
percent volume point, and then rises back up to the no-delivery point. Horse-
power requirement increases with volume. Because horsepower increases rapidly
with capacity, there is a danger of overloading the motor if system resistance
is not accurately estimated. Forward-curved-blade fans are designed to handle
large volumes of air at low pressures. The fan speeds ere relatively low, end
the pressures developed by forward-curved-blade fans are generally insuffi-
cient for emission control system applications. These fans are used exten-
sively in heating, ventilating, and air conditioning applications.
5.1.3 Straight-Blade Fan
Straight-blade fans are the simplest of all centrifugal fans. The fan
usually has 5 to 12 blades, which are generally attached to the rotor by a
solid steel backplate or a spider built up from the hub. The rotors are
relatively large in diameter.
Figure 27 shows the performance characteristics of the straight-blade
fan. The static pressure of this fan rises sharply from free delivery to a
maximum point near no delivery, where it falls off. Mechanical efficiency
rises rapidly from no delivery to a maximum near maximum pressure, and then
drops slowly as the fan capacity approaches free delivery.
52
-------
VOLUME
Figure 26. Typical characteristics curves for a forward-curved-blade fan.
53
-------
VOLUME
Figure 27. Typical characteristics curves for a straight-blade fan.
54
-------
The straight-blade fan can be used in exhaust systems handling gas
streams that are contaminated with dusts and fumes. Various blade designs and
scroll designs have been developed for specific dust-handling applications.
5.1.4 Backward-Inclined Blade Fan
The two types of backward-inclined blades in the centrifugal fan class
are air foil blades and flat blades.
5.1.5 Rim-Type Wheel Fans
With abrasive material, or materiel that tends to stick, a rim-type wheel
provides more structural integrity; some of these have back plates on them.
Literally dozens of rim-type fans are on the market. Each fan typically has
at least six blades. The radial tip tends to develop more static pressure so
it can develop the same gas flow as the straight-blade radial at higher pres-
sure drop.
5.2 FORCED VERSUS INDUCED DRAFT
The terms "forced draft" and "induced draft" come from boiler technology,
where they refer to either forcing air through the boiler (with a blower) or
pulling air through the boiler with a fan located on the exhaust side. Figure
28 illustrates the meaning of these terms in the context of air pollution
control ventilation systems. Essentially, the control device takes the place
of the boiler. Two considerations are important in the use of forced-draft
systems:
1. Whether the fan is exposed to cleaned gas or dirty gas
2. Whether the control device is under pressure or suction
The suitability of the centrifugal fan for dirty applications has already
been discussed. The axial fan would clearly only be suitable in an induced-
draft system. The forced-draft system is generally preferred in large ap-
plications, particularly those controlled by a fabric filter, because control
costs are lower, even when the higher fan cost and maintenance are taken into
account. Many large (i.e., 300,000 cfm and greater) systems treat relatively
clean gas because the process gas has been diluted with indraft air. Thus,
55
-------
DUCT
SOURCE
C>
TO
STACK OR
MONITOR
CONTROL
EQUIPMENT(UNDER PRESSURE)
FORCED-DRAFT
FAN (Dirty)
DUCT
YsOURCE /
u
CONTROL
EQUIPMENT(UNDER SUCTION)
INDUCED- DRAFT
Figure 28. Basic principle of induced versus forced draft.
56
-------
even the "dirty" gas inlet to the fan is relatively low in dust concentra-
tions, i.e., 1.0 gr/scf or less. Also, this type of system does not usually
require a stack. Systems that use scrubbers or electrostatic precipitators,
on the other hand, usually have induced-draft fans. In scrubber systems with
variable pressure drops, the induced-draft fan is especially needed to control
pressure drop.
Forced-draft fans that are used on the "dirty" gas side are usually
larger and rotate at a slower speed than fans on the clean side to cut down on
the abrasion of the fan blades.
5.3 FAN REQUIREMENTS FOR EMISSION CONTROL SYSTEM APPLICATIONS
Basically, a fan can develop static pressure without delivering much gas
volume, or it can deliver very little static pressure at high gas volume.
Unfortunately, it cannot do both at the same time. Any given fan cannot
perform beyond the limitation of its operating curve.
Increasing the rotation speed and the gas volume through the system
doesn't shift the system resistance curve, but it permits the fan to overcome
more resistance and move more gas through the system by shifting the fan
operating curve. A fan is basically a volumetric energy machine. A certain
gas volume is carried between each blade. Therefore, if the number of times
the blades pass the outlet is increased, more ges is moved and at an increase
in pressure. Any increase in fan rotation speed increases fan horsepower
requirements. Two ways to control capacity are 1) to put in dampers to waste
static pressure across the damper to reduce gas flow, and 2) to reduce fan
rotation speed. If dampers are added, the fan is still expending energy to
develop pressure drop; therefore, reducing fan rotation speed is more effi-
cient because it develops a lower horsepower and uses less energy than a
damper. Table 6 summarizes the basic fan laws.
In the design of fans for emission control system applications, con-
siderations must be given to the nature of the gases being handled. Emission
control system fans may be subjected to one or more of the following operating
conditions:
57
-------
TABLE 6. BASIC FAN LAWS
Variable
When speed changes
When density changes
Volume
Varies directly with speed ratio
Does not change
Pressure
Varies with square of speed ratio
RPM 2
Horsepower
P2 =
Varies with cube of speed ratio
3
HP2 = HP1
Varies directly with density ratio
P2 =
Varies directly with density ratio
= Hpi (TT)
JL U i
58
-------
0 High temperatures
0 Corrosive gases
0 Dust-laden gases
0 Presence of abrasive particles
0 Presence of explosive materials in the gases
Special construction materials can provide protection against high tem-
peratures end corrosive properties. Bronze alloys are used for handling
sulfuric acid fumes and other sulfates, halogen acids, various organic gases,
and mercury compounds. Stainless steel is the most commonly used corrosion-
resistant metal for impellers and fan housings. Protective coatings such as
bisonite, cadmium plating, hot galvanizing, and rubber covering provide re-
sistance to corrosive gases. Depending on the particular application, soft,
medium, or firm rubber can be bonded to the metal. A good bond will yield an
adhesive strength of 700 pounds per square inch. Rubber-covered fans have
proved exceptionally durable.
When a fan must handle explosive gases, the construction material should
be such that it does not produce a spark if accidentally struck by other
metal—e.g., bronze and aluminum alloys. In most applications, it is prefer-
able to combust explosive gases prior to entering the ductwork. In the case
of a BOF controlled by the closed-hood system, however, the gas is not com-
busted, so the inherent heating value rising from the contained carbon mon-
oxide can be retained.
Fans handling dust-laden gases must be protected from wear due to abra-
sion. When wear is expected, some manufacturers equip their fans with wear
strips and weld beads. The strips consist of a thick, cross-hatched hardened
floor plate welded to the blade at the centerplate or backplate, and the plate
is built up at the edges with weld beads. The inertia of the dust particles
carries them toward the backplate or centerplate, where the wear plate with-
stands most of the abrasion. The weld beads, which are angled along the edge
of the wear plate, break up the particle flow and prevent impingement and
scouring. For severe abrasion conditions, fans can be equipped with full-
blade liners or heavy-duty rotors with thick wear plates bolted or welded to
the full face of the blades. The fan construction is such that worn wear
plates can be replaced on site.
59
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5.4 FAN ARRANGEMENTS
Whenever possible, the fan should be installed on the clean side of the
emission control system, where it will be subjected to less severe operating
conditions. Plants with multiple emission control systems generally have
separate fans for each control system. If a fen is shared by more than one
emission control system, fan operation can be difficult to control. At such
installations, the fan may operate at less than optimum conditions because it
may be subjected to fluctuating loads.
For applications where fan load is expected to vary widely and the peak
fan load is relatively large, multiple fens may provide better control over
the system operation. A parallel fan system will increase reliability and
flexibility. The performance of two fans in parallel can be predicted by
combining the ordinates and abscissas of the pressure-volume curves of both
fans. At low loads, the parallel system can also be operated as a single fan
system. Individual fan loads can be adjusted to optimize the performance of
the system at a given load.
5.5 FAN DRIVES
Large centrifugal fans are generally driven by three-phase, alternat-
ing-current motors. The two principal types of motors used for driving fans
are 1) the squirrel-cage induction motor, and 2) the wound-rotor induction
motor. Although both motor types are self-starting, special starting controls
are needed in many applications to limit starting time to acceptable values.
Both types operate at rated load with very little slip.
The squirrel-cage motor takes its name from the rotor construction.
Among the various standard designs, the one designated as Design B is usually
used on fans. This motor is suitable for continuous operation at rated load,
and its starting current is relatively low.
Wound-rotor motors, also known as slip-ring motors, can provide adjust-
able-speed drive if desired. Speed reductions below 50 percent are not recom-
mended, however. Top speed and efficiency are about the seme as for the
squirrel-cage motor.
60
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5.6 FAN CONTROLS
The throughput of fans may be changed by varying the speed or by changing
the pressure condition at the inlet and/or outlet. Variable inlet vanes
provide control on the inlet side of the fan, and dampers can be used on
either the inlet or outlet side. Each of the fan control methods entails a
loss in efficiency over most of the operating range. Because most large-ca-
pacity fans are ordinarily driven by constant-speed motors, it is usually not
possible to control the fan by varying the speed. Variable inlet vanes can
provide gradual load adjustment; however, these must be purchased with the
fan. Dampers offer flexibility of location and control. A fan system with
dampers can be designed to meet a wide range of load fluctuations. Table 7
lists the basic damper types, and the following subsections provide brief
descriptions of the major types.
5.6.1 Louvre
Louvre or multiblade dampers may be of the opposed-blade or the paral-
lel-blade design (Figures 29a and 29b). An overall view of a parallel-blade,
multilouvre damper is shown in Figure 29c. Parallel-blade louvre dampers can
be closed tight, but they offer little modulation ability. Conversely, op-
posed-blade louvre dampers offer excellent modulation ability, but they cannot
be closed as tightly as the parallel-blade louvre dampers can. Louvre dampers
may be used to regulate and isolate gas flow. For isolation, two dampers can
be used together and sealed by pressurizing the chamber formed by the ductwork
between the dampers with a seal-air fan. A single damper may be used for gas
flow regulation.
5.6.2 Guillotine Damper
A guillotine damper may be a top-entry or bottom-entry design, with or
without seal air (Figure 30). Guillotine dampers for system isolation may be
equipped with a seal-air blower, single- or double-bladed, to pressurize the
sealing space and thus ensure against gas leakage past the damper.
5.6.3 Butterfly Damper
Butterfly dampers are often used for secondary duct runs (Figure 31).
