Industrial process ventilation

Industrial facilities generate airborne contaminants that ordinary comfort HVAC cannot handle: welding fume, solvent vapours, metal and wood dust, mists from machining, and heat from furnaces and ovens. Industrial process ventilation removes these at the source or dilutes them to safe levels, protecting workers and, in many cases, preventing fires and explosions.

This guide covers the core concepts used to design and evaluate industrial ventilation systems in Canada. The standard practical reference throughout North America is the ACGIH manual Industrial Ventilation: A Manual of Recommended Practice for Design, which most of the typical values below come from.

Regulations that apply

In Canada, workplace exposure to airborne contaminants is regulated provincially (or federally for federally regulated workplaces). Each province sets occupational exposure limits (OELs), commonly based on ACGIH Threshold Limit Values, though some provinces set their own. The employer is responsible for keeping exposures below these limits.

Relevant regulations and standards include:

• Provincial occupational health and safety regulations, such as the Alberta OHS Code, BC's Occupational Health and Safety Regulation administered by WorkSafeBC, and Ontario's regulations under the Occupational Health and Safety Act.
• The National Building Code and National Fire Code (and provincial versions) for building, fire separation and hazardous process requirements.
• NFPA 91 for exhaust systems conveying vapours, mists and particulate solids.
• NFPA 652 and related standards such as NFPA 654 and NFPA 664 for combustible dust hazards. Fire codes in many jurisdictions reference these.
• NFPA 33 for spray finishing operations.
• CSA Z94.4 where respiratory protection is used as a supplementary control.

Requirements vary by province and by authority having jurisdiction, so the regulatory basis should be confirmed at the start of every project.

Hierarchy of controls

Ventilation is an engineering control, which sits above administrative controls and personal protective equipment in the hierarchy of controls. Before designing ventilation, it is worth asking whether the hazard can be eliminated or substituted, for example by switching to a water-based coating or enclosing a process. Where it cannot, local exhaust is usually preferred over dilution because it removes the contaminant before it reaches the worker's breathing zone.

Local exhaust ventilation

A local exhaust ventilation (LEV) system has five main parts: the hood, the ductwork, the air cleaning device, the fan and the exhaust stack.

Hoods and capture velocity

The hood is the most important part of the system. If it does not capture the contaminant, nothing downstream matters. Hoods range from full enclosures, such as glove boxes and spray booths, to booths with one open face, to capturing hoods placed near an open source.

Capture velocity is the air velocity at the point of contaminant release needed to overcome its motion and draw it into the hood. ACGIH gives typical ranges:

Capture velocity drops off rapidly with distance from the hood face. For a simple capturing hood, the airflow needed increases roughly with the square of the distance to the source. Moving a hood from 300 mm to 600 mm away can require several times the airflow for the same capture. Flanges, enclosures and keeping hoods close to the source are the cheapest ways to improve performance.

Duct transport velocity

For particulate, the duct must keep air moving fast enough that dust does not settle and build up. Typical minimum transport velocities from ACGIH include:

Settled dust in ducts is both a performance problem and, for combustible dusts, a fire and explosion hazard. Branch entries at shallow angles, long-radius elbows and avoiding unnecessary blast gates help maintain velocity and reduce pressure losses.

Air cleaning and discharge

Air cleaners include cartridge and baghouse dust collectors, cyclones, wet scrubbers, mist collectors and activated carbon or thermal oxidizers for vapours. Combustible dust collectors need explosion protection such as venting, isolation or suppression under NFPA standards. Exhaust stacks should discharge vertically and high enough to avoid re-entrainment into air intakes. See odour control and exhaust re-entrainment.

Corrosive exhaust streams need appropriate duct and fan materials. Our guide to materials and coatings for corrosive exhaust covers common options.

Dilution ventilation

Dilution, or general ventilation, mixes clean air with contaminated air to lower concentrations. It is suited to low-toxicity contaminants released in small quantities, spread out over a large area, at a fairly uniform rate, and where workers are not close to the source. It is not suitable for highly toxic materials or for point sources near workers. Dilution is also commonly used for heat removal in foundries, mills and other hot processes.

The required dilution airflow depends on the generation rate, the exposure limit and a safety factor that accounts for incomplete mixing.

Make-up air

Every litre of air exhausted must be replaced. Without make-up air, buildings go negative, doors become hard to open, hoods lose capture, combustion appliances backdraft and cold air leaks in at the perimeter. In Canada, heating make-up air is often the largest energy cost of an industrial ventilation system. Direct-fired gas make-up air units are common because of their high efficiency. See make-up air units.

Testing and maintenance

Industrial ventilation systems degrade as filters load, ducts accumulate material, fans wear and hoods are modified. Good practice includes:

1. Measuring hood static pressure and airflow at commissioning and recording baseline values.
2. Installing static pressure gauges at each hood so operators can spot changes.
3. Periodic airflow testing and capture verification, such as smoke testing at hoods.
4. Inspecting ducts for buildup, especially for combustible dust.
5. Rebalancing when hoods are added, removed or relocated.

Air balancing of exhaust branches is often the fastest way to restore capture in a system that has been extended over the years.

Key takeaways

• Industrial ventilation is regulated through provincial occupational health and safety rules plus building, fire and NFPA standards.
• Local exhaust at the source is usually preferred over dilution ventilation.
• Hood design and placement determine capture, and airflow requirements rise sharply with distance from the source.
• Ducts conveying dust need adequate transport velocity to prevent buildup and fire hazards.
• Make-up air is essential and is often the largest energy cost in a Canadian facility.
• Baseline testing and ongoing monitoring keep systems performing as processes change.