Vacuum flask

China Product


Theory of operation

A practical vacuum flask is a bottle made of metal, glass, 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. Using vacuum as an insulator avoids heat transfer by conduction or convection. 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 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, later plastics. Inevitably, most heat loss takes place through this stopper. bamboo salad bowl

Purpose and uses melamine ware

Food and drink stoneware dinner plates

Vacuum flasks are used to maintain their contents (often but not always liquid) at a temperature higher or lower than ambient temperature, while retaining the ambient pressure of approximately 1 Atmosphere (14.7 Psi). Domestically and in the food industry, they are often used to keep food and drink either cold or hot. A typical domestic vacuum flask will keep liquid cool for about 24 hours, and warm for up to 8.

Laboratory and industry

In laboratories and industry, vacuum flasks are often used to store liquids which become gaseous at well below ambient temperature, such as oxygen and 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 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.

Modifications

Several applications rely on the use of double Dewar 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 Dewar flask include the vapor-cooled radiation shield and the vapor-cooled neck, which both help to reduce evaporation from the flask.

Gallery

Sketch of vacuum flask, from a U.S. patent, 1907

A large stainless steel Dewar flask of liquid nitrogen, used to supply a cryogenic freezer (for storing laboratory samples at about -197C).

References

^ http://www.cryogenicsociety.org/resources/cryo_central/history_of_cryogenics/

Further reading

Burger, R., U.S. Patent 872,795, "Double walled vessel with a space for a vacuum between the walls", December 3, 1907.

Sella, Andrea (August 2008). "Dewar's Flask". Chemistry World: 75. http://www.rsc.org/chemistryworld/Issues/2008/August/DewarsFlask.asp. Retrieved 2008-08-30. 

Categories: Containers | Scottish inventions | Cryogenics | Vacuum | 1892 introductions