HOT PROCESSES

NOO]) LOS

3.9 HOT PROCESSES

Design of hooding for hot processes requires different considerations than design for cold processesY24) When sig- nificant quantities of heat are transferred to the air above and around the process by conduction and convection, a thermal draft is created which causes an upward air current with air velocities as high as 400 fpm. The design of the hood and exhaust rate must take this thermal draft into consideration.

3.9.1 Circular High Canopy Hoods: As the heated air rises, it mixes turbulently with the surrounding air. This results in an increasing air column diameter and volumetric flow rate. The diameter of the column (see Figure 3-20) can be approximated by:

Dc = 0.5X~88 [3.8)

where:

Dc

=

column diameter at hood face

Xc

=

^{Y + Z =} the distance from the hypothetical point source to the hood face, ft

y distance from the process surface to the hood face, ft

z

=

distance from the process surface to the hypo- thetical point source, ft

"z" can be calculated from:

[3.9]

where:

bo_s

=

diameter of hot source, ft.

The velocity of the rising hot air column can be calculated from:

v

= 8(A )0.33 (Llt)042

I s

X

^{O.25 [3.10]}

where:

c

VI

=

velocity of hot air column at the hood face, fpm As

=

area of the hot source, ft2

bot = the temperature difference between the hot

source and the ambient air, F

Xc

=

Y + Z = the distance from the hypothetical point source to the hood face, ft.

The diameter of the hood face must be larger than the diameter of the rising hot air column to assure complete capture. The hood diameter is calculated from:

where:

Dr = diameter ofthe hood face, ft Total hood air flow rate is

where:

Qt

=

total volume entering hood, cfm

[3.11]

[3.12]

Vr

=

velocity of hot air column at the hood face, fpm Ac

=

area of the hot air column at the hood face, ft2 Vr

=

the required air velocity through the remaining

hood area, fpm

Af

=

total area of hood face, ft2 EXAMPLE PROBLEM

Given: 4.0 ft diameter melting pot (Da) 1000 F metal temperature 100 F ambient temperature

Circular canopy hood located 10ft above pot (y) Calculate xc:

Xc = Y + z = y + (2Ds) 1.138 Xc

=

1

a

⁺ (2 X 4) 1 .138 Xc= 10.7ft

Calculate the diameter of the hot air column at the hood face:

Dc

=

^{0.5 Xc 0.88}

Dc = 0.5(20.7)°88 Dc

=

^{7.2 ft}

Calculate the velocity ofthe hot air column at the hood face:

v

= 8(A )0.33 (bot)042 I s (X)0.25 As = 0.251tDc² As

=

0.251t(4.2)2 As

=

^{12.6 ft2}

bot = 1 000 - 1 00 = 900 F

v

= 8(1.26)033 (900)°.42

I (20.7)025

v =

(8)(2.31) (17.4)

I (2.13)

VI = 151 fpm

Calculate diameter of hood face:

Of = Dc + 0.8y Of = 7.2 = 0.8(10) Of = 15.2 ft

Calculate total hood airflow rate Of = VfAe + Vr(Af -Ad Ac = 0.25nDe2

Ac = 0.25n(7.2)2 Ac = 41 ft2 AI = 0.25nDf² AI = 0.25n(7.2)2 Af = 181 ft2

Of= 151(41) + 100(181-41) Of = 10,290 cfm

3.9.2 Rectangular High Canopy Hoods: Hot air col- umns from sources which are not circular may be better controlled by a rectangular canopy hood. Hood air flow calculations are performed in the same manner as for circular hoods except the dimensions ofthe hot air column at the hood (and the hood dimensions) are determined by considering both the length and width of the source. Equations 3.8, 3.9, and 3.11 are used individually to determine length and width of the hot air column and the hood. The remaining values are calculated in the same manner as for the circular hood.

EXAMPLE PROBLEM

Given: 2.5 ft x 4 ft rectangular melting furnace 700 F metal temperature

80 F ambient temperature

Rectangular canopy hood located 8 ft above furnace (y)

Calculate Xc for each furnace dimension.

X_C2.5 = Y + Z25 = Y + (2Ds^{25) 1.138}

= 8 + (2 X 2.5)1138

= 14.2 ft

Xc4 = 8 + (2

x

4) 1.138

= 18.7ft

Calculate the width of the hot air column at the hood face.

De2.5

=

^0.5 Xe2.5⁰.88

= 0.5(14.2)088

=

^{5.2 ft}

De4.0 = 0.5(18.7)°·88

=

^{6.6 ft}

Calculate the velocity of the hot air column at the hood face.

v = 8 (A )0.33 (""1)°.42

I s (X

c)0.25 As

=

2.5 x 4

=

10 ft2

~t = 700 - 80 = 620 F Xc = Xc2.5 = 14.2 ft

Note: X_c25 is used rather than _{X c40} as it is smaller and as such will yield a slightly larger Vr which results in a margin alsafety.

V

=

8(10)0.33 (620)°42 (14.2)025

= 8 (2.1) (14.9) 1.9

= 132 fpm

Calculate hood face dimensions.

Hood width

=

De2.5 + 0.8y

=

^(5.2) + 0.8(8)

= 11.6 ft Hood length

=

^DC4.0 + O.By

= 6.6 + 0.8(8)

=

^{13.0 ft}

Calculate the total hood air flow rate.

