In this article
Industrial ventilation does two jobs in a factory. It keeps the working environment tolerable by removing heat and stale air, and it protects people from dust, fumes, vapours and mists produced by the process. In hot Nigerian factory buildings, ventilation also has a direct effect on comfort and productivity. Yet many systems are put together from a few roof fans and some ducting without much design, and then struggle to do either job well.
This guide covers the design basics: the difference between general and local exhaust ventilation, how hoods, ducts and fans work together, how to choose extraction fans, and how to keep a system performing. It is written for plant managers, health and safety staff and engineers planning new or upgraded systems.
General ventilation versus local exhaust ventilation
General (dilution) ventilation
General ventilation moves large volumes of air through a building to remove heat and dilute airborne contaminants. It uses roof extract fans, wall fans, louvres and sometimes supply fans. It suits:
- Removing heat from process areas, boiler houses, compressor rooms and warehouses.
- Low-hazard contaminants released at low rates over a wide area.
- Supplementing local extraction.
Local exhaust ventilation (LEV)
LEV captures contaminants at or near their source, before they spread into the workroom. Examples include extraction hoods over welding benches, dust extraction on sanding, cutting and grinding machines, fume cupboards, tank-edge extraction and bag-filling extraction.
Where a substance is hazardous, dilution is not usually enough; it lets the contaminant pass through the worker’s breathing zone first. LEV is the right tool. The UK Health and Safety Executive’s guidance HSG258, Controlling airborne contaminants at work, is one of the most practical references available and is widely used outside the UK.
Designing local exhaust ventilation
An LEV system has five main parts: the hood, the ductwork, an air cleaner (where needed), the fan and the discharge. The hood is where most systems succeed or fail.
Choosing the right hood
HSG258 groups hoods into three types:
| Hood type | How it works | Examples |
|---|---|---|
| Enclosing | The process is inside the hood | Booths, fume cupboards, enclosed machines |
| Receiving | Catches contaminant thrown towards it by the process | Canopies over hot processes, grinding wheel hoods |
| Capturing | Draws contaminated air in from outside the hood | Slot hoods, flexible arms, welding extraction |
HSG258 is clear that enclosures are always more effective than capturing or receiving hoods. Enclose the process as far as the work allows.
The limits of capturing hoods
Capturing hoods are popular but often disappointing. HSG258 explains why: air velocity in front of a hood falls very quickly with distance, to about one-tenth of the face velocity at one hood diameter from the face. In practice, capturing hoods usually work only when the source is within about two hood diameters. Cross-draughts from doors, fans and moving vehicles can destroy the capture zone altogether.
The lesson for designers and users: bring the hood close to the source, keep it there, and control draughts. A large pedestal fan blowing on workers in a hot shop can undo a well-designed extraction hood.
Receiving hoods for hot processes
For a canopy over a hot process, HSG258 says the extraction must empty the hood of contaminated air at least as fast as the rising plume fills it. Undersized canopies spill fume around their edges.
Ductwork
- Transport velocity. In dust systems, air must move fast enough in the duct to keep particles airborne; otherwise dust settles and blocks ducts. Designers specify transport velocity for the material handled.
- Balancing. Multi-branch systems need the right flow at each hood. HSG258 describes adjusting flows with tapers or dampers during design and commissioning.
- Smooth layout. Use gentle bends and angled branch entries rather than sharp elbows and right-angle tees.
- Materials. Galvanised or stainless steel for most duties; special materials for corrosive fumes.
- Access. Cleaning doors in dust ducts and test points for measurements.
Air cleaning and discharge
Dust collectors, filters or scrubbers remove contaminants before air is discharged. Discharge stacks should release air where it will not re-enter the building through windows, doors or air intakes. Provide make-up air too: a building that is extracting air must let the same amount in, or doors become hard to open and extraction performance falls.
