# Fume Capture Design Methodology

More stringent environmental regulations are requiring significant improve­ments in both air pollution and the workplace environment. As a result, fume control systems must be designed to operate efficiently. Equation (13.75) rep­resents the general equation for overall system performance:

V

^capture ~ ^cleaning 100

 N — n 1 1 — ^cleaaing ^capture ‘ Jcapturej 100
 Overall
 (13.75)
 Figure 13.31 is a pictorial representation of the scaling technique and the calculation procedure. Figure 13.32 shows a typical application of this tech­nique to calculate a plume flow rate from an electric furnace scrap-charging process. Although the technique seems very simple and straightforward, its successful application requires experienced judgment and careful attention to detail during the critical data analysis stage.

 Where Coverall = overall fume control system performance ^capture = percentage of total fume generated which is captured by control System ^cleaning = percentage of fume collected by gas cleaning relative to total fume load to fume system

 Flow rate = Vclocitv x Area = (Hcight/Timc) x (Dia)2 x 3.14/4 = <9.76 — 3.66)/( 1/9 S X 13 frames) x (9.45)* x 3.14/4 = 296

 Process.

Sample Problem

Consider a fume control system with an overall hood fume capture effi­ciency of 60% and a fume cleaning efficiency of 99%. Calculate overall fume control system performance.

Solution The overall fume control system performance is (60)(99)

= 59.4%

100

The technology in the fume capture field is not well developed, and perfor­mances of many capture systems are low and typically may be in the 30% to 60% Range. There is a paucity of fundamental research and development in the fume capture field. In contrast, hundreds of million of dollars have been spent on re­search and development activities in the gas-cleaning area, which is mature and well developed. It is not uncommon to specify and to measure gas-cleaning equip­ment performances of over 99.9% collection efficiency. As shown in Eq. (13.75), the overall fume control system performance is determined by the product of the capture efficiency and the gas-cleaning efficiency. This equation clearly shows the need to improve the efficiency of capture of the fume at the source in order to ob­tain significant improvements in the overall fume control system performance.

Fume capture techniques can be classified into three specific types as follows:

1. Source capture or direct evacuation

2. Low-level or close-capture hood

3. Canopy or remote-capture hood

 Low-Level Or Close-capture Hood

 Canopy or remote capture hood

 FIGURE 13.33 Classification of fume capture techniques.

 /

A pictorial representation of the different fume capture techniques is shown in Fig. 13.33.

Table 13.17 lists some of the important considerations for the different fume capture techniques. From the point of view of cost effectiveness, the usual preference is source collection or a low-level hood, provided an accept­able scheme can be developed within the process, operating, and layout con­straints. The cost of fume control systems is almost a direct function of the gas volume being handled. Hence, the lower volume requirements for the source capture or low-level hood approach often results in significant capital and op­erating cost savings for the fume control system.

The use of a source capture or a direct evacuation system is the most positive form of fume capture. A well-designed system can operate at high fume capture efficiencies. For many of these systems, the captured gas temperature for the pro­cessing operation is very high (1000-1500 °C), and gas cooling may be required

TABLE 13.17 Comparison of Fume Capture Techniques

 Source capture Low-level hood Canopy hood Gas volume to be treated Lowest Low Very high Gas temperatures Very high Medium/low Low Fume loadings Very high High Low Ln-plant environment Excellent Good/excellent Often poor (cross-drafts) Process interference May be significant May be significant Not significant Layout constraints May be significant May be significant Not significant System costs Lowest Medium/high Highest

Ahead of the gas-cleaning equipment. Specific processing information, such as feed rates, chemical composition of feed, operating levels, etc., is required in or­der to define the necessary design parameters for the direct-evacuation fume sys­tem. A typical example of the processing information required for sizing the fume system for an electric steelmaking furnace is shown in Table 13.18. From a completed questionnaire, design calculations can be carried out to calculate gas flows, gas temperatures, and gas compositions. These calculations form the basis for the subsequent sizing of direct-evacuation fume systems.

TABLE 13.18 Canopy-Hood Design Equations

 Reference

 Sketch

Equations

 ACGIH1’’ ACGIH1 * ACGIH1* Danielson1

 Canopy Hood

Open type

Q = 1.4PHV Q = flow rate

P = perimeter of tank H = height of hood above tank

V = velocity (50-500 FPM)

Two sides enclosed

Q ={W+ L)HV W, L = open sides of hood

Three sides enclosed

 . ‘ ‘ H : ‘ ‘ !1 ‘ Rsourcel

Q = WHV W = open side of hood

Contained flow angle of air column 12°—20°

Typical design flow angle = 15°

Poini source ‘

 Canopy Hoot!

Face velocity hood = 150-200 FPM

 Hemeon lf!

