Table of Contents
Axial vs Centrifugal Fans: Complete HVAC Fan Selection Comparison
Quick Verdict
Axial versus centrifugal fan selection is primarily determined by system pressure requirements.
Bottom Line: If your system pressure is below 500 Pa (2" WG), axial fans offer lower cost and simpler installation for ventilation, cooling, and exhaust applications. If system pressure exceeds 500 Pa, centrifugal fans are required—they're not an optional upgrade but a necessity for overcoming resistance in ducted HVAC systems. Most commercial air handling units use centrifugal fans because coils, filters, and ductwork create pressures that axial fans cannot generate.
At-a-Glance Comparison Table
| Feature | Axial Fan | Centrifugal Fan | Winner |
|---|---|---|---|
| Maximum Pressure | ~500 Pa (2" WG) | 3000+ Pa (12"+ WG) | Centrifugal |
| Peak Efficiency | 70-85% (vaneaxial) | 60-85% (depends on type) | Tie |
| Noise Level | Higher (same pressure) | Lower | Centrifugal |
| First Cost | Lower (30-50% less) | Higher | Axial |
| Space Required | Inline (less floor space) | 90° turn (more floor space) | Axial |
| Airflow Direction | Straight through | 90° discharge | Axial |
| Stall Characteristics | Abrupt, dangerous | Gradual | Centrifugal |
| Best For | Exhaust, cooling, ventilation | AHUs, ducted systems | — |
How Axial and Centrifugal Fans Work
Understanding airflow mechanics explains each fan type's capabilities.
Technical Note: The fundamental difference is airflow direction relative to the impeller. Axial fans move air parallel to the shaft; centrifugal fans turn air 90° from intake to discharge. This geometric difference determines pressure capability and application suitability.
Axial Fan Operation
Axial fans use propeller-style blades mounted on a hub:
Airflow Path:
- Air enters parallel to the rotating shaft
- Angled blades deflect and accelerate air
- Air exits parallel to shaft (same direction as entry)
- No 90° turn; straight-through flow
Pressure Generation:
- Blades impart velocity to air
- Limited velocity-to-pressure conversion
- Maximum pressure rise ~500 Pa per stage
- Higher pressure requires multiple stages (inefficient)
Axial Fan Types:
| Type | Pressure (Pa) | Efficiency | Application |
|---|---|---|---|
| Propeller | <125 | 45-60% | Wall/roof exhaust |
| Tube axial | <375 | 55-70% | Inline ventilation |
| Vaneaxial | <750 | 70-85% | Tunnels, industrial |
Centrifugal Fan Operation
Centrifugal fans use an impeller in a scroll housing:
Airflow Path:
- Air enters axially through the impeller inlet (eye)
- Impeller accelerates air radially outward
- Scroll housing converts velocity to static pressure
- Air exits perpendicular to entry (90° turn)
Pressure Generation:
- Centrifugal acceleration provides initial pressure
- Scroll housing expands, converting velocity to pressure
- Much higher pressure capability than axial
- Single stage can achieve 3000+ Pa
Centrifugal Fan Types:
| Type | Pressure (Pa) | Efficiency | Application |
|---|---|---|---|
| Forward-curved | 500-1500 | 55-65% | Residential, FCUs |
| Backward-curved | 750-2500 | 75-82% | Commercial AHUs |
| Backward-inclined | 750-2500 | 70-80% | Industrial, dirty air |
| Airfoil | 1000-3000 | 80-85% | Premium AHUs |
| Radial | 1500-4000 | 60-70% | Dust collection |
Pressure Capability: The Determining Factor
System pressure is the primary selection criterion.
Axial Fan Pressure Limits
Axial fans have inherent pressure limitations:
Where:
- = air density
- = blade tip speed
- = pressure coefficient (~0.5 for axial)
Practical limits by type:
| Axial Type | Max Static Pressure | Stall Point |
|---|---|---|
| Propeller | 125 Pa (0.5" WG) | ~100 Pa |
| Tube axial | 375 Pa (1.5" WG) | ~300 Pa |
| Vaneaxial | 750 Pa (3" WG) | ~600 Pa |
What happens at high pressure:
- Fan approaches stall condition
- Efficiency drops dramatically
- Noise increases (turbulence)
- Airflow becomes unstable
- Potential motor overload
Centrifugal Fan Pressure Range
Centrifugal fans achieve much higher pressures:
Pressure capability by type:
| Centrifugal Type | Typical SP Range | Max SP Possible |
|---|---|---|
| Forward-curved | 250-750 Pa | 1500 Pa |
| Backward-curved | 500-1500 Pa | 2500 Pa |
| Airfoil | 750-2000 Pa | 3000 Pa |
| Radial/Paddle | 1000-2500 Pa | 4000 Pa |
Why higher pressure is possible:
- Centrifugal acceleration provides primary pressure
- Scroll housing converts velocity efficiently
- Blade design optimized for pressure rise
- Gradual stall characteristics allow higher loading
System Pressure Determines Choice
Verdict: Pressure Capability
Winner: Centrifugal — When system pressure matters, centrifugal fans are essential. Axial fans simply cannot generate pressures required for typical ducted HVAC systems.
