Vacuum jugs are used domestically to keep beverages hot or cold for a long time and are used for many purposes in the industry.
A vacuum jug consists of two containers, one placed inside the other and connected at the neck. The gap between the two containers is partially evacuated with air, creating a partial vacuum that reduces heat transfer or convection. Heat transfer due to thermal radiation can be minimized by silver plating the surface of the jug facing the gap, but this can be a problem if the contents of the kettle or the surrounding environment are very hot; therefore, vacuum kettles usually keep the contents in the water below the boiling point. Most heat transfer is through the neck and opening of the kettle without a vacuum. Vacuum jugs are usually made of metal, borosilicate glass, foam or plastic, and their openings are plugged with cork or polyethylene plastic. Vacuum tanks are often used as insulated shipping containers.
Very large or very long vacuum jugs sometimes cannot fully support the inner jug from just the neck, so a spacer between the inner and outer shell provides additional support. These baffles act as thermal bridges and partially reduce the insulating properties of the kettle around the area where the baffles come in contact with the inner surface.
Some technical applications, such as NMR and MRI machines, rely on the use of double vacuum tanks. These kettles have two vacuum sections. The inner tank is filled with liquid helium, the outer tank is filled with liquid nitrogen, and there is a vacuum part in the middle. In this way the loss of precious helium is limited.
Other improvements to the vacuum canister include a vapour-cooled radiation shield and a vapour-cooled neck, both of which help reduce the vaporization of the vacuum canister.
Vacuum jugs have a risk of implosion, especially glass containers that can accidentally shatter under a vacuum. Chips, scratches, or cracks can be the starting point for dangerous container failure, especially when the container temperature changes rapidly (when adding hot or cold liquids). Proper preparation of the Dewar by tempering prior to use is recommended to maintain and optimize the functionality of the unit. Glass vacuum pots are usually mounted in a metal base, with cylinders contained in or coated with mesh, aluminum or plastic to aid in handling, protect them from physical damage, and contain debris if they break.
Also, cryogenic storage dewars are usually pressurized and can explode if a pressure relief valve is not used.
Thermal expansion must be considered when designing a vacuum tank. The outer and inner walls are exposed to different temperatures and will expand at different rates. The vacuum tank may rupture due to the difference in thermal expansion between the outer and inner walls. Expansion joints are commonly used in tubular vacuum jugs to avoid rupture and maintain vacuum integrity.