.af-calculator-container {
max-width: 600px;
margin: 20px auto;
padding: 25px;
background-color: #f9f9f9;
border: 1px solid #e0e0e0;
border-radius: 8px;
font-family: Arial, sans-serif;
box-shadow: 0 4px 6px rgba(0,0,0,0.05);
}
.af-calculator-container h2 {
text-align: center;
margin-bottom: 20px;
color: #333;
}
.af-form-group {
margin-bottom: 15px;
}
.af-form-group label {
display: block;
margin-bottom: 5px;
font-weight: bold;
color: #555;
}
.af-form-group input, .af-form-group select {
width: 100%;
padding: 10px;
border: 1px solid #ccc;
border-radius: 4px;
box-sizing: border-box;
font-size: 16px;
}
.af-calculate-btn {
width: 100%;
padding: 12px;
background-color: #0073aa;
color: white;
border: none;
border-radius: 4px;
font-size: 18px;
cursor: pointer;
transition: background 0.3s;
}
.af-calculate-btn:hover {
background-color: #005177;
}
.af-result-box {
margin-top: 20px;
padding: 15px;
background-color: #e8f4f8;
border-left: 5px solid #0073aa;
display: none;
}
.af-result-item {
margin: 10px 0;
font-size: 18px;
color: #333;
display: flex;
justify-content: space-between;
border-bottom: 1px solid #dcdcdc;
padding-bottom: 5px;
}
.af-result-item span {
font-weight: bold;
color: #0073aa;
}
.hidden {
display: none;
}
function toggleShapeInputs() {
var shape = document.getElementById('ductShape').value;
var rectDiv = document.getElementById('rectInputs');
var roundDiv = document.getElementById('roundInputs');
if (shape === 'rectangular') {
rectDiv.style.display = 'block';
roundDiv.style.display = 'none';
} else {
rectDiv.style.display = 'none';
roundDiv.style.display = 'block';
}
// Hide results when changing inputs
document.getElementById('afResult').style.display = 'none';
}
function calculateAirFlow() {
// Get Inputs
var velocity = parseFloat(document.getElementById('airVelocity').value);
var shape = document.getElementById('ductShape').value;
var areaSqFt = 0;
var cfm = 0;
// Validation: Velocity
if (isNaN(velocity) || velocity < 0) {
alert("Please enter a valid Air Velocity (FPM).");
return;
}
// Calculation based on shape
if (shape === 'rectangular') {
var widthIn = parseFloat(document.getElementById('ductWidth').value);
var heightIn = parseFloat(document.getElementById('ductHeight').value);
if (isNaN(widthIn) || widthIn <= 0 || isNaN(heightIn) || heightIn <= 0) {
alert("Please enter valid duct dimensions.");
return;
}
// Convert inches to sq ft
// Area = (Width * Height) / 144
areaSqFt = (widthIn * heightIn) / 144;
} else {
var diameterIn = parseFloat(document.getElementById('ductDiameter').value);
if (isNaN(diameterIn) || diameterIn <= 0) {
alert("Please enter a valid duct diameter.");
return;
}
// Convert inches to sq ft
// Radius in inches = Diameter / 2
// Area = pi * r^2
// Convert to sq ft: divide result by 144
var radius = diameterIn / 2;
areaSqFt = (Math.PI * Math.pow(radius, 2)) / 144;
}
// Final CFM Calculation: Q = V * A
cfm = velocity * areaSqFt;
// Display Results
document.getElementById('resArea').innerHTML = areaSqFt.toFixed(3) + " sq. ft.";
document.getElementById('resCFM').innerHTML = cfm.toFixed(2) + " CFM";
document.getElementById('afResult').style.display = 'block';
}
Understanding Air Flow Rate Calculation
Calculating the air flow rate is a fundamental aspect of HVAC system design and maintenance. Whether you are balancing a residential air conditioning system or designing industrial ventilation, knowing the Cubic Feet per Minute (CFM) is essential for ensuring efficient air distribution and comfort.
The Air Flow Formula
The core physics behind calculating air flow comes down to the continuity equation. The most common formula used by engineers and HVAC technicians is:
Q = V × A
- Q represents the Air Flow Rate (typically in CFM – Cubic Feet per Minute).
- V represents the Air Velocity (typically in FPM – Feet per Minute).
- A represents the Cross-Sectional Area of the duct (typically in Square Feet – ft²).
How to Calculate Cross-Sectional Area
Because ductwork typically comes in inches, but the CFM formula requires square feet, a unit conversion is necessary. The calculator above handles this automatically, but here is the manual math:
1. Rectangular Ducts
For a rectangular or square duct, the area is calculated by multiplying width by height (in inches) and dividing by 144 (since there are 144 square inches in a square foot).
Formula: Area (ft²) = (Width inches × Height inches) / 144
2. Round Ducts
For circular ducts, the geometry uses the radius (half of the diameter). Just like the rectangular calculation, the final result in square inches must be divided by 144.
Formula: Area (ft²) = (π × (Diameter inches / 2)²) / 144
Example Calculation
Imagine you have a rectangular duct measuring 20 inches by 12 inches. An anemometer measurement shows the air moving at a velocity of 800 FPM.
- Calculate Area: (20 × 12) = 240 sq. inches.
- Convert to Sq. Ft: 240 / 144 = 1.667 sq. ft.
- Calculate CFM: 800 FPM × 1.667 sq. ft = 1,333.6 CFM.
Why is Accurate Air Flow Calculation Important?
Incorrect air flow can lead to significant problems in building management:
- Energy Efficiency: Systems with too much airflow waste energy, while too little airflow forces the system to run longer to satisfy the thermostat.
- Equipment Longevity: Low airflow can cause evaporator coils to freeze in the summer or heat exchangers to overheat in the winter.
- Comfort: Proper CFM ensures that conditioned air reaches all parts of the building evenly.