They are mounted by a center shaft that crosses the duct, and the damper plate
61
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TABLE 7. BASIC DAMPER TYPES
Generic
Specific
Common designs
Louvre
Guillotine
Parallel-blade multilouvre
Opposed-blade multilouvre
Top-entry guillotine
Top-entry guillotine/seal-air
Bottom-entry guillotine
Bottom-entry guillotine/seal-air
Single louvre
Double louvre
Double louvre/seal-air
Single louvre
Double louvre
Double louvre/seal-air
Butterfly
Blanking plate
62
-------
\
\
(a)
(b)
(c)
Courtesy: Frisch Division, DAYCO Company. Chicago, Illinois.
Figure 29. Louvre damper: (a) parallel-blade multilouvre; (b) opposed-
blade multilouvre; (c) view of parallel-blade, multilouvre damper showing
linkage.
63
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11
(a)
DAMPER BLADE
STAINLESS STEEL
SEALS
COVER PLATE
PRESSURIZED CHAMBER
GAS FLOW
0 0
PRESSURIZED
CHAMBER
FAN INLET
COVER PLATE
(b) (c)
Courtesy: Frisch Division, DAYCO Company, Chicago, Illinois.
Figure 30. Guillotine damper: (a) simplified cross-sectional view of a
guillotine damper; (b) guillotine isolation damper using seal air;
(c) top-entry type guillotine damper, showing operation.
64
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BUTTERFLY
(a)
(b)
Courtesy: Frisch Division, DAYCO Company, Chicago, Illinois.
Figure 31. Butterfly damper: (a) simplified cross-sectional view of a
butterfly damper; (b) butterfly damper showing hand operator.
65
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rotates about the shaft from a plane parallel to the gas flow (open) to a
plane perpendicular to the gas flow (closed). Butterfly dampers are used most
often to regulate gas flow, but they have been used for opening and closing
off ducts and fan systems.
5.6.4 Blanking-Plate Damper
The most basic damper is the simple blank-off plate. Blanking plates are
essential when a process must be isolated for the protection of the mainte-
nance crew. Should it become necessary for persons to enter any section of
the ductwork, the blanking plate ensures isolation of that section. When used
in conjunction with positive-ventilation air purge, the blanking plates ensure
the safety of personnel.
Blanking plates are similar to guillotine dampers in that they cut across
the duct opening; however, the track design for a blanking plate is intended
only to guide the plate as it is put in place and bolted down.
5.7 FAN SIZING
The air (base) horsepower requirement of a fan can be calculated by
Equation 17:
Air hp = Qh/6356 (Eq. 17)
where Q = inlet volume, ft3/min
h = total static pressure rise, in. H20
Fan brake horsepower can be obtained by dividing the air horsepower by
the mechanical efficiency of the fan. Mechanical efficiency for most centri-
fugal fan operating points will be 50 to 65 percent. Fan size can also be
determined from the fan manufacturer's catalog. Manufacturers' catalogs
generally include multirating tables that give the operating parameter ranges
for different fan models. Many larger fans, however, are custom-built and
thus not found in multirating tables. If the inspector needs to do a detailed
analysis on a custom-built fan, he/she must obtain the fan ratings and per-
formance curves from either the manufacturer or the plant.
66
-------
The multirating tables are generally based on standard conditions. When
air is not at standard conditions, corrections must be made to volume, pres-
sure, and horsepower corrections to select a fan at an "equivalent" volume and
pressure. For a manufacturer's rating table to be used, the fan requirements
must be converted to the density used in the ratings. The density correction
is generally made by using the ratio of air density at standard conditions to
the actual density of the air at the fan inlet. Table 8 provides air density
correction factors at various temperatures and altitudes. Table 9 lists the
densities of various common gases. It is usually sufficient to assume the
density of air because it is the predominant gas, but in some cases (e.g.,
saturated gas from a scrubber), a density correction for composition may be
necessary. A further density correction is theoretically required because the
air at the fan inlet is under suction, which lowers the density- For example,
the correction factor from -60 in. H?0 to sea level is about 1.18. Before a
fan is chosen from the multirating tables, the following adjustments should be
made:
1. Determine density factor (d = 0.075 for air at 70°F and 29.92 in.
Hg)
2. If the gas flow is indicated at standard conditions, convert it to
actual fan conditions:
where Q = actual volume of air entering the fan
A - barometric pressure corresponding to fan
site altitude, psia
T = inlet temperature, °F
Vscfm = volume of air at standard conditions
(70°F and 14.7 psia)
3. Multiply static pressure by the density factor, d
4. Using corrected static pressure and actual gas flow, Q, select fan
from multirating tables
5. Divide the fan bhp selected in Step 4 by the density factor.
67
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TABLE 8. AIR DENSITY CORRECTION FACTOR, d
Altitude, ft.
Barometer, in
Air temp. , °F
Hg
Wg
-40
0
40
70
100
150
200
250
300
350
400
450
500
550
600
700
800
900
1000
-1000
31.02
422.2
1.31
1.19
1.10
1.04
.98
.90
.83
.77
.72
.68
.64
.60
.57
.54
.52
.47
.44
.40
.37
Sea
level
29.92
407.5
1.26
1.15
1.06
1.00
0.95
0.87
0.80
0.75
0.70
0.65
0.62
0.58
0.55
0.53
0.50
0.46
0.42
0.39
0.36
1000
28.86
392.8
1.22
1.11
1.02
0.96
0.92
0.84
0.77
0.72
0.67
0.62
0.60
0.56
0.53
0.51
0.48
0.44
0.40
0.37
0.35
2000
27.82
378.6
1.17
1.07
0.99
0.93
0.88
0.81
0.74
0.70
0.65
0.60
0.57
0.54
0.51
0.49
0.46
0.43
0.39
0.36
0.33
3000
26.82
365.0
1.13
1.03
0.95
0.89
0.85
0.78
0.71
0.67
0.62
0.58
0.55
0.52
0.49
0.47
0.45
0.41
0.37
0.35
0.32
4000
25.84
351.7
1.09
0.99
0.92
0.86
0.81
0.75
0.69
0.64
0.60
0.56
0.53
0.50
0.47
0.45
0.43
0.39
0.36
0.33
0.31
5000
24.90
338.9
1.05
0.95
0.88
0.83
0.78
0.72
0.66
0.62
0.58
0.54
0.51
0.48
0.45
0.44
0.41
0.38
0.35
0.32
0.30
6000
23.98
326.4
1.01
0.91
0.85
0.80
0.75
0.69
0.64
0.60
0.56
0.52
0.49
0.46
0.44
0.42
0.40
0.37
0.33
0.31
0.29
7000
23.09
314.3
0.97
0.89
0.82
0.77
0.73
0.67
0.62
0.58
0.54
0.51
0.48
0.45
0.43
0.41
0.39
0.35
0.32
0.30
0.28
8000
22.22
302.1
0.93
0.85
0.79
0.74
0.70
0.65
0.60
0.56
0.52
0.49
0.46
0.43
0.41
0.39
0.37
0.34
0.31
0.29
0.27
9000
21.39
291.1
0.90
0.82
0.76
0.71
0.68
0.62
0.57
0.58
0.50
0.57
0.44
0.42
0.39
0.38
0.35
0.33
0.30
0.28
0.26
10,000
20.58
280.1
0.87
0.79
0.73
0.69
0.65
0.60
0.55
0.51
0.48
0.45
0.42
0.40
0.38
0.36
0.34
0.32
0.29
0.27
0.25
en
co
Standard air density, sea level, 70°F = 0.075 lb/ft3.
-------
TABLE 9. DENSITY OF COMMON GASES
Gas
Hydrogen
Oxygen
CO
co2
N2
Benzene
Ammonia
so2
Water vapor
Air
lb/ft3
0.0052
0.0828
0.072
0.1146
0.0728
0.2017-
0.0446
0.1697
0.0466
0.075
69
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The fan brake horespower obtained from the above calculations is divided
by the motor efficiency to obtain motor horsepower. Motor efficiency for
three-phase motors such as those used on large systems is typically 85 to 90
percent.
Usually, fan horsepower is designed for so-called cold-start conditions,
in which case the density correction factor is not used. This enables the
motor end fan to pull the full quantity of "cold" (i.e., ambient) dry air upon
startup without overloed. After startup, when the air reaches operating
conditions of temperature and humidity, it is less dense and the horsepower
load is reduced.
70
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SECTION 6
VENTILATION SYSTEM INSPECTION
Careful preparation and planning are vital to a successful inspection
and evaluation of ventilation systems. An inspection will be meaningful only
if the inspector knows what information he/she wants to collect and is famil-
iar with the equipment at the site. Time invested in a file review will
reduce the inspector's field time and that of the source representative.
Also, if the inspector can obtain all the required data during the inspec-
tion, later time-consuming efforts to secure missing data can be avoided.
Furthermore, if the inspector has performed his/her homework, the plant
personnel are more likely to view the inspector as a professional and to
provide the information and cooperation the Agency needs. This section
presents guidelines to assist the inspector in conducting a successful in-
spection.
6.1 PREPARING FOR INSPECTION
When inspecting a ventilation system, the inspector must record the data
on site for later use in evaluating compliance practices. The following
items will help to ensure that the inspection is complete and that the perti-
nent information is obtained while the inspector is on site:
Plot Plan
The plot plan should show entrances, major buildings, and the process
area to scale and include other appropriate details that provide orien-
tation.
Equipment Drawings
Photographs or sketches of the equipment configuration are useful for
reference or comparison. These should show major process and control
equipment for easy reference at a later date.
71
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Process Flowsheet and Equipment Checklist
These should provide the inspector with a clear idea of the operating
procedures, factors affecting emissions, a listing of necessary data to
collect for determining compliance, and data collection methodology.
Before beginning the inspection, the inspector should review the work-
sheets and process flows with the plant's representative at the plant to
assure that the information obtained during the file review is accurate and
up to date. This also informs the plant representative of the inspection
procedures so that he/she can assist the inspector in collecting information.
6.2 SAFETY CONSIDERATIONS
To aovid injury while conducting a thorough inspection, the inspector
must:
0 Wear the requisite safety equipment
0 Be aware of the safety hazards
0 Respect the company's safety procedures
0 Never become overconfident
The last point is particularly important. A relatively new inspector or
engineer may begin to feel like an "oldtimer" after the first few trips to
the plant, and this can be dangerous. For safety reasons, the inspector
should make it a practice to stay with the plant escort.
6.2.1 Safety Equipment
For proper fit, the inspector should have his/her own safety equipment.
The following equipment is recommended:
Hard hat
0 Safety glasses with side shields or full-cover goggles
0 Steel-toed safety shoes
0 Fire-resistant pants and jacket
0 OSHA-approved respirator (fit-tested)
0 Heavy-duty gloves
Although incidents may seem ulikely, when the inspector's attention is
often focused on observing emissions and/or operating procedures, he/she can
easily become unmindful of potential hazards. Part of the. preparation of the
observation procedures should be to review the location of the observation
point, any required movements, the expected activities in the immediate
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vicinity, and the availability of an escape route. It is generally advisable
to stay as far away from moving equipment as possible.
Unfortunately, a thorough inspection entails some risk. Respect for the
hazards, familiarity with operations, and constant concern for safety will
minimize the chance of an unpleasant or fatal accident.
