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A vacuum flask (also known as a Dewar flask, Dewar bottle or thermos) is an insulating storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's 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. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which significantly reduces heat transfer by conduction or convection.
Vacuum flasks are used domestically to keep beverages hot or cold for extended periods of time and for many purposes in industry.
The vacuum flask was designed and invented by Scottish scientist Sir James Dewar in 1892 as a result of his research in the field of cryogenics and is sometimes called a Dewar flask in his honour. While performing experiments in determining the specific heat of the element of palladium, Dewar formed a brass chamber that he enclosed in another chamber to keep the palladium at its desired temperature. He evacuated the air between the two chambers creating a partial vacuum to keep the temperature of the contents stable. Through the need for this insulated container James Dewar created the vacuum flask, which became a significant tool for chemical experiments and also became a common household item. The flask was later developed using new materials such as glass and aluminum; however, Dewar refused to patent his invention.
Dewar's design was quickly transformed into a commercial item in 1904 as two German glassblowers (one of whom was Reinhold Burger) discovered that it could be used to keep cold drinks cold and warm drinks warm. The Dewar flask design was never patented, but the German men who discovered the commercial use for the product renamed it Thermos and claimed the rights to the commercial product and the trademark to the name. The manufacturing and performance of the Thermos bottle was significantly improved and refined by the Viennese inventor and merchant Gustav R. Paalen, who designed various types for domestic use which he patented and distributed widely through his Thermos Bottle Companies also in the US. and Canada. The name later became a genericized trademark after the term "thermos" became the household name for such a liquid container. The vacuum flask went on to be used for many different types of scientific experiments and the commercial “Thermos” was transformed into a common item. "Thermos" remains a registered trademark in some countries but was declared a genericized trademark in the US in 1963 since it is colloquially synonymous with vacuum flasks in general. However there are other vacuum flasks.
After the German glassblowers determined the commercial uses for the Dewar flask, the technology was sold to the Thermos company who used it to mass-produce vacuum flasks for at-home use. Over time, the company expanded the size, shapes and materials of these consumer products, primarily used for carrying coffee on the go and carrying liquids on camping trips to keep them either hot or cold. Eventually other manufacturers produced similar products for consumer use.
The vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a partial-vacuum which reduces heat conduction or convection. Heat transfer by thermal radiation may be minimized by silvering flask surfaces facing the gap but can become problematic if the flask's contents or surroundings are very hot; hence vacuum flasks usually hold contents below the boiling point of water. Most heat transfer occurs through the neck and opening of the flask, where there is no vacuum. Vacuum flasks are usually made of metal, borosilicate glass, foam or plastic and have their opening stoppered with cork or polyethylene plastic. Vacuum flasks are often used as insulated shipping containers.
Extremely large or long vacuum flasks sometimes cannot fully support the inner flask from the neck alone, so additional support is provided by spacers between the interior and exterior shell. These spacers act as a thermal bridge and partially reduce the insulating properties of the flask around the area where the spacer contacts the interior surface.
Several technological applications, such as NMR and MRI machines, rely on the use of double vacuum flasks. These flasks have two vacuum sections. The inner flask contains liquid helium and the outer flask contains liquid nitrogen, with one vacuum section in between. The loss of precious helium is limited in this way.
Other improvements to the vacuum flask include the vapour-cooled radiation shield and the vapour-cooled neck, both of which help to reduce evaporation from the flask.
Research and industry
In laboratories and industry, vacuum flasks are often used to hold liquefied gases (often LN2) for flash freezing, sample preparation and other processes where maintaining an extreme low temperature is desired. Larger vacuum flasks store liquids that 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 eventually shattering the flask. The insulation of the vacuum flask results in a very slow "boil" and thus the contents remain liquid for long periods without refrigeration equipment.
Vacuum flasks have been used to house standard cells and ovenized Zener diodes, along with their printed circuit board, in precision voltage-regulating devices used as electrical standards. The flask helped with controlling the Zener temperature over a long time span and was used to reduce variations of the output voltage of the Zener standard owing to temperature fluctuation to within a few parts per million.
