Ductwork Cfm Calculator

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Ductwork CFM Calculator

Rectangular Round
function toggleDuctInputs() { var rectInputs = document.getElementById('rectangularInputs'); var rectInputs2 = document.getElementById('rectangularInputs2′); var roundInputs = document.getElementById('roundInputs'); var ductShapeRect = document.getElementById('ductShapeRect'); if (ductShapeRect.checked) { rectInputs.style.display = 'block'; rectInputs2.style.display = 'block'; roundInputs.style.display = 'none'; } else { rectInputs.style.display = 'none'; rectInputs2.style.display = 'none'; roundInputs.style.display = 'block'; } } function calculateCFM() { var ductShapeRect = document.getElementById('ductShapeRect').checked; var ductWidth = parseFloat(document.getElementById('ductWidth').value); var ductHeight = parseFloat(document.getElementById('ductHeight').value); var ductDiameter = parseFloat(document.getElementById('ductDiameter').value); var airVelocity = parseFloat(document.getElementById('airVelocity').value); var resultDiv = document.getElementById('result'); resultDiv.innerHTML = "; // Clear previous results if (isNaN(airVelocity) || airVelocity <= 0) { resultDiv.innerHTML = 'Please enter a valid Air Velocity (FPM) greater than zero.'; return; } var ductAreaSqFt; if (ductShapeRect) { if (isNaN(ductWidth) || ductWidth <= 0 || isNaN(ductHeight) || ductHeight <= 0) { resultDiv.innerHTML = 'Please enter valid Duct Width and Height (inches) greater than zero.'; return; } // Area in square inches: width * height // Convert to square feet: (width * height) / 144 ductAreaSqFt = (ductWidth * ductHeight) / 144; } else { // Round duct if (isNaN(ductDiameter) || ductDiameter <= 0) { resultDiv.innerHTML = 'Please enter a valid Duct Diameter (inches) greater than zero.'; return; } // Area in square inches: π * (diameter/2)^2 // Convert to square feet: (π * (diameter/2)^2) / 144 ductAreaSqFt = Math.PI * Math.pow((ductDiameter / 2), 2) / 144; } var cfm = ductAreaSqFt * airVelocity; resultDiv.innerHTML = '

Calculated CFM:

'; resultDiv.innerHTML += '' + cfm.toFixed(2) + ' CFM'; resultDiv.innerHTML += '(Cubic Feet per Minute)'; } // Initialize input visibility on page load window.onload = toggleDuctInputs;

Understanding CFM in Ductwork Design

Cubic Feet per Minute (CFM) is a fundamental metric in heating, ventilation, and air conditioning (HVAC) systems. It quantifies the volume of air moving through a space or duct per minute. Understanding and accurately calculating CFM is crucial for designing an efficient, comfortable, and healthy indoor environment.

What is CFM and Why is it Important?

CFM represents the volumetric flow rate of air. In ductwork, it tells you how much air is being delivered to or removed from a specific area. Correct CFM is vital for several reasons:

  • Comfort: Adequate airflow ensures consistent temperatures throughout a space, preventing hot or cold spots.
  • Efficiency: Properly sized ducts and balanced CFM reduce the workload on your HVAC system, leading to lower energy consumption and utility bills.
  • System Longevity: An HVAC system struggling with insufficient or excessive airflow can experience premature wear and tear, shortening its lifespan.
  • Indoor Air Quality (IAQ): Proper ventilation (measured in CFM) is essential for removing pollutants, allergens, and stale air, bringing in fresh outdoor air.
  • Noise Control: Air moving too fast through ducts can create whistling or rushing noises, while too slow can lead to poor distribution.

How the Ductwork CFM Calculator Works

This calculator determines the CFM based on the physical dimensions of your ductwork and the velocity of the air moving through it. The core formula is:

CFM = Duct Area (square feet) × Air Velocity (Feet Per Minute)

Calculating Duct Area:

Since duct dimensions are typically given in inches, they must first be converted to square feet for the CFM calculation. The calculator handles this conversion automatically:

  • For Rectangular Ducts:

    Duct Area (sq ft) = (Width (inches) × Height (inches)) / 144

    The division by 144 converts square inches (12 inches × 12 inches = 144 square inches) into square feet.

  • For Round Ducts:

    Duct Area (sq ft) = (π × (Diameter (inches) / 2)²) / 144

    Here, π (pi ≈ 3.14159) is used to calculate the area of a circle, and again, the result is divided by 144 to convert from square inches to square feet.

Air Velocity (FPM):

Air velocity is measured in Feet Per Minute (FPM) and represents how fast the air is traveling through the duct. Typical air velocities vary depending on the section of the ductwork and the application:

  • Main Trunks: Often range from 700-1200 FPM.
  • Branch Ducts: Typically 500-900 FPM.
  • Supply/Return Grilles: Usually lower, around 300-700 FPM, to minimize drafts and noise.

The ideal air velocity is a balance between delivering sufficient airflow and avoiding excessive noise or static pressure loss.

Using the Calculator

  1. Select Duct Shape: Choose whether your duct is rectangular or round.
  2. Enter Dimensions:
    • For rectangular ducts, input the Width and Height in inches.
    • For round ducts, input the Diameter in inches.
  3. Enter Air Velocity: Input the air velocity in Feet Per Minute (FPM). If you don't know the exact velocity, you can use typical values for your duct type as a starting point.
  4. Click "Calculate CFM": The calculator will instantly display the CFM for your specified ductwork.

Example Calculations:

  • Rectangular Duct: A duct with a width of 12 inches, a height of 8 inches, and an air velocity of 700 FPM.
    • Area = (12 * 8) / 144 = 0.6667 sq ft
    • CFM = 0.6667 * 700 = 466.69 CFM
  • Round Duct: A duct with a diameter of 10 inches and an air velocity of 800 FPM.
    • Area = (π * (10/2)²) / 144 = (3.14159 * 25) / 144 = 0.5454 sq ft
    • CFM = 0.5454 * 800 = 436.32 CFM

Limitations and Considerations

While this calculator provides a theoretical CFM based on ideal conditions, actual airflow in a real-world HVAC system can be influenced by several factors:

  • Duct Leakage: Imperfectly sealed ducts can lose a significant amount of air.
  • Fittings and Bends: Elbows, transitions, and other fittings create resistance and reduce airflow.
  • Filters and Coils: Dirty filters or restrictive coils can impede airflow.
  • Static Pressure: The resistance to airflow within the duct system, which the fan must overcome.
  • Fan Performance: The actual CFM delivered by an HVAC fan can vary based on static pressure and motor efficiency.

For precise HVAC system design and balancing, it is always recommended to consult with a qualified HVAC professional who can perform detailed load calculations and airflow measurements.

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