6.3 ONSITE COMPANY-INSPECTOR INTERACTION
The success of an inspection depends greatly on the interaction between
the inspector and plant representative. Upon arrival at the plant, the
inspector should be prepared to discuss the following:
0 Authority for the inspection
0 Agency organization
0 Scope, timing, and organization of the inspection (preferred in-
spection agenda)
0 Treatment of confidential data
It is also important to inquire about the operational status of the
equipment to be inspected and the kinds and frequencies of any malfunctions.
If equipment is not operating at or near normal conditions, the inspector
should note the reasons and when the plant expects it to be operating nor-
mally (for followup inspection scheduling).
Before collecting any data, the inspector should observe process opera-
tions for a while to become familiar with process variations, wind patterns,
plume characteristics, etc.
6.4 INSPECTION PROCEDURES
The inspection and testing of large ventilation systems present some
problems not normally associated with smaller ventilation systems. Most of
the problems are associated with access to the ductwork and the physical size
of the system. In many instances, access limits the measurements that can be
taken.
The first criterion in the evaluation of the performance of large sys-
tems is to determine whether capture is proper at the source. Poor lighting
sometimes limits visual observation of capture capabilities and makes it
difficult to evaluate the effectiveness of the system. The second criterion
is to determine the integrity of the ductwork to the control equipment.
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Again, access conditions may limit this visual inspection to what can be
observed from the ground or from a nearby roof. Nevertheless, excessive
inleakage is occurring and the need for maintenance should be determinable.
6.4.1 Duct System Inspection
The inspector should have the following before he/she begins the inspec-
tion:
0 Knowledge of the operations in the process.
0 Knowledge of the physical/chemical nature of the process charge
materials.
0 A layout of the operations showing the controlled and uncontrolled
areas.
0 A line sketch showing the elevation(s) and layout of the ductwork;
the locations of the collector, fan, control equipment; and the
flow patterns.
0 Layout(s) and sketch(es) of the duct size (length and diameter)
showing the main duct, branch or interconnecting ducts, and their
respective flow (actual cubic feet per minute); temperature (°F) of
actual gas flow; slope ratio for transition points of branch duct-
ing to the main or interconnecting ducts and their respective
angles; type and thickness of duct lining; and the location, type
and size of blast gates and dampers and how they are controlled.
0 Copies of plant tests for velocity, pressure drop, and temperature,
and the production or process rate during the test.
0 The production or process rate during the inspection.
During the inspection, the inspector should note the condition of the
duct (e.g., erosion, corrosion, rusted-through openings), flanges, expansion
joints, fits of swing-away joints, etc., all of which affect the workability
of a ducting system.
He/she should check to see that any emergency air inlet dampers, such as
those located in the duct on electric arc furnace systems, are closed. These
are designed to open when temperatures are high, and they should close auto-
matically and stay closed during normal operation.
For inspection purposes, the ventilation system is defined as the duct-
work leading from the emission points to the control devices. It is recom-
mended that static pressure taps be made throughout the length of this
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ductwork to provide data on air inleakage and ductwork plugging, two of the
major problems with ventilation systems. Both will change the static pres-
sure and temperature profiles of the ductwork system. If a problem is indi-
cated, the inspector should carefully examine the ventilation system to
pinpoint the problem.
Duct blockage, which is characterized by a large increase in static
pressure between the blockage and fan, reduces hood face velocity and results
in failure of the hooding to capture fugitive emissions. Blockage typically
occurs in ductwork bends and cooling loops and may be attributable to insuf-
ficient duct velocity (improper duct sizing), sedimentation of dust par-
ticles, or excessive cooling of the gas stream (which tends to change the
particulate matter into sticky particles that deposit in the duct).
Air inleakage can be a major contributor to excessive cooling of the gas
stream. Although air inleakage may not occur at a single point, it is char-
acterized by lower static pressures and lower temperatures downstream of the
inleakage points. Excessive inleakage may result in fugitive emissions due
to decreased collection efficiency at the emission point and, as noted above,
increased potential for duct blockage.
The inspector should check the face velocity and positioning of all
fugitive hooding. Improper positioning of hooding or hood damage can reduce
capture efficiency. Also, a negative pressure of at least 24 to 49 Pa (0.1
to 0.2 in. HpO) should be maintained at fugitive and process emission points
to accommodate any surges in gas volume and emissions.
6.4.2 Fan Inspection
Lower duct static pressures can also indicate an undersized or underper-
forming fan. The fan system components should be inspected for wear or
corrosion and excessive dust buildup or grease accumulation removed. Fan
couplings should be inspected for loose bolts or misalignments. Bearings
should be clean and lubricated. Shaft seals should be inspected for leakage.
Although a certain amount of leakage is tolerable, excessive leakage may
indicate a need for seal replacement.
A major factor to be checked during an inspection is fan vibration.
Vibration is a function of fan speed. Normal vibration amplitudes at dif-
ferent fan speeds are as follows:
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Normal vibration
Speed, rpm amplitude, in.
400 0.003
800 0.002
1200 0.0013
1800 0.0008
3600 0.0005
Although vibration amplitudes up to 2.5 times the normal value are accept-
able, corrective measures are required when this limit is exceeded.
Excessive vibration can result from several things, including material
buildup on a blade or bearing wear. Excessive vibration requires immediate
attention, as it may quickly lead to catastrophic failure of the fan. An
out-of-balance fan wheel rotating at 300 to 700 rpm can fracture the shaft
and break through the housing. This can damage other equipment in the area
and endanger the welfare of people in the immediate vicinity. The inspector
should vacate the area immediately and promptly report any severe vibration
to the plant.
Performance tests may be necessary to ensure the proper fan operation
after major maintenance, including rebalancing the fan wheel if any repairs
were made to rotating components. Using a pitot tube and manometer, an
inspector can measure velocity pressures at various points in the duct and
calculate the volume flow rate from these readings after correcting them for
density at the operating temperature and pressure. Fan horsepower can be
calculated from readings of voltage and current supplied to the motor, but it
also must be corrected for actual density. The horsepower and static pres-
sure should be plotted on the fan's original characteristic curves for com-
parison.
Each fan has a set of isolation sleeves, the size of which depends on
the fan size. Cracks or tears in the inlet sleeves permit in-draft air to be
pulled in by the fan, which reduces suction from the hood. In high-tempera-
ture operations, inlet sleeves are made of an asbestos compound. In lower-
temperature operations, they are made of neoprene type rubber. Rubber
sleeves should never be painted; any type of paint will attack rubber.
Fans are designed to rotate in a given direction. If electrical connec-
tions are reversed, the fan will still move air, but not efficiently. Fan
rotation direction should be confirmed against design direction.
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Fan curves cannot be used directly on a scrubber that has water droplet
carryover, as the water droplets represent a mass that the fan has to accel-
erate. A wet fan indicates that a mist eliminator isn't working properly.
This will require the fan to have higher horsepower, and the fan curve will
inaccurately project that more gas is going through the system than there
actually is. Uncondensed moisture in the gas stream also must be corrected
because it changes the density. At some sources, a Fyrite test kit can be
used to check the change in oxygen content throughout the system. Severe
inleakage will cause a sudden jump in oxygen content. This technique cannot
be used for an ambient system.
6.4.3 Gas Flow Check
Gas flow is a useful parameter in evaluating system performance because
it provides an indication of the capture velocity at the hood(s). In gen-
eral, as the quantity of the gas moved through the fan increases, the re-
quired horsepower also increases. This increase is reflected by an increase
in motor current, which is often measured in a control room. Although motor
current sometimes can be measured by a portable clamp-on ammeter, inexperi-
enced personnel should not attempt this kind of measurement, particularly at
high operating voltages. The current flow is a useful indicator for all
types of systems, regardless of the flow control method (i.e., damper type or
speed control). Static pressure across a fan operating at a fixed speed and
without inlet spin-vane dampers also will indicate gas flow; gas flow de-
creases as static pressure or resistance increases.
The use of fan curves is one method of determining gas flow. This
requires the measurement of fan rotation speed, gas temperature, and motor
horsepower or static pressure across the fan. This method cannot be used if
inlet spin vane dampers are used.
The use of a pitot traverse is a more traditional approach to deter-
mining gas velocity. In this method (outlined in EPA Reference Methods 1 and
2) a pitot tube is used to measure velocity profile across the duct and the
known cross-sectional area of the duct for calculating gas volume through the
system. Again, the limiting factor may be access to the ductwork. Long,
difficult-to-handle pitot tubes may be required to obtain the measurements in
large ducts.
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Once gas volume is estimated, the average hood capture velocity may be
estimated by comparing the measured gas volume with the cross-sectional area
of the hood. This indicates whether the average hood face velocity is ade-
quate to provide the desired capture efficiency. When multiple-hoods are
interconnected, the flow in each branch may have to be measured to establish
the proper flow balance from each hood. The difficulty with this method is
that it provides only an average hood face velocity, which may be substan-
tially different from localized velocities in large ventilation hoods.
Pitot tubes generally will not accurately indicate hood face velocity at
the relatively low velocity encountered at most ventilation hoods (250 to 750
fpm). Therefore, other types of instrumentation must be used such as hot-
wire and low-pressure gauges. Hand-held instruments such as vane and propel-
ler-type anemometers cannot be used to measure localized face velocity.
Hot-wire anemometers may be attached to long probes for measurement of face
velocity at various points across the hood face, but the results would have
to be corrected for the additional wire length (resistance), which would
affect the readings. Again, access would have to be provided for these
tests, which could be impractical or unsafe during source operation. A test
of this nature, however, would establish minimum and maximum hood face veloc-
ities, flow distribution, and average hood face velocity, whereas measurement
of the gas volume/hood opening area provides only an average velocity.
6.4.4 Visual Observations
If no measurements are made, inspection is limited to visual observation
of the system performance, including hood and duct integrity, the effects of
cross-drafts, and operational procedures that may affect the capture effi-
ciency of the ventilation system. Hoods should remain intact and as close to
the source as possible, not only to capture emissions, but also to reduce gas
volume requirements. Ducts should be checked for damage, which can cause
pressure losses, wear, and inleakage. Cross-drafts should be minimal. If
the system is not designed to operate with cross-drafts, it may not perform
satisfactorily. Wind direction and open doors and windows should be noted.
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6.5 OPERATION AND MAINTENANCE (O&M) CONSIDERATIONS
Because the fan systems used in large ventilation systems represent
substantial capital outlay and are costly to operate, it is in the best
interest of the plants to maintain this equipment in a fashion that lowers
the incidence of failures that lead to excessive downtime and extends the
useful life of the equipment. The fan system should be considered as includ-
ing the fan, the motor and drive systems, the inlet and outlet duct systems,
and any flow-control dampers used to control the quantity of gas being moved
through the ventilation system.
The successful operation and maintenance of the fan system on large
industrial ventilation systems does not depend on any one item, but on the
proper design and operation of several components. Although some items are
more critical than others, all must operate as a unit for the ventilation
system to deliver the desired gas volume most efficiently.
