Emission Source Characterization

13.4.2.1 Design Methodology

The first essential step in the design of a fume control system and selection of gas-cleaning equipment is the characterization of the fume emission source. Design procedures which can be used for new and existing industrial plants fol­low. The characterization of fume emission sources includes parameters such as plume flow rates (m3/s), plume geometry (m), source heat flux (J/s), physical and chemical characteristics of particulates, fume loadings (mg/m3), etc.

For a new process plant, calculations can be carried out using the heat re­lease and plume flow rate equations outlined in Table 13.16 from a paper by Bender.11 For the theory to be valid, the hood must be more than two source di­ameters (or widths for line sources) above the source, and the temperature differ­ence must be less than 110 °C. Experimental results have also been obtained for the case of hood plume eccentricity. These results account for cross drafts which occur within most industrial buildings. The physical and chemical characteristics of the fume and the fume loadings are obtained from published or available data of similar installations or established through laboratory or pilot-plant scale tests.12 If exhaust volume requirements must be established accurately, small — scale modeling can be used to augment and calibrate the analytical approach.

For an existing process plant, the designer has the opportunity to take measurements of the fume or plume flow rates in the field. There are two basic approaches which can be adopted. For the first approach, the fume source can be totally enclosed, and a temporary duct and fan system in­stalled to capture the contaminant. For this approach, standard techniques can be used to measure gas flow rates, gas compositions, gas temperatures, and fume loadings. From the collected fume samples, the physical and chemical characteristics can be established using standard techniques. For most applications, this approach is not practical and not very cost effec­tive. For the second approach, one of three field measurement techniques, described next, can be used to evaluate plume flow rates and source heat fl uxes.

13.4.2.2 Propeller Anemometer Technique

Fume velocity can be measured at the roof truss level by using several pro­peller-type anemometers mounted on a grid. The output from the anemome­ters can be connected to a recorder located at the operating floor level. Experience has shown that six to eight anemometers are usually sufficient to give a good description of the plume velocity. This approach has the advan­tage of obtaining both a velocity profile and an average velocity for the plume. The combined information of velocity distribution and observed plume size provides the necessary design parameters to ensure a satisfactory performance for the designed hood. Figure 13.29 is a plot of average plume flow rates mea­sured at roof truss level as a function of time for a typical tapping operation on an electric steelmaking furnace. To carry out such velocity measurements as a standard method is not recommended because fume and dust tend to harm the propeller bearings, making the anemometer inoperative after a num­ber of tests.

	Dimensions	Point jet	Line jet	Point plume	Line plume
Characterizing		“Massless”	“Massless”	Buoyancy	Buoyancy
Source	Assumed	Momentum flux	Momentum flux	Flux — consr.	Flux COIiSf.
Quantity	Uniform	M Q (m^/s^)	M — Q (ni’Vs^)	F ■::= Q	F QSi. Ai.(rrr,/s’i
Volume Flow rare Q	M ’ S	/8 MY1/2 [—J 17,12	1 1/2 4h) z	6 Tra/’1 8 FaV ^ ^ ^ ^ “YTTV ‘
		= “ma*^2			” FfTrux"’^7’
Mass flux G	G	Qpo	Qpo	Qpo	QP()
“Massless,? momentum flux M	M4 S-	,M = const.	M = const.	Пi’-WT^ _ «Lx Vb2 ")	.11,0′; I I “ A ^ 7r/ ^ B ”max ,u — c

Kin. energy flux (power)

Gmj

Pof 7зг

„ft

= PoMmax VПт/3-

Г-" (Г

5 F18aF

6a V 5тг 0.093 (6/5)az

0.1 Sй

{2! JTT)az

Centerline m Velocity nrmx — F. ntrainmeni cunsr. A Length scale B m
M y/2i 2irj AZ
0.068
0.057 2az
(4/ Утг)с

Relationship between buoyancy flux and pollutant flux:

Po~ P T-T0 Buoyancy A = G = G———

Grainloading Pn

Pn

Concentration of pollutant С = G

Buoyancy flux F —■ (Amal/2) • Q = const.

Pollutant flux (Cmax/2) • Q = const.

For pure plumes the distribution of velocity, buoyancy, and concentration is the same.

