AIR POLLUTION CONTROL TECHNOLOGY HANDBOOK - CHAPTER 20 pps

Hood and Ductwork Design 20.1 INTRODUCTION The design of hoods and ductwork is often a very important part of air-pollution control.. 20.1 Here Q is the volumetric flow rate, V is the ve

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Hood and Ductwork Design

20.1 INTRODUCTION

The design of hoods and ductwork is often a very important part of air-pollution control Hoods and the air exhausted must be adequate to prevent escape of con-taminants to the atmosphere On the other hand, the air exhausted through a hood must usually be treated to remove the contaminant Therefore, to keep capital and operating costs of control equipment to a minimum, no more air should be exhausted than necessary for complete capture By designing the hood with the minimum openings necessary, the air quantity to be exhausted can often be decreased When an exhaust system must exhaust gases from a number of points, it is important that the ductwork sizing and layout be carefully engineered for the proper flow and velocity in all branches, both for conveying velocity and pressure drop It

is axiomatic that the pressure drop through each branch at its design flow must be the same from its intake to the point of common junction If it is not, the flow will redistribute itself in operation to create the same pressure drop in each branch When handling abrasive dusts, duct wear can be quite severe Situations are known to have occurred where ¼-in.-thick elbows have been worn through in

2 weeks time in poorly designed exhaust systems Therefore, when handling abrasive dusts, the duct routing should be as simple and direct as possible Bends, where necessary, should be gradual with long radii Velocity should be kept as low as possible, consistent with keeping the dust particles in suspension Where bends in the ductwork are necessary and rapid wear is encountered, designs can be developed which employ easily replaceable wear pads on the bends These pads can be thick metal, made of wear-resistant steel, castings, hard-surfaced with special abrasion-resistant alloys such as stellite, or have thick rubber-coated linings

A number of good reference sources are available for the design of hoods and ductwork and should be consulted before design of an elaborate dust-collecting network of ducts Among the better general references recommended are: Dalla Valle,1 Hemeon,2 American Conference of Governmental Industrial Hygienists,3

ASHRAE Handbook,4 and Buonicore and Davis.5

A recent article by R H King6 describes the proper installation of fans for optimal system performance A concise summary of exhaust system design was prepared by Brandt.7 While it is an excellent article on hood and duct design, it was written in 1945 Thus, it was written more from the standpoint of industrial venti-lation rather than pollution control The information presented is still accurate and useful However, it must be understood that statements concerning control velocity 20

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which relate to capturing sufficient contaminants to prevent a worker health hazard, must now be interpreted as meaning capture of contaminants sufficient to prevent pollution of air

The principle in design of hoods and ducts is to choose a velocity that fits the situation For both hoods and ducts, the velocity must be sufficient to overcome the pressure drop For hoods, the material must be drawn up and into the duct For ducts, the velocity must be sufficient to keep any particles suspended Once the operating velocity is set, the flow is governed by the following law for volumetric flow

(20.1) Here Q is the volumetric flow rate, V is the velocity chosen, and A is the required area of flow Once a velocity is chosen for a hood, the area required to cover the source from which the pollutant must be removed determines the flow rate The flow rate needed in a duct sets the area of the duct

20.2 HOOD DESIGN

The purpose of a hood is to collect contaminants from a workplace Significant amounts of air are also drawn into the hood The air flow is set by the distance between the source and the hood and by the pressure loss created by the air entering the duct A sophisticated approach is given by Goodfellow.3 Brandt7 lists the fol-lowing four major groups of hoods and designates the relationship governing the flow into the hood

1 Enclosing hoods

2 Rectangular or round hoods

3 Slot hoods

4 Canopy hoods

In each case the minimum control velocity must first be selected Table 20.1 is

a guide for the selection of this control velocity

20.2.1 F LOW R ELATIONSHIP FOR THE V ARIOUS T YPES OF H OODS

20.2.1.1 Enclosing Hoods

An enclosing hood completely shuts off the ineffective outside area Paint booths and laboratory hoods are typical examples of an enclosing hood Equation 20.1 describes this type of hood where the area, A, is the area of any opening into the hood This opening is necessary to assure air flow However, the area should be kept

to a minimum for good performance

20.2.1.2 Rectangular or Round Hoods

This type of hood is used for welding, stone surfacing, cleaning and degreasing, and drilling The shape of the velocity pattern in front of these hoods determines the

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velocity into the hood The total air flow entering the hood is determined by a point

at x distance from an imagined suction point Thus the air velocity V is measured

at a distance x from this suction point The area A is essentially a spherical surface with area = 4πx2 = 12.57x2 Equation 20.1 then becomes,

