function calculateBoilOff() {
// 1. Get Input Values
var heatLeak = parseFloat(document.getElementById("heatLeak").value);
var heatLeakUnit = document.getElementById("heatLeakUnit").value;
var latentHeat = parseFloat(document.getElementById("latentHeat").value);
var latentHeatUnit = document.getElementById("latentHeatUnit").value;
var density = parseFloat(document.getElementById("liquidDensity").value);
var densityUnit = document.getElementById("liquidDensityUnit").value;
var volume = parseFloat(document.getElementById("tankVolume").value);
var volumeUnit = document.getElementById("tankVolumeUnit").value;
// Validation
if (isNaN(heatLeak) || isNaN(latentHeat) || isNaN(density) || isNaN(volume) || latentHeat === 0 || density === 0) {
alert("Please enter valid numeric values for all fields. Latent heat and density cannot be zero.");
return;
}
// 2. Normalize to Standard SI Units (Watts, J/kg, kg/m³, m³)
// Heat Leak -> Watts (J/s)
var qWatts = 0;
if (heatLeakUnit === "watts") qWatts = heatLeak;
else if (heatLeakUnit === "kw") qWatts = heatLeak * 1000;
else if (heatLeakUnit === "btu") qWatts = heatLeak * 0.293071; // Btu/hr to Watts
// Latent Heat -> J/kg
var lJoulesKg = 0;
if (latentHeatUnit === "kjkg") lJoulesKg = latentHeat * 1000;
else if (latentHeatUnit === "btulb") lJoulesKg = latentHeat * 2326; // Btu/lb to J/kg
// Density -> kg/m³
var rhoKgM3 = 0;
if (densityUnit === "kgm3") rhoKgM3 = density;
else if (densityUnit === "lbft3") rhoKgM3 = density * 16.0185;
// Volume -> m³
var volM3 = 0;
if (volumeUnit === "m3") volM3 = volume;
else if (volumeUnit === "liters") volM3 = volume / 1000;
else if (volumeUnit === "gallons") volM3 = volume * 0.00378541;
// 3. Perform Physics Calculations
// Mass Flow Rate (kg/s) = Q (W) / L (J/kg)
var massRateKgSec = qWatts / lJoulesKg;
// Convert to hourly rates
var massRateKgHr = massRateKgSec * 3600;
var massRateLbHr = massRateKgHr * 2.20462;
// Volume Rate (Liquid) per day
var massRateKgDay = massRateKgSec * 86400;
var volRateLiquidM3Day = massRateKgDay / rhoKgM3;
var volRateLiquidLitersDay = volRateLiquidM3Day * 1000;
// Total Initial Mass
var totalMassKg = volM3 * rhoKgM3;
// Boil-off Rate (BOR) % per day
// BOR = (Mass Loss per Day / Total Mass) * 100
var borPercent = (massRateKgDay / totalMassKg) * 100;
// Energy Loss Rate is essentially the input Q in kW
var energyLossKW = qWatts / 1000;
// 4. Display Results
document.getElementById("results").style.display = "block";
document.getElementById("resMass").innerHTML = massRateKgHr.toFixed(3) + " kg/hr (" + massRateLbHr.toFixed(3) + " lb/hr)";
document.getElementById("resVolLiquid").innerHTML = volRateLiquidLitersDay.toFixed(2) + " Liters/day";
document.getElementById("resEnergy").innerHTML = energyLossKW.toFixed(4) + " kW";
document.getElementById("resPercent").innerHTML = borPercent.toFixed(4) + " % / day";
}
Understanding Boil-off Rate (BOR) in Cryogenics
The management of Boil-off Gas (BOG) is a critical aspect of storing and transporting cryogenic liquids such as Liquefied Natural Gas (LNG), Liquid Nitrogen (LIN), and Liquid Oxygen (LOX). Even with high-performance vacuum insulation, some heat inevitably penetrates the storage tank, causing a portion of the liquid to evaporate.
The Boil-off Rate (BOR) Calculator helps engineers and operators estimate the amount of product lost due to heat ingress and determine the efficiency of storage tanks.
How to Calculate Boil-off Rate
The calculation relies on the thermodynamics of phase change. The fundamental principle is that the heat energy entering the tank (Heat Leak) is consumed by the liquid's phase change from liquid to gas. The specific amount of energy required for this change is the Latent Heat of Vaporization.
Mass Boil-off Rate (ṁ) = Q / L
Where:
ṁ = Mass flow rate of boil-off gas (kg/s)
Q = Heat leak rate into the tank (Watts or J/s)
L = Latent heat of vaporization of the liquid (J/kg)
Calculating BOR Percentage
In the industry, BOR is most often expressed as a percentage of the tank's total volume (or mass) lost per day.
BOR (%) = (Mass Evaporated in 24h / Total Liquid Mass Capacity) × 100
Typical Parameters for Common Cryogens
To use the calculator effectively, you can use these approximate reference values (at atmospheric pressure):
Cryogen
Density (kg/m³)
Latent Heat (kJ/kg)
Boiling Point (°C)
Liquid Nitrogen (LIN)
808
199
-196
Liquid Oxygen (LOX)
1141
213
-183
LNG (Methane)
~450 (varies)
~510
-162
Liquid Hydrogen
71
446
-253
Factors Affecting Boil-off Rate
Insulation Quality: The effectiveness of the vacuum, perlite, or multi-layer insulation (MLI) directly dictates the Heat Leak (Q).
Surface Area: Larger tanks have a better volume-to-surface-area ratio, generally resulting in a lower BOR percentage compared to smaller tanks.
Ambient Temperature: Higher outside temperatures increase the temperature gradient, driving more heat into the tank.
Tank Pressure: Keeping the tank at a higher pressure can suppress boiling temporarily, but the thermodynamics eventually reach equilibrium.
Why Monitor BOR?
Economic Loss: For high-value commodities like Helium or Hydrogen, a high boil-off rate represents significant financial loss.
Safety: BOG increases pressure within the tank. If not vented or re-liquefied, it poses an explosion risk.
Aging Equipment: An increasing BOR over time is a primary indicator of insulation vacuum failure (softening of the vacuum).