Choosing industrial ventilation fans
The US Department of Energy’s fan system sourcebook describes the main fan types and where they suit:
| Fan type | Characteristics | Typical uses |
|---|---|---|
| Propeller | High airflow at low pressure, little ductwork | Wall and roof general ventilation |
| Tubeaxial | Medium pressure, high airflow, ducted | Ducted general extraction, spray booths |
| Vaneaxial | Medium to high pressure, compact | Ducted systems needing more pressure |
| Centrifugal, backward-inclined | Efficient, higher pressure; can collect build-up in dirty air | Clean-air extraction, after a dust collector |
| Centrifugal, radial blade | Rugged, tolerates dust and high temperature | Dirty air streams, material handling |
| Centrifugal, forward-curved | Quiet, low to medium flow at low pressure; clean air only | Air handling units, small systems |
Fan curves and system curves
A fan does not deliver a fixed airflow. It operates where its performance curve meets the system resistance curve. The DOE sourcebook advises selecting fans to operate near their best efficiency point, and avoiding the unstable region of the fan curve. To do that, the designer needs a realistic estimate of system pressure loss through hoods, ducts, filters and stack.
System effect
Poor inlet and outlet conditions reduce fan performance below what the catalogue promises. The DOE sourcebook notes that an elbow close to a fan outlet can reduce delivered flow by up to 30 per cent. Allow straight duct runs at the fan inlet and outlet as the fan maker recommends.
Do not oversize
Adding generous margins seems safe but the DOE warns that oversized fans waste energy, make more noise and are often throttled with dampers, which reduces reliability. Design for the real requirement with a sensible, not excessive, allowance.
Controlling energy use
Fans often run continuously, so their energy use adds up, particularly on generator power. The DOE sourcebook highlights the fan laws: airflow varies with speed, pressure with the square of speed, and power with the cube of speed. A modest reduction in fan speed therefore gives a disproportionately large reduction in power.
- Speed control with variable speed drives is generally the most efficient way to vary airflow.
- Outlet dampers change system resistance but save little energy.
- Belt drives allow speed to be set by changing pulleys; check belt tension and condition regularly.
- Switch off extraction serving idle processes, using dampers to isolate unused hoods on multi-branch systems where the design allows.
Commissioning, testing and maintenance
Commissioning
HSG258 says commissioning should show the system achieves its design performance, and the commissioning report becomes the benchmark for future checks. Record airflows, hood face velocities, static pressures and fan data.
User manual and logbook
HSG258 recommends the supplier provides a user manual explaining the system, its performance benchmarks and component replacement schedules, and that a logbook is kept for checks and maintenance.
Airflow indicators
Fit simple airflow indicators, such as a manometer, at hoods so operators can see when extraction is working. HSG258 says the indicator must show simply and clearly when airflow is adequate.
Thorough examination
In the UK, HSE describes the thorough examination and test of LEV as a three-stage check done at least every 14 months, with a few exceptions. Nigeria does not have an identical published requirement, but periodic examination against the commissioning benchmark is good practice for any extraction system protecting workers.
Routine maintenance
- Inspect hoods and ducts for damage and blockage.
- Empty dust collectors and replace filters.
- Check fan belts, bearings and vibration. The DOE recommends replacing all belts in a set together.
- Clean fan impellers in dusty or humid air streams to prevent imbalance.
- Compare airflow readings with the commissioning benchmark.
Common mistakes to avoid
- Relying on roof fans to deal with fume or dust that should be captured at source.
- Fitting capturing hoods that sit too far from the work to have any real effect.
- Using pedestal fans for comfort that blow straight across extraction hoods.
- Forgetting make-up air, so the building runs under negative pressure and extraction falls off.
- Selecting fans from a catalogue without calculating system resistance.
- Mounting an elbow directly on the fan outlet.
- Discharging extracted air where it is drawn back in through doors, windows or air intakes.
- Commissioning without recording airflows, so there is no benchmark when performance drops.
FAQ
Is a roof fan enough for welding fume?
Usually not on its own. General ventilation removes heat and dilutes fume, but local extraction at the weld is the effective way to protect the welder.
Why does our extraction hood not seem to work?
Common causes are hoods too far from the source, cross-draughts, blocked filters, leaking or damaged ducts, or a fan running below design speed. Compare readings with the commissioning report.
Axial or centrifugal fan?
Axial fans suit high airflow at low pressure, such as general ventilation. Centrifugal fans suit higher pressure and dirty air, such as ducted dust extraction.
Can a VSD save energy on extraction fans?
Often, where airflow demand varies. Because power varies with the cube of speed, even small speed reductions can make a noticeable difference.
Talk to Maven about your ventilation system
Maven Technical Services designs and installs industrial HVAC, ventilation and extraction systems, along with the ductwork, steel supports and electrical work they need. If your factory is too hot, extraction is not working, or you are planning a new process area, speak to Maven’s engineers about a system designed around the job it has to do.
