What a fan does
A fan adds a small amount of pressure to air so it will move through a space, a duct or a piece of equipment. Fans are different from compressors because they handle large volumes at low pressure rise. In plants they appear everywhere: ventilating buildings, cooling machinery, pulling dusty air to a collector, supplying combustion air to burners, drying products and moving process gases.
Fans fall into two main families according to how air travels through the impeller. Understanding that one difference explains most of what follows.
Axial fans
In an axial fan, air enters and leaves along the axis of the shaft, much like a propeller or a household desk fan. The blades push air straight through.
Strengths. Axial fans can move a large volume of air in a compact package, and they are usually efficient when the system resistance is low. They can be mounted in a wall or duct with little extra housing.
Limits. They generally cope less well with high resistance, such as long duct runs, tight filters or dampers. Under high resistance some designs can become unstable or noisy. Dirty or sticky air can build up on the blades.
Common types.
- Propeller fans for wall and roof ventilation, with low pressure capability.
- Tube-axial fans with a cylindrical housing, used for ducted ventilation and some process cooling.
- Vane-axial fans with guide vanes that straighten the airflow, able to handle higher pressures within the axial family.
Typical uses. General building ventilation, cooling towers and large heat exchangers, tunnel and mine ventilation, and spaces where moving a great deal of air through little resistance is the aim.
Centrifugal fans
In a centrifugal fan, air enters along the shaft and is thrown outward by the spinning impeller into a scroll-shaped housing, leaving at a right angle. The spinning builds pressure.
Strengths. Centrifugal fans typically handle higher system resistance, so they suit ducted systems, filters, dust collectors and processes where air must be forced through something.
Limits. They are usually larger and heavier for a given airflow, and the housing needs space for the discharge.
Common impeller shapes.
- Backward-inclined or backward-curved blades. Generally efficient and stable, often chosen for clean air and many general duties.
- Radial blades. Straight, simple and sturdy. They are often chosen for dust-laden or abrasive air, because material tends to shed rather than stick.
- Forward-curved blades. Compact and quiet at modest pressures, found in many air handling units and packaged equipment. They are typically less efficient than backward-curved designs.
Typical uses. Dust collection and fume extraction, ovens and dryers, combustion air, material conveying, process exhaust, and air handling units serving large buildings.
Other types worth knowing
- Mixed-flow fans sit between the two families, with some axial and some radial movement. They offer moderate pressure with a compact shape.
- Roof and inline exhausters package a fan into a housing designed for a specific installation.
- Blowers is a term often used for fans or positive-displacement machines built to deliver higher pressure than a typical fan.
- Jet or induction fans push air along a space, such as a tunnel or car park, instead of through a duct.
Matching fan type to duty
A rough guide, not a rule:
| Duty | Typical lean |
|---|---|
| General ventilation with little resistance | Axial |
| Ducted supply or exhaust | Centrifugal or vane-axial |
| Dust and fume extraction | Centrifugal, often radial blade types for dusty air |
| Cooling of coils or towers | Axial |
| Combustion and process air | Centrifugal |
Real choices depend on the details of the system, covered in fan selection, which looks at airflow, pressure and noise in order.
The environment matters too
Air that is hot, corrosive, damp or carries dust changes the construction needed. Motors may sit outside the airstream, shafts may need cooling or sealing, and materials may need to resist corrosion. Where flammable gases or combustible dust are present, the equipment must be suitable for the hazard classification of the area, which is a specialist matter. The notes on combustible dust explain why this is taken seriously.
Drives and control
Most fans are driven by electric motors, either directly on the shaft or through belt drives. Direct drive is simpler and tends to need less upkeep. Belt drives allow speed adjustment by changing sheaves but need tension and wear checks. Increasingly, variable speed drives let operators slow a fan when demand is low, which usually cuts energy use considerably, a point developed in fan energy use.
Upkeep in brief
Fans are simple machines but suffer from a few predictable problems: dirt buildup that unbalances the impeller, worn bearings, loose or worn belts and corroded parts. Regular inspection of vibration, noise and temperature catches most problems early. Always isolate and lock out the fan before working near the impeller, because a fan can start or windmill unexpectedly, and follow the manufacturer’s manual and local safety rules.
In short
Axial fans excel at moving big volumes against little resistance; centrifugal fans build pressure to push air through systems. Within those groups, the blade shape and construction set how well the fan copes with clean air, dust, heat or corrosion. Knowing the family tells you where to look first.