Vz = velocity at distance Z (ft) Z = height above point source = effective height (ft)

 Tkourcel

 = Y + 2b Y — distance from source to hood (ft) B = width of source (ft) H = rate sensible heat transfer to air column (BTU/min) Qz = total airflow rate (ft3/min) D = diameter of air column (ft) H hood diameter (m) Fisenbarrh1L> W = furnace diameter (m) P = theoretical point Hood face velocity = 0.5-1.0 m/s

 Qz = 7.4г3/2 L/H’ D = zaM/2 H = 0.437 C0-98 A = 2.58 W138

 Canopy Ho(|d

 /

 E !

4

._ W__ ‘

І

[source f

The advantages of low-level hoods are listed in Table 13.17. The first step is to verify that the general principle of local capture of emissions is acceptable and feasible for the process. The next step is to establish the most efficient hood geom­etry. In most cases, this involves a balancing of the degree of process interference tolerable against the degree of emission source enclosure required.

The factors affecting the performance of a local exhaust system are well known.18 For fume control, an added factor is the effect of heat release or buoyancy. Important design parameters are process heat release and the size and geometry of air-supply openings and their location relative to major surfaces of the enclosure. The location of the fume off-take is usually only of secondary importance.

Figure 13.34 gives an example of a local exhaust hood above a vessel holding a hot product. There are three different design approaches which can be used to establish the exhaust volume requirement for the low-level hood. These three approaches, in order of increasing accuracy, are as follows:

1. Rule of thumb

2. Analytical approach applying the three fluid mechanics equations

3. Small-scale modeling tests

For the rule-of-thumb approach, the control velocities required through A And,42 to prevent fume emissions from the hood are obtained from standard references.16-18 The exhaust required to control emissions from the opening A Is greater when the pot is empty. The short-circuiting of flow Q2 is not a seri­ous problem as long as the clearance A2 is small. These rule-of-thumb esti­mates can be improved by applying the three well-established fluid mechanics equations governing conservation of mass, energy, and momentum to the fume collection process.20 These equations simply state that “flows in” must equal “flows out” and that all flow forces (pressures) must be balanced at all times. Unfortunately, a high degree of inaccuracy still exists in this approach because of the large number of assumptions regarding the expected flow field in the hood. For industrial applications in which complex hoods are needed, it

 Local exhaust hood

Opening for addition of product A

___ containing

Hot product

Is essential to carry out a small-scale model study in order to establish the min­imum exhaust volume required to control fume emissions.

For modeling, the similitude laws governing modeling must be followed. The topics of dynamic similitude and theory of models are discussed in most text­books on fluid mechanics,20 and only the resulting equations are discussed here.

In order to create dynamic physical models, it is necessary that the follow­ing criteria be met:

1. Exact geometric similitude between the model and the real-world prototype

2. An identical balance of forces between the model and the real-world prototype

In the set of conservation equations described earlier, the Reynolds number and the Froude number must be the same for the model and the prototype. Since most industrial operations involve turbulent flow for which the Reynolds number dependence is insignificant, part of the dynamic similarity criteria can be achieved simply by ensuring that the flow in the model is also turbulent. For processes involving hot gases (i. e., buoyancy driving forces), the Froude number similarity yields the required prototype exhaust rate as follows.

Froudemodel = Froudepr0t0typf

 L
 (13.76) P
 RP0~P
 L P ‘
 V2

Where

V = representative velocity (L /0) gc = gravitational acceleration (L /9) P0 = ambient fluid density (M/L3) p = representative fluid density (M/L3)

L = representative dimension

Ail parameters must be in consistent units.

A representative velocity can be taken as the ratio of the hood off-take

Flow rate to the hood off-take cross-sectional area. For this basis, the proto­

Type off-take flow rate is obtained from the following equation.

^ ^ [(P0-P)/P)lp/2 <;/>

Qp = Qn,———- — TT77-xS’V — (13.77)

L(p0-p)p]m

^ [(To-TVT]1/2

= Qn,————— f^x5 ’ !13-78>

((To — T)/T]n ~

Where

Q = volumetric flow rate (L3/ 0)

S = model scale (e. g., for a model 1/100 the size of the prototype, S = 100) T = representative gas temperature T0 = ambient gas temperature

This equation can be rewritten by substituting for the temperatures to give the following equation:

 / Pmtfp 1/3 5/3 Pmj
 : 13.79)

 QP = Qn

Where

Q = heat flux rate (H/0)

D = characteristic dimension (L)

If the model test uses the same fluid medium as the prototype at similar ambi­ent temperatures, Eq. (13.79) can be simplified:

 FQp] 1/3 F DP) 5/3 (Dm)
 (13.80)

 QP = Qr

In this context, it is important to note that a model test simulating the oper­ation of an air pollution control scheme can also be modeled in water. For some air pollution problems, an air model might become quite large in order to en­sure modeling the turbulent nature of the prototype flow rate. For some applica­tions, a water model can be used which will give a reasonable scale size.

