Fan Law Calculator

Initial Fan State

New Fan State

Tip: The Fan Laws (or Affinity Laws) describe the relationship between fan speed, airflow, pressure, and power consumption.

Resulting Fan Performance

New Flow Rate

–

CFM

New Pressure

–

in.wg.

New Power

–

BHP

function calculateFanLaws() { var rpm1 = parseFloat(document.getElementById('rpm1').value); var rpm2 = parseFloat(document.getElementById('rpm2').value); var cfm1 = parseFloat(document.getElementById('cfm1').value); var sp1 = parseFloat(document.getElementById('sp1').value); var bhp1 = parseFloat(document.getElementById('bhp1').value); if (!rpm1 || !rpm2 || rpm1 === 0) { alert("Please enter both Initial Speed (RPM1) and New Speed (RPM2) to calculate."); return; } var ratio = rpm2 / rpm1; var resultsDiv = document.getElementById('fan-results'); resultsDiv.style.display = 'block'; // Law 1: Flow if (!isNaN(cfm1)) { var cfm2 = cfm1 * ratio; document.getElementById('res-cfm').innerText = cfm2.toLocaleString(undefined, {maximumFractionDigits: 2}); } else { document.getElementById('res-cfm').innerText = "N/A"; } // Law 2: Pressure if (!isNaN(sp1)) { var sp2 = sp1 * Math.pow(ratio, 2); document.getElementById('res-sp').innerText = sp2.toLocaleString(undefined, {maximumFractionDigits: 3}); } else { document.getElementById('res-sp').innerText = "N/A"; } // Law 3: Power if (!isNaN(bhp1)) { var bhp2 = bhp1 * Math.pow(ratio, 3); document.getElementById('res-bhp').innerText = bhp2.toLocaleString(undefined, {maximumFractionDigits: 3}); } else { document.getElementById('res-bhp').innerText = "N/A"; } }

Understanding the Fan Affinity Laws

The Fan Laws, also known as Affinity Laws, are a set of mathematical formulas used to predict the performance of a centrifugal fan when its rotational speed (RPM) or impeller diameter changes. These laws are critical for HVAC engineers, industrial technicians, and energy auditors when evaluating the impact of Variable Frequency Drives (VFDs) or pulley changes.

The Three Primary Fan Laws

1. Law 1: Airflow (CFM) is proportional to Speed. If you double the fan speed, you double the volume of air moving through the system.
   Formula: CFM₂ = CFM₁ × (RPM₂ / RPM₁)
2. Law 2: Static Pressure is proportional to the square of Speed. If you double the fan speed, the pressure capability increases by four times.
   Formula: SP₂ = SP₁ × (RPM₂ / RPM₁)²
3. Law 3: Power (BHP) is proportional to the cube of Speed. This is the most significant law for energy savings. Doubling the fan speed increases the power requirement by eight times. Conversely, slowing a fan down slightly can lead to massive energy reductions.
   Formula: BHP₂ = BHP₁ × (RPM₂ / RPM₁)³

Practical Example: Speed Reduction for Energy Savings

Imagine a fan running at 1,000 RPM, moving 10,000 CFM, at 2.0 in.wg. static pressure, consuming 10 HP. If you reduce the speed to 800 RPM (an 80% speed reduction):

• New CFM: 10,000 × (0.8) = 8,000 CFM
• New Pressure: 2.0 × (0.8)² = 1.28 in.wg.
• New Power: 10 × (0.8)³ = 5.12 HP

By reducing the speed by only 20%, you have cut the power consumption nearly in half! This demonstrates why VFDs are a popular investment for industrial ventilation and air handling units.

When to Use This Calculator

This calculator is perfect for:

• Pulley Changes: Determining what happens to your airflow if you change the belt-drive ratio.
• VFD Tuning: Estimating energy savings when slowing down a motor to match actual demand.
• System Balancing: Calculating how much more speed is required to reach a target CFM in an existing duct system.

Important Note: These laws assume that the system resistance (ductwork) remains constant and that the air density does not change significantly. In real-world applications, factors like motor efficiency and air temperature may slightly alter the actual outcomes.