Total thermal energy entering the system in Watts (W).
Difference between riser and downcomer temperature (°C or K).
Thermal capacity of the fluid. Water ≈ 4186 J/kg·K.
Density of the fluid. Water ≈ 997 kg/m³.
Calculation Results
Mass Flow Rate:0.0000 kg/s
Mass Flow Rate (Hourly):0.00 kg/hr
Volumetric Flow Rate:0.0000 m³/s
Flow Rate (LPM):0.00 L/min
Flow Rate (GPM):0.00 gal/min
function calculateThermosiphon() {
// 1. Get Input Values
var Q = document.getElementById('heatLoad').value;
var dT = document.getElementById('deltaT').value;
var Cp = document.getElementById('specificHeat').value;
var rho = document.getElementById('fluidDensity').value;
// 2. Validate Inputs
if (Q === "" || dT === "" || Cp === "" || rho === "") {
alert("Please fill in all fields.");
return;
}
var qVal = parseFloat(Q);
var dtVal = parseFloat(dT);
var cpVal = parseFloat(Cp);
var rhoVal = parseFloat(rho);
if (qVal < 0 || dtVal <= 0 || cpVal <= 0 || rhoVal m_dot = Q / (Cp * dT)
// Mass Flow Rate (kg/s)
var massFlowKgS = qVal / (cpVal * dtVal);
// Mass Flow Rate (kg/hr)
var massFlowKgH = massFlowKgS * 3600;
// Volumetric Flow Rate (m³/s) = Mass Flow / Density
var volFlowM3S = massFlowKgS / rhoVal;
// Volumetric Flow Rate (Liters per Minute)
// 1 m³/s = 60,000 L/min
var volFlowLPM = volFlowM3S * 60000;
// Volumetric Flow Rate (Gallons per Minute – US)
// 1 L/min = 0.264172 GPM
var volFlowGPM = volFlowLPM * 0.264172;
// 4. Update UI
document.getElementById('resMassFlowSec').innerHTML = massFlowKgS.toFixed(4) + " kg/s";
document.getElementById('resMassFlowHour').innerHTML = massFlowKgH.toFixed(2) + " kg/hr";
document.getElementById('resVolFlowSec').innerHTML = volFlowM3S.toExponential(4) + " m³/s";
document.getElementById('resVolFlowLPM').innerHTML = volFlowLPM.toFixed(2) + " L/min";
document.getElementById('resVolFlowGPM').innerHTML = volFlowGPM.toFixed(2) + " gal/min";
// Show results
document.getElementById('resultsBox').className = "results-section visible";
}
function resetCalculator() {
document.getElementById('heatLoad').value = "";
document.getElementById('deltaT').value = "";
document.getElementById('specificHeat').value = "4186";
document.getElementById('fluidDensity').value = "997";
document.getElementById('resultsBox').className = "results-section";
}
Understanding Thermosiphon Flow Rates
A thermosiphon (or thermosyphon) is a method of passive heat exchange, based on natural convection, which circulates a fluid without the need for a mechanical pump. Calculating the flow rate is critical for designing efficient solar water heaters, cooling systems for electronics, and industrial reboilers.
The circulation occurs because fluids change density when their temperature changes. As fluid in the collector or heat source warms up (increasing temperature), it becomes less dense and rises. Conversely, the fluid in the heat sink cools down (decreasing temperature), becomes denser, and sinks. This creates a continuous loop known as a thermosiphon.
How the Calculator Works
This calculator estimates the flow rate required to maintain a specific thermal equilibrium based on the First Law of Thermodynamics (Energy Balance). While a full hydraulic analysis involves complex friction factors and pipe geometry, the thermal balance method tells you exactly how much fluid must circulate to remove a given Heat Load ($Q$) at a specific Temperature Difference ($\Delta T$).
The Mathematical Formula
The mass flow rate is derived from the fundamental heat transfer equation:
$\dot{m} = \frac{Q}{C_p \times \Delta T}$
Where:
$\dot{m}$ = Mass flow rate (kg/s)
$Q$ = Heat input or load (Watts)
$C_p$ = Specific heat capacity of the fluid (J/kg·K)
$\Delta T$ = Temperature difference between the hot and cold legs (°C or K)
Once the mass flow rate is determined, the volumetric flow rate is calculated using the fluid density ($\rho$):
$\dot{V} = \frac{\dot{m}}{\rho}$
Key Inputs Explained
Heat Load (Q): The amount of energy being transferred into the fluid per second. In a solar heater, this is the solar energy absorbed. In electronics cooling, it is the waste heat generated by the component.
Temperature Difference ($\Delta T$): This is the driving force of the efficiency. A larger $\Delta T$ implies a lower flow rate is required to remove the same amount of heat, but in a thermosiphon, the flow rate itself is generated by this temperature difference.
Specific Heat ($C_p$): A physical property of the fluid representing how much energy is needed to raise 1kg of the fluid by 1 degree. Water has a very high $C_p$ (~4186 J/kg·K), making it an excellent heat transfer fluid.
Density ($\rho$): The mass per unit volume. For water at standard temperatures, 997 kg/m³ is a standard approximation.
Optimization Tips
To maximize thermosiphon performance, ensure vertical separation between the heat source and the heat sink. Minimizing flow restriction (using larger diameter pipes and fewer bends) allows the buoyancy forces to generate higher flow rates, resulting in a lower temperature difference and better cooling performance.