*Calculation assumes standard conditions (14.7 psia, 60°F). Standard air density used: 0.0763 lb/ft³.
Understanding the SCFM to Mass Flow Conversion
In fluid dynamics and engineering, converting SCFM (Standard Cubic Feet per Minute) to a Mass Flow Rate is a critical calculation for sizing pipes, selecting compressors, and monitoring fuel consumption. While SCFM measures volume corrected to standard conditions, mass flow rate measures the actual weight of the gas moving through the system.
Why Convert SCFM to Mass Flow?
Volumetric flow (SCFM) can be misleading because gases are compressible. The amount of "stuff" (molecules) is best represented by mass. Mass flow is essential for:
Combustion control: Ensuring the correct stoichiometric ratio of fuel to air.
Chemical reactions: Balancing molar ratios in process engineering.
Inventory management: Gases like natural gas or nitrogen are often sold by mass (or energy content derived from mass), even if metered in volume.
The Conversion Formula
To convert SCFM to Mass Flow Rate, you need the density of the gas at standard conditions. The general formula is:
Mass Flow (lb/min) = SCFM × Densitystd
Since the density of a specific gas relates to the density of air via Specific Gravity ($SG$), the practical formula used in this calculator is:
Mass Flow (lb/min) = SCFM × 0.0763 × SG
Where:
SCFM: The volumetric flow rate at standard conditions.
0.0763: The density of air (lb/ft³) at standard conditions (14.7 psia and 60°F).
SG: Specific Gravity of the gas (Air = 1.0).
Common Gas Specific Gravities
Gas
Specific Gravity (SG)
Approx. Density @ Std (lb/ft³)
Air
1.00
0.0763
Natural Gas
~0.55 – 0.60
0.042 – 0.046
Nitrogen
0.967
0.0738
Propane
1.52
0.116
Example Calculation
Let's say you have a flow of 500 SCFM of Natural Gas (SG = 0.60).
Identify Standard Air Density: 0.0763 lb/ft³.
Calculate Gas Density: $0.0763 \times 0.60 = 0.04578 \text{ lb/ft}^3$.
Calculate Mass Flow: $500 \times 0.04578 = 22.89 \text{ lb/min}$.
function updateSpecificGravity() {
var selectBox = document.getElementById("gasSelect");
var sgInput = document.getElementById("sgInput");
var selectedValue = selectBox.value;
if (selectedValue !== "custom") {
sgInput.value = selectedValue;
// Briefly highlight the change
sgInput.style.backgroundColor = "#e8f0fe";
setTimeout(function() {
sgInput.style.backgroundColor = "#fff";
}, 300);
} else {
sgInput.value = "";
sgInput.placeholder = "Enter SG value";
sgInput.focus();
}
}
function calculateMassFlow() {
// 1. Get input values
var scfmVal = parseFloat(document.getElementById("scfmInput").value);
var sgVal = parseFloat(document.getElementById("sgInput").value);
// 2. Validation
if (isNaN(scfmVal) || scfmVal < 0) {
alert("Please enter a valid positive number for SCFM.");
return;
}
if (isNaN(sgVal) || sgVal <= 0) {
alert("Please enter a valid Specific Gravity (SG).");
return;
}
// 3. Constants
// Standard density of air at 14.7 psia & 60°F is approx 0.0763 lb/ft^3
var airDensityStd = 0.0763;
// 4. Calculations
// Mass Flow (lb/min) = SCFM * Density_Air * SG
var massFlowLbMin = scfmVal * airDensityStd * sgVal;
// Mass Flow (lb/hr) = lb/min * 60
var massFlowLbHr = massFlowLbMin * 60;
// Mass Flow (kg/hr) = lb/hr * 0.45359237
var massFlowKgHr = massFlowLbHr * 0.45359237;
// 5. Display Results
document.getElementById("resLbMin").innerHTML = massFlowLbMin.toFixed(4);
document.getElementById("resLbHr").innerHTML = massFlowLbHr.toFixed(2);
document.getElementById("resKgHr").innerHTML = massFlowKgHr.toFixed(2);
// Show result box
document.getElementById("resultBox").style.display = "block";
}