Tools › Building Systems › Ventilation and Air Quality › Industrial Ventilation and Exhaust Hoods › Example
Two-hood local exhaust system (flanged slot over a bench, canopy over a hot process): hood flows, branch sizing, junction balance by design, fan pressure and the MIL-HDBK-1003/17C xylene dilution case
| Tool | Industrial Ventilation and Exhaust Hoods |
| Source | Naval Facilities Engineering Command (1996). Industrial Ventilation Systems. MIL-HDBK-1003/17C, Department of the Navy, Alexandria, VA. Appendix A (design calculation examples, pp. 108–115), Appendix B (dilution ventilation for xylene, p. 116) and Appendix D (non-standard air, pp. 118–119); Hagopian, J. H., and Bastress, E. K. (1976). Recommended Industrial Ventilation Guidelines. DHEW (NIOSH) Publication No. 76-162, Cincinnati, OH (hood relations, Figures 5 to 7, pp. 34–36); Occupational Safety and Health Administration (1999). OSHA Technical Manual, Section III, Chapter 3: Ventilation Investigation (entry loss, coefficient of entry, dilution and stack rules). |
| Level | Professional |
Problem
A flanged slot hood 4 ft long over a bench must capture at 150 ft/min 1 ft from its face, and a four-sided canopy of 14 ft perimeter sits 2 ft above a hot process needing 75 ft/min. The slot branch (12 ft, ΣC 0.5, tapered takeoff F_h 0.25, slot velocity 2,000 ft/min with a slot loss of 1.78 slot VP) runs at a 2,000 ft/min transport velocity and the canopy branch (20 ft, ΣC 0.9, F_h 0.25) at 2,500 ft/min; the main runs 8 ft (ΣC 0.15) to the second junction and 30 ft (ΣC 0.3) to a 3.0 in. w.g. air cleaner, then 10 ft of outlet duct and a 20-ft, 16-in stack 30 ft above grade on a 20-ft building in a 15 mph wind; fan efficiency 0.65, motor 0.90; galvanized duct, standard air. Find each hood's flow, the branch sizes, velocity pressures and hood static pressures, balance the junction by design, size the main, find the fan static pressure and brake horsepower, check the stack, and find the dilution flow for 2 pints per hour of xylene (1.79 lb/h, MW 106.2) to 100 ppm with a mixing factor of 3 in a 3,000 ft³ room, the purge time to 10 ppm and the flow for 25 percent of the 1.1 percent LEL.
Inputs to Enter
- Air 0.075 lb/ft³, galvanized duct (ε 0.0003 ft), balance by design, slot velocity 2,000 ft/min, slot loss factor 1.78
- Hood 1 Bench slot: flanged slot, V_c 150 ft/min, X 1.0 ft, L 4 ft, V_t 2,000 ft/min, branch 12 ft, ΣC 0.5, F_h 0.25; Hood 2 Canopy: V_c 75 ft/min, H 2.0 ft, P 14 ft, V_t 2,500 ft/min, branch 20 ft, ΣC 0.9, F_h 0.25; branch diameters sized by the tool
- Main M1 8 ft ΣC 0.15, M2 30 ft ΣC 0.3 (sized); air cleaner 3.0 in. w.g.; outlet duct 10 ft ΣC 0.15; stack 16 in, 20 ft, exit coefficient 1.0; fan efficiency 0.65, motor 0.90; stack top 30 ft, building 20 ft, intake 20 ft, wind 15 mph
- Dilution: 1.79 lb/h evaporated, MW 106.2, 70 °F, limit 100 ppm (xylene PEL), K 3, room 3,000 ft³, C_0 0, 60 min, purge to 10 ppm, LEL 1.1 %, SF 4, B 1; sensible heat 20,000 Btu/h at a 10 °F rise; makeup: no other supply, 68 °F indoors, 10 °F outdoors
Answers
- Hood flows: Q_1 = 2.6 × 4 × 150 × 1 = 1,560 cfm; Q_2 = 1.4 × 14 × 2 × 75 = 2,940 cfm
- Branch 1: 12.0 in computed, 11 in chosen, V = 2,364 ft/min, VP = 0.348 in. w.g.; slot area 0.78 ft², slot loss 1.78 × 0.249 = 0.443; h_e = 0.530, SP_h = 0.878 in. w.g., C_e = 0.629; friction 0.080, fittings 0.174 → 1.132 in. w.g. at the main. Branch 2: 14 in, 2,859 ft/min after correction, SP_h 0.636, 1.237 in. w.g. at the main
- Junction 2: main 1.238 vs branch 1.145 in. w.g., ratio 1.081, correction √1.081 = 1.0396, Q_2 raised to 3,056 cfm; main M1 11 in (0.106 in. w.g.), M2 18 in at 2,612 ft/min with an acceleration term of 0.077 in. w.g.; SP entering the air cleaner 1.576 in. w.g.
- Fan: Q = 4,616 cfm, SP_in 4.576, VP_in 0.425, SP_out 0.344, VP_out 0.680 in. w.g.; FTP = 5.175, FSP = 4.495 in. w.g.; air power 3.77 hp, brake 5.79 hp, motor input 6.44 hp; stack exit 3,306 ft/min (≥ 3,000 and 2.5 × the wind), stack top 30 ft = 1.5 × 20 ft
- Dilution: vapour 0.1087 cfm (387 ft³/lb-mol), Q = 1,087 cfm at K = 1 (MIL-HDBK-1003/17C Appendix B prints 1,088), 3,260 cfm at K = 3; τ = 2.76 min, purge 100 → 10 ppm in 6.36 min; LEL flow 39.5 cfm (handbook 40); heat 1,850 cfm; makeup 4,616 cfm, heating 289,500 Btu/h (84.8 kW)
Notes
The tool's default inputs are this system; press Submit. The two-hood system is illustrative (stated on the page); its arithmetic is checked by test_industrial_ventilation.php (320 checks) against an independent numpy / scipy script (References/Industrial-Ventilation/checks/) and the method against the MIL-HDBK-1003/17C Appendix A plating-hood examples (3,375 cfm, 15-in duct at 2,751 ft/min, hood SP 1.09 in. w.g.; 6,000 / 4,560 / 5,400 cfm cases), the Appendix B xylene dilution (1,088 and 40 cfm), Appendix D (268 acfm, FSP 4.02 in. w.g.) and the 29 CFR 1910.94(c)(6)(ii) toluene example (8,564 ft³ per gallon).