Industrial Exhaust System Design: Define the Process Before the Fan

An industrial exhaust system should capture a contaminant where it forms and carry it safely to an air cleaner or a discharge point. The design starts with the process, not a fan catalogue: what is released, where, how and how often. Choose the fan first and you are guessing at everything that decides whether it works. This guide covers capture versus air changes, duct and filter losses, the hazardous processes that need specialist input, and the discharge and commissioning scope to write into the contract.

1. Contaminant capture versus air changes

General (dilution) ventilation mixes clean air into the whole room and removes it again. It suits heat and low-level, widely spread contaminants. Local exhaust ventilation captures the contaminant at the source, before it reaches the operator's breathing zone. NBC 2016 Part 8 gives recommended air changes and contaminant limits for ventilation air, but a room air-change figure does not size the capture of a fume plume at a welding bench or a powder tip. That airflow depends on the contaminant, how it is released, the distance from the hood and the cross-draughts around it.

Start with a process survey. Walk the line with operations and maintenance staff, mark each source on a current plan, and record it against this checklist.

The hood follows from the release. An enclosing hood surrounds the source; a capturing hood draws the contaminant towards an opening; a receiving hood sits in the path of a plume that already moves in a known direction, such as the heat rising off a hot tank. Ask suppliers to show each hood position and the capture basis behind it, and how the operator reaches the work without blocking it.

2. Duct and filter losses decide the fan

• Velocity and size. Airflow equals duct velocity multiplied by duct area (Q = V × A). A duct that is too large lets dust settle; one that is too small raises resistance and fan energy.
• System resistance. Set the flow at each hood, combine the branches that run together, then add the resistance of duct length, elbows, branches, dampers, the air cleaner and the stack. Select the fan at that airflow and pressure, not by a headline motor rating.
• Filters load up. A filter's pressure drop rises as it collects dust, so the fan has to deliver the design airflow at the filter's design final pressure drop, not only when the filter is clean. A differential pressure gauge across each filter tells operators when to clean or change it.
• Balance by design. Dampers can trim branches, but they cannot fix a poorly placed hood or an undersized fan.
• Materials. Galvanised sheet is common; corrosive vapours, hot air or abrasive dust may need other materials or linings. The specification should state the material for each section.
• Air cleaning. Choose it for the contaminant and for where the air goes next: cyclones and bag filters for particles, a treatment method suited to the gas for vapours and gases.
• Make-up air. Every cubic metre exhausted must be replaced. Plan where replacement air enters so it does not blow across the hoods, keep the intended pressure relationship with neighbouring rooms, and include the heating or cooling load of that air in the HVAC design.
• Energy. Exhaust fans run long hours. Variable frequency drives on fans are among the measures in the Bureau of Energy Efficiency's retrofit guidance, but they need a control sequence that keeps capture and room pressure at part load.

The ventilation and air-change calculator gives a first general-ventilation airflow for a room; capture at each hood still has to be designed from the process.

3. Hazardous processes need specialist requirements

Some processes put the exhaust system itself at risk. These are not decisions for the ventilation contractor alone; the process owner, a specialist and, where licensing applies, the authority decide them.

• Flammable vapours and gases (painting, solvent cleaning, fuel handling). The area is classified into hazardous zones, and fan motors, controls and lighting must be suitable for the zone. In premises covered by the Petroleum Rules, 2002, rule 102 requires electrical apparatus in those areas to be approved by the Chief Controller of Explosives at the Petroleum and Explosives Safety Organisation (PESO). The airflow should keep vapour concentrations well below the flammable range.
• Combustible dusts (flour, sugar, wood, some plastics, and metal powders such as aluminium). Collectors and ducts can carry an explosion risk; explosion venting or isolation, earthing and bonding, and duct velocities that stop dust settling are specialist decisions.
• Toxic or corrosive fumes (plating, pickling, chemical processing). Corrosion-resistant ducts and fans, scrubbing where required, and ductwork inside the building kept under negative pressure so any leak is inward.
• Hot processes (furnaces, ovens, heat treatment). High-temperature materials, allowance for expansion, and fire separation where ducts pass through fire-rated walls.
• Commercial kitchens. Grease-laden exhaust is a fire risk, with its own filtration, cleaning access and suppression requirements; see our guide to kitchen exhaust and fire suppression.

4. Discharge and commissioning scope

Discharge the cleaned air away from air intakes, openable windows and neighbouring buildings, at a height and velocity that carry it clear of the roof. Where the Air (Prevention and Control of Pollution) Act, 1981 applies, the State Pollution Control Board's consent can specify the control equipment and the chimney the plant must install, so design to the consent conditions; our CTE and CTO guide covers the process.

Write the acceptance checks into the purchase scope before work starts:

1. Testing, adjusting and balancing (TAB). Measure the airflow at each hood and branch and the fan duty, then adjust to the agreed design basis.
2. Capture checks. Show with smoke visualisation that each hood captures under real working conditions, with doors and nearby equipment operating.
3. Baseline readings. Record the static pressure at each hood and across each filter; they are the reference for every later check.
4. Fan and interlock checks. Rotation, motor current, vibration, and any interlock that stops a process starting without its exhaust.
5. Handover. Design airflow for each capture point, fan and filter details with pressure-drop limits, as-built drawings, operation and maintenance instructions, and who maintains filters and ducts.

Check access before approving the layout: staff need a safe route to filters, dampers and fans. Then use this table to scope each process before the fan is chosen.

Exhaust, general ventilation and make-up air belong in one design with the rest of the HVAC; our industrial HVAC design checks cover the wider review, and warehouse ventilation for expansion covers general ventilation in storage buildings. Design and installation sit under HVAC and mechanical (see ventilation, pressurisation and exhaust), commissioning and TAB under testing and commissioning, and a new factory or expansion can take it as part of a turnkey MEPF scope or factory MEPF. To scope yours, share the layout, operating schedule and existing HVAC data through a scope request; more planning guides are in Insights.

Sources and how to verify

• Bureau of Indian Standards, National Building Code of India 2016 (SP 7:2016), Part 8 Building Services: Section 1 (recommended air changes, contaminant limits for ventilation air) and Section 3 (mechanical ventilation for industries); summarised in the BIS Guide for Using NBC 2016.
• Bureau of Energy Efficiency, retrofit and energy-efficiency publications, including Energy Efficiency Solutions for Existing Buildings: Commercial (prepared by TERI for BEE, 2025), which lists variable frequency drives for fans among HVAC retrofit measures.
• Petroleum and Explosives Safety Organisation, Approval of Ex (electrical) equipment for use in hazardous areas: rule 102 of the Petroleum Rules, 2002, zone definitions and the IS/IEC 60079 series.
• India Code, Air (Prevention and Control of Pollution) Act, 1981: section 21, consent of the State Board and conditions on control equipment and chimneys.

Confirm locally: your consent conditions on emissions and stacks, the hazardous-area classification by a competent person, any licence your materials need, and the fire authority's requirements for kitchen and special-hazard systems.