📊 Required Hood Airflow
📐 Hood Face Velocity
Calculation Breakdown
Full Calculation Table
| Parameter | Value | Result |
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How to Use the Spot Ventilation Calculator
Choose a plain opening, flanged opening, slot hood, flanged slot or canopy hood. Flanged hoods need less airflow than the same opening without a flange.
Enter the distance from the source to the hood, plus the opening area for a plain or flanged hood, the slot length and width for a slot, or the perimeter for a canopy.
Choose a target capture velocity that matches how forcefully the contaminant is released into the air.
Click Calculate Hood Airflow to see the required exhaust airflow and the velocity through the hood opening.
Why distance matters more than almost anything else
Spot ventilation, also called local exhaust ventilation, creates enough air velocity at the contaminant source to pull it into a hood before it reaches the room. The Canadian Centre for Occupational Health and Safety ranks enclosing hoods as preferred whenever possible and describes capturing hoods, which sit next to a source without surrounding it, as the type this calculator covers. It also notes that the airflow needed varies with the square of the distance from the source, so moving a hood from 2 in to 4 in away takes four times the airflow.
The hood equations used here
The tool uses the standard industrial ventilation relationships. For a plain opening, Q = V × (10X² + A), where V is the capture velocity, X the distance from the opening and A its area. A flanged opening uses 0.75 of that. A slot hood uses Q = 3.7 × L × V × X, where L is the slot length, and 2.6 × L × V × X when it is flanged. A canopy over a source uses Q = 1.4 × P × D × V, with P the perimeter and D the height above the source. The constants come from decades of testing, not first principles, and the plain-opening equation applies within about 1.5 hood diameters of the face.
Worked example: a 0.09 m² plain opening 0.3 m from the source with a capture velocity of 0.75 m/s needs Q = 0.75 × (10 × 0.3² + 0.09) = 0.74 m³/s, which is 742 L/s or about 1,570 CFM. Doubling the distance to 0.6 m raises it to 2,768 L/s, 3.7 times as much. Adding a flange cuts the original 742 L/s to 557 L/s.
Choosing the right capture velocity
What matters is how the contaminant enters the air, not how toxic it is. A solvent evaporating quietly from a tank needs far less than chips thrown off a grinding wheel, even if the grinding dust is less hazardous. The presets follow the commonly published ranges: 0.25 to 0.5 m/s for quiet release, 0.5 to 1 m/s for low-velocity release, 1 to 2.5 m/s for active generation and 2.5 to 10 m/s for high-velocity release. Use the upper end of a range for toxic materials or a drafty room. For plant-wide process ventilation rather than a single source, the industrial HVAC calculator covers the wider picture.
Slot hoods and canopy hoods
A slot hood suits a long, narrow source such as a tank edge. The slot should be no wider than 0.2 times its length, and the air should pass through it fast enough to draw evenly along its length. The OSHA Technical Manual defines slot velocity as total flow divided by slot area and gives 2,000 fpm (about 10 m/s) as the usual value. The same chapter describes a canopy hood as an overhead hood that receives rising hot air or gas and calls it inappropriate for toxic materials, so use the canopy option for hot or non-toxic sources only.
After the hood: duct, fan and makeup air
Once the hood airflow is known, size the duct and fan with the exhaust fan sizing calculator, and use the duct velocity pressure calculator to work out the velocity pressure that sets the hood entry loss. Check whether the building needs dedicated makeup air to replace what the hood removes with the makeup air calculator. For laboratory fume hoods, see the laboratory ventilation calculator.
When a hood isn't practical
Some sources move around too much, or a hood would get in the way of the work. Dilution ventilation is the fallback, at the cost of moving far more air for the same protection. Compare the two with the dilution ventilation calculator.
Frequently Asked Questions
Capture velocity is the air speed needed at the point where the contaminant is released to overcome room air currents and carry it into the hood. It is measured at the source, not at the hood face, which is why the distance between source and hood drives the calculation. The Canadian Centre for Occupational Health and Safety gives about 0.5 m/s (100 fpm) as a general figure when there is little cross-draft, with grinding and other fast releases needing 5 to 10 times that.
For an open hood, the airflow needed grows with the square of the distance from the source, because the suction spreads out in all directions in front of the opening. Moving a hood from 2 in to 4 in away takes four times the airflow for the same capture. This tool's plain-opening equation includes a hood area term, so the increase is a little smaller when the opening is large compared with the distance. A slot hood grows in direct proportion to distance, not its square. Moving the hood closer is almost always cheaper than a bigger fan.
The commonly published ranges follow how the contaminant is released. Released with almost no velocity into quiet air, such as evaporation from a tank, use 0.25 to 0.5 m/s (50 to 100 fpm). Released at low velocity into moderately still air, such as welding or low-speed transfers, use 0.5 to 1 m/s (100 to 200 fpm). Active generation into rapid air motion, such as spray painting, takes 1 to 2.5 m/s (200 to 500 fpm). Fast releases like grinding take 2.5 to 10 m/s (500 to 2,000 fpm). Within a range, use the upper end for toxic materials, cross-drafts or a small hood.
A flange is a flat collar around the opening that stops the hood drawing air from behind it, which does no capture work. A flanged opening needs about 75 percent of the airflow of the same plain opening to give the same capture velocity, and a flanged slot needs about 70 percent of a plain slot. The effect holds only if the flange is wide enough, so size it at least as wide as the hood opening.