For economic reasons, most single fan installations are limited to gas
volumes of approximately 1.0 to 1.3 million acfm at nominal static pressure
drops. If greater gas volumes are needed than can be economically delivered
by a single fan, two or more fans arranged in parallel may be used. This
arrangement allows the use of several smaller "off-the-shelf" fans, which are
less costly than custom engineered fans; however, when multiple fans are
used, care must be taken to see that the ductwork and fans match, particu-
larly when fans of different sizes are used. This approach permits the use
of smaller individual fan drive motors and also allows the ventilation system
to remain partly operational in the event of the failure of one of the fans
or fan drive motor systems.
Although many components in the fan system may be subject to failure,
two areas are of the greatest concern: 1) improper balance and the resulting
excessive vibration of the fan, and 2) failure of the drive motor system.
Either can result in expensive repairs or replacements. Proper design and a
preventive maintenance program can reduce the incidence of such failure.
Some drive systems are quite large and require motor sizes in excess of
500 hp (up to as high as 8000). The initial purchase is generally propor-
tional to the motor size. On the other hand, the larger systems tend to be
more efficient, and energy savings over the life of the unit may offset the
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initial cost. Proper matching of motor size with horsepower requirements
also will usually increase motor efficiency.
Most drive motors are at their highest efficiency and lowest power
factor losses when operated at 80 to 90 percent of their maximum rated load.
If motors are always operated at 100 percent of rated load, they may require
more maintenance and may not have sufficient reserve for occasional periods
when more horsepower is needed. The "service factor" rating of the motor,
which usually ranges between 1.0 and 1.15, is an indicator of the sturdiness
of the motor and its ability to run at higher than rated load for an extended
period of time without experiencing damage.
Because many of these fans are used in systems that control fugitive
emissions varying in gas temperature (and gas volume to be captured), the
maximum and minimum gas temperatures must be considered in sizing the fan
motor. If the fan is sized only for gas conditions at the maximum operating
temperatures, problems with motor overload may occur when gas temperature de-
creases. This results in a denser gas, and more energy is required to move
it. Eventually, such continual overloading of the motor may lead to burnout
of the motor windings and motor failure if efforts are not made to minimize
the problem.
Motor startup is also related to motor overload. The larger the motor
size is, the higher the operating voltage required to keep the current flow
at a reasonable level. Operating voltage will typically be 440 to 460 for
motors having 350 hp or less (but in some cases up to 500 hp). For larger
motors, operating voltages generally increase to 2130 or 4260. When a fan
motor is started "across the line," however, it can draw 6 to 7 times its
normal operating current (regardless of voltage) during acceleration of the
fan wheel to its normal rotation speed. Although this current surge is of
short duration and diminishes as the fan approaches its running speed, it
could damage the motor on startup with a cold gas stream and cause circuit
breakers to trip, both at the fan and in other areas of the plant that might
be affected if the surge of current into the fan circuit were to reduce
operating voltages to unacceptably low voltages and cause undervoltage trips.
Two methods that can be used to reduce the possibility of circuit
breaker trips or damage to the motor upon startup are reduced-voltage starts
and closed-damper starts. Reduced-voltage starts allow the fan wheel to
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accelerate to a portion of its rotation speed, and then full voltage is
applied! to accelerate the fan wheel and gas to full speed. Although the
current will still surge upon startup, the levels of surge should be more
acceptable, particularly if other circuits are involved. Closed-damper
starts allow the motor to accelerate the fan wheel without simultaneously
moving the gas; much of the current surge results from accelerated gas flow
through the system. Opening the dampers gradually to allow gas to flow after
the fan wheel is rotating permits the flow of current to be controlled.
A third method is to control the rotation speed. This method is similar
to the reduced-voltage start. The primary difference is that this method
uses a variable-speed motor or a transmission coupling, which allows variable
fan rotation speeds. A gradual increase in rotation speed controls the
current surge.
Excessively high currents in the motor create heat in the windings,
which can destroy the winding insulation and result in the loss of windings
because of short-circuiting. Heat buildup in the motor can be a major reason
for motor failure. Other reasons for heat buildup in the motor include
improper or restricted ventilation due to the location of the motor or exces-
sive dust buildup. Motors must be kept clean to maintain the flow of cooling
air through them. Additional cooling considerations may be necessary if fans
are located where ambient temperatures are high. Normally, solid-state
controls for variable-speed motors also must be protected from high tempera-
tures.
Transmission of the motor energy to the fan shaft is usually accom-
plished by direct drive, by V-belts, or through a variable-speed trans-
mission. The belts must be tensioned properly and kept free of grease or oil
to prevent slippage and belt damage. The fluid levels of fluid-drive trans-
missions for variable-speed operation must be maintained (and possibly
cooled) for reliable operation.
Most fan shafts are supported by bearings at the fan housing and at the
drive connections. Worn bearings can cause excessive fan vibration and
increase energy costs. Bearings for smaller fans can be installed with
grease seals and a grease fitting for routine lubrication. The bearing
lubrication should be checked at least daily to ensure that it is adequate.
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On fans operating with high-temperature gas streams, it is particularly
important to keep the bearings cool to prevent breakdown of the lubricant.
Heat can be transmitted through the shaft to the bearing assembly.
Although most fans are designed with some inleakage of air around the
shaft to the fan housing, this may not be enough for adequate cooling. In
this case, heat fins may be installed on the fan shaft where it exits from
the fan. These fins rotate with the shaft and provide extra cooling surface
by conducting heat away from the bearings. In situations where this approach
is inadequate, a continuous lubrication system that utilizes circulating oil
can be applied. This system, which consists of a pump, circulation lines, a
filter, and a cooling system (usually water), simultaneously provides bear-
ings with continuous lubrication and cooling. More complex than simple
grease lubrication, this system has the disadvantage of requiring continuous
operation.
On large fan systems, the use of vibration monitoring equipment can
prevent continued operation of a fan that is unbalanced or has worn bearings.
If a severely vibrating fan is allowed to continue operating, a "fan explo-
sion" could occur when the fan is no longer able to withstand the stresses
and simply comes apart during operation. This can be extremely dangerous to
personnel working in the area and also costly to repair.
All fan systems are equipped with inspection hatches and the fans should
be inspected through these hatches at least annually to evaluate the severity
of fan wheel wear. Generally, evidence of fan blade wear appears on the
edges of the fan wheel, on the side opposite the inlet. The chance of fan
wear and blade buildup is greater on fans that are installed in the gas
stream prior to the air pollution control equipment than on fans placed down-
stream of the control equipment, and they should be checked more frequently
for wear and vibration. When blades are replaced, the fan usually must be
rebalanced.
Particulate buildup on fan blades can lead to improper fan balance.
Because the buildup on fan wheels tends to be relatively uniform, it does not
affect the fan wheel balanced until it flakes off, at which time a substan-
tial change in fan balance can occur. This, in turn, leads to fan vibration
and bearing wear, and can eventually cause fan wheel failure. This buildup
problem can be severe when the dust is sticky or oily or if the fan follows a
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wet scrubber, in which case wet gas conditions enhance the buildup problems.
(Water can be applied to a fan that is used after the scrubber to prevent
particulate buildup.) The higher the rotation speed of fans, the more sus-
ceptible they are to the severe inbalance problems of particulate buildup.
Fans with linings should be inspected as often or more often than other
fans to ascertain that the lining is still intact. Care should be taken not
to damage the lining during inspection. Because fans are most often lined
for corrosion resistance, these linings must be kept in good condition to
prevent excessive fan wheel or housing corrosion. The key to the successful
application of lining is good surface preparation, which seems to be as much
an art as a science.
Another area of concern is the delivery of proper gas flow through the
ventilation system. This flow control is usually accomplished through the
use of dampers or by controlling the rotation of the fan speed. The latter
method is the most energy-efficient. The quantity of gas delivered by a fan
is proportional to the change in rotation speed, but the horsepower required
changes by a cubic relationship. Thus, very small changes in rotation speed
can result in significant changes in horsepower. The initial cost of the
speed control equipment will offset the energy savings somewhat.
The next most efficient method of controlling gas flow is the use of
inlet spin vane dampers. As the vanes close, gas is spun in the direction of
the fan wheel rotation. The quantity of gas delivered by the fan is a func-
tion of the difference in inlet and outlet gas velocity. Because the gas is
spinning in the direction of rotation, the difference in velocity is smaller
and the quantity of gas delivered is smaller with no waste in the static
pressure developed by the fan. These dampers should be checked periodically
to guard against improper functioning and excessive wear of the vanes.
The least efficient but simplest method of controlling gas flow involves
the use of blade-type dampers. These dampers place additional resistance
(variable) into the system to control the gas flow. Even at reduced flow
rates, the fan still delivers a high static pressure and thus requires more
energy to move the reduced gas volume. This method is also the simplest to
maintain unless excessive wear occurs.
Air inleakage is one reason why systems may not perform as designed.
Air can flow into the system wherever a leak occurs, e.g., through a hole in
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the ductwork, an expansion joint, or open branches that should be closed.
The fan will pull gas through the point of least resistance, and this can
leave the ventilation system with inadequate draft. Any damage should be
repaired before it leads to inadequate control of emissions, corrosion (lead-
ing to increased air inleakage), and higher energy costs for transporting
unwanted dilution air though the system.
Ductwork design also affects how much gas the system will deliver. The
shortest, straightest duct is the most desirable for the design, operation,
and maintainance of the system. On large systems, rectangular ducts are
often easier to install than round ones because it is easier to weld large
flat sections of steel; however, particulate matter has a tendency to build
up in the corners of rectangular ducts and close off usable area. For this
reason rectangular ducts may require periodic cleanout. On the other hand,
some fans (e.g., double-inlet centrifugal fans) are more adaptable to rectan-
gular ducts.
Improper design of the inlet design of the ductwork can cause a swirling
gas flow to the fan, which creates an effect similar to that obtained from
inlet spin vane dampers. This can cause reduced gas flow and inadequate
capture by the ventilation system. This is particularly true if smooth
transitions are not provided to the fan, in which case flow-straightening
vanes or redesign of the ductwork may be required.
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SECTION 7
TOTAL FURNACE ENCLOSURES
This section discusses total furnace enclosures in the metallurgical
industries. It is assumed that the reader is generally familiar with the
metallurgical processes that use these enclosures. The basic ventilation
principles outlined in previous sections are valid for these special appli-
cations.
7.1 ELECTRIC ARC FURNACES
The recent trend in electric arc furnace (EAF) emission control is to
totally enclose the furnace operations. This system allows the collection
of both primary and secondary emissions from EAF operations. Several
smaller furnaces (<100 tons) have installed total furnace enclosures. The
essential features of these enclosures are sliding doors that create access
for the crane and scrap charging bucket and an air curtain to block the
escape of fumes from the roof when the roof doors are parted for crane cable
access during charging. The advantages of total furnace enclosure are:
s
° Effective fume capture.
