What is a fan performance curve?

A fan performance curve shows the relationship between airflow (CFM or m³/h) and static pressure (inches WG or Pa) a fan can deliver. As airflow increases, pressure decreases. The curve also shows power consumption and efficiency at different operating points. Manufacturers provide these curves for proper fan selection.

What is the fan operating point?

The operating point is where the fan curve intersects the system resistance curve—it shows actual airflow and pressure in your specific system. The system curve follows the square law: pressure varies with the square of flow rate (P ∝ Q²). Finding the operating point is essential for verifying fan selection.

What is Best Efficiency Point (BEP) for fans?

BEP is the operating point where the fan achieves maximum efficiency, typically shown on performance curves. Operating near BEP (within ±10%) minimizes energy consumption and extends fan life. Operating far from BEP increases vibration, noise, and wear. Select fans where your operating point falls near BEP.

How do VFDs (Variable Frequency Drives) affect fan curves?

VFDs adjust fan speed, shifting the entire performance curve according to affinity laws: Flow ∝ Speed, Pressure ∝ Speed², Power ∝ Speed³. Reducing speed 20% reduces power by ~49%, making VFDs extremely energy-efficient for variable-load systems. Each speed creates a new fan curve.

What is the fan surge zone and why avoid it?

The surge zone is the unstable region at low flow rates where the fan curve may have a peak or inflection. Operating here causes flow reversal, pressure pulsations, noise, and vibration that can damage the fan. Ensure your system operating point is always in the stable region to the right of any curve peak.

How do I compare multiple fans for selection?

Plot multiple fan curves with your system curve. Compare: operating point efficiency, noise levels, motor power requirements, first cost, and maintenance needs. The best fan operates near its BEP at your design conditions with adequate margin for future capacity and lowest total cost of ownership.

Fan/System Curve Calculator

Interactive tool to analyze fan performance curves, system resistance curves, and determine the operating point. Essential for HVAC fan selection and VFD optimization.

ASHRAE HandbookAMCA 201/210Interactive CurvesVFD Simulation

Fan & System Configuration

Define fan characteristics and system resistance to find the operating point where curves intersect.

About Fan Curves

Fan and system curves are fundamental tools for HVAC fan selection. The operating point occurs where the fan performance curve intersects the system resistance curve.

Key Concepts

Fan Curve: Shows static pressure vs. airflow - characteristic of the fan
System Curve: Shows pressure loss vs. airflow - follows P = K × Q²
Operating Point: Where curves intersect - actual fan operation
BEP (Best Efficiency Point): Where fan operates most efficiently

Fan Affinity Laws

Flow ∝ Speed¹ (Q₂/Q₁ = N₂/N₁)
Pressure ∝ Speed² (P₂/P₁ = (N₂/N₁)²)
Power ∝ Speed³ (HP₂/HP₁ = (N₂/N₁)³)

Key Formulas

System Curve::

P = K \times Q^2

Air Horsepower::

P_h = \frac{Q \times SP}{6356}

Brake Horsepower::

BHP = \frac{P_h}{\eta}

Fan/System Curve Calculator

Fan Curve

System Curve

VFD/Speed Control

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