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HVAC & Airflow

Fume Hood Sash Airflow Calculator

Exhaust airflow for a lab fume hood by sash opening.

Enter your values

ⓘ Live results

Results
Required exhaust airflow
600 CFM
Sash open area
6 sq ft
At full sash (say 2x height)
1,200 CFM
VAV note
Variable air volume hoods track this in real time as the sash moves

Quick answer

Laboratory fume hoods typically hold 80-120 FPM face velocity at the sash opening; airflow must rise as the sash is opened wider.

CFM = sash width x sash height x face velocity

How to use the Fume Hood Sash Airflow Calculator

  1. 1Enter the sash width and current opening height.
  2. 2Enter your lab's target face velocity, commonly 100 FPM.
  3. 3Use a VAV hood controller so airflow tracks sash position automatically for energy savings.

Worked example

Using the values the calculator loads with:

Inputs

  • Sash width4 ft
  • Sash opening height1.5 ft
  • Target face velocity100 FPM

Results

  • Required exhaust airflow600 CFM
  • Sash open area6 sq ft
  • At full sash (say 2x height)1,200 CFM
  • VAV noteVariable air volume hoods track this in real time as the sash moves

Frequently asked questions

Constant air volume vs VAV?

CAV hoods hold airflow constant and face velocity varies with sash position; VAV hoods hold face velocity constant and vary airflow, saving energy.

What if velocity is too low?

Contaminants can escape into the room; check for obstructions, sash position and exhaust fan performance.

About this calculator

Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.