Fan Laws Calculator

How It Works

1. Enter the Known Operating Point

   Input the current airflow (CFM), total or static pressure (in. w.g.), brake horsepower (BHP), and fan speed (RPM). These values typically come from the fan performance data sheet, a commissioning report, or field measurements.

2. Select the Analysis Mode

   Choose Speed Change mode to model a VFD installation, sheave change, or motor replacement at a different RPM. Choose System Estimate mode to evaluate a new flow and pressure point on the system.

3. Enter the New Condition

   For speed change, enter the new RPM. For system estimate mode, enter the new desired flow (CFM) and optionally a new static pressure; if you leave the pressure blank, it scales along the original system curve.

4. Review the calculator

   The calculator shows predicted CFM, pressure, BHP, estimated electrical kW, and annual cost difference. Use measured kW, actual utility rates, operating profiles, and installed cost before treating the result as a VFD business case.

Built For

• Building engineers screening possible energy reduction from slowing a constant-speed supply air fan before gathering field kW and curve data
• HVAC technicians predicting the new airflow after changing the fan sheave from a 10" to an 8" driven pulley
• Plant maintenance teams troubleshooting why airflow dropped after replacing air filters with a higher-pressure-drop MERV rating
• Mechanical engineers evaluating whether an existing fan can handle increased airflow demand from a building addition by increasing speed

Features & Capabilities

Fan Affinity-Law Calculator

Applies the three local relationships: Q varies with N, SP varies with N squared, and BHP varies with N cubed. Source pointers are included, but the app does not reproduce or certify a proprietary standard calculation.

Energy Context

Converts shaft BHP to estimated electrical kW using entered motor/drive efficiency, then applies user-entered electricity cost and operating hours. Actual utility bills, measured kW, and VFD losses can change savings materially.

System Curve Estimate

Models the parabolic system curve (SP = k * Q^2) and shows proportional BHP at a new flow/pressure point. This is a screening estimate, not a full fan-curve intersection analysis.

Dual Operating Mode

Speed change mode for VFD or sheave changes, and system estimate mode for evaluating new flow/pressure conditions. Each mode shows the relevant inputs and outputs for that specific scenario.

Assumptions

• Air density is constant between the original and new operating conditions. If temperature or altitude changes significantly, density corrections must be applied separately.
• The system curve follows the standard parabolic relationship (P = k * Q^2), which assumes all system resistance is turbulent (friction and fitting losses). Systems with significant laminar flow components will deviate.
• Fan and system efficiency are assumed unchanged unless the user updates the known point or checks the manufacturer curve.
• BHP is treated as shaft horsepower; estimated electrical kW uses BHP * 0.7457 divided by entered motor/drive efficiency.

Limitations

• Only flags local low-speed screens; it does not model fan stall or surge behavior or prove stable operation.
• Does not account for motor efficiency changes at different speeds or loads. VFD energy savings calculations assume constant motor and drive efficiency, which is optimistic at very low speeds.
• Cannot model systems with multiple fans in series or parallel without additional analysis of the combined fan curve.

References

• AMCA Publication 201-23 - Fans and Systems source pointer.
• ASHRAE Handbook - HVAC Systems and Equipment, Fans chapter source pointer.
• DOE Improving Fan System Performance: A Sourcebook for Industry.

Frequently Asked Questions