Calculations

How to Calculate Boil-Off Rate, Cooldown Time, and Vaporizer Capacity for Cryogenic and LNG Tanks

Worked, unit-checked formulas for the five calculations that drive cryogenic and LNG equipment design: heat leak, boil-off, cooldown, vaporizer duty, and pump power.

Cryogenic storage engineering lives between 77 K (liquid nitrogen) and 111 K (LNG), and every design decision traces back to five calculations: heat leak through the insulation, boil-off rate, cooldown time and fluid consumption, vaporizer duty, and pump power. Work in SI units throughout: heat leak in watts, latent heat in kJ/kg, mass flow in kg/h, temperature in kelvin. The property values you need most are LNG density near 450 kg/m³, latent heat of vaporization near 510 kJ/kg, and liquid nitrogen at 807 kg/m³ and 199 kJ/kg. Get these five calculations right and sizing, purchasing, and troubleshooting all follow from the same numbers.

Boil-off rate is the anchor metric. BOR in percent per day equals (Q × 86,400 × 100) / (m × h_fg), where Q is total heat leak in watts, m is stored liquid mass in kg, and h_fg is latent heat in J/kg. Worked example: a 100 m³ LNG tank holds 45,000 kg at 450 kg/m³. With 600 W of heat leak, daily evaporation is 600 × 86,400 / 510,000 = 101.6 kg/day, so BOR = 101.6 / 45,000 = 0.23 percent per day. The Tank Boil-Off Rate calculator runs this directly; the only hard input is Q, which comes from the insulation calculation below or from vendor test data.

Heat leak through insulation follows Q = k_eff × A × ΔT / t. Evacuated perlite runs 0.0015 to 0.002 W/m·K, while multilayer insulation under high vacuum reaches 0.00005 W/m·K. Example: a vacuum jacketed vessel with 120 m² of cold surface, a 0.25 m perlite annulus, and ΔT of 189 K (300 K ambient to 111 K LNG) gives Q = 0.0015 × 120 × 189 / 0.25 = 136 W. The catch is vacuum quality: at 100 microns of annulus pressure, gas conduction raises effective conductivity roughly tenfold, turning a 136 W design into a 1.4 kW problem. The Vacuum Jacket Leak Rate calculator converts measured pressure rise into a heat leak trend before boil-off tells you the hard way.

Cooldown consumes both time and fluid. Stainless steel releases about 90 kJ/kg cooling from 300 K to 77 K, so on latent heat alone (199 kJ/kg) one kg of liquid nitrogen cools only 2.2 kg of steel. Route the cold vapor through the equipment and its sensible heat nearly doubles that to 4 kg or more. A 20,000 kg piping system therefore needs roughly 9,000 kg of LN2 with poor vapor use, or about 5,000 kg with good use. Time is set by the allowable ramp: 9 percent nickel steel and 304L are commonly limited to 25 to 50 K/h, so 300 K to 111 K takes at least 4 to 8 hours. The Cryogenic Cooldown Time calculator handles both sides.

Vaporizer duty is Q = m_dot × (h_fg + cp,gas × ΔT). For LNG regasified from 111 K to 278 K, that is 510 kJ/kg latent plus roughly 2.2 kJ/kg·K × 167 K = 367 kJ/kg sensible, about 880 kJ/kg total. A 2,000 kg/h send-out therefore needs 2,000 × 880 / 3,600 = 489 kW of heat input. Ambient air vaporizers lose capacity as frost builds, typically 30 to 40 percent after 4 to 8 hours of continuous duty, so size at 1.3 to 1.5 times the steady requirement or install switching pairs. The Vaporizer Capacity calculator converts flow, fluid, and outlet temperature into required duty and derated area.

Pump shaft power is P = Q_v × ΔP / η, with Q_v in m³/s and ΔP in Pa. Worked example: 100 m³/h of LNG raised 12 bar gives (100 / 3,600) × 1,200,000 = 33.3 kW hydraulic; at 65 percent efficiency the shaft draw is 51 kW. Inefficiency matters twice in cryogenic service, because the lost 18 kW enters the liquid and flashes vapor at 18,000 / 510,000 = 0.035 kg/s, about 127 kg/h of extra boil-off to vent or re-liquefy. Keep 2 to 3 K of subcooling ahead of the impeller to protect NPSH. The Cryogenic Pump Energy Load calculator totals shaft power, fluid heat input, and flash generation in one pass.

Two more calculations round out the set. Transfer loss combines flash from pressure letdown, line cooldown, and displaced vapor; a 40 m³ trailer offload through a warm 50 m line typically loses 0.3 to 0.8 percent of cargo, which the LNG Transfer Loss calculator itemizes stream by stream. Relief loads for the fire case follow API 521, Q = 21,000 × F × A^0.82 in BTU/h with wetted area in ft², then required relieving flow is W = Q / h_fg; run this before picking an orifice letter, and see the Pressure Relief Sizing Cost calculator when budgeting the hardware. Chain everything together: heat leak feeds boil-off, boil-off feeds relief and venting, and one bad input propagates through all of it.

Published 2026-07-02.