Fan Performance

Fundamental Performance Principles

Fan performance represents the relationship between airflow delivery, pressure development, power consumption, and efficiency across the fan’s operating range. Performance is governed by fluid mechanics principles, specifically the conversion of mechanical shaft power into kinetic and potential energy of the airstream.

The fundamental energy transfer in a fan occurs through:

Mechanical Energy Conversion:

• Shaft power converts to air velocity (kinetic energy)
• Velocity pressure converts to static pressure (potential energy)
• Energy losses occur through friction, turbulence, and recirculation
• Performance depends on blade geometry, impeller design, and housing configuration

Operating Point Determination: Fan performance results from the intersection of the fan curve (supply capability) and system curve (resistance demand). This operating point establishes actual flow, pressure, power, and efficiency under specific conditions.

Fan Performance Curves

Performance curves graphically represent fan behavior across the operating range. Standard curves include pressure-flow, power-flow, and efficiency-flow relationships.

Pressure-Flow Characteristics

The pressure-flow curve shows total pressure (TP), static pressure (SP), and velocity pressure (VP) as functions of volumetric flow rate:

Total Pressure Curve:

• Represents maximum theoretical performance
• Includes both static and velocity pressure components
• TP = SP + VP
• Used for fan selection and rating

Static Pressure Curve:

• Represents useful pressure available to overcome system resistance
• Excludes velocity pressure at fan outlet
• Primary curve for system matching
• Determines actual operating point

Velocity Pressure:

• Dynamic pressure component at fan discharge
• VP = (V²)/4005 (in. wg) where V is velocity in fpm
• Recovered through discharge ductwork
• Significant in high-velocity applications

Curve Shape Analysis

Fan curve shape indicates performance characteristics and suitable applications:

Stall and Surge Regions:

• Stall occurs at low flow when blade angle of attack exceeds critical value
• Surge involves flow reversal and system instability
• Operating points should remain right of peak pressure point
• Safety margin typically 10-15% from surge point

Fan Laws

Fan laws (affinity laws) predict performance changes with speed, size, or density variations. These relationships derive from dimensional analysis and similarity principles.

Speed Variation (Same Fan, Variable Speed)

For a given fan operating at different speeds with constant density:

Flow Rate:

Q₂/Q₁ = N₂/N₁

Flow varies directly with speed ratio.

Pressure:

P₂/P₁ = (N₂/N₁)²

Pressure varies with square of speed ratio.

Power:

W₂/W₁ = (N₂/N₁)³

Power varies with cube of speed ratio.

Where:

• Q = volumetric flow rate (cfm)
• P = pressure (in. wg)
• W = shaft power (hp)
• N = rotational speed (rpm)
• Subscripts 1 and 2 denote initial and new conditions

Example Application: A fan delivering 10,000 cfm at 0.5 in. wg static pressure and 5 hp at 1200 rpm:

• At 1500 rpm: Q = 12,500 cfm, SP = 0.781 in. wg, W = 9.77 hp
• At 900 rpm: Q = 7,500 cfm, SP = 0.281 in. wg, W = 2.11 hp

Size Variation (Geometric Similarity)

For geometrically similar fans of different sizes at same speed and density:

Flow Rate:

Q₂/Q₁ = (D₂/D₁)³

Pressure:

P₂/P₁ = (D₂/D₁)²

Power:

W₂/W₁ = (D₂/D₁)⁵

Where D represents characteristic dimension (typically impeller diameter).

Density Variation

For same fan and speed with different air densities:

Flow Rate:

Q₂ = Q₁ (constant with density)

Pressure:

P₂/P₁ = ρ₂/ρ₁

Power:

W₂/W₁ = ρ₂/ρ₁

Where ρ is air density (lb/ft³).

Density Correction: Standard air density: ρ = 0.075 lb/ft³ at 70°F, sea level

Actual density:

ρ = (Pb/Pstd) × (Tstd/T) × 0.075 lb/ft³

Where:

• Pb = barometric pressure (in. Hg)
• Pstd = 29.92 in. Hg
• Tstd = 530°R (70°F)
• T = actual temperature (°R)

Fan Efficiency

Efficiency quantifies energy conversion effectiveness, relating useful air power output to mechanical shaft power input.

