Fan Performance

Fan performance characteristics determine how effectively fans move air through HVAC systems. Understanding performance curves, efficiency factors, and operating behavior enables optimal fan selection and system operation.

Fan Performance Curves

Fan Curve Components

A complete fan performance curve includes:

1. Pressure-Volume Curve: Static or total pressure vs. CFM
2. Power Curve: BHP vs. CFM
3. Efficiency Curve: Mechanical efficiency vs. CFM
4. Operating Region: Stable operation zone

Static Pressure vs. Total Pressure

Total Pressure: $$P_T = P_S + P_V$$

Velocity Pressure: $$P_V = \frac{\rho V^2}{2g} = \left(\frac{V}{4005}\right)^2$$ (inches w.g., standard air)

Fan Total Pressure (FTP): $$FTP = TP_{outlet} - TP_{inlet}$$

Fan Static Pressure (FSP): $$FSP = FTP - VP_{outlet}$$

Curve Shapes by Fan Type

Operating Point

System Curve

The system resistance curve represents duct and component losses:

$$\Delta P_{sys} = K_{sys} \times Q^2$$

Where $K_{sys}$ is the system resistance coefficient.

Finding Operating Point

The fan operates where fan curve intersects system curve:

$$P_{fan}(Q) = P_{sys}(Q)$$

This intersection point defines:

• Delivered airflow (CFM)
• Developed pressure
• Required power
• Operating efficiency

Operating Point Shifts

Increased System Resistance:

• System curve steepens
• Operating point moves left on fan curve
• Airflow decreases
• Pressure increases

Decreased System Resistance:

• System curve flattens
• Operating point moves right
• Airflow increases
• Pressure decreases

Fan Efficiency

Definitions

Total Efficiency: $$\eta_T = \frac{Q \times P_T}{6356 \times BHP}$$ (Q in CFM, P in in. w.g.)

Static Efficiency: $$\eta_S = \frac{Q \times P_S}{6356 \times BHP}$$

Peak Efficiency Point

Maximum efficiency occurs at specific flow rate:

• Design near peak efficiency
• Avoid operation far from peak
• Efficiency drops at extremes

Typical Peak Efficiencies:

Brake Horsepower

Power at the fan shaft:

$$BHP = \frac{Q \times P_T}{6356 \times \eta_T}$$

Include motor efficiency for electrical input:

$$kW_{input} = \frac{BHP \times 0.746}{\eta_{motor}}$$

Surge and Stall

Centrifugal Fan Surge

Surge occurs when operating left of peak pressure:

• Periodic flow reversal
• Noise and vibration
• Potential damage
• Unstable operation

Avoid operation in surge region (typically left 1/3 of curve).

Axial Fan Stall

Stall occurs at high pressure/low flow:

• Blade flow separation
• Sharp efficiency drop
• Noise increase
• Performance instability

Stall region visible as curve “dip” on axial fan performance.

Selection Guidelines

Safe Operating Range:

• Centrifugal: Right of peak pressure
• Axial: Right of stall dip
• Allow margin for system variations

Performance at Non-Standard Conditions

Altitude Effects

At elevation, air density decreases:

$$\rho_{alt} = \rho_{std} \times \frac{P_{baro}}{P_{std}}$$

Fan delivers same CFM but:

• Reduced pressure capability
• Reduced power consumption
• Same mass flow requires higher CFM

Temperature Effects

Hot air is less dense:

$$\rho_T = \rho_{std} \times \frac{T_{std}}{T_{actual}}$$ (absolute temps)

Correct published curves for actual density.

Correction Example

Given: Fan curve at standard conditions Need: Performance at 5,000 ft, 120°F

Density ratio: $$\frac{\rho_{actual}}{\rho_{std}} = 0.83 \times \frac{530}{580} = 0.76$$

Corrected pressure: Published × 0.76 Corrected power: Published × 0.76

Multiple Fan Operation

Fans in Parallel

Same pressure, combined flow:

$$Q_{total} = Q_1 + Q_2$$ $$P_{total} = P_{individual}$$

Requirements:

• Similar fans recommended
• System curve determines actual split
• Ensure both operate in stable region

Fans in Series

Same flow, combined pressure:

$$P_{total} = P_1 + P_2$$ $$Q_{total} = Q_{individual}$$

Considerations:

• Second fan handles heated air
• Derate second fan for temperature
• Used for high-pressure systems

Performance Testing

AMCA Standard Testing

AMCA 210/ASHRAE 51 specifies test methods:

• Multi-nozzle chamber
• Pitot traverse
• Various configurations

Test conditions:

• Standard air: 0.075 lb/ft³
• Sea level pressure
• 70°F temperature

Field Testing

Verify installed performance:

• Pitot traverse in duct
• Measure motor amps
• Calculate actual CFM and BHP
• Compare to published curves

Performance Verification

$$CFM = V_{avg} \times A_{duct}$$

$$BHP = \frac{V \times I \times PF \times \eta_{motor}}{746}$$ (single phase)

Compare actual to design, accounting for system effect.

Specification Considerations

Selecting Operating Point

1. Calculate system CFM requirement
2. Determine system static pressure
3. Add safety factor (10-20%)
4. Plot on fan curve
5. Verify efficiency and power
6. Ensure stable operation region

Rating Tolerances

AMCA certified fans meet tolerance requirements:

• Airflow: ±10% at rated pressure
• Pressure: ±10% at rated flow
• Peak efficiency: -5%

Understanding fan performance characteristics enables selection of fans that efficiently deliver required airflow at design pressure while operating reliably within their stable performance range.