Efficiency Comparison
Efficiency affects operating cost and motor sizing.
Axial Fan Efficiency
| Axial Type | Peak Efficiency | Efficient Range |
|---|---|---|
| Propeller | 45-60% | 30-50% of free air |
| Tube axial | 55-70% | 40-60% of peak |
| Vaneaxial | 70-85% | 50-70% of peak |
Efficiency characteristics:
- Narrow efficient operating range
- Efficiency drops sharply near stall
- Peak efficiency at specific pressure/flow point
- Guide vanes (vaneaxial) recover swirl losses
Centrifugal Fan Efficiency
| Centrifugal Type | Peak Efficiency | Efficient Range |
|---|---|---|
| Forward-curved | 55-65% | Wide but low peak |
| Backward-curved | 75-82% | 50-80% of peak |
| Airfoil | 80-85% | 60-85% of peak |
| Radial | 60-70% | Wide, tolerant |
Efficiency characteristics:
- Backward-curved/airfoil have highest efficiency
- Wider efficient operating range than axial
- Gradual efficiency reduction off-peak
- Non-overloading motor characteristic (BC/BI/AF)
Motor Power Characteristics
Axial fans: Power increases with airflow; motor can overload at low system pressure (free delivery).
Forward-curved centrifugal: Power increases with airflow; motor can overload at free delivery.
Backward-curved/airfoil centrifugal: Self-limiting; power peaks then decreases toward free delivery—"non-overloading."
Field Tip: For systems where pressure may vary (dirty filters, damper modulation), specify backward-curved centrifugal fans. Their non-overloading characteristic prevents motor burnout that can occur with forward-curved or axial fans when system pressure drops unexpectedly.
Energy Cost Example
Verdict: Efficiency
Winner: Tie at Peak — Both types achieve similar peak efficiency (70-85% for best designs). Centrifugal fans maintain efficiency over wider operating range and offer non-overloading characteristics important for variable systems.
Noise Comparison
Acoustic performance matters for occupied spaces.
Axial Fan Noise Characteristics
Noise sources:
- Blade passage frequency (prominent tone)
- Turbulent interaction with housing
- Tip vortex noise
- Flow separation near stall
Typical sound levels:
| Axial Type | Specific Sound Power (dB) | Character |
|---|---|---|
| Propeller | High (75-85 Lw) | Broadband + tones |
| Tube axial | Medium-high (70-80 Lw) | Tones dominate |
| Vaneaxial | Medium (65-75 Lw) | Reduced tones |
Noise concerns:
- Strong blade passage tones difficult to attenuate
- Noise increases rapidly near stall
- Low-frequency rumble common
- Fan should not be near occupied spaces without treatment
Centrifugal Fan Noise Characteristics
Noise sources:
- Blade passing scroll cutoff (tone)
- Turbulent flow in scroll
- Inlet turbulence
- Discharge velocity noise
Typical sound levels:
| Centrifugal Type | Specific Sound Power (dB) | Character |
|---|---|---|
| Forward-curved | Low-medium (60-70 Lw) | Broadband |
| Backward-curved | Medium (65-75 Lw) | Moderate tones |
| Airfoil | Low-medium (60-72 Lw) | Smooth spectrum |
Noise advantages:
- Generally lower sound levels at equivalent duty
- More amenable to acoustic treatment
- Less prominent tones
- Better high-frequency characteristics
Noise-Sensitive Applications
For NC 35-40 requirements (typical offices):
- Centrifugal fans strongly preferred
- Sound attenuators effective on discharge/return
- Fan speed reduction (VFD) helps significantly
For NC <35 requirements (recording studios, hospitals):
- Backward-curved centrifugal essential
- Low discharge velocity design
- Extensive sound attenuation required
Verdict: Noise
Winner: Centrifugal — Lower noise levels and better acoustic characteristics make centrifugal fans the default choice for noise-sensitive applications.
Cost Analysis
First Cost Comparison
| Airflow (CFM) | Axial Cost | Centrifugal Cost | Centrifugal Premium |
|---|---|---|---|
| 5,000 | $400-800 | $800-1,500 | 80-100% |
| 10,000 | $800-1,500 | $1,500-3,000 | 80-100% |
| 25,000 | $2,000-4,000 | $4,000-8,000 | 100% |
| 50,000 | $5,000-10,000 | $10,000-20,000 | 100% |
Why centrifugal costs more:
- More complex impeller manufacturing
- Scroll housing fabrication
- Higher-quality bearings for radial loads
- More robust motor mounting
Installation Cost
| Factor | Axial | Centrifugal |
|---|---|---|
| Mounting | Simple inline | Requires base/platform |
| Transitions | Inlet/outlet cones | Flexible connections |
| Electrical | Direct | Often belt-drive |
| Space | Compact inline | Floor space for scroll |
| Typical labor | $500-1,500 | $1,000-3,000 |
Lifecycle Cost Comparison
Verdict: Cost
Winner: Depends — Axial wins on first cost; centrifugal often wins on lifecycle cost due to efficiency. For low-pressure applications where both work, lifecycle analysis determines the winner.
