Fan Laws Performance Calculator

This industrial-grade calculator applies the standard Fan Affinity Laws to predict changes in performance when modifying fan speed (RPM), impeller diameter, or air density. It features exhaustive step-by-step physics validation, Tip Speed Safety Analysis, and Motor Overload Protection warnings.

1. System Configuration

Setup

2. Baseline Performance (Original)

Current Operating Point
CFM
in.wg
BHP
Current Parameters
RPM
in
lb/ft³

3. Target Conditions (New)

Modify any parameter below to calculate the new performance. Unchanged fields will default to original values.

Proposed Changes
RPM
in
lb/ft³

Engineering Insights: Fan Laws

1. The "Cubic Law" of Power

The most critical takeaway from the Fan Laws is the relationship between Speed and Power. While Flow increases linearly with speed, Power increases by the cube of the speed ratio.

$$ \frac{P_2}{P_1} = \left(\frac{N_2}{N_1}\right)^3 $$

Example: Increasing fan speed by just 10% (1.1x) increases power consumption by 33% ($1.1^3 = 1.33$). Doubling the speed requires 8x the power. Always verify motor capacity before speeding up a fan!

2. Density & Altitude Effects

Fan Laws also account for air density ($\rho$). Fans are constant volume machines, but mass flow and power vary linearly with density.

  • High Altitude: Air is less dense. A fan moves the same CFM, but generates less Pressure and uses less Power.
  • High Temperature: Hot air is less dense. Similar effect to altitude.
  • Cold Start: Cold air is dense. A fan sized for hot operation may overload the motor during a cold startup (winter).

3. Impeller Diameter Changes

Changing the wheel diameter (trimming or replacing) has a drastic effect. Power varies with the 5th power of the diameter ratio.

$$ \frac{P_2}{P_1} = \left(\frac{D_2}{D_1}\right)^5 $$

Even a small increase in diameter can catastrophically overload a drive system.