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Calcimator

Local Exhaust Design Calculator

Calculate required hood airflow, duct size, and face velocity for local exhaust ventilation systems using ACGIH capture velocity principles.

Inputs

Results

Required airflow (CFM)

975

Face velocity (fpm)325
Calculated duct dia. (in)7.1
Standard duct size (in)7.5
Actual duct velocity (fpm)3,178
Hood entry loss (in. wg)0.59
Required M3Hr1,656.53
How to Use This Calculator
  1. Enter Capture velocity (fpm), Distance to source (ft), and Hood width (ft).
  2. Set Hood height (ft), Flanged? (1=Yes, 0=No), and Design duct velocity (fpm).
  3. Review the Required airflow (CFM) result.
  4. Use Face velocity (fpm) and Calculated duct dia. (in) to inform your decision.

What each input means

Capture velocity (fpm)
Required capture velocity at the contaminant source. ACGIH recommends 50-100 fpm for low-velocity releases, 100-200 fpm for active generation, 200-500 fpm for high-velocity releases.
Distance to source (ft)
Distance from the hood face to the point of contaminant generation.
Hood width (ft)
Width of the hood opening in feet.
Hood height (ft)
Height of the hood opening in feet.
Flanged? (1=Yes, 0=No)
Flanged hoods reduce required airflow by ~25% by preventing air from being drawn from behind the hood.
Design duct velocity (fpm)
Target duct transport velocity. 3500-4000 fpm for light dust, 4000-4500 for medium dust, 4500+ for heavy particles.

What each result means

Required airflow (CFM)
Total exhaust airflow needed at the hood.
Face velocity (fpm)
Average air velocity across the hood opening.
Calculated duct dia. (in)
Exact duct diameter calculated from airflow and duct velocity.
Standard duct size (in)
Next standard duct diameter rounded up to nearest 0.5 inch.
Actual duct velocity (fpm)
Actual velocity in the standard-sized duct.
Hood entry loss (in. wg)
Estimated static pressure loss at the hood entry.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Capture velocity (fpm) = 100, Distance to source (ft) = 1, Hood width (ft) = 2, Hood height (ft) = 1.5 = 6 input(s) provided
  2. Calculate Required airflow
    975 = 975
  3. Calculate Face velocity
    325 = 325
  4. Calculate Calculated duct dia.
    Calculated duct dia. = ductDiameterFt * 12
    7.1 = 7.1

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