Vacuum flask
From Wikipedia, the free encyclopedia
- This article is about the vacuum-insulated flask. For the flask used in filtration under vacuum, see Büchner flask.
A vacuum flask is a vessel which keeps its contents hotter or cooler than their environment by interposing an evacuated region to provide thermal insulation between the contents and the environment. Note that the vacuum is used for thermal insulation; the contents are not in vacuum conditions.
The vacuum flask was invented by physicist and chemist James Dewar in 1892. A practical vacuum flask is a bottle made of glass, metal, or plastic with hollow walls; the narrow region between the inner and outer wall is evacuated of air. It can also be considered to be two thin-walled bottles nested one inside the other, and sealed together at their necks. The vacuum flask is also known as a Dewar flask after its inventor.
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[edit] Theory of operation
A vacuum does not conduct heat by conduction or convection, but only by radiation. Radiative heat loss can be minimized by applying a reflective coating to surfaces: Dewar used silver.
The contents of the flask reach thermal equilibrium with the inner wall; the wall is thin, with low thermal capacity, so does not exchange much heat with the contents, affecting their temperature little. At the temperatures for which vacuum flasks are used (usually below the boiling point of water), and with the use of reflective coatings, there is little radiative (infrared) transfer.
The flask must in practice have an opening for contents to be added and removed; as a vacuum cannot be maintained at the opening, a stopper made of thermally insulating material must be used, originally cork, later plastics. Most heat loss takes place through the stopper.
[edit] Purpose and uses
Vacuum flasks are used to maintain their contents, often but not always liquid, at a temperature higher or lower than ambient temperature.
[edit] Domestic and food industry use
They are often used to keep food and drink either cold or hot. A typical domestic vacuum flask, usually referred to generically by the name of the first company to produce them commercially, Thermos, will keep liquid cool for about 8 hours, and warm for 24.
[edit] Laboratory and industrial use
Vacuum flasks are often used to store liquids which become gaseous at well below ambient temperature, e.g., oxygen, nitrogen, etc.; in this case, the leakage of heat into the extremely cold interior of the bottle results in a slow "boiling-off" of the liquid so that a narrow unstoppered opening, or a stoppered opening protected by a pressure relief valve, is necessary to prevent pressure from building up and shattering the flask. The excellent insulation of the Dewar flask results in a very slow "boil" and thus the contents remain liquid for a long time without the need for expensive refrigeration equipment.
[edit] History
The first vacuum flasks for commercial use were made in 1904 when a German company, Thermos GmbH, was formed. "Thermos", their tradename for their flasks, remains a registered trademark in some countries but was declared a genericized trademark in the US in 1963 as it is colloquially synonymous with vacuum flasks in general—in fact it is far more common to speak of a domestic thermos than a vacuum flask.
[edit] See also
[edit] External links
- Burger, R., U.S. Patent 872795, "Double walled vessel with a space for a vacuum between the walls", December 3, 1907.
- Thermos LLC Major manufacturer of Vacuum flasks for consumers
- Stanley Major manufacturer of Vacuum flasks for consumers
- The Gourmet Kitchen Major retailer of Vacuum flasks for consumersde:Isolierkanne
es:Termo eo:Termoso nl:Thermosfles ja:魔法瓶 pl:Termos fi:Termospullo sv:Termos uk:Термос
