Calculate Thermal Power (kW)
Calculate Flow Rate (L/min)
Calculate Outlet Temperature (°C)
Water (Standard)
Oil (Thermal)
Glycol (50% Solution)
Air (Dry, Sea Level)
Custom Specific Heat
Default is for Water. Change fluid type above to auto-fill.
Result
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Understanding Flow Rate and Thermodynamics
This Flow Rate and Temperature Calculator uses the fundamental principles of thermodynamics to determine the relationship between fluid flow, temperature change, and thermal energy (power). This is essential for engineers designing heating systems, heat exchangers, chillers, or hydronic piping loops.
The Fundamental Formula
The calculation is based on the specific heat equation:
P = (Q / 60) × ρ × Cp × ΔT
Where:
P = Thermal Power or Heat Transfer Rate (kW)
Q = Volumetric Flow Rate (L/min)
ρ (rho) = Fluid Density (assumed ~1 kg/L for water-based fluids in this simplified tool)
Cp = Specific Heat Capacity (kJ/kg·°C)
ΔT = Temperature Difference (Tout – Tin) in °C
Specific Heat Capacity Values
Different fluids absorb heat at different rates. Water is extremely efficient at carrying heat, which is why it has a high specific heat capacity.
Water: ~4.186 kJ/kg·°C
Glycol (50%): ~3.5 kJ/kg·°C
Thermal Oil: ~1.67 kJ/kg·°C
Air: ~1.005 kJ/kg·°C (Note: Density of air is much lower, approx 0.0012 kg/L)
How to Use This Calculator
Select Calculation Mode: Choose whether you want to solve for Power, Flow Rate, or Output Temperature.
Choose Fluid: Select the fluid medium. This automatically updates the Specific Heat Capacity.
Enter Variables: Input the known values. For example, if sizing a heater, input your Flow Rate and desired Temperature Rise to find the required Power (kW).
// Initial Setup
window.onload = function() {
updateFormVisibility();
};
function updateSpecificHeat() {
var fluidSelect = document.getElementById("fluidType");
var cpInput = document.getElementById("specificHeat");
var selectedValue = fluidSelect.value;
if (selectedValue !== "custom") {
cpInput.value = selectedValue;
}
}
function updateFormVisibility() {
var mode = document.getElementById("calculationMode").value;
var groupPower = document.getElementById("group-power");
var groupFlow = document.getElementById("group-flow");
var groupTempOut = document.getElementById("group-tempOut");
// Reset display
groupPower.style.display = "block";
groupFlow.style.display = "block";
groupTempOut.style.display = "block";
if (mode === "power") {
groupPower.style.display = "none";
} else if (mode === "flow") {
groupFlow.style.display = "none";
} else if (mode === "tempOut") {
groupTempOut.style.display = "none";
}
// Hide result on mode change
document.getElementById("resultBox").style.display = "none";
}
function calculateThermalPhysics() {
// Inputs
var mode = document.getElementById("calculationMode").value;
var cp = parseFloat(document.getElementById("specificHeat").value);
var tIn = parseFloat(document.getElementById("tempIn").value);
var tOut = parseFloat(document.getElementById("tempOut").value);
var flow = parseFloat(document.getElementById("flowRate").value);
var power = parseFloat(document.getElementById("thermalPower").value);
var fluidType = document.getElementById("fluidType").value;
// Density Assumption
// For simplified calc, we assume 1 kg/L for liquids.
// If Air is selected, we adjust density roughly to 0.001225 kg/L (at sea level 15C).
var density = 1.0;
if (fluidType === '1.005') { // Air identifier from value
density = 0.001225;
}
var resultBox = document.getElementById("resultBox");
var mainResult = document.getElementById("mainResult");
var subResult = document.getElementById("subResult");
var resultLabel = document.getElementById("resultLabel");
// Validation helpers
function isValid(num) {
return !isNaN(num) && isFinite(num);
}
var calculatedValue = 0;
var deltaT = 0;
if (mode === "power") {
// Calculate Power (kW)
// Formula: P (kW) = (Flow (L/min) / 60) * Density (kg/L) * Cp (kJ/kgC) * DeltaT (C)
if (!isValid(flow) || !isValid(tIn) || !isValid(tOut) || !isValid(cp)) {
alert("Please enter valid numbers for Flow, Inlet Temp, and Outlet Temp.");
return;
}
deltaT = tOut – tIn;
// Mass Flow Rate (kg/s) = (L/min / 60) * density
var mDot = (flow / 60) * density;
calculatedValue = mDot * cp * deltaT;
resultLabel.innerHTML = "Required Heat Power";
mainResult.innerHTML = calculatedValue.toFixed(2) + " kW";
subResult.innerHTML = "Temperature Change: " + deltaT.toFixed(1) + " °CMass Flow: " + (mDot*3600).toFixed(1) + " kg/hr";
}
else if (mode === "flow") {
// Calculate Flow Rate (L/min)
// Formula: Q = (P * 60) / (Density * Cp * DeltaT)
if (!isValid(power) || !isValid(tIn) || !isValid(tOut) || !isValid(cp)) {
alert("Please enter valid numbers for Power, Inlet Temp, and Outlet Temp.");
return;
}
deltaT = tOut – tIn;
if (deltaT === 0) {
alert("Temperature difference cannot be zero for this calculation.");
return;
}
// P = mDot * cp * dT => mDot = P / (cp * dT)
var mDotReq = power / (cp * deltaT); // kg/s
// Q (L/min) = (mDotReq / density) * 60
calculatedValue = (mDotReq / density) * 60;
resultLabel.innerHTML = "Required Flow Rate";
mainResult.innerHTML = Math.abs(calculatedValue).toFixed(2) + " L/min";
subResult.innerHTML = "Based on ΔT of " + deltaT.toFixed(1) + " °C and Power of " + power + " kW";
}
else if (mode === "tempOut") {
// Calculate Outlet Temp
// Formula: Tout = Tin + (P * 60) / (Q * Density * Cp)
// Or Tout = Tin + (P / (mDot * Cp))
if (!isValid(power) || !isValid(flow) || !isValid(tIn) || !isValid(cp)) {
alert("Please enter valid numbers for Power, Flow Rate, and Inlet Temp.");
return;
}
if (flow === 0) {
alert("Flow rate cannot be zero.");
return;
}
var mDot = (flow / 60) * density; // kg/s
var tempChange = power / (mDot * cp);
calculatedValue = tIn + tempChange;
resultLabel.innerHTML = "Outlet Temperature";
mainResult.innerHTML = calculatedValue.toFixed(2) + " °C";
subResult.innerHTML = "Temperature Rise: " + tempChange.toFixed(2) + " °C";
}
resultBox.style.display = "block";
}