Axial Fan: How It Works, Explosion Proof & Repair Guide

What Does Axial Fan Mean

The term axial describes the direction of airflow through the fan. Air enters and exits along the same axis as the rotating shaft, moving in a straight line from inlet to outlet. This is in direct contrast to a centrifugal fan, where air enters axially but is discharged radially at 90 degrees to the shaft.

The name comes from the Latin word axis, meaning the central line around which something rotates. Every component in an axial fan, from the hub to the blade tips, is designed to accelerate air in this single axial direction. This geometry makes axial fans exceptionally efficient at moving large volumes of air through a relatively short, straight duct path.

How Does an Axial Fan Work

An axial fan works on the same aerodynamic principle as an aircraft wing. Each blade is an aerofoil section set at a specific pitch angle relative to the plane of rotation. As the motor drives the hub and blades in rotation, the leading edge of each blade cuts through the air. The angled blade surface creates a pressure difference between the two faces: higher pressure on the rear face and lower pressure on the front face. This pressure difference accelerates air rearward, producing thrust in the axial direction.

Key Components and Their Roles

• Motor: Drives the hub and blades. Most industrial axial fans use three-phase induction motors rated from 0.09 kW to over 90 kW. The motor may be internal (tube axial design) or external (panel fan design).
• Hub: The central boss connecting blades to the motor shaft. Hub-to-tip ratios typically range from 0.3 to 0.6, with larger hub ratios used in higher-pressure applications.
• Blades: Usually 2 to 12 in number, made from aluminium alloy, glass-fibre reinforced plastic, or steel depending on duty. Blade pitch angle determines the duty point: steeper angles give more pressure and airflow but require more motor power.
• Casing or Ring: Surrounds the blade tips to minimise tip clearance losses. Typical tip clearances are 0.5 to 1.5 percent of blade tip diameter. Reducing this gap from 3 percent to 1 percent can improve efficiency by 5 to 8 percentage points.
• Inlet Bell or Bellmouth: A smoothly contoured inlet that reduces turbulence entering the blade plane, improving efficiency by 3 to 6 percent compared to a square-edged inlet.
• Guide Vanes (optional): Inlet guide vanes pre-swirl air before it reaches the blades to improve efficiency or control flow. Outlet guide vanes remove swirl from the discharge stream, recovering kinetic energy as static pressure.

The Three Fan Laws

The performance of any axial fan obeys the fan affinity laws, which are essential for sizing and speed control decisions:

• Flow varies directly with speed: Doubling fan speed doubles airflow volume.
• Pressure varies with the square of speed: Doubling speed increases static pressure by a factor of 4.
• Power varies with the cube of speed: Doubling speed increases power consumption by a factor of 8. This is the key reason that reducing fan speed by 20 percent cuts power consumption by nearly 50 percent, which is why variable frequency drives (VFDs) deliver such significant energy savings in axial fan applications.

Are Axial Fans Explosion Proof

Standard axial fans are not explosion proof. A standard fan uses conventional materials and construction that can generate sparks from motor windings, bearing failures, or blade-to-casing contact, any of which is sufficient to ignite a flammable atmosphere.

However, explosion-proof axial fans are manufactured specifically for hazardous area installations. These units are designed and tested to prevent ignition of specified flammable gas or dust atmospheres under both normal operating conditions and defined fault conditions.

What Makes an Axial Fan Explosion Proof

• ATEX or IECEx certified motor: The motor uses flameproof (Ex d) or increased safety (Ex e) construction. A flameproof motor contains any internal ignition within the motor enclosure, preventing flame propagation to the surrounding atmosphere.
• Non-sparking blade material: Blades are made from aluminium alloy or non-sparking composite materials. A blade-tip strike against the casing must not produce a spark capable of igniting the surrounding atmosphere.
• Antistatic construction: Fan components are bonded and earthed to prevent electrostatic charge accumulation, which is particularly important in dust-handling applications.
• Controlled surface temperatures: The motor and all external surfaces are rated to a temperature class (T1 to T6 under ATEX), where T6 limits the maximum surface temperature to 85 degrees Celsius, ensuring it stays below the auto-ignition temperature of the target gas or dust.

Hazardous Area Classification for Axial Fan Selection

For most industrial explosion-proof ventilation applications, Zone 1 or Zone 2 classification applies and a Category 2 ATEX-certified axial fan is the appropriate selection. Always verify the specific gas group (IIA, IIB, or IIC) and temperature class with the area hazardous classification report before ordering.

How to Repair an Axial Fan

Most axial fan failures fall into four categories: bearing failure, motor winding failure, blade damage, and impeller imbalance. A systematic diagnosis approach prevents replacing parts unnecessarily and ensures the root cause is addressed rather than just the symptom.

