Please enter valid numeric values greater than zero.
Required Water Flow Rate:
Liters per Minute:0 L/min
Cubic Meters per Hour:0 m³/h
US Gallons per Minute:0 GPM
Imperial Gallons per Minute:0 IGPM
Understanding Cooling Water Flow Rate
Calculating the correct cooling water flow rate is fundamental to the design and operation of chillers, heat exchangers, cooling towers, and industrial process cooling systems. Without adequate flow, heat cannot be efficiently removed, leading to equipment overheating, shutdowns, or reduced efficiency.
The Physics Behind the Calculation
The flow rate calculation is based on the fundamental thermodynamics equation of heat transfer:
Q = m × Cp × ΔT
Where:
Q = Heat Load (Cooling Capacity)
m = Mass flow rate
Cp = Specific heat capacity of the fluid
ΔT = Temperature difference between inlet and outlet
Common "Rules of Thumb"
Engineers often use simplified formulas for quick estimates involving standard water:
1. The "500 Rule" (Imperial Units)
When working with BTU/hr and Fahrenheit, the formula simplifies to:
GPM = Heat Load (BTU/hr) / (500 × ΔT °F)
Note: The constant 500 is derived from: 8.33 lbs/gal (density) × 60 min/hr × 1 BTU/lb°F (specific heat).
2. Metric Estimation
For Kilowatts (kW) and Celsius:
L/min ≈ (kW × 14.3) / ΔT °C
This is derived from water's specific heat capacity of approximately 4.186 kJ/kg·K.
Why Delta T (ΔT) Matters
The ΔT is the design temperature change across your heat exchanger. Standard commercial chiller systems typically design for a 10°F to 12°F (approx. 5.5°C to 6.7°C) temperature drop. Industrial processes might demand higher or lower ΔT depending on the equipment sensitivity.
If your actual flow rate is lower than calculated, the ΔT will increase (water leaves hotter). If the flow rate is higher, the ΔT will decrease (water leaves cooler), but pumping energy is wasted.
Common Cooling Load Units
Unit
Equivalent in kW
Common Usage
1 Ton (TR)
~3.517 kW
US HVAC & Refrigeration
1000 BTU/hr
~0.293 kW
Small AC units, heaters
1 kcal/hr
~0.00116 kW
Older metric systems
function calculateFlowRate() {
// 1. Get Input Values
var heatLoad = parseFloat(document.getElementById('heatLoad').value);
var loadUnit = document.getElementById('loadUnit').value;
var deltaT = parseFloat(document.getElementById('deltaT').value);
var tempUnit = document.getElementById('tempUnit').value;
var specificHeat = parseFloat(document.getElementById('fluidType').value); // kJ/kg.C
var errorMsg = document.getElementById('errorMsg');
var resultsDiv = document.getElementById('results');
// 2. Validation
if (isNaN(heatLoad) || isNaN(deltaT) || heatLoad <= 0 || deltaT <= 0) {
errorMsg.style.display = 'block';
resultsDiv.style.display = 'none';
return;
} else {
errorMsg.style.display = 'none';
resultsDiv.style.display = 'block';
}
// 3. Normalize to Base Units (kW and Celsius)
var loadKW = 0;
// Convert Load to kW
if (loadUnit === 'kw') {
loadKW = heatLoad;
} else if (loadUnit === 'tons') {
loadKW = heatLoad * 3.51685; // 1 Ton = 3.51685 kW
} else if (loadUnit === 'btu') {
loadKW = heatLoad / 3412.14; // 1 kW = 3412.14 BTU/hr
} else if (loadUnit === 'kcal') {
loadKW = heatLoad / 860; // 1 kW = 860 kcal/hr
}
// Convert Delta T to Celsius
var deltaTC = 0;
if (tempUnit === 'c') {
deltaTC = deltaT;
} else {
// Delta T conversion is different from absolute temp conversion.
// A difference of 1.8°F = 1°C.
deltaTC = deltaT / 1.8;
}
// 4. Calculate Mass Flow Rate (kg/s)
// Formula: Q (kW) = m (kg/s) * Cp (kJ/kg.C) * DeltaT (C)
// Therefore: m = Q / (Cp * DeltaT)
var massFlowKgSec = loadKW / (specificHeat * deltaTC);
// 5. Convert to Volume Flow Rates
// Assuming density of water/glycol mix is approx 1 kg/L for general estimation
// (Changes slightly with temp/glycol but negligible for general sizing)
var lPerSec = massFlowKgSec; // 1 kg ~ 1 L
var lPerMin = lPerSec * 60;
var m3PerHr = lPerMin * 60 / 1000;
var gpm = lPerMin * 0.264172; // US Gallons
var igpm = lPerMin * 0.219969; // Imperial Gallons
// 6. Display Results
document.getElementById('resLpm').innerText = lPerMin.toFixed(2) + " L/min";
document.getElementById('resM3h').innerText = m3PerHr.toFixed(2) + " m³/h";
document.getElementById('resGpm').innerText = gpm.toFixed(2) + " GPM";
document.getElementById('resIgpm').innerText = igpm.toFixed(2) + " IGPM";
}