0 Low volumes of air handled as emissions collected at the source.
0 Capture of both primary and secondary emissions.
0 Access for furnace maintenance.
0 Lower noise levels outside the enclosure
0 Lower capital and operating costs.
0 Better working environment and lower roof temperature in the EAF
shop.
0 Minimal effect of cross winds because the entire operation is
enclosed.
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0 Lower maintenance of cranes and other equipment because of reduced
dust-fall within the shop.
Figure 32 illustrates a typical furnace enclosure.
7.1.1 Description
A typical enclosure should have adequate space around the furnace to
provide a suitable working area and reduce the effect of heat on the enclo-
sure structure and furnace components. The enclosure structure is generally
lined with hi-rib aluminized sheeting and joints sealed with closure strips.
Bi-parting vertical doors in front of the furnace are controlled by air
cylinders to allow the overhead crane to enter the enclosure. A rectangular
roof slot provides clearance for crane cables when the crane is operated
within the enclosure. This slot is closed by roof doors that are pneumati-
cally operated. An air curtain under the roof slot area contains and guides
the emissions to the pickup hood located opposite the air curtain nozzles.
The air curtain fan is usually located on the roof of the furnace transformer
room. Removable roof panels above the roof doors allow access for furnace
maintenance. The furnace operations, air curtain, and pickup hood can be
observed safely from the control room. The control room usually has a full
glass window that forms a part of the enclosure wall. Doors to the enclosure
allow access to the furnace for oxygen lancing and for taking metal and slag
samples.
To charge the furnace, the crane operator positions the charging bucket
in front of the enclosure and aligns the crane cable in front of the roof
slot. The furnace operator swings the furnace roof aside and opens the
front vertical bi-parting doors and the roof doors of the enclosure. The
crane operator then brings the charging bucket into the enclosure. At
ground level, a furnace operator guides the crane operator in positioning
the charge bucket directly over the furnace. The crane operator dumps the
scrap by opening the bottom of the bucket and then reverses the crane out of
the enclosure. The control room operator closes the vertical and roof doors
and swings the furnace roof in position to seal the furnace. The melting
operation commences after the furnace roof is in position. During the
entire period when the bi-parting doors and roof doors are open, the air
curtain is in operation along with the enclosure exhaust. During melting,
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00
LOCAL TAP HOOD
AIR CURTAIN
PICKUP HOOD
BI-PARTING DOORS
TO BAGHOUSE
CRANE RAIL LEVEL
FURNACE LEVEL
GROUND LEVEL
Figure 32. Typical furnace enclosure.
(Not to scale)
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oxygen lancing, deslagging, and sampling, the doors are closed and the
enclosure is exhausted to a gas cleaning device, generally a baghouse.
A stack in the fourth hole position of the furnace roof leads the
emissions generated during melting and refining closer to the exhaust hood.
Depending on the design of the enclosure, the furnace tapping occurs either
within or outside the enclosure. In most cases a transfer car brings the
teeming ladle into the tapping position underneath the furnace. If tapping
takes place outside the enclosure, a local hood is provided to capture
tapping emissions. In the design where tapping takes place within the
enclosure, the enclosure exhaust evacuates the tapping emissions.
7.1.2 Design Considerations
Charging, melting, refining, and tapping emissions occur during the
operation of the EAF. The maximum emissions (85 - 90%) occur during melt-
i p
down or refining. Direct shell evacuation or side draft hoods adequately
control these primary emissions. Several techniques including canopy hoods
and building evacuation control secondary emissions. Although canopy hoods
have few operating restrictions, they have the disadvantages of high volume
requirements, high capital, and high operating costs. Crosswinds within the
shop affect capture efficiency. Local hoods have provided limited success
in collecting tapping fumes. A compromise solution to the high volumes
required for canopy hood systems is a modified canopy approach where fixed
curtain walls form a partial enclosure around the furnace to act as a
chimney to direct charging emissions to the canopy hood, while local hoods
are utilized to control tapping emissions.
The ultimate control of furnace emissions involves furnace enclosure
technology. On small electric furnaces, complete emissions control can be
achieved by collecting emissions at the source by adopting furnace enclosure
technology. Emissions generated during all phases of the EAF operations can
be withdrawn from the enclosure using relatively low flow volumes. If
emission volumes are high as a result of the use of UHP furnaces, oxyfuel
burners, or larger furnaces, however, additional primary controls such as a
DSE (fourth hole) system may be required. The approximate volume of air
required for a furnace enclosure in a small furnace will be equivalent to
the volume required by a side-draft hood system; a larger furnace will
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IP
require an amount equivalent to the volume required by a DSE system. The
actual air flow required will depend on many factors such as furnace size,
oxygen lancing rate, exhaust-duct position, scrap cleanliness, furnace
power, types of steel produced, operating practices, and enclosure volume.
7.1.3 Retrofit Installations
Because each retrofit installation is unique, enclosures must be
tailored to meet individual requirements, furnace practices, and shop
layout. Examples of two retrofit EAF enclosures in the United States are
found at the North Star Steel Company and Birdsboro Corporation.
North Star Steel Company--
This company has two 60-ton furnaces that are controlled by DSE and a
roof canopy system. The capture efficiency of the canopy system was low
during charging and tapping because of the flat design of the roof canopy
and insufficient extraction volume. Each furnace was retrofitted with an
enclosure. The furnace enclosure is a steel structure c-lad with ribbed
aluminized sheets and sealed. Mechanical bi-parting side doors and roof
doors allow entry of the crane for charging. The bi-parting vertical doors
are closed during charging and an air curtain seals the roof. The entire
available flow is used to extract the charging emissions through a high-
level evacuation duct. Emissions from melting, oxygen lancing, and slagging
operations are controlled by a DSE system. During tapping, mechanical
bi-parting doors on the tapside of the enclosure are opened to permit entry
of the tap ladle. These doors have a top horizontal roof with a slot for
cables holding the tap ladle. Once the ladle is positioned, the bi-parting
doors close to contain the fumes. During tapping, fume is evacuated by a
high-level duct. On the charge side, the mechanical roof door, the roof
section complete with air curtain, and the charge section evacuation duct
can all be mechanically wheeled aside for maintenance operations such as
replacement of furnace parts and removal of the shell and roof. Salient
19
data on this plant are as follows:
Furnaces: Two: 15-ft diameter; 60-ton capacity each
Transformer: 30,000 to 33,000 kVA
89
-------
Meltdown rate: 55 tons/hour
02 consumption: 300 ft3/ton
Approximate enclosure
size: 40 x 36 x 31 ft high
Extraction volume: 170,000 acfm per enclosure
Air curtain volume: 6000 acfm
Figure 33 shows the enclosure at the North Star Steel Company.
Birdsboro Corporation--
This new facility initially designed with a side-draft hood for primary
emission control had to be redesigned to control both primary and secondary
emissions. The furnace enclosure design allowed an acceptable solution with
an exhaust volume of 75,000 acfm for a 40-ton furnace. Tapping and charging
are carried out from the same side. A mechanical hood car moves away from
the enclosure to provide an opening for the scrap bucket and tap ladle.
Three cylinder-operated movable roof doors seal the crane cable entry slot.
These doors are linked to dampers in the air-curtain duct system. The air
curtains seal the opening when the doors are opened for charging or tapping.
Overhead removable panels at the enclosure roof level aid maintenance
operations. In the initial startup, the enclosure was not effective during
lancing. Measures adopted to overcome this problem were better sealing of
the furnsce enclosure, upgrading the air curtain fan, adjusting air nozzles
to deflect the lancing emissions into the extraction duct, and installing a
] Q
deflector plate. Salient data on this shop are as follows:
Furnaces: One: 13-ft, 6 in. diameter; 40 tons capacity
Transformer: 10,000 kVA
Maximum meltdown rate: 18.6 tons/hour
Oxygen lancing: 250 scfm (design)
Enclosure size
(approximate): 36 x 30 x 28 ft high
Extraction volume: 75,000 acfm (design); 81,600 acfm
operating
90
-------
Figure 33. Furnace enclosure at North Star Steel Company.
91
-------
Temperature: Normal 150°F; Maximum 275°F
Air curtain volume: 9,900 acfm (design) increased to
11,700 acfm.
Figure 34 shows the furnace enclosure at Birdsboro Corporation.
7.1.4 New Installations
Some of the new EAF installations required to meet NSPS have adopted
furnace enclosures for smaller furnaces (<100 tons). Table 10 is a list of
the essential features of new EAF shops designed for furnace enclosures.
Plant 3 has additional pickup points including a local tapping hood, an
additional hood to evacuate tundish lancing emissions, and a scavenging hood
located above the roof slot of the enclosure, all connected to a common
baghouse. Balancing the draft for such multiple pickup points is difficult,
but adjustments are made with operating experience. A typical flow balance
during various phases of the EAF operations is shown in Table 11. Dampers
on each hood control the flow.
7.1.5 Inspection
To achieve good evacuation, the enclosure should be well sealed. This
will depend on the integrity of the enclosure. The inspector should observe
and note the general condition of the enclosure walls in regard to any loose
sealing strips, misalignment of the bi-parting doors (which creates large
gaps), holes in the panels, damaged or warped panels, open access doors, and
any other openings that reduce the capture efficiency of the enclosure.
During scrap charging and at other periods when the air curtain is operated,
the inspector should observe the air curtain as it diverts the thermally-
driven plume into the pickup hood. Improperly placed air nozzles in the air
curtain may not completely seal the roof door area and thus allow emissions
to escape. The air curtain operation can be safely observed from the
control room. The performance of the enclosure can be assessed by simultane-
ous observations by two inspectors, one inside the control room and the
other outside the enclosure, during all phases of a heat cycle. The inspector
in the control room should observe and note the fit and closure of the roof
doors, ?nd the buildup of emissions within the enclosure and how effectively
they are evacuated at various stages of the furnace operation. The inspector
92
-------
EXHAUST DUCT
REMOVABLE PANELS
OVERHEAD MAINTENANCE ACCESS
ROOF DOORS
NORTH
CHARGING AND
TAPPING HOOD CAR
LADLE ADDITIVE CHUTE
ROOF AND HOOD CAR
CONTROL PANEL
FURNACE CONTROL PANEL
EAST ROLL-UP DOOR
SOUTH ROLL-UP DOOR
FURNACE CONTROL PANEL
Figure 34. Furnace enclosure at Birdsboro Corporation.
93
-------
TABLE 10. DATA ON EAF PLANTS DESIGNED FOR TOTAL FURNACE ENCLOSURE20'21
Plant 1
Plant 2
Plant 3
Number of furnaces
Furnace size
Transformer
Type of steel made
Meltdown rate
Approximate enclosure
size
Extraction volume
Air curtains at the
roof slot
Other features
1
12-ft,-6 in diameter;
28-32 tons/heat
16,800 kVA
Specialty
s20 tons/hour
42 x 51 x 35 ft
150,000 acfm
Yes
Vertical bi-parting
doors and roof slot with
doors for crane cable
access.
