Fan Types
HVAC systems employ various fan types to move air through ducts, equipment, and spaces. Understanding fan construction, performance characteristics, and application suitability enables appropriate selection for each application’s requirements.
Centrifugal Fans
Centrifugal fans accelerate air radially outward from a rotating impeller, converting velocity energy to pressure through the scroll housing.
Forward-Curved (FC) Centrifugal
Construction:
- Many shallow blades (24-64)
- Blades curve in rotation direction
- Compact scroll housing
- Lower tip speeds
Characteristics:
| Parameter | Typical Value |
|---|---|
| Efficiency | 60-70% |
| Pressure | Low-medium (to 5" w.g.) |
| Volume | High relative to size |
| Speed | Lower RPM |
| Noise | Higher at low frequency |
Applications:
- Packaged HVAC equipment
- Residential furnaces
- Small air handling units
- Clean air applications
Advantages: Compact, economical, quiet at design point Limitations: Unstable left of peak, not for dirty air
Backward-Curved (BC) Centrifugal
Construction:
- Fewer blades (10-16)
- Blades curve against rotation
- Self-limiting power characteristic
- Available as backward-inclined (flat) or airfoil
Characteristics:
| Parameter | Typical Value |
|---|---|
| Efficiency | 75-85% (airfoil higher) |
| Pressure | Medium-high (to 10" w.g.) |
| Volume | Moderate |
| Speed | Higher RPM |
| Noise | Lower, higher frequency |
Applications:
- Central air handling units
- Clean room systems
- Variable volume systems
- Energy-critical applications
Advantages: High efficiency, non-overloading power Limitations: Higher first cost, limited particle tolerance
Radial Blade Centrifugal
Construction:
- Straight radial blades (6-12)
- Robust, simple construction
- Self-cleaning characteristics
- Heavy-duty housings available
Characteristics:
| Parameter | Typical Value |
|---|---|
| Efficiency | 55-70% |
| Pressure | High (to 20" w.g.) |
| Volume | Lower |
| Speed | Higher RPM |
| Noise | Higher |
Applications:
- Material handling
- Dust collection
- Industrial exhaust
- Corrosive/erosive environments
Advantages: Handles particulates, high pressure capability Limitations: Lower efficiency, higher noise
Axial Fans
Axial fans move air parallel to the shaft axis, providing high volume at lower pressure.
Propeller Fans
Construction:
- Simple blade/hub assembly
- No inlet or outlet housing
- Direct or belt drive
- 2-8 blades typical
Characteristics:
| Parameter | Typical Value |
|---|---|
| Efficiency | 40-60% |
| Pressure | Very low (<0.5" w.g.) |
| Volume | High |
| Speed | Variable |
| Noise | Moderate-high |
Applications:
- General ventilation
- Cooling towers
- Condenser fans
- Agricultural ventilation
Advantages: Simple, economical, high airflow Limitations: Very low pressure capability
Tube-Axial Fans
Construction:
- Propeller in cylindrical housing
- Straightens airflow somewhat
- May include inlet bell
- Motor in airstream or external
Characteristics:
| Parameter | Typical Value |
|---|---|
| Efficiency | 50-65% |
| Pressure | Low-medium (to 2" w.g.) |
| Volume | High |
| Speed | Higher RPM |
| Noise | Moderate |
Applications:
- Duct boosters
- Exhaust systems
- Paint spray booths
- Parking garage ventilation
Advantages: Compact, good volume/pressure balance Limitations: Air swirl without straightener
Vane-Axial Fans
Construction:
- Propeller with guide vanes
- Inlet and/or outlet vanes
- Removes rotational swirl
- Higher pressure capability
Characteristics:
| Parameter | Typical Value |
|---|---|
| Efficiency | 65-80% |
| Pressure | Medium (to 6" w.g.) |
| Volume | High |
| Speed | Higher RPM |
| Noise | Lower with proper vanes |
Applications:
- Large HVAC systems
- Tunnel ventilation
- Mine ventilation
- Industrial processes
Advantages: High efficiency, compact, adjustable pitch available Limitations: Higher cost, noise at off-design
Mixed-Flow Fans
Mixed-flow fans combine centrifugal and axial characteristics.
Construction:
- Angled impeller (between axial and radial)
- Compact cylindrical housing
- Combines volume with pressure
Characteristics:
- Efficiency: 70-80%
- Pressure: Medium (2-6" w.g.)
- Compact footprint
- Lower noise than equivalent axial
Applications:
- Inline duct applications
- Space-constrained installations
- Retrofit situations
- Parking structures
Specialty Fans
Plenum Fans (Plug Fans)
Centrifugal impeller without scroll housing:
- Discharges directly into plenum
- Multiple fans in parallel
- Easy maintenance access
- Lower system effect losses
Inline Centrifugal
Centrifugal fan with axial-style mounting:
- Belt or direct drive
- Maintains duct continuity
- Higher pressure than tube-axial
- Good efficiency
Roof/Wall Exhausters
Specialized mounting for building exhaust:
- Centrifugal: upblast/downblast
- Axial: propeller or tube type
- Weather protection
- Various drive arrangements
Ceiling Fans
Low-speed circulation fans:
- Large diameter, low RPM
- Destratification applications
- Supplemental cooling effect
- Visible in occupied space
Fan Comparison Summary
| Type | Efficiency | Pressure | Volume | Best Application |
|---|---|---|---|---|
| FC Centrifugal | 60-70% | Low-Med | High | Packaged equipment |
| BC Centrifugal | 75-85% | Med-High | Med | Central AHU |
| Radial | 55-70% | High | Low | Material handling |
| Propeller | 40-60% | Very Low | High | Ventilation, condensers |
| Tube-Axial | 50-65% | Low-Med | High | Duct boosters |
| Vane-Axial | 65-80% | Medium | High | Large HVAC, tunnels |
| Mixed-Flow | 70-80% | Medium | Med-High | Inline applications |
Selection Considerations
Operating Point Matching
Select fan type based on system requirements:
- High volume, low pressure → Propeller or FC
- Medium volume and pressure → BC centrifugal or vane-axial
- High pressure → Radial or BC centrifugal
- Space constraints → Mixed-flow or inline
Efficiency Priority
For energy-critical applications:
- Backward-curved airfoil centrifugal
- Vane-axial
- Backward-inclined centrifugal
- Mixed-flow
Particulate Environments
For dirty or corrosive air:
- Radial blade centrifugal
- Tube-axial with coated blades
- Avoid FC centrifugal and airfoil
Understanding fan type characteristics enables optimal selection for each HVAC application’s specific requirements of airflow, pressure, efficiency, and operating environment.