Application-Specific Recommendations
When to Choose Axial Fans
Use axial fans when:
- System pressure below 375 Pa (tube axial) or 125 Pa (propeller)
- High airflow volume required (>50,000 CFM economical)
- Inline installation preferred (no floor space)
- Straight-through airflow needed
- Budget is primary concern
- Simple ventilation without extensive ductwork
- Cooling applications (condensers, cooling towers)
Typical Axial Applications:
- Wall and roof exhaust fans
- Parking garage ventilation
- Tunnel and mine ventilation
- Condenser and cooling tower fans
- Agricultural ventilation
- Data center hot aisle containment
- Through-wall heat transfer units
- Emergency smoke exhaust (large tunnels)
When to Choose Centrifugal Fans
Use centrifugal fans when:
- System pressure exceeds 500 Pa
- Ducted supply or return system
- Air handling unit application
- Noise is a concern
- Stable operation across varying conditions needed
- Non-overloading motor characteristic required
- High efficiency is priority
- Variable speed operation planned
Typical Centrifugal Applications:
- Air handling units (standard choice)
- Central station ventilation
- Clean room supply
- Laboratory fume hood exhaust
- Commercial kitchen exhaust (grease-laden)
- Dust collection systems
- Paint booth ventilation
- Data center cooling (CRAC/CRAH units)
- VAV and constant volume systems
Selection Warning: Never select an axial fan for a system requiring pressure above its capability. Operating an axial fan near stall causes: (1) severe noise and vibration, (2) rapid efficiency decline, (3) potential motor burnout, and (4) premature bearing failure. If pressure exceeds 500 Pa, use centrifugal—there's no alternative.
Fan Curves and Selection
Reading Fan Performance Curves
Both fan types are selected using manufacturer performance curves plotting:
- Static pressure (vertical axis) vs. airflow (horizontal axis)
- Multiple curves for different speeds or blade pitches
- Efficiency contours showing best operating zone
- BHP/power curves
Axial Fan Curve Characteristics
Key features:
- Steep curve drops off rapidly at high pressure
- "Stall region" where curve becomes unstable
- Narrow efficient operating band
- Power increases toward free delivery
Selection caution: Never select axial fan in stall region—look for "operating limit" line on curves.
Centrifugal Fan Curve Characteristics
Key features:
- Smoother curve from shutoff to free delivery
- No abrupt stall (gradual reduction)
- Wider efficient operating band
- BC/BI curves show self-limiting power
Selection flexibility: Can select anywhere on curve without stall concern.
Common Mistakes to Avoid
| Mistake | Impact | Prevention |
|---|---|---|
| Selecting axial for high-pressure system | Stall, noise, failure | Verify system pressure vs. fan capability |
| Ignoring future filter loading | Fan operates off-design as pressure increases | Size for dirty filter pressure |
| Forward-curved for variable system | Motor overload at low resistance | Use backward-curved for variable systems |
| Undersizing for altitude | Reduced air density reduces pressure capability | Apply altitude correction factors |
| Ignoring system effect | Actual pressure higher than calculated | Include inlet/outlet losses |
| Selecting at peak efficiency only | No margin for variations | Select with 10-15% margin |
Related Tools
Use these calculators for fan selection:
- Duct Pressure Loss Calculator - Calculate system pressure
- Duct Sizing Calculator - Size distribution ductwork
- Fresh Air Flow Calculator - Determine ventilation airflow
Key Takeaways
- Pressure determines choice: Axial for <500 Pa; centrifugal required for higher pressures
- Axial strengths: High volume, low pressure, lower cost, inline mounting
- Centrifugal strengths: Higher pressure, quieter, wider efficient range, non-overloading options
- AHUs use centrifugal: Filters + coils + ducts exceed axial capability
- Don't force axial: Operating near stall causes noise, efficiency loss, and failure
Further Reading
- Understanding Duct Pressure Loss - System pressure calculation
- Understanding Duct Sizing - Ductwork design fundamentals
- Equal Friction vs Velocity Reduction - Duct sizing methods
References & Standards
- ASHRAE Handbook—HVAC Systems and Equipment: Chapter 21, Fans
- AMCA 201: Fans and Systems
- AMCA 210/ASHRAE 51: Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating
- AMCA 300: Reverberant Room Method for Sound Testing of Fans
Disclaimer: This comparison provides general technical guidance. Actual fan performance depends on specific installation conditions and system characteristics. Always consult manufacturer data and verify selections with qualified engineers before final specification.