Local Exhaust Capture Velocity Calculator

Work out local exhaust capture velocity instantly with clear inputs, formula shown and shareable results.

About Local Exhaust Capture Velocity Calculator

For a hood drawing on a source at distance X, the classic relation Q = V x (10 X² + A) reflects that air converges from a hemisphere in front of the opening. Adding a flange blocks air drawn from behind and reduces the required flow by about twenty-five percent for the same capture velocity, which is the cheapest improvement available.

Formulas

Capture airflow

Q = coefficient x V x (10 X² + A)

V: Capture velocity, m/s
X: Distance to source, m
A: Hood face area, m²

Flange effect

Flanged hoods use a coefficient of 0.75

Worked example

With the default values below, Local Exhaust Capture Velocity Calculator returns Exhaust airflow required of 0.4500. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hood face areaminimum 0.001 m²: 0.3 m²
Distance from hood to sourceminimum 0.01 m: 0.3 m
Required capture velocityminimum 0.05 m/s: 0.5 m/s
Hood typeone of Plain opening, freely suspended, Flanged opening: Flanged opening

Results

Exhaust airflow required: 0.4500 m³/s
Airflow in cubic feet per minute: 953 CFM
Resulting hood face velocity: 1.500 m/s
Flange coefficient applied: 0.75

Step by step

1Values used
2Capture airflow
3Flange effect
4Exhaust airflow required= 0.4500 m³/s
5Airflow in cubic feet per minute= 953 CFM
6Resulting hood face velocity= 1.500 m/s
7Flange coefficient applied= 0.75

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Local exhaust ventilation must be designed by a competent engineer to a recognised industrial ventilation standard and commissioned by measurement.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Local exhaust ventilation must be designed by a competent engineer to a recognised industrial ventilation standard and commissioned by measurement.

Frequently Asked Questions

How is the Local Exhaust Capture Velocity figure worked out?

Q = coefficient x V x (10 X² + A). For a hood drawing on a source at distance X, the classic relation Q = V x (10 X² + A) reflects that air converges from a hemisphere in front of the opening.

Why does distance matter so much?

The 10X² term means airflow rises with the square of distance. Doubling the standoff quadruples the exhaust needed, so bringing the hood closer is the single most effective change.

What values do I need to enter?

This calculator takes 4 inputs: Hood face area, Distance from hood to source, Required capture velocity, Hood type. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.