Vacuum Flask - Design

Design

The vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which prevents heat transfer by conduction or convection. Heat transfer by thermal radiation may be minimized by silvering flask surfaces facing the gap, but can become problematic if the flask's contents or surroundings are very hot; hence vacuum flasks usually hold contents below water's boiling point. Most heat transfer occurs through the flask's neck and opening, where a vacuum is not present. Vacuum flasks are usually made of metal, glass, foam, or plastic, and have their opening stoppered with cork or plastic. Vacuum flasks are often used as insulated shipping containers.

Several applications rely on the use of double vacuum flasks, such as NMR and MRI machines. These flasks have two vacuum sections. The flasks contain liquid helium in the inside flask and liquid nitrogen in the outer flask, with one vacuum section in between. The loss of expensive helium is limited in this way.

Other improvements to the vacuum flask include the vapor-cooled radiation shield and the vapor-cooled neck, which both help to reduce evaporation from the flask.