Boiloff

Boiloff refers to the loss of liquid due to heat input causing the liquid to vaporize into gas. In cryogenics, the term boil-off rate (BOR) quantifies how much liquid vaporizes over a given time. It is influenced by heat load, container design, insulation, and venting systems.

In cryogenic storage, liquids such as liquid nitrogen, oxygen, hydrogen, or helium are kept at very low temperatures in dewars. Heat entering the vessel from conduction, convection, or radiation raises the liquid toward its boiling point, producing boil-off gas that must be vented to prevent pressure buildup. Dewars are designed with vacuum insulation and reflective shielding, and some systems employ capture or recondensation of boil-off gas to improve efficiency or safety.

In aerospace and space applications, boil-off of cryogenic propellants such as liquid hydrogen or liquid oxygen occurs in storage tanks exposed to ambient conditions. Boil-off gas is often vented to avoid overpressure, and mission designers may use active cooling, reliquefaction, or gas utilization strategies to minimize losses and preserve propellant margins.

In the liquefied natural gas (LNG) industry, boil-off gas arises from heat ingress in LNG storage and transport systems. The gas may be burned as fuel on site or routed to reliquefaction or gas processing facilities. Reducing boil-off gas improves energy efficiency, safety, and environmental performance.

Measurement and control: BOR is typically expressed as mass or volume per time (e.g., kg/day, L/day) or as a percentage of stored volume per day. Factors affecting BOR include insulation quality, tank geometry, fill level, ambient temperature, and venting strategy. Safety considerations emphasize proper ventilation and monitoring to prevent oxygen deficiency or flammable gas accumulation.