Industrial ventilation: duties, airflow rates and equipment

In an industrial building, ventilation is not about comfort. It removes the pollutants generated by the activity — dust, solvent vapours, welding fumes, flour, combustion gases — and keeps the atmosphere within limits compatible with operator health and site safety.

It is also the item most often neglected after handover. An installation that was correct at commissioning can lose a large share of its airflow within two or three years, with no alarm to signal it.

A duty, not a comfort feature

Moroccan labour law requires the employer to keep workplaces sanitary and to protect employees against risks arising from the atmosphere of those workplaces. In practice this means three obligations: renew the air, capture pollutants where they are emitted, and be able to demonstrate that the installation works.

The last point is the one most often missing. An airflow measurement report at commissioning, followed by periodic measurements, is the only usable evidence during an inspection or after an incident.

General ventilation and capture at source

These are two distinct functions, and one does not replace the other.

General ventilation dilutes pollutants in the volume of the building and renews the air. It suits diffuse, low-toxicity contamination.

Capture at source extracts the pollutant at the exact point of emission, before it reaches the operator's breathing zone: extraction arm at a welding station, hood over a treatment bath, nozzle on a machining centre, downdraught bench.

The design rule is constant: the closer the capture point is to the source, the lower the airflow required. Doubling the distance between the extraction inlet and the emission point can multiply the required airflow considerably. A well-placed capture point therefore consumes less than an oversized general ventilation system, while protecting better.

How airflow rates are determined

Airflow is not set by air change rate alone. It follows from several inputs:

• The nature and quantity of pollutant emitted per unit of time.
• The occupational exposure limit applicable to that pollutant.
• The number of operators and their position relative to the sources.
• The heat gains to be removed, where ventilation also cools the space.
• Make-up air supply, without which extraction puts the building under negative pressure and throttles itself.

That last point is the most common cause of underperformance found on site: extraction is installed, air supply is forgotten. The fan runs, actual airflow falls well below the rated figure, and doors become hard to open.

Air handling units

An AHU filters, heats, cools and, depending on requirements, humidifies or dehumidifies the supply air. It becomes necessary as soon as fresh air must be treated before introduction: controlled-environment workshops, food processing, pharmaceuticals, electronics.

What determines its performance over time is the filter class and accessibility, casing airtightness, the presence of heat recovery on the extract air, and the quality of the control system. An AHU driven by a simple time clock consumes needlessly during quiet periods.

Special cases

• Explosive atmospheres (ATEX): ventilation directly reduces risk by keeping concentrations below the flammability threshold. Equipment installed in a classified zone must be certified for that zone.
• Commercial kitchens: extraction airflow is calculated from the rating and type of appliances under the hood. Grease filters and duct cleaning are fire prevention measures.
• Cleanrooms: control depends on air change rates, terminal filter class and maintaining a pressure cascade between rooms.
• Enclosed car parks: extraction sized for carbon monoxide, controlled by sensors.

Maintenance: where the airflow disappears

A ventilation system loses performance in three ways. Filters load up and pressure drop rises. Ducts silt up, particularly with grease or fibrous dust. Belts slacken and fan speed falls.

The minimum programme:

• Filter checks and replacement based on measured pressure drop, not on the calendar alone.
• Verification of belt tension and bearing condition.
• Duct cleaning, at a frequency matched to the pollutant carried.
• Airflow measurement at the terminals, at least annually, with a written report.

Signs that ventilation is undersized

Condensation on cold surfaces and glazing. Odours lingering at the end of the day. Dust deposits on ceilings and light fittings. Outward-opening doors that are hard to push. Operators reporting headaches at the end of a shift. None of these proves anything on its own, but together they justify a measurement campaign before drawing conclusions.

An airflow measurement costs little against a production stoppage or a recognised occupational illness. It is the starting point of any serious diagnosis.