16-ft diameter;
60 tons/heat
33,600 kVA
Carbon steel
=72 tons/hour
34 x 53 x 44 ft (curved
at the top, dome shaped)
175 to 200,000 acfm at
150°F
Yes
Vertical bi-parting doors
and roof slot with doors
for crane cable access.
1
15-ft diameter;
55 tons/heat
17,000 kVA
Medium-carbon steels
=16 tons/hour
Not available
150,000 acfm at 130°F
Yes
Vertical bi-parting doors
and roof slot with doors
for crane cable access.
A separate tapping hood
and a scavenging hood above
the crane slot, evacuated
to the same baghouse.
Each furnace has a separate enclosure.
-------
TABLE 11. FLOW BALANCING OF A TYPICAL FURNACE ENCLOSURE WITH
ADDITIONAL PICKUP HOODS, CONNECTED TO A COMMON BAGHOUSE
(150,000 acfm)
Operating Mode
Furnace charging
Melting and refining
Tapping
Lancing at Lancing
Station
Roof
scavenging
hood
73,000 acfm
73,000 acfm
73,000 acfm
43,000 acfm
Furnace
enclosure
pickup hood
77,000 acfm
77,000 acfm
22,000 acfm
77,000 acfm
Local
tapping
hood
Closed
Closed
55,000 acfm
Closed
Lancing
hood
Closed
Closed
Closed
30,000 acfm
NOTE: Lancing hood cannot be operated during charging or when air curtain is
operated, and during tapping. Roof scavenging hood can evacuate at a
higher rate than 73,000 acfm, if needed.
outside the enclosure should observe and note the shop layout, any leakage
of emissions from the enclosure, crossdrafts inside the shop, emissions
fromancillary operations, additional pickup points, and the overall flow
balance under different operating modes. Although crosswinds generally do
not affect the performance of an enclosure, emissions that build up near the
shop roof above the enclosure will escape the building. If a separate
scavenger hood is provided above the enclosure, its capture efficiency
should be observed.
7.2 BASIC OXYGEN FURNACES
Primary emissions from basic oxygen furnaces are captured in specially
designed hoods. The hood is erected above the mouth of the vessel to
control the primary emissions when the vessel is in the vertical position.
The primary hood is not as effective, however, when the vessel is tilted for
various operations such as charging, sampling, tapping, and deslagging.
Additional hoods or enclosures are required to control these secondary
emissions. The trend is toward total enclosures.
95
-------
7.2.1 Description
The vessel is enclosed on all sides, with a door or bi-parting doors in
the front to facilitate scrap and hot metal charging. Since these
operations occur at and above the vessel, natural convection will permit a
plume of hot dusty gases to escape during charging. To reduce this
possibility, charging should occur as close to the vessel as possible and
under the hood. A separate charging hood is provided within the enclosure
for capturing the charging emissions. The enclosure can extend partially or
completely at the tap side. Tapping is carried out at and below the level
of the vessel, and the hot dusty gases have a tendency to escape in the
natural draft induced by the process heat. In the newer designs, a permanent
tap hood is installed at the back of the enclosure. Figure 35 shows a
typical arrangement of a BOF vessel enclosure. The furnace enclosure
extends below the charging floor, and the only openings are for the ladle
car. These openings can be effectively reduced by the addition of vertical
shields on the end of the ladle car.
In the bottom blown BOF (Q-BOP), oxygen is blown through tuyeres.
During charging and turndown, gas (either oxygen or nitrogen) must be blown
through the tuyeres to prevent liquid metal, slag, or solids from entering
and clogging the tuyeres. This generates heavy emissions and makes capture
of the secondary emissions more difficult; hence, invariably all the Q-BOP
furnaces are completely enclosed.
7.2.2 Gas Cleaning Systems
The gas cleaning system employed to control secondary emissions may be
an extension of the primary control system. One hood designed to collect
charging emissions and another for collecting tapping emissions could be
ducted to the same primary gas cleaning system. The gas flow would be
adjusted for the different demands of the heat cycle. Another alternative
is to duct the charging and tapping hoods in the furnace enclosure to a
secondary control unit, generally a bag filter. Figure 36 indicates a
schematic of the secondary control system at Kaiser Steel in Fontana,
Cal i form' a.
96
-------
SECONDARY
HOT METAL CHARGMG
FURNACE CHARGMG
(Rctractebto)
SLAG POT
HOOD
LADLE
WATER COOLED HOOD
HOOD TRANSFER CAR
ADJUSTABLE SKRT
TAPPING EMISSIONS DUCT
SEAL RING
FURNACE ENCLOSURE
OPERATING FLOOR
TEEMING LADLE
SHOP AM
INDRAFT DURMG
SLAGGMGA
TAPPING
Courtesy:
Figure 35. Typical BOF furnace enclosure.
Pennsylvania Engineering Corporation.
-------
HOT METAL
TRANSFER
1
SKIMMING
1
HOT METAL
TRANSFER
2
\ /
SLAG
SKIMMING
2
FURNACE
CHARGING HOOD
1
\
FURNACE
CHARGING HOOD
2
A
V
TAPPING HOOD
V
TAPPING HOOD
BAGHOUSE
1,020,000 m3/hr
MAXIMUM TEMPERATURE 230°C
EXHAUST FANS
Figure 36. Schematic of basic oxygen secondary emission control system
of Kaiser Steel-Fontana.
98
-------
A common baghouse with a I,020,000-m3/h (600,000 acfm) design flow
serves not only the charging and tapping hoods within the furnace enclosure
but other areas such as the hot metal transfer and slag skimming stations.
In the closed-hood system that intermittently handles a flammable gas
(CO), use of the primary system to control the secondary sources could
result in an explosion.
At the Q-BOP plant of Republic Steel in Chicago, which has a suppressed
combustion or closed-hood system, charging emissions are safely collected by
diverting the emissions to the primary system of the nonoperating vessel.
The shop has two vessels, and one furnace is operated at a time. Figure 37
shows the schematic arrangement of the gas cleaning operation. When both
gas cleaning system fans are operated, a flow rate of 634,000 m3/h (373,000
??
acfm) is available at the charging hood during hot metal charging. Fumes
captured in the charging hood bypass the quencher and pass directly to the
venturi scrubber. The design pressure drop of the venturi during furnace
22
charging is 218 cm (56 inches) water column.
Table 12 lists the BOF/Q-BOP shops that utilize the furnace enclosure
for emission control.
7.2.3 Inspection
The integrity of the furnace enclosure is important in obtaining good
evacuation within the enclosure. The inspector should observe and note the
condition of the bi-parting doors to detect any misalignments or warpage
that leads to large gaps between the doors. The inspector should also check
for gaps between the water-cooled sections of the duct work and inspect the
condition of the pickup hoods within the enclosure and all the associated
ductwork leading to the cleaning unit.
During scrap charging, the vessel tilt angle should be estimated along
with the effective capture efficiency of charging emissions in the secondary
charging hood. During hot metal charging the tilt angle and the pouring
rate of hot metal play an important part in the effective capture of the
secondary emissions. The primary hood capture and enclosure evacuation
should be observed during slopping. In top-blown furnaces, lanse hole
emissions or fume rollout (puffing) from the primary hood indicate in-
sufficient draft at the primary hood. This will increase the load on the
secondary controls within the enclosure.
99
-------
o
o
NO. 1
Q-BOP
FURNACE CLOSURE
K
MBH
Ic
s
s->
1
L,
r«
-t
»-/
.^
)
fr>
*•
)
X
1
1
z
_1
SECONDARY
HOOD NO. 1
SHUTOFF NO. 1
OPEN j^BELL VALVE NO. 1
£4"
CLOSED" —^ 1 ^~
~^" '><' "^ W
QUENCHER wn , r^
90% OPEN N0- ] SCNR(
QUENCHER N0 2 SCR
20% OPEN^ NC
-* Q -*P
OPEN^. ~ If ^
V gj -I
SHUTOFF BELL VALVE NO. 2
CLOSED NO. 2
SECONDARY
HOOD NO. 2
D
FAN
NO. 1
FAN
NO. 2
D
NO. 2
Q-BOP
FURNACE CLOSURE
STACK NO. 1
STACK NO. 2
Figure 37. Schematic of Q-BOP secondary (charging) emissions control system of
Republic Steel, Chicago.
-------
TABLE 12. BOF/Q-BOP SHOPS UTILIZING FURNACE ENCLOSURES'
Company and location
Type of enclosure
Type of gas cleaning
Startup date
Republic Steel Corp.
Chicago, Illinois
Bethlehem Steel Corp.
Burns Harbor, Ind.
CF&I Steel Corp.
Pueblo, Colorado
Kaiser Steel Corp.
Fontana, California
Granite City Steel Div.
Granite City, Illinois
Rouge Steel Co.
Dearborn, Michigan
Sharon Steel Corp.
Farrell, Pennsylvania
4-sided enclosure with
mechanized biparting
charging doors
4-sided enclosure with
mechanized biparting
charging doors
3-sided enclosure with-
out charging doors
4-sided enclosure with
single mechanized
charging door
4-sided enclosure with
single mechanized
charging door
3-sided enclosure with-
out charging doors
4-sided enclosure with
single mechanized
charging door
High-energy scrubber (420,000
acfm at 143°F)
High-energy scrubber (350,000
acfm at 145°F)
Fabric filter (540,000 acfm at
275°F)
Fabric filter (500,000 acfm at
600°F)
Electrostatic precipitator
(900,000 acfm at 550°F)
Electrostatic precipitator
(1,050,000 acfm at 500°F)
High-energy scrubber (320,000
acfm at 168°F)
1976-77
1977
1978
1978
1980
1981
1982
Source: Pennsylvania Engineering Corporation.
-------
The inspector should observe and note the shop layout, gas cleaning
equipment, and multiple-pickup points connected to the same gas cleaning
source and become familiar with the flow-balancing scheme under different
operating modes. This is very important as each plant is unique and equip-
ment operation varies from plant to plant.
102
-------
SECTION 8
SPECIAL APPLICATIONS
The basic principles of ventilation outlined in previous sections are
valid for all applications. This section discusses some additional con-
siderations of interest in common control systems in the metallurgical indus-
tries. It is assumed that the reader is already generally familiar with the
processes involved.
*
8.1 COKE OVEN SHEDS
8.1.1 Description
The coke oven shed for control of coke oven pushing emissions is a
special kind of hooding. Many variations in configuration are possible
(Figure 38) but all are designed to meet three basic objectives:
0 To contain excessive emissions during the brief (45-second) period
of a single push; cooling and exhausting of these emissions take
place over a longer period.
0 To arrest the upward travel of the plume and to roll the plume
within the shed; heavier particles fall out to the ground.
0 To contain all of the emissions from the coke side of the battery,
including door emissions, coke spillage emissions, and hot car
emissions.
Shed control systems require ducting, fans, and a control device, usu-
ally a fabric filter. This discussion is limited to the ventilation princi-
ples and O&M considerations associated with the shed itself.
