Dewar flask
Encyclopedia
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 (trademark
ed), 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.
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
. For instance, when filled with a hot liquid, the vessel will not allow the heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
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
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Sir James Dewar
James Dewar
Sir James Dewar FRS was a Scottish chemist and physicist. He is probably best-known today for his invention of the Dewar flask, which he used in conjunction with extensive research into the liquefaction of gases...
(1842–1923).
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
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
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
; the near vacuum
Vacuum
In everyday usage, 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". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
minimizes conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
and convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
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
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
. Dewar himself used silver
Silver
Silver is a metallic 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...
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
Cork (material)
Cork is an impermeable, buoyant material, a prime-subset of bark tissue that is harvested for commercial use primarily from Quercus suber , which is endemic to southwest Europe and northwest Africa...
are often used to further reduce heat loss.
A very common use of the Dewar flask in laboratories is the storage of liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
; 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
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
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
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
equipment.
A Thermos bottle (trademark
Trademark
A trademark, trade mark, or trade-mark is a distinctive sign or indicator used by an individual, business organization, or other legal entity to identify that the products or services to consumers with which the trademark appears originate from a unique source, and to distinguish its products or...
ed), or vacuum flask
Vacuum flask
A vacuum flask is an insulating storage vessel which keeps its contents hotter or cooler than its surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck...
(generic) used to keep coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...
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
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
or insulated metal are also popular.
Patents
- Burger, R., , "Double walled vessel with a space for a vacuum between the walls", December 3, 1907.