Diagnosing the Fault Before Disassembly

• Fan runs but airflow is reduced: Check blade pitch angle (adjustable pitch fans), inspect for blade deposits or damage, measure motor current and compare to nameplate full-load current. A motor drawing significantly below nameplate current suggests blade stall or disconnected blades.
• Excessive noise or vibration: Measure vibration using a handheld vibration meter. ISO 14694 sets acceptable vibration limits for industrial fans. A reading above 4.5 mm/s RMS on the bearing housing indicates a problem requiring investigation. Common causes are bearing wear, blade damage, or impeller contamination causing imbalance.
• Motor trips on overload: Measure motor winding resistance between each phase with a multimeter. Resistance should be balanced within 2 percent between phases. Measure insulation resistance to earth with a 500 V megohmmeter. A reading below 1 megohm indicates a compromised winding requiring rewinding or motor replacement.
• Fan fails to start: Check supply voltage at the motor terminals under load. A voltage drop greater than 5 percent from nameplate voltage causes starting torque reduction. Check contactor contacts and overload relay settings.

Step-by-Step Bearing Replacement

Bearing replacement is the most common axial fan repair. The procedure below applies to most direct-drive tube axial fans:

1. Isolate and lock out the power supply. Confirm the supply is dead with a voltage tester at the motor terminals. Apply a lockout-tagout padlock. Wait for the impeller to come to a complete stop.
2. Remove the fan from its mounting. Support the fan assembly, remove fixing bolts, and lower the unit to a clean work area. Note the orientation of any wiring before disconnecting.
3. Remove the impeller from the motor shaft. Mark the impeller hub and shaft with a reference line before removal to preserve balance. Use a correctly sized gear puller rather than striking the hub, which can damage shaft threads or bearing housings.
4. Remove the old bearings. Use a bearing puller to extract both the drive-end and non-drive-end bearings. Never use heat to free a stuck bearing on an aluminium housing, as this will distort the housing bore.
5. Clean and inspect the shaft and housing bores. Measure the shaft diameter and housing bore diameter with a micrometer. Compare to the bearing manufacturer tolerance tables. A shaft that is more than 0.02 mm undersize indicates wear and requires shaft repair or replacement before fitting new bearings.
6. Fit the new bearings correctly. Use bearings of identical designation to the originals (confirm the stamped code on the old bearing outer ring). Heat new bearings in a bearing heater to 80 to 100 degrees Celsius and slide onto the shaft by hand. Never strike a bearing with a hammer directly. Allow bearings to cool completely before reassembly.
7. Reassemble and check impeller clearance. Refit the impeller to the shaft, aligning the reference marks made before removal. Check tip clearance at four positions around the casing with a feeler gauge. Clearance should be uniform within 0.5 mm. Uneven clearance indicates shaft misalignment or a bent shaft.
8. Run the fan and verify performance. After restoring power, measure vibration at the bearing housings within the first 30 minutes of operation. Vibration should be below 2.8 mm/s RMS for a newly repaired fan in good condition. Measure motor current on all three phases and confirm readings are balanced and within nameplate limits.

Blade Repair and Replacement

Minor surface erosion on aluminium blades can be repaired with epoxy-based filler compounds, but any blade with a crack, a significant bend, or a missing section must be replaced rather than repaired. A cracked blade will propagate to failure under cyclic fatigue loading and can cause catastrophic impeller disintegration.

When replacing one blade on a multi-blade impeller, always replace all blades as a matched set and rebalance the complete impeller assembly on a dynamic balancing machine to ISO 21940-11 Grade G6.3. An improperly balanced impeller generating vibration at running frequency will destroy the new bearings within weeks.

When to Replace Rather Than Repair

Selecting the Right Axial Fan for Your Application

Choosing an axial fan correctly at the specification stage avoids the most common causes of premature failure and poor performance. The following parameters must be defined before any fan can be sized.

• Required airflow volume (m3/h or CFM): Calculate from the space volume and required air changes per hour, or from the heat load to be removed using Q = P divided by (rho x Cp x delta T), where P is heat load in watts, rho is air density, Cp is specific heat, and delta T is allowable temperature rise.
• System resistance (Pa or inches WG): Sum all duct and fitting pressure losses at design flow. Axial fans are well suited to systems below 500 Pa total static pressure. Above 800 Pa, a centrifugal fan or multi-stage axial design should be considered.
• Air temperature and density: Fan performance curves are typically published at 20 degrees Celsius and standard atmospheric pressure (1.2 kg/m3 air density). At 100 degrees Celsius, air density is approximately 0.95 kg/m3, reducing fan static pressure and power by the same ratio. Always correct published performance to actual site conditions.
• Environment classification: Confirm whether a standard, weatherproof (IP55 or higher), or explosion-proof rated unit is required based on the hazardous area classification report and local installation standards.
• Noise constraints: Axial fans are louder than centrifugal fans at equivalent duty points. If the installation is near occupied spaces, obtain octave-band sound power data from the manufacturer and model the predicted sound pressure level at the nearest sensitive receptor before finalising the selection.