Figure 39 presents a general illustration of the thermal expansion of
hot gases from a push that the shed must accommodate. In the original shed
designs, a duct ran along the length of the shed, and the exhaust suction was
distributed along the entire length. These sheds required high flow rates
(300,000 acfm or more) to achieve reasonable face velocity. Newer designs
103
-------
Figure 38. Various shed configurations.
104
-------
1MIN
2MIN
3MIN
o
tn
BASE AMBIENT TEMP.
THERMAL EXPANSION
/v
4MIN
TIME
Figure 39. Thermal expansion of hot gases from a push.
23
-------
entail a partition approach in which dampers are used to concentrate the
suction in the segment of the shed where the push is occurring. In either
design, an inherent design principle is the holding of it within the large
confines of the shed until it can be fully exhausted. The design relies on
the theory of fully exhausting a given push before the next push occurs.
Because containment of the plume is an important factor, several dif-
ferent shed configurations have been offered to maximize containment. A
common feature of all designs is an enlarged upper portion of the shed, which
serves as a plenum (see Figure 38). The containment volume must be designed
to match the size of the push and pushing rate expected at the battery.
A potential weak point in shed control is the influence of wind cur-
rents, especially at the ends of the shed. Even in the absence of wind
disruption, emissions from "dirty pushes," i.e., heavy plumes, have a tend-
ency to "roll out" from under the shed near the ends. The only two solutions
to this problem are
1) to close the ends insofar as possible to interrupt wind currents, and 2)
to extend the overall shed structure beyond the ends of the battery. Sheds
can be designed to extend beyond the battery toward the quench tower to
capture emissions during travel of the hot car.
8.1.2 Inspection
In the inspection of a coke-side shed, the inspector should consider the
following:
1. The integrity of the shed structure and any missing roof panels or
holes in the roof.
2. Leakage between the battery top and the shed interface.
3. Integrity and cleanliness of the baffle plates and heat shields.
4. Effect of cross-drafts and heavy-particle fallout.
5. The time required for the plume to clear under the shed.
6. Efficacy of capture and evacuation of emissions resulting from
pushes from the ovens near the ends of the shed.
7. The greenness of the push; a shed that can contain clean pushes
will not necessarily be able to contain the much larger gas volume
generated during a dirty push.
106
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8.2 ELECTRIC ARC FURNACE VENTILATION
8.2.1 Description
Canopy hoods are widely used to control secondary emissions from elec-
tric arc furnace (EAF) refining. The advantages of these hoods are that they
do not interface with the furnace; they provide ventilation during charging,
tapping, and slagging off; they do not affect furnace metallurgy; and their
maintenance costs are low. The disadvantages are that they require high air
volume; they are subject to cross-flow air currents that interface with fume
control; and the plume can interfere with crane operator's line of vision
during charging and tapping.
The need for high air volumes can be overcome by combining the canopy
with a partial enclosure, as illustrated in Figure 40. The three-sided
enclosure reduces the effect of cross-flow interference and directs the plume
toward the hood. As shown earlier in Figure 6 and explained in the discus-
sion regarding coke oven sheds, another approach is to use a baffled hood
that concentrates suction towards the heaviest area of emissions.
The ultimate expansion of this approach is total furnace enclosure which
was previously discussed in Section 7.
A complicating factor in EAF ventilation is the control system required
to combine fourth-hole evacuation and a canopy hood on the same fan system.
A balanced draft is difficult to achieve, and it must be established by trial
and error. For example, the canopy hood is often dampered off when the
furnace cover is in place so that suction will be concentrated on the fourth-
hole takeoff. In this configuration, the control system directs total system
suction to the canopy hood when the cover is off for charging and when the
furnace is tilted for tapping. In some cases, however, the charging emission
plume is still rising toward the hood while the cover is being quickly re-
placed. Thus, when the cover has been replaced and suction is taken off the
canopy hood, the rising plume is not captured. In such an arrangement, it
would be desirable either to replace the cover more slowly or to build an
electrical delay into the automatic damper control system.
107
-------
SLAGGING HOOO-
5LAG POT"
Figure 40. Electric arc furnace utilizing partial enclosure,
Source: U.S. Environmental Protection Agency, Office of Air
Quality Planning and Standards, Research Triangle
Park, North Carolina 27711
108
-------
Figure 41 shows the basic types of ventilation systems used on electric
arc furnaces. Figure 42 illustrates a popular approach in which the fourth-
hole direct evacuation is combined with a canopy hood. In some cases, a
third stage of capture is included by closing off the roof monitors in the
area of the furnace and connecting a take-off duct to the roof. This third
stage captures smoke that has escaped the canopy.
In the combined direct evacuation canopy system, baghouse temperature is
controlled by mixing the cool gas from the canopy with the hot gas from the
furnace. A cool-air emergency inlet damper is usually located prior to the
baghouse. If this damper remains open, it will cause a ventilation "short-
circuit" and rob the hood of suction.
As shown in Figure 43, two or more furnaces are typically combined on a
system, which can produce complex flow balancing requirements. Table 13
presents an example of the ideal flow balance on a two-furnace system.
Actually, flows through various duct sections must be monitored to confirm
their desirability. All elements of the system must be kept in good condi-
tion to maintain the ideal balance. Misalignment of the gap in the DSE duct
(see Figure 43) will result in the indraft of more combustion air at this
point and lower suction elsewhere. Figure 44 summarizes total flow rate
requirements for various EAF control approaches and indicates the rapid
increase in flow requirements with the move from direct evacuation to build-
ing evacuation. These data are based on averages of various systems and
should be used only as a general indicator of reasonable flow requirements.
Compromises were made in most plants where hoods had to be retrofitted.
The furnace and bay configuration, clearance for canopy hoods above the crane
gantry level, and ductwork layout became important factors because they could
mitigate the efficiency of the canopy hood. Failure to consider activities
that increase the fume generation rate, such as oxygen blowing and oxyfuel
burners, during the initial design of the hoods can cause the system to be
overloaded and capture efficiency to be reduced.
8.2.2 Inspection
The inspector should consider the following when inspecting EAF's:
109
-------
FOURTH HOLE
SIDE DRAFT
COMBINATION HOOD
CANOPY HOOD
Figure 41. Ventilation systems for electric arc furnaces.
-------
CLEANED GAS LOUVERS
t
CANOPY HOOD
DIRECT SHELL EVACUATION
ROOF TAP (WATER COOLED)
FAN
L
COMBUSTION AIR GAP
V
BAGHOUSE
\7\7\7
COLLECTED
DUST
Figure 42. Combined direct shell evacuation with canopy hood.
in
-------
POLLUTION CONTROL SYSTEM
CANOPY
HOOD
SLAG POT
HOOD
Figure 43. Multiple pickup points vented to common control device.
112
-------
TABLE 13. EXAMPLE OF FLOW BALANCING OF MULTIPLE EVACUATION
SYSTEM ON ELECTRIC ARC FURNACE
Item
Furnace 1 charging
Furnace 1 slagging hood leakage
Furnace 1 tapping hood leakage
Furnace 1 direct shell leakage
Furnace 2 oxygen lancing
Furance 2 slagging (simultaneous with
oxygen blowing)
Furnace 2 canopy leakage
Furnace 2 tapping hood leakage
Total system capacity
Air flow, cfm
800,000
12,000
18,000
25,000
265,000
118,000
44,000
18,000
1,000,000
113
-------
1200
100 200 300 400 500 600
H = SUM OF HEAT SIZES IN SHOP, tons
700
Figure 44. Flow rate required for electric arc furnace control.
-------
1. The integrity of the canopy hoods, the side-draft hood, and all the
duct work leading to the cleaning unit. In the case of an enclosed
or sealed roof, any large holes or missing panels will reduce the
capture efficiency.
2. Additional thermal currents produced by ancillary operations such
as overflow of slag and/or metal on dumping ladles should be noted.
These operations increase the load on the evacuation system.
3. Furnaces and bay layout have an important bearing on the capture
efficiency of the hoods.
4. Multiple-furnace operations, additional pickup points, and the
overall flow balance under different modes of operation should be
observed.
5. Crosswinds inside the shop can severely disrupt the capture effi-
ciency of the hoods.
8.3 BLAST FURNACE CASTHOUSE CONTROL
8.3.1 Description
24
Although recently developed technology does not use ventilation for
control of casting emissions, numerous ventilation-based systems are in-
stalled or planned on blast furnace casthouses. Sources of fugitive emis-
sions in the casthouse are the tap hole, trough, skimmer, runners, hot metal
spouts, and slag spouts. These various sources require a multiple hooding
system or total building evacuation. Local control is favored because total
flow requirements are less; however, the design of many older casthouses
precludes their use. Table 14 lists the typical flow volume requirements for
a large blast furnace casthouse system.
These systems require flow balancing, proper design, and the other
factors discussed earlier. Also, the frequent removal and replacement of
close-fitting hoods or runner covers invite both damage and misplacement.
Because of the proximity of the hooding to the molten metal, frequent main-
tenance of the refractory linings and runner covers is required. Large blast
furnaces with multiple tapholes and casthouses require a carefully designed
flow balancing and damper system for switching from one casthouse to the
other. Such systems have been designed for full evacuation of one taphole at
a time. If damper settings are such that two tapholes are evacuated simulta-
neously, capture efficiency will be curtailed.
115
-------
TABLE 14. BLAST FURNACE CASTHOUSE TYPICAL VOLUME REQUIREMENTS
Location
Taphole
Skimmer
Iron runner
Hot metal spouts
Slag spouts
Total
Volume
requirements ,
cfm
70,000
35,000
70,000
75,000
50,000
300,000
Volume
requirements ,
m3/min
1978
989
1978
2119
1413
8477
116
-------
8.3.2 Inspection
When inspecting blast furnace casthouse systems, the inspector should
consider the following:
1. The performance of the taphole hood during various stages of the
tap (i.e., immediately after taphole opening, during heavy metal
flow, and during slag and metal flow). The capture efficiency of
the hoods (covers) at the trough, skimmer, runners, and slag and
metal spouts also should be observed during various stages of the
cast and under different conditions, such as high and low metal
temperature and high and low slag basicity.
2. When there are multiple casthouses, the flow balance between the
casthouses should be observed. The flow is sometimes equally split
between the casthouses, which significantly reduces the capture
efficiency.
3. The furnace layout, casthouse layout, and ductwork location have an
important bearing on the efficiency of the evacuation system.
4. Cross-currents inside the casthouse mitigate the hood capture. It
is common practice to remove and replace the siding in the cast-
house, depending on the season. A system designed for a closed
structure is not likely to perform adequately if the structure is
opened.
8.4 CONTROL SYSTEMS ON BASIC OXYGEN FURNACES (BOF's)
8.4.1 Description
Primary capture systems on BOF's use a specially designed hood over the
mouth of the furnace. This usually operates effectively when the furnace is
in an upright position; however, when the furnace is tilted for charging,
tapping, or testing, emissions generally escape the hood (for the reasons
discussed earlier). Capture of these secondary emissions requires additional
hooding or enclosures. The total enclosure of BOF was previously discussed
in Section 7.