U(r) A (r) C(r)

From source heat flux Q, the buoyancy flux F Is

F = J A(r)u(r) dA — ———

J o C„T0p0

C = local pollutant concentration, m/s2, ft/s

C, nax = plume centerline concentration, taken at hood face when used for nondimensionalizing, m/s2, ft/s2

C„ = specific heat of ambient fluid, J/g K, BTU/fb °R

Q = plume flow rate, nrVs, ft:,/s

Q = source heat flux, J/s, BTU/s

R = radial coordinate, m, ft

T = local plume temperature, K, °R

T0 = temperature of ambient fluid, K, °R

It = local vertical plume velocity component, m/s, ft/s

A = local plume buoyancy, m/s2, ft/s2

P = local plume fluid density, g/m lb/ft3

P, = hood exhaust fluid density, g/m lb/ft3

P0 = ambient fluid density, g/m3, lb/ft3

O X

O G3

0 15 .30 45 60 75 90 105 120 135 150 165 189 195 210 Time (s) ——— ► FIGURE 13.29 Average plume flow rate at roof truss level for a tapping operation.

13.4.2.3 Stopwatch Technique

The stopwatch technique for determining emission volume flow rate is based on measuring with a stopwatch the elapsed time for fume to rise be­tween two known levels (e. g., Zj, Z2). For this test procedure to be valid, the test must be carried out in a region where the rising fume clearly exhibits buoyancy-dominated plume behavior. The calculation procedure depends on a good estimate of the location of the virtual origin of the plume and the heat release for the process.

(13.73)

At the Z2 level, the plum volumetric flow rate is given by

7 7 1 2 —	[ 1 -a ‘
T	U-2/3J

Qz, = 0.026

I 13.74)

Where A = (Z2 — Zx)/t , the observed maximum plume velocity, is equal to

Z,-Z, :

Where QZi is obtained by substituting the formulas for Q from Eq. (13.73) and

This technique permits estimation of the volumetric flow rate at any level above a source, provided that the result is matched to the gravitational fume acceleration terms applicable near the source. The result of such an analysis is shown in Fig. 13.30. The emission flow rate from an electric arc tapping pro­cess has been estimated at any level above the steel ladle using the stopwatch technique in conjunction with the plume theory.

The nature of the preceding analysis does not permit the application of the technique to design of local capture hoods but rather to the design of remote or canopy fume hoods. For this approach to be valid, the hoods must usually be at least two source diameters away from the emission source.

Average plume velocity (m/s)

Tapping plume flow rates x 105 (m /h) Roof truss level 0.5 0.8 1 2 4 8 10

I 2 3 4 r _________ I_________ L________ I______

20

1-S

10

FIGURE 13.30 Average plume velocity and flow rate during an electric furnace tapping operation.

13.4.2.4 Plume Photographic Scaling Technique

For an existing industrial process plant, Goodfellow and Bender13 have developed a simple, expedient, and low-cost technique for establishing the average plume flow rate and plume geometry. Once the flow rate is known, the heat flux for the source can be established easily through off-gas temper­ature measurements. This technique is called the plume photographic scaling technique or the movie scaling technique. This technique has been used suc­cessfully to solve numerous fume hood design problems in Canada, the United States, and elsewhere around the world.2-J This approach is preferred over the earlier two approaches because it is simple, cost effective, and accu­rate.

The plume photographic scaling technique is based on proper illumina­tion of the fume (preferably by oblique-angle back lighting) and the inclu­sion in the scene of an object suitable for movie scaling. During the early development of this technique, an accurate average velocity of heavily con­taminated turbulent flows was obtained by taking 8-mm or 16-mm color movies of the emission. The movie camera found most suitable for this pur­pose used a standard 18 frames per second. Although high-speed movie cameras have been used, it was concluded that they are unsuitable for this work.

Recent developments in the field-testing procedure have demonstrated that a motor-driven 35-mm camera at up to three frames per second produces superior results to a movie camera.14’1’’ The analysis of the film is carried out by scaling the distance that fume advances from one consecutive photograph to the next. The diameter of the plume as a function of the distance above the source can be scaled directly off the photograph.

TU

TU H—— H

Time

• n

TU

Tn

Reference plane

Photograph

Plume Ladle

S = Distance Traveled = z2 — D = Diameter

Te = Frame exposure time 4 Ta = Frame advance time Note: Frames are shot in sequence at up to three frames per second with a motor-driven 35mm camera. FIGURE 13.31 Plume photographic scaling technique and calculation procedure.

T = Time = Tf, + TU + TUl Span

A = Plume cross-sectional area — D2

	D

V = Velocity = y-

Q = Plume flow rate At reference plane = VM

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