Efficiency Definitions

Total Efficiency:

ηt = (Q × TP)/(6356 × W)

Static Efficiency:

ηs = (Q × SP)/(6356 × W)

Where:

• η = efficiency (decimal)
• Q = flow rate (cfm)
• TP, SP = total or static pressure (in. wg)
• W = shaft power (hp)
• 6356 = conversion constant (ft-lbf/min per hp)

Mechanical Efficiency: Accounts for bearing friction, seal losses, and mechanical drive losses:

ηm = Wair/Wshaft

Typically 95-98% for properly maintained fans.

Peak Efficiency Operation

Efficiency varies across the operating range, reaching maximum at the design point:

Characteristics:

• Peak efficiency occurs at specific flow-pressure combination
• Operating significantly off peak reduces energy efficiency
• Variable speed fans maintain higher efficiency across load range
• Oversized fans operate inefficiently at reduced flow

Typical Efficiency Values:

Fan Power Calculations

Power calculations determine electrical and mechanical energy requirements.

Shaft Power

Theoretical air power:

Wair = (Q × P)/6356

Actual shaft power accounting for efficiency:

Wshaft = (Q × P)/(6356 × η)

Motor Power

Motor power must account for drive losses and safety factor:

Direct Drive:

Wmotor = Wshaft/ηm × SF

Belt Drive:

Wmotor = Wshaft/(ηm × ηbelt) × SF

Where:

• ηm = motor efficiency (0.90-0.96 for premium efficiency)
• ηbelt = belt drive efficiency (0.95-0.98)
• SF = service factor (typically 1.15-1.25)

Electrical Power:

kW = (Wmotor × 0.746)/PF

Where PF = power factor (0.85-0.90 typical).

Power Curve Characteristics

Power curves show distinct characteristics based on fan type:

Non-Overloading (Backward-Curved, Airfoil):

• Power peaks near design point
• Power decreases at higher flows
• Motor cannot overload across operating range
• Preferred for variable speed applications

Overloading (Forward-Curved):

• Power increases continuously with flow
• Motor can overload at high flows
• Requires careful motor sizing
• Suitable for constant volume applications

System Matching and Operating Point

The operating point occurs where fan curve intersects system curve.

System Curve

System resistance varies with flow squared:

Psystem = K × Q²

Where K is system resistance coefficient determined by:

K = Pdesign/Q²design

Operating Point Stability

Stable Operation:

• System curve slope exceeds fan curve slope
• Single intersection point
• Pressure increases with decreased flow
• Self-regulating behavior

Unstable Operation:

• Fan curve slope exceeds system curve slope
• Potential for multiple operating points
• Surge and stall regions
• Control instability

Multiple Fan Operation

Parallel Operation:

• Flow rates add at constant pressure
• Combined curve: Q = Q₁ + Q₂ at each pressure
• Used for redundancy and capacity modulation
• Requires similar fan characteristics

Series Operation:

• Pressures add at constant flow
• Combined curve: P = P₁ + P₂ at each flow
• Used for high-pressure applications
• Less common in HVAC systems

System Effect Factors

System effect factors account for installation conditions affecting fan performance.

AMCA System Effect Loss

AMCA 201 provides system effect loss coefficients for non-ideal installations:

Inlet Conditions:

• No inlet duct: SEF 0-0.05 in. wg
• Inlet elbow close-coupled: SEF 0.10-0.30 in. wg
• Inlet obstruction: SEF 0.05-0.50 in. wg

Outlet Conditions:

• No outlet duct: SEF 0-0.10 in. wg
• Outlet elbow close-coupled: SEF 0.20-0.60 in. wg
• Outlet obstruction: SEF 0.15-0.75 in. wg

Correction Method:

Prequired = Psystem + SEF

Select fan based on required pressure including system effects.

