How to Calculate Air Flow Rate in Duct

by

Airflow Rate Calculator

This calculator helps you determine the airflow rate in a duct, a crucial measurement for HVAC system performance. Airflow rate is typically measured in Cubic Feet per Minute (CFM) or Cubic Meters per Hour (CMH).

.calculator-container { font-family: Arial, sans-serif; border: 1px solid #ccc; padding: 20px; border-radius: 8px; max-width: 400px; margin: 20px auto; background-color: #f9f9f9; } .input-section { margin-bottom: 15px; } .input-section label { display: block; margin-bottom: 5px; font-weight: bold; color: #333; } .input-section input[type="number"] { width: calc(100% – 22px); padding: 10px; border: 1px solid #ccc; border-radius: 4px; box-sizing: border-box; /* Include padding and border in the element's total width and height */ } button { background-color: #4CAF50; color: white; padding: 12px 20px; border: none; border-radius: 4px; cursor: pointer; font-size: 16px; width: 100%; } button:hover { background-color: #45a049; } .result-section { margin-top: 20px; padding: 15px; border: 1px solid #d4edda; background-color: #d4edda; color: #155724; border-radius: 4px; text-align: center; font-size: 1.1em; font-weight: bold; } function calculateAirflow() { var ductWidth = parseFloat(document.getElementById("ductWidth").value); var ductHeight = parseFloat(document.getElementById("ductHeight").value); var airVelocity = parseFloat(document.getElementById("airVelocity").value); var resultElement = document.getElementById("result"); // Validate inputs if (isNaN(ductWidth) || isNaN(ductHeight) || isNaN(airVelocity) || ductWidth <= 0 || ductHeight <= 0 || airVelocity <= 0) { resultElement.textContent = "Please enter valid positive numbers for all fields."; return; } // Calculate duct area in square feet // Width and Height are in inches, convert to feet by dividing by 12 var ductAreaSqFt = (ductWidth / 12) * (ductHeight / 12); // Calculate airflow rate in CFM // Airflow Rate (CFM) = Duct Area (sq ft) * Air Velocity (FPM) var airflowCFM = ductAreaSqFt * airVelocity; // Display the result resultElement.textContent = "Calculated Airflow Rate: " + airflowCFM.toFixed(2) + " CFM"; }

Understanding Airflow Rate in Ductwork

Airflow rate is a fundamental metric in heating, ventilation, and air conditioning (HVAC) systems. It quantifies the volume of air moving through a duct or a specific space over a unit of time. The most common unit for measuring airflow rate in residential and light commercial HVAC is Cubic Feet per Minute (CFM).

Why is Airflow Rate Important?

  • System Efficiency: Proper airflow ensures that your HVAC system operates at its intended efficiency. Too little airflow can strain components, while too much can lead to noise and discomfort.
  • Comfort: Correct airflow distribution is essential for maintaining consistent temperatures throughout a building.
  • Air Quality: Adequate airflow helps in circulating fresh air and filtering out pollutants, improving indoor air quality.
  • System Sizing: When designing or assessing an HVAC system, airflow rate calculations are critical for selecting appropriately sized equipment (like fans and air handlers) and ductwork.

How to Calculate Airflow Rate

The basic formula to calculate airflow rate is:

Airflow Rate = Duct Area × Air Velocity

Let's break down the components:

  • Duct Area: This is the cross-sectional area of the duct through which air is flowing. For a rectangular duct, it's calculated by multiplying its width by its height. For a circular duct, it's calculated using the formula for the area of a circle (πr²). In our calculator, we assume a rectangular duct.
  • Air Velocity: This is the speed at which the air is moving within the duct. It's typically measured in Feet Per Minute (FPM). Air velocity can be measured using specialized tools like an anemometer.

Units and Conversions

It's crucial to ensure your units are consistent. In the formula Airflow Rate = Duct Area × Air Velocity:

  • If Duct Area is in square feet (ft²) and Air Velocity is in Feet Per Minute (FPM), then Airflow Rate will be in Cubic Feet per Minute (CFM).
  • If your duct dimensions are in inches, you must convert them to feet by dividing by 12 before calculating the area.

Example Calculation:

Let's say you have a rectangular duct with the following dimensions:

  • Duct Width = 12 inches
  • Duct Height = 8 inches
  • Measured Air Velocity = 500 FPM

First, convert the dimensions from inches to feet:

  • Width in feet = 12 inches / 12 = 1 foot
  • Height in feet = 8 inches / 12 = 0.667 feet

Next, calculate the duct's cross-sectional area in square feet:

  • Duct Area = 1 foot × 0.667 feet = 0.667 sq ft

Finally, calculate the airflow rate:

  • Airflow Rate (CFM) = Duct Area (sq ft) × Air Velocity (FPM)
  • Airflow Rate (CFM) = 0.667 sq ft × 500 FPM = 333.5 CFM

Therefore, the airflow rate in this duct is approximately 333.5 CFM. This value is essential for HVAC technicians to diagnose issues, balance air distribution, and ensure the system performs optimally.

Leave a Comment