- Dewar flask
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See also: Vacuum flask
A Dewar flask is a vessel designed to provide very good thermal insulation. For instance, when filled with a hot liquid, the vessel will not allow the heat to easily escape, and the liquid will stay hot for far longer than in a typical container. The Dewar flask was named after its inventor, the Scottish physicist Sir James Dewar (1842–1923).
In form, a Dewar flask is a glass or metal bottle, with a double-layer construction; in fact, it can be considered to be two thin-walled bottles nested one inside the other, and sealed together at the neck. The narrow space between is evacuated almost entirely of air; the near vacuum minimizes conduction and convection of heat.
The inner surfaces of the outer bottle, and the outer surface of the inner bottle, have a metallic or similarly reflective coating to prevent heat from being transmitted via radiation. Dewar himself used silver for this purpose. The open end of the flask is reduced to a small diameter relative to the body of the flask, reducing the only opening by which heat loss can occur. The design of the Dewar flask is so effective that what little heat transport does occur is almost entirely via the neck and stopper; insulating materials such as cork are often used to further reduce heat loss.
A very common use of the Dewar flask in laboratories is the storage of liquid nitrogen; in this case, the leakage of heat into the extremely cold interior of the bottle results in a slow boiling-off of the liquid (a pressure relief valve is provided to prevent pressure from building up). The excellent insulation of the Dewar flask results in a very slow boil and thus the nitrogen lasts a long time without the need for expensive refrigeration equipment.
A Thermos bottle (trademarked), or vacuum flask (generic) used to keep coffee or other beverages hot, is usually based on the design of a Dewar flask; since glass dewar flasks are fragile and somewhat expensive to fabricate, other constructions based on plastic or insulated metal are also popular.
See also
Patents
- Burger, R., U.S. Patent 872,795, "Double walled vessel with a space for a vacuum between the walls", December 3, 1907.
Categories:- Cryogenics
- Scottish inventions
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