Vacuum flask

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Vacuum flask

A vacuum flask is a storage vessel or insulated shipping container

Insulated shipping container

Insulated shipping containers are a type of packaging used to ship temperature sensitive products such as foods, pharmaceuticals, and chemicals....
which keeps its contents hotter or cooler than their environment without the need to modify the pressure, by interposing an evacuated region to provide thermal insulation

Thermal insulation

The term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer....
between the contents and the environment.

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A vacuum flask is a storage vessel or insulated shipping container

Insulated shipping container

Thermal insulation

Vacuum

A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty....
referred to is used for thermal insulation; the contents are not in vacuum conditions.

The vacuum flask was invented by Scottish physicist

Physicist

A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many Physics#Major fields of physics spanning all length scales: from atom particles of which all ordinary matter is made to the behavior of the material Universe as a whole ....
and chemist

Chemist

A chemist is a scientist trained in the science of chemistry. Chemists study the composition of matter and its properties such as density, acidity, size and shape....
Sir James Dewar in 1892 and is sometimes referred to as a Dewar flask after its inventor. The first vacuum flasks for commercial use were made in 1904 when a German

Germany

Germany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium, and the Netherlands....
company, Thermos GmbH, was formed. Thermos, their tradename for their flasks, remains a registered trademark in some countries but was declared a genericized trademark

Genericized trademark

A genericized trademark is a trademark or brand name that has become the colloquialism or generic description for a general class of Good or Service , rather than the specific meaning intended by the trademark's holder....
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.

Theory of operation

A practical vacuum flask is a bottle made of glass

Glass

Glass generally refers to a Hardness, brittle, transparency amorphous solid, such as that used for windows, many Glass Bottles, or eyewear, including, but not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovite , or aluminium oxynitride....
, metal

Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
, or plastic

Plastic

Plastic is the general common term for a wide range of synthetic or semisynthetic organic chemistry solid materials suitable for the manufacture of industrial products....
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.

Using vacuum as an insulator avoids heat transfer by conduction

Heat conduction

Heat conduction or thermal conduction is the spontaneous heat transfer through matter, from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences....
or convection

Convection

Convection in the most general terms refers to the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer....
. Radiative

Thermal radiation

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb....
heat loss can be minimized by applying a reflective coating to surfaces: Dewar used silver

Silver

Silver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal....
.

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 infrared (radiative) transfer.

The flask must, in practice, have an opening for contents to be added and removed. A vacuum cannot be maintained at the opening; therefore, a stopper made of insulating material must be used, originally cork

Cork (material)

Cork material is a prime-subset of generic Cork cambium, harvested for commercial use primarily from the Cork Oak tree, Quercus suber, with Portugal producing 50% of cork worldwide....
, later plastics. Inevitably, most heat loss takes place through the stopper.

Purpose and uses

Vacuum flasks are used to maintain their contents (often but not always liquid

Liquid

Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material....
).

Vacuum flask

Theory of operation

Purpose and uses

See also

Further reading