(20.2)

By experiment Dalla Valle1 noted that the contour shape actually changes and flattens slightly in front of the hood and, thus, he modified this theoretical relationship

to the following,

(20.3)

where

x = distance in feet along hood centerline from the face of the hood to the point where the air velocity is V ft/min

a = area in ft2 of the hood opening

This equation is applied to the centerline or axial velocity and not to the velocity

at any point Consequently Dalla Valle1 or Brandt7 should be consulted before applying this equation

20.2.1.3 Slot Hoods

Slot hoods are lateral hoods recommended for ventilation at tanks used for degreas-ing, pickldegreas-ing, or plating These hoods are an extreme form of rectangular hood with

50 ft or more of hood opening and are very narrow, with as little as 4 in of height

TABLE 20.1

Minimum Recommended Control Velocities

Condition of Release

of Contaminant

Example of Process

or Operations

Minimum Control Velocity (ft/min)

Released with no significant

velocity into quiet air

Released with low initial velocity

into moderately quiet air

Spray paint booths, welding, dumping of dry materials into containers

100–200 Released with considerable

velocity or into zone of rapid air

movement

Spray painting in small booths with high pressure, active barrel or container filling, conveyor loading

200–500

Released with high velocity or into

zone of rapid air movement

Grinding, abrasive blasting, surfacing operations on rock

500–2000

Q= 12 57x V2

Q=(10x2+a V)

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The opening can be visualized as cylindrical surface with an area of 2πrL Hence Equation 20.1 becomes

(20.4) where

r = radius in ft or distance from source of suction to point where the air velocity

is V

L = length of cylinder in ft

Brandt7 modifies this equation to account for the actual noncylindrical form of the cylinders The modified expression is

(20.5) where

L = length of hood in ft

W = tank width in ft (distance from hood to remotest source of contamination)

k = constant

k = 6.28 for a freely suspended slot hood with cylindrical contours

k = 3.27 for freely suspended slot hoods

k = 2.8 for slot hoods adjacent to tank tops

k = 1.5 if slot hood has flanges or other restricting surfaces at right angles to each other so that air can enter from only one quadrant

20.2.1.4 Canopy Hoods

Canopy hoods are a class of rectangular or round hoods They are used for tanks and furnaces They are more effective if the contaminate air is warmer than the surrounding air In this case Brandt7 recommends that the area A be replaced by PY where,

P = the perimeter of the hood face in ft

Y = the perpendicular distance in ft from the hood face to the top of the tank Therefore,

(20.6) where V = the average velocity through the opening between the hood edge and the tank

Della Valle1 found that for canopy hoods located between 3.5 and 4 ft above the source of contamination, the velocity at the top edge of the tank was about 0.7 of the average velocity Equation 20.6 was then modified to

(20.7)

Q=2πrL=6 28 rL

Q= 1 4 PDV

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where V is now the minimum control velocity given in Table 20.1 Note that the constant could be as low as 1.0 if the canopy hood is close to the surface from which this contaminant source is coming Also, the constant could be greater than 1.4 if the distance between the hood and the contaminant source is much greater than 4 ft

20.3 DUCT DESIGN

Ducts have the purpose of carrying the contaminated air from a hood, a piece of process equipment, or another piece of control equipment to another piece of equip-ment or to the discharge stack Ducts can be water cooled, refractory lined, or made

of stainless steel or plain carbon steel For corrosive materials in the air or temper-atures in the range of 1150 to 1500°F, stainless steel is used At lower temperature and noncorrosive materials in the air, plain carbon steel may be used

20.3.1 S ELECTION OF M INIMUM D UCT V ELOCITY

The ductwork, if carrying particulates, must be designed to keep the particulates in suspension This means that carrying velocities must be sufficiently high to prevent settling of the largest particles being conveyed An empirical formula recommended

by Brandt7 in use for estimating duct velocity required to prevent settling is

(20.8)

where

V = duct velocity in feet per minute

S = specific gravity of the particle

d = diameter in inches of the largest particle to be conveyed

The above equation has been developed for use with ambient air While it considers the effect of density of the particle, it ignores the density of the conveying gas Where the gas density is considerably different from sea level ambient air, the need to alter the equation could be anticipated Although the velocity chosen by Equation 20.8 is to convey particulates, it is generally desirable in exhaust ductwork

to avoid long horizontal runs where possible and to provide some slope to the essentially horizontal portions of the ductwork In addition, moist and sticky partic-ulates can produce duct buildup, and the velocities predicted by the above equation are not adequate to prevent duct-wall caking in such situations Higher duct veloc-ities, providing frequent duct cleanouts, and fluorocarbon-sheet lining of the duct are practices employed in such situations

