Understanding the relationship between air flow rate and pressure drop is crucial in various HVAC (Heating, Ventilation, and Air Conditioning) and industrial applications. Air flow rate, often measured in cubic feet per minute (CFM) or liters per second (L/s), represents the volume of air moving through a system per unit of time. Pressure drop, typically measured in inches of water column (in. H₂O) or Pascals (Pa), is the reduction in air pressure that occurs as air moves through ducts, filters, or other components due to friction and turbulence.
This calculator helps you estimate the pressure drop across a system given its air flow rate, or vice versa, based on fundamental fluid dynamics principles. The relationship is generally non-linear; as air flow rate increases, pressure drop tends to increase more rapidly. This is often described by the Darcy-Weisbach equation for turbulent flow, which considers factors like duct length, diameter, roughness, and air velocity. For simpler estimations, we often rely on charts and empirical data specific to different components and duct materials.
Key Factors Influencing Pressure Drop:
Air Flow Rate: Higher flow rates lead to greater pressure drop.
Duct Size and Shape: Smaller or longer ducts, and those with many bends or transitions, increase resistance.
Duct Material and Roughness: Rougher interior surfaces create more friction.
System Components: Filters, dampers, grilles, and coils all add to the overall pressure drop.
Air Density: While often assumed constant, changes in temperature and altitude can affect air density and thus pressure drop.
Accurate calculation of pressure drop is essential for proper system design, ensuring that fans can deliver the required air flow without excessive energy consumption or noise. It also helps in identifying potential issues like clogged filters or undersized ducts.
Air Flow Rate and Pressure Drop Calculator
CFM
L/s
inches
cm
feet
meters
lb/ft³
kg/m³
function calculatePressureDrop() {
var airFlowRate = parseFloat(document.getElementById("airFlowRate").value);
var flowUnit = document.getElementById("flowUnit").value;
var ductDiameter = parseFloat(document.getElementById("ductDiameter").value);
var diameterUnit = document.getElementById("diameterUnit").value;
var ductLength = parseFloat(document.getElementById("ductLength").value);
var lengthUnit = document.getElementById("lengthUnit").value;
var airDensity = parseFloat(document.getElementById("airDensity").value);
var densityUnit = document.getElementById("densityUnit").value;
var frictionFactor = parseFloat(document.getElementById("frictionFactor").value);
var resultElement = document.getElementById("result");
resultElement.innerHTML = ""; // Clear previous results
if (isNaN(airFlowRate) || isNaN(ductDiameter) || isNaN(ductLength) || isNaN(airDensity) || isNaN(frictionFactor)) {
resultElement.innerHTML = "Please enter valid numbers for all fields.";
return;
}
// Unit conversions to a common base (e.g., metric for internal calculations)
var flowRate_m3_s;
if (flowUnit === "cfm") {
flowRate_m3_s = airFlowRate * 0.000471947; // CFM to m³/s
} else { // L/s
flowRate_m3_s = airFlowRate * 0.001; // L/s to m³/s
}
var diameter_m;
if (diameterUnit === "in") {
diameter_m = ductDiameter * 0.0254; // inches to meters
} else { // cm
diameter_m = ductDiameter * 0.01; // cm to meters
}
var length_m;
if (lengthUnit === "ft") {
length_m = ductLength * 0.3048; // feet to meters
} else { // m
length_m = ductLength; // already in meters
}
var density_kg_m3;
if (densityUnit === "lb_ft3") {
density_kg_m3 = airDensity * 16.0185; // lb/ft³ to kg/m³
} else { // kg_m3
density_kg_m3 = airDensity; // already in kg/m³
}
// Calculate velocity (v = Q / A)
var area_m2 = Math.PI * Math.pow(diameter_m / 2, 2);
var velocity_m_s = flowRate_m3_s / area_m2;
// Calculate pressure drop using Darcy-Weisbach equation: ΔP = f * (L/D) * (ρ * v² / 2)
// This formula gives pressure in Pascals (Pa)
var pressureDrop_Pa = frictionFactor * (length_m / diameter_m) * (density_kg_m3 * Math.pow(velocity_m_s, 2) / 2);
// Convert result to inches of water column (in. H₂O) for common HVAC units
// 1 in. H₂O ≈ 248.84 Pa
var pressureDrop_inH2O = pressureDrop_Pa / 248.84;
resultElement.innerHTML = "Estimated Pressure Drop: " + pressureDrop_inH2O.toFixed(4) + " in. H₂O";
resultElement.innerHTML += "(Approximately " + pressureDrop_Pa.toFixed(2) + " Pa)";
}