One notable use was by Guildline Instruments, of Canada, in their Transvolt, model 9154B, saturated standard cell, which is an electrical voltage standard. Here a silvered vacuum flask was encased in foam insulation and, using a large glass vacuum plug, held the saturated cell. The output of the device was 1.018 volts and was held to within a few parts per million.
The principle of the vacuum flask makes it ideal for storing certain types of rocket fuel, and NASA used it extensively in the propellant tanks of the Saturn launch vehicles in the 1960s and 1970s. 
The design and shape of the Dewar flask was used as a model for optical experiments based on the idea that the shape of the two compartments with the space in between is similar to the way the light hits the eye. The vacuum flask has also been part of experiments using it as the capacitor of different chemicals in order to keep them at a consistent temperature.
The industrial Dewar flask is the base for a device used to passively insulate medical shipments. Most vaccines are sensitive to heat and require a cold chain system to keep them at stable, near freezing temperatures. The Arktek device uses eight one-litre ice blocks to hold vaccines at under 10 °C.
Vacuum flasks are at risk of implosion hazard and glass vessels under vacuum in particular may shatter unexpectedly. Chips, scratches or cracks can be a starting point for dangerous vessel failure, especially when the vessel temperature changes rapidly (when hot or cold liquid is added). Proper preparation of the Dewar vacuum flask by tempering prior to use is advised to maintain and optimize the functioning of the unit. Glass vacuum flasks are usually fitted into a metal base with the cylinder contained in or coated with mesh, aluminum or plastic to aid in handling, protect from physical damage and to contain fragments should they break.
In addition, cryogenic storage dewars are usually pressurized and may explode if pressure relief valves are not used.
The rate of heat (energy) loss through a vacuum flask can be analyzed thermodynamically, starting from the second TdS relation:
Assuming constant pressure throughout the process,
Rearranging the equation in terms of the temperature of the outside surface of the vacuum flask's inner wall,
- Tsurr is the temperature of the surrounding air
- ∆S is the change in specific entropy of stainless steel
- cp is the specific heat capacity of stainless steel
- Tc is the temperature of the liquid contained within the flask
- Tb' is the temperature of the outside surface of the vacuum flask's inner wall
Now consider the general expression for heat loss due to radiation:
In the case of the vacuum flask,
Substituting our earlier expression for Tb',
- Q'0 is the rate of heat transfer by radiation through the vacuum portion of the flask
- Ain is the surface area of the outside of the inner wall of the flask
- εs.s. is the emissivity of stainless steel
- σ is the Stefan–Boltzmann constant
Assuming that the outer surface of the inner wall and the inner surface of the outer wall of the vacuum flask are coated with polished silver to minimize heat loss due to radiation, we can say that the rate of heat absorption by the inner surface of the outer wall is equal to the absorptivity of polished silver times the heat radiated by the outer surface of the inner wall,
In order for energy balance to be maintained, the heat lost through the outer surface of the outer wall must be equal to the heat absorbed by the inner surface of the outer wall,
Since the absorptivity of polished silver is the same as its emissivity, we can write
We must also consider the rate of heat loss through the lid of the vacuum flask (assuming it is made of polypropylene, a common plastic) where there is no vacuum inside the material. In this area, the three heat transfer modes of conduction, convection, and radiation are present. Therefore, the rate of heat loss through the lid is,
- k is the thermal conductivity of air
- h is the convective heat transfer coefficient of free air
- εp.p is the emissivity of polypropylene
- Alid is the outer surface area of the lid
- cpp.p. is the specific heat capacity of polypropylene
- ∆Sp.p. is the specific entropy of polypropylene
- ∆x is the distance over which conduction across the temperature gradient takes place
Now we have an expression for the total rate of heat loss, which is the sum of the rate of heat loss through the walls of the vacuum flask and the rate of heat loss through the lid,
where we substitute each of the expressions for each component into the equation.
The rate of entropy generation of this process can also be calculated, starting from entropy balance:
Written in rate form,
Assuming a steady-state process,
Since there is no heat added to the system,
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