Core Calculations in Fume and Dust Extraction Systems

Sizing an extraction system follows a specific sequence of calculation steps; skip any one of them and everything downstream rests on a flawed foundation.

1. Required Velocity

The first step is establishing the required capture velocity or hood face velocity based on how the contaminant is released (see the velocity ranges in ACGIH-type references). This is the base value everything else in the design rests on.

2. Required Flow

Working from the target velocity and hood geometry, the required volumetric flow rate (Q) is calculated.

3. Transport Velocity

Air velocity inside the duct needs to stay high enough to keep particulate suspended; otherwise dust settles in the duct, creating blockage and fire risk. The required transport velocity depends on the material's density, and the commonly cited ranges vary somewhat between references — so rather than relying on a single fixed number, a value verified against a current engineering reference (ACGIH tables, for example) for the specific material should be used.

4. Total Pressure Loss

Duct friction loss (dependent on length, diameter, velocity, and internal roughness), elbow/transition losses, hood entry loss, and filter resistance (both clean and partially loaded conditions) are summed to find the system's total static pressure loss.

5. Fan Selection

With the calculated Q and total pressure, a fan is selected whose manufacturer curve satisfies that point while remaining within its efficient operating range.

Conclusion

This calculation chain provides a starting design point; confirming actual field performance is completed with measurement at commissioning. Calculation and measurement aren't alternatives to each other — they're two steps that complete one process.

Frequently Asked Questions

Which single variable matters most? — None on its own is sufficient; the sequence of calculations matters as a whole.

Why does transport velocity matter? — At low velocity, dust settles in the duct, creating blockage and fire risk.

References

●        ACGIH — Industrial Ventilation: A Manual of Recommended Practice for Design.

●        General local exhaust ventilation (LEV) design practice.