Water-cooled primary hoods are sometimes used to generate steam because
the gas temperatures in the hood are initially over 3000°F. These hoods are
exposed to the most severe operating conditions of all the process applica-
tions discussed—fluctuating high temperatures and the possibility of physi-
cal damage from cranes or ejections of material from the furnace. The hood
may be elevated several feet above the furnace to allow indraft of combustion
air (open), or they may be closely mated with the furnace to conserve the
117
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carbon monoxide inherent in the exhaust gas (closed). Figure 45 illustrates
these BOF hood arrangements.
Many different hood constructions have been tried to cope with the
severe operating conditions. The membrane or "tube-bar" type is usually
favored today. Table 15 presents the performance characteristics of various
hood constructions. Table 16 lists the various hood types that are used for
full combustion and limited combustion systems in the United States, Canada,
25
and Mexico.
Control of secondary emissions is achieved by local hoods or enclosures.
Local hoods are generally ineffective because they are too far from the
source, are too small to encompass the emissions, and because the evacuation
rate is usually insufficient. Figure 46 illustrates a simple external hood-
ing arrangement for charging emissions.
Figure 47 illustrates the Gaw damper approach, which enables the primary
hood to function as a secondary hood for charging emissions. The movable
damper increases hood suction by restricting the hood face area. Maintenance
of the damper is high because of the harsh environment. As in the case of
the local hood discussed previously, effective use of the Gaw damper requires
a slow pouring rate of charged metal to decrease fume generation and a mini-
mum tilt angle of the vessel to generally direct the fumes toward the hood.
8.4.2 Inspection
When inspecting BOF ventilation systems, the inspector should consider
the following:
1. Primary hood capture efficiency during operations such as slopping.
2. The tilt angle and hot metal pour rate during hot metal charging,
which greatly affects the capture efficiency.
3. The performance and capture of secondary controls.
4. Emissions from the lance hole or fume rollout (puffing) from the
primary hood, which indicate insufficient draft on the hood.
5. In closed hood systems, the snug fit of the hood skirt at the
furnace. (Indraft of air above the design level will overload the
fan and cause the variable-throat scrubber to open, which reduces
pressure drop.)
118
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RELIEF OOOft
RELIEF DOOM
— GAS COOLER
WATER-COOLED FLANGES
SANO SEAL
MOVEABLE SKIRT
REMOVABLE HOOD
FIXED HOOD
— GAS COOLER
LANCE
WATER-COOLED FLANGES *
SANO SEAL
REMOVABLE HOOD
FIXED HOOD
NO SKffTT HERE
VESSEL
CLOSED
VESSEL
OPEN
Figure 45. BOF hood arrangements.
Courtesy: BOF Steelmaking, Volume III. Published by American Institute of Mining, Metallurgical
and Petroleum Engineers.
-------
TABLE 15. PERFORMANCE CHARACTERISTICS OF DIFFERENT BOF HOOD CONSTRUCTIONS
25
Initial cost
Ability to take
high tempera-
ture
Ability to take
temperature
change
Resistance to
slag buildup
Resistance to
scaling
Maintenance Cost
Refractory-
lined
Lowest
Poor
Poor
Poor
--
Very high
Water-
cooled
plate
panels
Low
Fair
Fair
Good
Poor
High
Formed
panels
Moderate
Good
Good
Good
Fair
Fair
Double
pass
High
Very good
Very good
Very good
Good
Low
Water wall
boiler
High
Very good
Very good
Fai r
Good
Fair
Membrane
High
Very good
Very good
Good
Good
Fair
TABLE 16. BOF HOOD CONSTRUCTION DESIGNS IN USE
IN THE UNITED STATES, CANADA, AND MEXICO25
Construction
Tube
Panel/jacket
Membrane
Not known
Total
Full combustion
Number
10
41
30
4
85
Percent
12
48
35
5
100
Limited combustion
Number
3
4
13
0
20
Percent
15
20
65
0
100
Total
Number
13
45
43
4
105
Percent
12
43
41
4
100
120
-------
FURNACE
CHARGING AISLE
--CRANE GIRDER
CANOPY HOOD
CHARGING
LADLE
Figure 46. Canopy hood concept for BOF charging emissions,
121
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CLOSURE PLATE CONCEPT
FURNACE
CHARGING AISLE
CRANE
GIRDER
CHARGING
LADLE
Figure 47. Gaw damper (closure plate) use in BOF control.
122
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8.5 BUILDING EVACUATION
The use of total building evacuation to control air pollution contamina-
tion or to improve the work area environment is normally a difficult and
costly approach.
The problem areas are:
1. Achieving the necessary number of air changes per hour (20 or more,
depending on the contaminants).
2. The ability to reach the specific work areas to supply these air
changes. (Some areas have dead air pockets, and either ducting or
forced ventilation has to be provided.)
3. The volume of air required. (A building 800 ft long, 80 ft wide,
and 40 ft high has a volume of 2,560,000 ft3. With 20 air changes
per hour, :850,000 scfm of air is required. If contaminants, open
doorways, heat emissions, and air currents caused by thermal proc-
esses or vehicle movement are added, the actual requirement fog
effective ventilation of such a building would be 3 to 4 x 10
scfm.)
4. The existing building structure not being designed for total en-
closure. (The weight of siding, ducting, and wind loads may re-
quire strengthening of the building columns and roof trusses and
the addition of more purlins, struts, and bracing.)
5. The location of process equipment and the flow patterns of mate-
rials not being conducive to the proper collection of emissions.
(Lighting may also be a problem.)
6. Need for redundancy of the fans. (Clean-out mechanisms should be
built into the ducting that are readily accessible and repairable.
Ducting is long and huge.)
A newly designed building in which process equipment is located specifi-
cally for total building evacuation can greatly reduce these problems.
A variation of building evacuation that does not involve local hooding
?
and ventilation systems is the roof-mounted electrostatic precipitator (REP).
This system has not yet been applied in the United States, but it has been
?fi
successfully used in Japan. The technique involves roof modification to
help channel the natural plume rise of BOF fugitive emissions into an REP,
which collects the process fugitive emissions. The system has no fan, and
pc
the plates are generally cleaned by use of a water spray. The REP specifi-
cation provided by Sumitomo Heavy Industries for controlling two BOF's, each
?fi
with 300-ton capacity, offers a design efficiency of 91.5 percent.
123
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8.6 COPPER CONVERTERS
8.6.1 Description
Capture of emissions from Peirce-Smith type copper converters presents
some unique problems. The converter is essentially a horizontal cylinder
with a circular opening on top. This cylinder is rotated around its longi-
tudinal axis so the opening can face toward the ladle. This rotation makes
it impossible to achieve an absolutely tight seal between the hood and the
converter. Also, when the converter is rotated for charging and tapping, the
opening is no longer under the hood, and fumes tend to escape into the con-
verter building. The heat and collisions with the ladle suspended from the
crane tend to cause the hood to warp, and eventually it does not fit well.
Figure 48 shows a converter and the primary hood.
At copper plants, the emissions from all converters (usually three to
five units) are ducted together to a common particulate control device and
sometimes to a sulfuric acid plant. Dampers in the individual breeching
control the draft on any single converter. These dampers require routine
maintenance to ensure proper functioning.
Primary converter hoods have sliding gates on the front side as shown in
Figure 48. These gates are really movable hood extensions that serve to
cover the converter opening and improve capture efficiency.
Some smelters have installed secondary hoods (Figure 49) or air curtain
systems to capture fumes when rotation causes the converter opening not to be
unde'~ the main hood. By providing a strong blast of air to blow fumes into a
suction hood, the air curtain provides an open area for locating the charging
and tapping ladle and the crane hooks and cables. Secondary hoods suspended
above the converter opening and ladle have the same weaknesses found in other
processes. For example, because of their distance above the converter and
ladle, they are affected by thermal and cross-drafts and are not effective
unless face velocities of the hood are high.
124
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(END VIEW)
en
(SIDE VIEW)
(1) SHELL; (2) HOOD; (3) AIR BAFFLE; (4) CONVERTER MOUTH; (5) ROLLERS;
(6) TURNING RINGS; (7) AIR - SUPPLY DUCT;
Reprinted from the Kirk-Othmer Encyclopedia of Chemical Technology, 3rd edition,
Fig. 9, Page 836. Copyright© John Wiley & Sons, Inc. 1979.
Figure 48. Peirce-Smith converter.
-------
TO SECONDARY
HOODING
MAIN DUCT
Figure 49. Secondary converter hood configuration.
126
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8.6.2 Inspection
Converter hoods are inspected by making a visual assessment of their
physical condition and the emissions that escape during the various converter
cycles. The hood and duct system should not contain any openings that allow
air to be drawn in and reduce the suction at the converter. The hood lip
should come within about 6 inches of the converter opening during blowing.
The hood gate or slide should be extended when the converter is in the up-
right position to ensure that the converter opening is covered. During
converter rollout, the draft should be reduced so that a high-suction air
flow can be maintained on the converters in the blowing mode.
127
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A-8
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing!
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSIOI
4. TITLE AND SUBTITLE
Performance Evaluation Guide for Large Flow
Ventilation Systems
5 REPORT DATE
May 1984
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
W. F. Kemner, R. W. Gerstle, and Y. M. Shah
B PERFORMING ORGANIZATION REPORT NO.
3760-1-119
9. PERFORMING ORGANIZATION NAME AND ADDRESS
PEDCo Environmental, Inc.
11499 Chester Rd.
Cincinnati, OH 45246
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-01-6310
Work Assignment 119
12. SPONSORING AGENCY NAME AND ADDRESS
U. S. Environmental Protection Agency
Control & Process Engineering
Stationary Source Compliance Division
Washington, D.C.
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
Project Officer for SSCD: Dwight Hlustick
16. ABSTRACT
Air pollution control systems in the primary metals industry, particularly the
steel and copper segments, rely on large capture and ventilation systems with flow
rates commonly in the range of 50,000 to 1,000,000 acfm and greater. These systems
are used primarily to control process fugitive emissions from various furnaces and
for building evacuation.
Because these systems are in integral feature of the compliance programs of the
industries involved, this manual was initiated to accomplish the following:
* To provide design and operation and maintenance guidelines to state and
local agency personnel who evaluate the performance of these systems.
* To provide a comprehensive treatment of the existing literature with
regard to technical and specific aspects of typical designs.
* To provide an easy-to-read technical manual on design and operation for the
use of inspectors.
Inasmuch as ventilation systems are highly complex from a design standpoint and
experience plays a major role in most designs, this manual should be considered an
introductory primer rather than a detailed design manual.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Ventilation
Industrial Engineering
Fans, Ducts, Hoods
Steel Industry
Copper Smelting
13A
13H
18. DISTRIBUTION STATEMENT
19 SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
146 p.
20 SECURITY CLASS (This page)
Unclassified
22. PRICE
A07
Form 2220-1 (R«v. 4-77) PREVIOUS EDITION is OBSOLETE
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