Installation Best Practices

Inlet Optimization:

• Provide straight duct minimum 2.5 diameters upstream
• Use gradual transitions (15° maximum included angle)
• Eliminate obstructions in inlet region
• Ensure uniform velocity profile

Outlet Optimization:

• Minimum 1.5 diameters straight duct downstream
• Avoid immediate elbows after discharge
• Use turning vanes in elbows
• Size outlet duct for 1.5-2.0 times fan outlet area

Performance Testing and Verification

Field Performance Testing

Field testing verifies actual performance against design specifications per AMCA 203.

Test Procedure:

1. Measure flow rate (pitot traverse or flow station)
2. Measure pressures (static, total, velocity)
3. Measure power input (true RMS power)
4. Calculate efficiency
5. Plot operating point on manufacturer curve

Acceptance Criteria:

• Flow within ±10% of design
• Pressure within ±5% of design
• Power within ±10% of calculated
• Efficiency within 5 points of rated

Common Performance Issues

Design Considerations

Fan Selection Criteria

Performance Requirements:

• Operating flow rate with appropriate safety margin (10-15%)
• Total system pressure including all losses and effects
• Required turndown range for variable volume systems
• Altitude and temperature correction factors

Efficiency Optimization:

• Select fan to operate at or near peak efficiency
• Avoid oversizing beyond necessary safety margin
• Consider variable speed for varying loads
• Evaluate life-cycle cost including energy consumption

Operating Range:

• Ensure operating point in stable region of curve
• Provide adequate margin from surge/stall regions
• Consider future system modifications
• Account for filter loading over time

Control Strategy Impact

Variable Speed Control:

• Fan laws apply for speed modulation
• Energy savings proportional to cube of speed reduction
• Maintains high efficiency across load range
• Preferred for VAV and variable load applications

Inlet Vane Control:

• Modifies fan curve by changing inlet swirl
• Less efficient than speed control
• Energy savings approximately proportional to square of flow reduction
• Lower first cost than VFD

Outlet Damper Control:

• Adds artificial resistance to system
• Least efficient control method
• Minimal energy savings
• Acceptable only for limited modulation range

Code and Standard References

AMCA Standards:

• AMCA 210: Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating
• AMCA 201: Fans and Systems
• AMCA 203: Field Performance Measurement of Fan Systems
• AMCA 300: Reverberant Room Method for Sound Testing of Fans

ASHRAE Standards:

• ASHRAE 51: Laboratory Methods of Testing Fans for Aerodynamic Performance Rating
• ASHRAE 90.1: Energy Standard for Buildings (fan power limitations)
• ASHRAE Handbook - HVAC Systems and Equipment (Chapter 21: Fans)

Performance Ratings:

• FEG (Fan Efficiency Grade) per AMCA 205
• FEI (Fan Energy Index) per ASHRAE 90.1 Appendix G
• Minimum efficiency requirements per energy codes

Advanced Performance Topics

Variable Frequency Drive Operation

VFD operation modifies fan curves per fan laws:

• Base curve at 100% speed (60 Hz)
• Proportional curves at reduced speeds
• Operating point follows system curve
• Energy reduction approximately cubic with speed

VFD Efficiency Impact:

• Drive losses 2-5% at full load
• Higher percentage losses at light loads
• Total system efficiency considers drive and motor losses
• Harmonics and power quality considerations

Altitude and Temperature Corrections

High altitude and elevated temperature reduce air density:

Performance Impact:

• Flow rate unchanged (volumetric basis)
• Pressure reduced proportionally to density ratio
• Power reduced proportionally to density ratio
• Fan selection based on standard conditions

Correction Procedure:

1. Calculate actual density at operating conditions
2. Correct specified pressure to standard density
3. Select fan based on corrected performance
4. Verify motor power adequate for actual conditions

Non-Standard Gas Applications

For gases other than air:

Pgas/Pair = ρgas/ρair

Wgas/Wair = ρgas/ρair

Molecular weight and temperature determine density ratio.

Performance Documentation

Complete fan performance documentation includes:

• Certified performance curves from manufacturer
• Operating point marked on curves
• System effect calculations
• Altitude and temperature corrections
• Motor selection and electrical data
• Installation requirements and clearances
• Acceptable operating range
• Maintenance and inspection intervals

Performance documentation ensures proper selection, installation, and operation throughout system life cycle.