function calculateAirFlow() {
// 1. Get DOM elements
var inputPress = document.getElementById("velocityPressure");
var inputDiam = document.getElementById("ductDiameter");
var resDiv = document.getElementById("resultsArea");
var errDiv = document.getElementById("errorDisplay");
var displayCFM = document.getElementById("resCFM");
var displayVel = document.getElementById("resVelocity");
var displayArea = document.getElementById("resArea");
// 2. Parse values
var pressure = parseFloat(inputPress.value);
var diameter = parseFloat(inputDiam.value);
// 3. Reset state
errDiv.style.display = "none";
resDiv.style.display = "none";
// 4. Validate Inputs
if (isNaN(pressure) || isNaN(diameter)) {
errDiv.innerText = "Please enter valid numbers for both fields.";
errDiv.style.display = "block";
return;
}
if (pressure < 0) {
errDiv.innerText = "Pressure differential cannot be negative for this calculation.";
errDiv.style.display = "block";
return;
}
if (diameter <= 0) {
errDiv.innerText = "Duct diameter must be greater than zero.";
errDiv.style.display = "block";
return;
}
// 5. Calculation Logic (Standard Air Conditions)
// Constant 4005 is derived from standard air density (0.075 lb/ft3)
// Formula: Velocity (FPM) = 4005 * sqrt(Pressure in in.wg)
var velocityFPM = 4005 * Math.sqrt(pressure);
// Calculate Area in Square Feet
// Area = pi * r^2. Diameter is in inches, so r = (d/2)/12 feet
var radiusFt = (diameter / 2) / 12;
var areaSqFt = Math.PI * Math.pow(radiusFt, 2);
// Calculate Flow Rate (CFM)
// CFM = Velocity (FPM) * Area (sq ft)
var flowCFM = velocityFPM * areaSqFt;
// 6. Display Results
displayCFM.innerText = flowCFM.toFixed(0) + " CFM";
displayVel.innerText = velocityFPM.toFixed(0) + " FPM";
displayArea.innerText = areaSqFt.toFixed(3) + " sq. ft.";
resDiv.style.display = "block";
}
Understanding Air Flow and Pressure Differential
Calculating air flow rate from pressure differential is a fundamental task in HVAC engineering, aerodynamics, and fluid dynamics. This calculation allows technicians to determine the volume of air moving through a duct or orifice by measuring the pressure drop or velocity pressure associated with the flow.
The Physics Behind the Calculator
This calculator utilizes the standard HVAC formula derived from Bernoulli's principle for standard air conditions (70°F, 29.92 in. Hg, dry air). The relationship between velocity and pressure is non-linear; as the pressure differential increases, the air velocity increases by the square root of that pressure.
Velocity Formula:
V = 4005 × √(ΔP)
Flow Rate Formula:
Q = V × A
Where:
V = Air Velocity in Feet Per Minute (FPM)
ΔP = Velocity Pressure in Inches of Water Column (in. wg)
4005 = Conversion constant for standard air density
Q = Air Flow Rate in Cubic Feet per Minute (CFM)
A = Cross-sectional Area of the duct in Square Feet (sq. ft.)
How to Use This Tool
To get an accurate Air Flow (CFM) reading, you need two specific measurements:
Velocity Pressure (ΔP): This is typically measured using a Pitot tube and a manometer. The Pitot tube subtracts the static pressure from the total pressure to isolate the velocity pressure. Enter this value in inches of water gauge.
Duct Diameter: Measure the internal diameter of the round duct where the pressure reading was taken. Enter this value in inches.
Why is the "4005" Constant Important?
The constant 4005 is widely used in the HVAC industry, but it assumes "Standard Air" density ($\rho = 0.075 \text{ lb/ft}^3$). If you are measuring air flow in extreme temperatures or at high altitudes, the air density changes, and the constant must be adjusted. For most standard building applications, 4005 provides a sufficiently accurate estimation.
Example Calculation
If you measure a velocity pressure of 0.25 in. wg in a 12-inch round duct: