Fan Law Calculator

About the Fan Law Calculator

The Fan Law Calculator applies the three affinity laws to predict how changes in fan speed or impeller diameter affect airflow, pressure, and power consumption. These laws are essential for HVAC design, industrial ventilation, and energy optimization, especially when you are comparing VFD settings, belt changes, or fan wheel swaps. It gives a quick estimate of whether a proposed speed change is even in the right operating range.

The fan laws state that airflow varies linearly with speed (Q ∝ N), pressure varies with the square (P ∝ N²), and power varies with the cube (HP ∝ N³). This cubic relationship means that a 20% speed reduction cuts power consumption by nearly 50% — making variable frequency drives (VFDs) one of the best energy investments in building systems.

Enter your current fan operating conditions and the new speed or diameter, and the calculator shows the new airflow, pressure, power, and efficiency. The comparison table reveals how sensitive each parameter is to speed changes.

When This Page Helps

Use this calculator when you need a first-pass estimate of what a speed change will do to airflow, pressure, and power before touching a VFD or fan sheave. It is useful for energy-savings studies, troubleshooting, and comparing how much performance you give up for a lower operating cost in HVAC or process fans.

How to Use the Inputs

1. Enter the current fan speed in RPM.
2. Enter the current airflow (CFM), static pressure (in. w.g.), and power (HP or kW).
3. Enter the new fan speed (or new impeller diameter for diameter changes).
4. Select whether you're changing speed or diameter.
5. Or use a preset for common HVAC scenarios.
6. Review the calculated new airflow, pressure, and power.
7. Check the energy savings comparison table.

Example Calculation

Tips & Best Practices

• The cube law means 50% speed = 12.5% power — always consider VFDs for variable-load fans.
• Fan laws assume constant system resistance — if you open/close dampers, the operating point shifts.
• Motor nameplate HP is the maximum; actual power depends on the operating point on the fan curve.
• For energy savings estimates, use blended speed hours, not just peak reduction.
• Backward-curved fans are more efficient than forward-curved but cost more — choose based on application.
• Always verify fan laws predictions with manufacturer's multi-speed performance data when available.

The Three Fan Affinity Laws Explained

Fan Law 1 (Flow): Doubling fan speed doubles airflow. This linear relationship is intuitive — spin the impeller faster and it moves more air proportionally. Fan Law 2 (Pressure): Doubling speed quadruples pressure because centrifugal force increases with the square of velocity. Fan Law 3 (Power): Doubling speed increases power by 8× (2³). This is because you're moving twice the air (Law 1) against four times the pressure (Law 2), requiring 8× the energy.

Variable Frequency Drives and Energy Savings

VFDs adjust motor speed electronically to match actual airflow demand. In VAV (Variable Air Volume) HVAC systems, fan speed is modulated based on duct static pressure, which responds to zone damper positions. At 60% airflow demand, the fan runs at ~60% speed, consuming only ~22% of full-speed power. Annual energy savings typically pay back VFD installation cost in 1-3 years.

Fan Selection and System Design

Proper fan selection requires matching the fan curve to the system curve at the design operating point. Oversized fans waste energy running throttled or at low speed. The best efficiency point (BEP) should fall near the most common operating condition. AMCA (Air Movement and Control Association) certified ratings ensure published performance data is accurate.

Frequently Asked Questions

• What are the fan affinity laws?

  Three relationships that predict fan performance changes with speed or diameter: (1) airflow ∝ speed, (2) pressure ∝ speed², (3) power ∝ speed³. They assume the system curve and fan efficiency remain constant.

• Why does the cube law make VFDs so cost-effective?

  Because power varies with the cube of speed, small speed reductions yield large power savings. Reducing fan speed by 20% cuts power by 49%. A VFD on a 10 HP fan running at 80% saves ~2.4 HP continuously, potentially $1,500+/year in electricity.

• When do fan laws not apply accurately?

  Fan laws are approximate when: (1) the system curve changes (e.g., dampers reposition), (2) the fan operates near stall/surge, (3) the speed change is very large (>±30%), or (4) Reynolds number effects become significant at very low speeds.

• Can I use fan laws for different diameter impellers?

  Yes, the same ratios apply with D₂/D₁ instead of N₂/N₁, but with an additional D ratio factor: Q ∝ D³, P ∝ D², HP ∝ D⁵ (when speed is held constant). This is used when resheaving or swapping wheels.

• What is a system curve?

  The system curve (P ∝ Q²) plots the pressure needed to push airflow through ductwork. The fan operating point is where the fan curve intersects the system curve. Fan laws slide the fan curve up/down along the system curve.

• How do I measure fan performance?

  Measure airflow with a pitot tube traverse or flow hood at the duct. Measure static pressure with a manometer at the fan inlet/outlet. Measure power with a power meter at the motor. Calculate efficiency as (CFM × SP) / (6356 × BHP).