Important conclusions can be drawn from the general modeling Eq. (13.79). The equation shows that the required prototype flow rates are di­rectly proportional to the model flow rates. For scaling, the equation shows that the prototype flow rate has a strong dependence on the accuracy of the model scale (5/3 power). Both of these parameters are easy to establish accu­rately. The flow rate is rather insensitive (varies as the 1/3 power) to the changes in the model and prototype heat flow rates, densities, and tempera­tures. This is desirable because an inaccuracy in the estimate of the model variable will have a rather small effect on the resulting prototype flow rate.

The use of canopy hoods or remote capture of fume is usually considered only after the rejection of source or local hood capture concepts. The common reasons for rejecting source or local hood capture are usually operating interfer­ence problems or layout constraints. In almost all cases, a canopy hood system represents an expensive fume collection approach from both capital and operating cost considerations. Remote capture depends on buoyant air currents to carry the contaminated gas to a canopy hood. The rising fume on its way to the hood is of­ten subjected to cross-drafts within the process buildings or deflected away from the hood by objects such as cranes. For many of these canopy systems, the capture efficiency of fume may be as low as 30-50%.

Canopy hoods have existed for a long time, and almost all industries have canopy-hood installations of some shape or size. Canopy design, until the mid — 1970s, was an art based on rule of thumb and highly empirical approaches. Ana­lytical work done to arrive at procedures for canopy-hood design is described in papers by Bender,11 Goodfellow and Bender,13 and Goodfellow.3

It is only recently that technology has developed so that canopy-hood efficiencies can be predicted and measured in a quantitative way. One of the difficulties in this area has been the use of different definitions for can — opy-hood efficiency. Goodfellow and Bender13 have proposed a standard
method for measuring canopy-hood performance which accounts for rhree parameters:

1. Ratio of spilled pollutant to total pollutant arriving at the hood

2. Ratio of hood suction to plume flow rate at the hood

3. Type of canopy hood

Hood types were classified into three groups.1-’

Type A—Ideal hood. Fume of the lowest concentration at the fringes of the plume spills first.

Type B—Actual hood. Falls in the intermediate range between type A and type C.

Type C—Worst hood. Spills fume of an average concentration.

The type C hood performance is due to turbulent mixing within the hood, which is caused by an inappropriate hood design or by objects below the hood face.

Figure 13.35 is a schematic of a canopy hood where

Q[ = hood suction flow rate (m3/s)

QH = plume flow rate at the hood face level (m3/s)

Qs = spilled plume flow rate at the hood (m3/s)

Gc = contaminant captured by hood (kg/s)

GH = contaminant arriving at hood (kg/s)

Gs = contaminant spilled (kg/s)

An equation can be developed which expresses the ratio of spilled con­taminant to total contaminant in terms of the ratio of hood suction to plume flow rate. This equation is as follows:

 Spilled
 Contaminant
 Contaminant FIGURE 13.35 Canopy hood schematic.

Where

X = 2 tor type A—ideal hood 1 < X < 2 for type B—actual hood X — 1 for type C—worst hood

Figure 13.36 is a plot of the preceding equation for the three types of hoods. The plot shows the curve for the actual and the worst hoods requiring a hood flow rate larger than the plume flow rate in order to get 99% fume capture.

The preceding results can be extended to relate canopy-hood performance to opacity. This is a significant step because air pollution legislation in many countries has a reference to opacity levels (i. e., a fume concentration level which must be met at the point of discharge of fume from a process building). Equations can be developed of this form:

 (13.82)

OP = 1 -(1 — OPmax)(1-(Q>/Q"))X,

Where

OP = observed opacity

OPmax = the maximum opacity observed for zero hood suction for an existing installation

 Figure 13.37 is a plot of relative roof monitor opacity as a function of fume hood suction. Following a similar approach to that used for low-level hoods, small-scale modeling is often pursued for the design of canopy hoods for a new facility or for modifications to an existing installation.19’21-24 Bender11 describes rests carried out 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ratio of hood suction to plume flow rate (Qj/Qh)

 ——- Worst hood: 0P= i-(i-OPmax)(i-e/QH) pollutant of average concentration is spilled first

 ——- Ideal hood: Op= i-(i-OPmax)U-Q/GH)J Pollutant of lowest concentration is spilled first

 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ratio of hood suction to plume flow rate (Qi/Q/j)

 FIGURE 13.37 Relative roof monitor opacity as a function of fume hood suction.

To examine the design of canopy hoods and describes a technique for the develop­ment of a fume hood design for fluctuating plume flow. The problems of recircula­tion and temporary storage of fume in hoods have also been examined. Test results have shown that a baffle plate arrangement for fume hoods can significantly im­prove the performance of canopy hoods under specific plume flow conditions.