What is the difference between dilution and local exhaust ventilation?

Dilution reduces contaminant concentration by mixing with fresh air, while local exhaust captures contaminants at their source before they spread into the breathing zone.

How is the required capture velocity determined?

It depends on the momentum of the contaminant being generated and the surrounding room air currents; higher momentum processes like grinding require higher capture velocities (up to 10 m/s).

Is a make-up air system mandatory for industrial exhaust?

Yes, to prevent negative pressure which can cause reverse draft in flues, doors to slam, and drastically reduce the performance of the exhaust fans.

IS 3103

: 1975

Code of Practice for Industrial Ventilation

CurrentSpecializedCode of PracticeBIMMEP · Functional Requirements in Buildings

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This code lays down the standard practices for designing and implementing natural and mechanical ventilation systems in industrial environments. It provides essential guidelines on local exhaust ventilation, dilution ventilation, and heat control to maintain occupational health, safety, and worker comfort.

Code of Practice for Industrial Ventilation

Quick Reference Values

capture velocity evaporation0.25 to 0.5 m/s

capture velocity welding plating0.5 to 1.0 m/s

capture velocity crushing grinding2.5 to 10.0 m/s

min distance exhaust to intake6 meters (typical thumb rule for contamination prevention)

Key Formulas

Q = V × A — Exhaust volume (Q) calculation where V is capture velocity and A is hood face area

Q = H / (Cp × ρ × ΔT) — Airflow required for sensible heat removal

Practical Notes

Always prioritize local exhaust ventilation (LEV) over general dilution ventilation for toxic dust and fumes to minimize worker exposure.

Ensure adequate make-up air is supplied to prevent the factory building from operating under severe negative pressure, which reduces exhaust fan efficiency and causes drafts.

Roof extractors and ridge ventilators are highly effective for natural ventilation in high-heat industries like foundries due to thermal buoyancy (stack effect).