Of = VfAe + Vr (Af - Ad Ae = (Dc25)(Dc⁴⁰⁾

= (5.2)(6.6)

=

^{34 ft2}

Af = (hood length)(hood width)

= (11.6)(13.0)

=

^{151 ft2}

Of

=

(151 )(34) + 100(151 - 34)

=

^{5134 + 11,700}

=

^{16,834 cfm}

3.9.3 Low Canopy Hoods: If the distance between the hood and the hot source does not exceed approximately the diameter ofthe source or 3 ft, whichever is smaller, the hood may be considered a low canopy hood. Under such conditions, the diameter or cross-section of the hot air column will be approximately the same as the source. The diameter or side dimensions ofthe hood therefore need only be I ft larger than the source.

The total flow rate for a circular low canopy hood is

where:

Ot =

total hood air flow, cfm Of

=

diameter of hood, ft

6t

=

difference between temperature of the hot source, and the ambient, F.

The total flow rate for a rectangular low hood is Ot = 6.2 b ^1.33 6t°.42

L where:

Ot

=

total hood air flow, cfm

L

=

length of the rectangular hood, ft b

=

width of the rectangular hood, ft

6t

=

difference between temperature of the hot source and the ambient, F.

REFERENCES

3.1. Brandt, A.D.: Industrial Health Engineering. John Wiley and Sons, New York (1947).

3.2. Kane, J.M.: Design of Exhaust Systems. Health and Ventilating 42:68 (November 1946).

3.3. Dalla Valle, J.M.: Exhaust Hoods. Industrial Press, New York (1946).

3.4. Silverman, L.: Velocity Characteristics of Narrow Ex- haust Slots. J. Ind. Hyg. Toxico\. 24:267 (November 1942).

3.5. Silverman, L.: Center-line Characteristics of Round Openings Under Suction. 1. Ind. Hyg. Toxico!. 24:259 (November 1942).

3.6. Piney, M.; Gill, F.; Gray,

c.;

et al.: Air Contaminant Control: the Case History Approach - Learning From the Past and Looking to the Future. In: Ventilation '88, 1. H.

Vincent, Ed., Pergammon Press, Oxford, U.K. (1989).

3.7. Ljungqvist, B.: Some Observations on the Interaction

Between Air Movements and the Dispersion of Pollu- tion. Document 08: 1979. Swedish Council for Build- ing Research, Stockholm, Sweden (1979).

3.8. Kim, T.; Flynn, M.R.: Airflow Pattern Around a Worker in a Uniform Freestream. Am. Ind. Hyg. As- soc. 1. 52:(7): 187-296 (1991).

3.9. George, D.K.; Flynn, M.R.; Goodman, R.: The Impact of Boundary Layer Separation on Local Exhaust De- sign and Worker Exposure. App!. Occup. Env. Hyg.

5:501-509 (1990).

3.10. Heriot, N.R.; Wilkinson, 1.: Laminar Flow Booths for the Control of Dust. Filtration and Separation 16:2:159-164 (1979).

3.11. Flynn, M.R.; Shelton, W.K.: Factors Affecting the Design of Local Exhaust Ventilation for the Control of Contaminants from Hand-held Sources. App\. Oc- cup. Env. Hyg. 5:707-714 (1990).

3.12. Tum Suden, K.D.; Flynn, M.R.; Goodman, R.: Com- puter Simulation in the Design of Local Exhaust Hoods for Shielded Metal Arc Welding. Am. Ind. Hyg.

Assoc. J., 51(3):115-126 (1990).

3.13. American Welding Society: Fumes and Gases in the Welding Environment. F. Y. Speight and H. C. Camp- bell, Eds. A WS, Miami, FL (1979).

3.14. American Society of Mechanical Engineers: Power Test Code 19.2.4: Liquid Column Gages. ASME (1942).

3.15. Hemeon, W.C.L.: Plant and Process Ventilation. In- dustrial Press, New York (1963).

3.16. Alden, 1.L.: Design of Industrial Exhaust System.

Industrial Press, New York (1939).

3.17. Rajhans, G.S.; Thompkins, R.W.: Critical Velocities of Mineral Dusts. Canadian Mining 1. (October 1967).

3.18. Djamgowz, O.T.; Ghoneim, S.A.A.: Determining the Pick-Up Air Velocity of Mineral Dusts. Canadian Mining J. (July 1974).

3.19. Baliff, 1.L.; Greenburg, L.; Stern, A.C.: Transport Velocities for Industrial Dusts - An Experimental Study. Ind. Hyg. Q. (December 1948).

3.20. Dalla Valle, J.M.: Determining Minimum Air Veloci- ties for Exhaust Systems. Heating, Piping and Air Conditioning (1932).

3.21. Hatch, T.F.: Economy in the Design of Exhaust Sys- tems.

3.22. Hughes, R.T.: Design Criteria for Plating Tank Push- Pull Ventilation. In: Ventilation '85. Elsevier Press, Amsterdam (1986).

3.23. Baturin, V.V.: Fundamentals ofindustrial Ventilation.

Pergamon Press, New York (1972).

3.24. U.S. Public Health Service: Air Pollution Engineering Manual. Publication No. 999-AP-40 (1973).

3.25. Burgess, W.A.; Ellenbecker, M.1.; Treitman, R.D.:

Ventilation for Control of the Work Environment,

John Wiley & Sons, New York (1989).

3.26. Braconnier, R: Bibliographic Review of Velocity Fields in the Vicinity of Local Exhaust Hoods. Am.

Ind. Hyg. Assoc. J., 49(4): 185-198 (1988).