Industrial Ventilation Hood CFM Calculator

Determine exhaust hood volumetric airflow (CFM), capture velocity, hood static entry loss, and transport duct sizing per ACGIH Industrial Ventilation standards.

Hood Configuration & Contaminant

Hood Geometry & Mounting:

Required Capture Velocity Vc (FPM):

Distance to Source X (inches):

Hood Face Width (inches):

Hood Face Height / Slot Length (in):

Duct Transport Standards

Contaminant Transport Minimum Velocity:

Ventilation Exhaust Sizing

Required Exhaust Airflow (Q)

-- CFM

-- round duct size

Hood Face Velocity

-- FPM

at hood opening

Entry Static Loss (he)

-- in. w.g.

Hood suction loss

Hood Opening Area: -- sq ft

Duct Transport Velocity: -- FPM

Flange Benefit: 25% CFM reduction

Sizing per ACGIH Industrial Ventilation equations ensures effective contaminant capture at the worker breathing zone.

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Frequently Asked Questions

Why does adding a flange to an exhaust hood reduce required airflow by 25%?

A plain duct opening draws air equally from all directions (a full 360° sphere), wasting fan CFM pulling clean air from behind the hood face. A flange blocks airflow from the rear, focusing suction entirely forward into a 180° hemisphere and slashing required fan power by 25%.

Why does capture velocity decrease with the square of distance (1/X²)?

In exhaust ventilation, suction velocity drops precipitously away from the opening because air expands radially across a widening hemisphere. At just one duct diameter away, capture velocity drops to only 10% of hood face velocity.

What is minimum transport velocity in industrial ductwork?

Minimum transport velocity is the continuous airspeed required inside ductwork to keep solid particulate suspended in the air stream. If velocity drops below threshold (e.g. 3,500 FPM for wood dust or 4,500 FPM for metal grindings), dust settles inside ducts, creating extreme fire and explosion hazards.