A vacuum flask is a special insulated storage bottle, which greatly increases the period over which its contents stay hot or cold than the surrounding environment of the flask. Discovered by Sir James Dewar in nineteenth-century England, the vacuum flask comprises two flasks, both sealed at the neck, one within the other. The liquids in the outer flask are heated above room temperature, while the contents of the inner flask are cooled below room temperature. Thus, a constant boiling of the contents inside the inner flask keeps them hot, whereas a constant cooling of the same contents in the outer flask slows down their boiling.
A vacuum flask has the desirable property to retain its heat even when the temperature is raised to a very high degree. Thus, it can be used in microwave ovens to keep food warm. The reason behind this heat retention is that the inner wall of the inner flask acts as a radiation barrier, preventing intense electromagnetic fields from entering the outer surface of the bottle. In addition, since no heat is lost from the inner wall of the bottle, the inside of the bottle is left free of heat loss. Such properties make vacuum flasks ideal for applications where the heat generation needs are large, such as heating metal or wrapping a bundle of dry cotton in a heated wax composition.
Thermos Flasks, in turn, uses the principles of the double exponential flushing cycle described by Claus. First, they contain a volume of liquid which has a higher temperature than the rest of the volume. When this first portion of the thermos is heated, it pulls out the heat energy from the surroundings, making the remaining part of the thermos cooler and less heat energy is absorbed. Thus, each time the temperature of the rest of the container is raised, more heat energy is released. Thus, each time the thermos is opened, the process is self-perpetuating.
In modern applications, such as for medical diagnostic equipment or industrial baking machines, vacuum flasks have undergone a variety of modifications. In general, it has been found that the principles of the double exponential flushing are also effective in these applications. This allows for the production of high-temperature steam through the use of convection and radiation barrier. The heat conduction from the center of the volume to the outer surface of the same still maintains high efficiency.
One aspect that may not be immediately apparent is the difference between a “closed” and “open” design. A closed vacuum flask is one in which the lid is tightly closed, so that any heat loss is not allowed to escape. As the heat concentration increases with the increase of the temperature, so does the thermal resistance. Thus, a closed system is one in which the lid can be kept closed without trapping heat energy.
The term “vacuum” is relative, and the same principles that are at work in a vacuum flask can also be seen working in an aluminum bowl. When the boiling water in the bowl’s interior boil to a boil evaporation of the steam is prevented by the presence of a vacuum between the surface of the bowl and the boiling water. At higher temperatures, evaporation becomes ineffective. With the help of a funnel, the coffee can be poured into the container without any danger of the steam escaping.