Dust extraction hoods and duct network for metal grinding, polishing and machine-tool enclosures (local exhaust ventilation)

01

Exhaust volume by wheel, disc and belt size, and face velocity at grinding enclosures

For dry grinding, polishing and buffing where exposure would exceed the permissible limits, OSHA's 29 CFR 1910.94(b) requires local exhaust and fixes minimum exhaust volumes per hood. For grinding and abrasive cut-off wheels on floor stands, pedestals and benches (Table G-4): 220 cfm for wheels up to 9 inches diameter (1½ inch wide), 390 cfm over 9 to 16 inches, 500 cfm over 16 to 19, 610 cfm over 19 to 24, 880 cfm over 24 to 30 and 1,200 cfm over 30 to 36 inches. [1]

Buffing, polishing and scratch-brush wheels need more air for the same diameter (Table G-5): 300 cfm up to 9 inches, 500 cfm over 9 to 16, 610 over 16 to 19, 740 over 19 to 24 and 1,040 over 24 to 30 inches. Grinding and polishing belts run from 220 cfm for belts up to 3 inches wide to 740 cfm for belts over 11 to 13 inches (Table G-9). Wider wheels scale by width: the regulation's own example is 4.5 ÷ 4 × 610 = 686, rounded to 690 cfm. [1]

Hood design is part of the rule: hoods must be placed so particles fall or are thrown into the hood in the direction of air flow, the adjustable tongue on bench and pedestal grinder hoods must be kept within ¼ inch of the wheel periphery, portable grinding is to be done in a partial enclosure with an average face velocity of at least 200 fpm where the work permits, and cradle and swing-frame work needs at least 150 fpm over the whole enclosure opening. [1]

HSE's HSG258 places grinding, cutting and abrasive blasting in its highest-energy release class, with capture velocities of 2.5 to more than 10 m/s, and notes these are difficult to control with capturing hoods; for enclosing hoods — which a machine-tool enclosure is — it asks for a face velocity of at least 0.4 m/s unless a lower value is shown to be effective. OSHA's Technical Manual adds that a hood should sit no more than 1.5 duct diameters from the source. [2] [3]

Editorial calculation: 220 cfm ≈ 374 m³/h, 610 cfm ≈ 1,036 m³/h and 1,200 cfm ≈ 2,039 m³/h (1 cfm = 1.699 m³/h); 200 fpm ≈ 1.02 m/s and 150 fpm ≈ 0.76 m/s, both above HSE's 0.4 m/s enclosure minimum. [1] [2]

List every hood for the supplier: machine type, wheel diameter and width or belt width, whether the work sits inside or outside the hood, and enclosure openings with their areas.

02

4,500 fpm in the branch, 3,500 fpm in the main, and what hood entry losses cost

OSHA's grinding rule recommends a minimum duct velocity of 4,500 fpm in the branch and 3,500 fpm in the main, and sets hood entry losses at 0.65 velocity pressure for a straight take-off and 0.45 velocity pressure for a tapered take-off (the vertical-spindle disc grinder hood has its own figure). [1]

HSE's Table 12 gives around 20 m/s for process dusts including grinding dust and around 25 m/s for large, aggregating or damp particles such as metal turnings. For multi-branch systems it asks for the required velocity to reach the air mover from the hood furthest away in distance or resistance, with enough fan power when the maximum number of hoods is in use; where unused hoods are isolated it lists a variable-speed fan drive holding constant duct static pressure among the options. [2]

OSHA's Technical Manual lists what pulls flow down in such a network: plugged or dented ducts, slipping fan belts, open access doors, holes in ducts, closed or stuck blast gates, and a fan turning backwards. At the fan it gives the six-and-three rule — six diameters of straight duct at the inlet and three at the outlet — and a 20% airflow loss for each 2½ diameters of straight duct between the fan inlet and an elbow. [3]

Editorial calculation: 4,500 fpm ≈ 22.9 m/s and 3,500 fpm ≈ 17.8 m/s, so OSHA's branch figure sits between HSE's 20 and 25 m/s classes. A 390 cfm hood (a 9–16 inch wheel) at 4,500 fpm needs a branch area of 390 ÷ 4,500 = 0.087 ft², a diameter of about 4.0 inches (101 mm). Assuming air at 1.2 kg/m³, velocity pressure at 22.9 m/s is 0.5 × 1.2 × 22.9² ≈ 314 Pa, so a straight take-off loses 0.65 × 314 ≈ 204 Pa at the hood and a tapered one 0.45 × 314 ≈ 141 Pa. [1] [2]

03

Which hood each grinding, buffing, belt and machine-tool source gets

04

Hood with adjustable tongue, branch pipe, header, blast gates, fan and stack

05

Exhaust volumes, duct velocities and static pressure to put in the quotation

06

List each hood with its machine and wheel or belt size

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machines: [bench or pedestal grinders; buffing wheels; belts; disc grinders; enclosed machine tools; hand sanding]
Sizes: [wheel diameter and width; belt width; enclosure openings]
Layout: [hood positions; distance to collector]
Ducts: [branch and main velocities; take-off shape]
Fan: [static pressure; variable speed]
Testing: [commissioning method; records]

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