Capture Flow (Local Exhaust)
Calculate the capture flow of an unflanged local exhaust hood, Q = V·(10·X² + A), from the capture velocity V (m/s), the source-to-hood distance X (m) and the hood face area A (m²). The result, in m³/s, is the flow needed for contaminated air to be drawn into the hood with enough velocity to overcome air currents and the contaminant's inertia. The Della Valle/ACGIH equation shows the flow grows with the square of distance — hoods should be as close to the source as possible. It is the basis of local exhaust ventilation design. Enter the capture velocity, the distance and the hood area.
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Capture flow rate (local exhaust ventilation)
Local exhaust ventilation (LEV) is the most efficient way to control airborne contaminants: instead of diluting them once they have spread, it captures them right at the source, before they reach the worker's breathing zone — think of a canopy hood over a degreasing tank, or a hood next to a bench grinder. The challenge is to create, at the position of the source, an air velocity (the capture velocity) high enough to draw the contaminant into the hood, overcoming room air currents and the momentum of the contaminant itself. For a simple unflanged hood, the Della Valle equation (adopted by the ACGIH) gives the required flow rate: Q = V·(10·X² + A), where V is the desired capture velocity at the source (m/s — typically 0.25 to 0.5 m/s for gases in still air, and up to 2.5 m/s or more for dust generated with energy), X is the distance from the source to the hood face (m) and A is the hood opening area (m²). The result is the flow rate in m³/s. The crucial insight sits in the 10·X² term: the required flow grows with the square of the distance — moving the hood just 2× farther from the source demands about 4× the flow (and more energy, bigger fans, higher cost). Hence the golden rule of LEV: keep the hood as close as possible to the source, and use flanges and enclosures, which cut the required flow by up to 25% by concentrating the suction where it matters. Enter the capture velocity, the distance and the hood area.
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