Calculate the required steam mass flow based on thermal power load and specific enthalpy.
kW
BTU/hr
Boiler HP
kcal/hr
The energy requirement of the process.
kJ/kg
BTU/lb
Energy released during condensation. (~2257 kJ/kg or ~970 BTU/lb at atm)
Required Steam Mass Flow Rate
0 kg/hr
0 lbs/hr
How to Calculate Steam Flow Rate
Determining the correct steam flow rate is critical for sizing boilers, control valves, and steam traps. The calculation relies on the relationship between the heat energy required by the process and the thermal energy stored within the steam (specifically, the enthalpy of evaporation or latent heat).
The Steam Flow Formula
The fundamental thermodynamic equation used to calculate steam mass flow rate is:
ṁ = Q / hfg
Where:
ṁ = Mass Flow Rate of Steam (kg/hr or lbs/hr)
Q = Heat Load or Energy Required (kJ/hr or BTU/hr)
hfg = Latent Heat of Evaporation (kJ/kg or BTU/lb)
Calculation Example
Suppose you have a heat exchanger requiring 500 kW of thermal power. The system operates at atmospheric pressure where the latent heat of steam is approximately 2257 kJ/kg.
First, convert the power from kW to kJ/hr (since 1 kW = 3600 kJ/hr):
Q = 500 kW × 3600 = 1,800,000 kJ/hr
hfg = 2257 kJ/kg
ṁ = 1,800,000 / 2257 = 797.5 kg/hr
Why Latent Heat Matters
The "Latent Heat" (hfg) varies depending on the steam pressure. As steam pressure increases, the latent heat decreases. This means that at higher pressures, you actually need a slightly higher mass flow rate of steam to deliver the same amount of latent thermal energy, although the steam is denser and occupies less volume.
function updateDefaultEnthalpy() {
var unit = document.getElementById('latentUnit').value;
var input = document.getElementById('latentHeat');
// Only update if the user hasn't typed a custom value or if it matches the other default
var currentVal = parseFloat(input.value);
if (unit === 'kjkg') {
if (isNaN(currentVal) || Math.abs(currentVal – 970) < 5) {
input.value = 2257;
}
} else if (unit === 'btulb') {
if (isNaN(currentVal) || Math.abs(currentVal – 2257) < 5) {
input.value = 970;
}
}
}
function calculateSteamFlow() {
// 1. Get Input Values
var heatLoad = parseFloat(document.getElementById('heatLoad').value);
var loadUnit = document.getElementById('loadUnit').value;
var latentHeat = parseFloat(document.getElementById('latentHeat').value);
var latentUnit = document.getElementById('latentUnit').value;
// 2. Validate Inputs
if (isNaN(heatLoad) || heatLoad <= 0) {
alert("Please enter a valid positive number for the Heat Load.");
return;
}
if (isNaN(latentHeat) || latentHeat <= 0) {
alert("Please enter a valid positive number for Latent Heat.");
return;
}
// 3. Normalize Heat Load to BTU/hr for calculation base
var heatInBTU = 0;
// Conversion factors
// 1 kW = 3412.142 BTU/hr
// 1 Boiler HP = 33475 BTU/hr
// 1 kcal/hr = 3.96567 BTU/hr
switch(loadUnit) {
case 'kW':
heatInBTU = heatLoad * 3412.142;
break;
case 'btu':
heatInBTU = heatLoad;
break;
case 'bhp':
heatInBTU = heatLoad * 33475;
break;
case 'kcal':
heatInBTU = heatLoad * 3.96567;
break;
}
// 4. Normalize Latent Heat to BTU/lb
var latentInBTUlb = 0;
// Conversion: 1 kJ/kg = 0.4299226 BTU/lb
switch(latentUnit) {
case 'kjkg':
latentInBTUlb = latentHeat * 0.4299226;
break;
case 'btulb':
latentInBTUlb = latentHeat;
break;
}
// 5. Calculate Flow Rate (Imperial: lbs/hr)
// Formula: Mass Flow (lbs/hr) = Heat Load (BTU/hr) / Latent Heat (BTU/lb)
var flowLbsHr = heatInBTU / latentInBTUlb;
// 6. Convert to Metric (kg/hr)
// 1 lb = 0.453592 kg
var flowKgHr = flowLbsHr * 0.453592;
// 7. Format and Display Results
var resultBox = document.getElementById('resultBox');
var metricDisplay = document.getElementById('metricResult');
var imperialDisplay = document.getElementById('imperialResult');
// Formatting numbers with commas
metricDisplay.innerHTML = flowKgHr.toLocaleString('en-US', {maximumFractionDigits: 1}) + " kg/hr";
imperialDisplay.innerHTML = flowLbsHr.toLocaleString('en-US', {maximumFractionDigits: 1}) + " lbs/hr";
// Show result box
resultBox.className = "result-box visible";
}