Table 20.2 should be consulted for determining the minimum duct velocity The area depends on the source of the air flow If the duct work originates from a hood, the flow rate will be determined from that hood as suggested in Section 20.2.1 If the duct work originates from a piece of process equipment or another piece of control equipment, that equipment will set the flow rate Knowing the flow rate from

=

+



15 700

1 ,

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either source and the desired velocity estimated from Table 20.2, for example, the area for flow can be determined from Equation 20.1 Then the duct work can be designed Fluid flow in the duct is described by the mechanical energy balance presented in the next section

20.3.2 T HE M ECHANICAL E NERGY B ALANCE

The following equation comes from Chapter 8 Here the ∆ represents the difference between the output and input value of the variable

(8.6)

In this equation, written for turbulent flow where, α = 1.0, the fundamental equation for enthalpy with no phase change or chemical reaction can be written

where

S = entropy

v = specific volume

Then from the definition of entropy, this equation can be rewritten

then

(20.9)

TABLE 20.2

Minimum Recommended Duct Velocities

Minimum Control Velocity (ft/min)

Vapors, gases, smoke,

fumes, very light dusts

polymers

3000

Large particles of heavy

moist materials







= +

2

dH=TdS+vdP

dH=dQ+vdP

P

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And then the mechanical energy balance results from substituting Equation 20.9 into Equation 8.6 above

(20.10)

This equation applies strictly to a reversible idealized process Because mechan-ical energy is dissipated into heat through friction, a term is added for friction The equation then can be used to describe real situations of fluid flow Equation 20.10 now becomes

(20.11)

where F is the added friction term Furthermore, for incompressible fluids,

(20.12)

where ρ is the fluid density Since WS is the shaft work done on a system, the equation can be rewritten to include the efficiency of the fan and motor used in this case If η is the efficiency, then

(20.13)

where W = the work done on the system by the fan whose efficiency is η

Now the mechanical energy balance becomes

(20.14)

The usual practice is to consider the velocity head U2, and the friction head, F, when making a calculation for the total pressure for a fan to encounter, discounting the potential energy loss due to change in elevation ∆z because it will be so small and the change in pressure ∆P will be small since the flow is nearly incompressible

20.3.2.1 Velocity Head

For the average velocity, U = V, the velocity head HV is

(20.15)

S P

P

C C

1

2

S P

P

C C

1

2

P P

1

2

ρ

WS= ηW

η ρ

2

g

2 in ft of fluid

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To convert to inches of H2O for the standard conditions of 70°F, 50% air

humidity, and 1 atm pressure, this equation becomes

(20.16)

The velocity head is now designated as the velocity pressure (VP) in inches of

H2O

20.3.2.2 Friction Head

The static pressure is sometimes called the friction pressure or friction head In ducts

the friction pressure is due to skin friction generated by flow and energy losses It

is also generated due to turbulence in bends, fittings, obstructions, and sudden

expansion and contractions The friction loss in smooth circular pipes and ducts can

be calculated from

(20.17)

where

f = friction factor

Dc= duct diameter

McCabe et al.8 report that the friction factor, f, can be calculated from the von

Karmen equation

(20.18) where NRe is the Reynolds Number,

(20.19)

A nomograph based on this type equation is given in Figure 20.1, friction losses for

air in circular ducts For noncircular, square ducts it is possible to use the hydraulic

radius concept

(20.20)

where

rH = hydraulic radius

S = cross-sectional area of the channel

LP = perimeter of the channel in contact with the fluid

2

D

V g

f

c















4

2

1

f = log (NRe f)−

µ

L

H p

≡

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Society of Heating Refrigeration and Air-Conditioning Engineers Inc., www.ashrae.org Reprinted by

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The diameter used in the Reynolds Number calculation is taken as four times

the hydraulic radius This concept is especially good for square ducts It is better to

use Figure 20.2, equivalent rectangular and circular ducts having equal pressure drop

(Crawford9), for nonsquare, rectangular ducts

The effect of bends, fittings, obstructions, and sudden expansion and contractions

can be accounted for through a relationship where the head loss is proportional to

the velocity in the pipe section squared

(20.21)

where Kx = proportionality constant

McCabe et al.8 list the following formulas for head loss due to a contraction HfC

(20.22) and the head loss due to an expansion, HfE,

(20.23)

FIGURE 20.2 Equivalent rectangular and circular ducts having equal pressure drop and flow

rate (With permission from Crawford, M., Air Pollution Control Theory, McGraw-Hill Book

Co., New York, 1976.)

g

c

2

S

C

D

U







0 4 1

S

E

U

D

= −







1

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