The purpose of a thermos container is to prevent the transfer of heat between the liquid inside and the external environment – this is how a Stanley cup and thermoses work, for example. So, whether to keep your coffee hot for longer, in a region with a mild temperature, or to keep your beer cold, on a typical summer day, the use of a thermos flask is suitable.
In this context, to understand how a Stanley cup and thermal containers from other brands work, it is necessary to keep in mind some concepts about heat propagation. First, it is worth remembering that “heat” is a form of energy (thermal energy) that spontaneously flows from a body with a higher temperature to another with a lower temperature. This transfer can happen in three ways: by conduction, by convection and by radiation.
Three ways of spreading heat
Conduction takes place in the presence of matter, when direct contact occurs. When exposing a metal object, for example, to a heat source, the molecules that make up the exposed section will be agitated by the incoming energy. In continuity, the agitation of the molecules of one section causes those of the next section to also agitate. In this way, the energy travels along the length of the item. If you touch the hot object, “heat” will also flow to your hand, due to direct contact.
Convection also takes place in the presence of matter. When you turn on an electric air heater inside a room, for example, the air in the lower part of the room will be heated. Because it is less dense and lighter, hot air rises. As warm air rises, colder (denser and heavier) air near the ceiling moves downwards. This process is continued until, gradually, the room is heated.
Radiation, on the other hand, in addition to displacing energy in the material medium, through particles, also allows this propagation in a vacuum, through electromagnetic waves. Infrared radiation, for example, originates from molecular vibration. When agitation is excessive, some of the energy is converted into light.
Thermal container components
Considering the forms of heat propagation, it is possible to understand how a Stanley cup and other thermos flasks work. Most versions have an inner and an outer chamber of plastic or metal, separated by two layers of glass with a vacuum between them. The glass is coated with a reflective layer.
In some containers, instead of glass, there are two layers of stainless steel, with a vacuum and a reflective layer between them. Typically, a tight, screw-on cap is added to the top of the bottle. See the image:
Therefore, the components of the thermal vessel prevent virtually all forms of heat transfer. The vacuum prevents conduction and the cap prevents air from entering or leaving the flask, preventing convection. Also, when radiation tries to make the liquid hot, the coating reflects it back. With this, the drink can stay hot for hours.
Of course, the mechanism also works to keep the drink cold for a few hours, as promised by Stanley’s thermos beer glass and Philco’s PTH01B and PTH01P models. Just as heat is temporarily prevented from escaping, heat cannot penetrate the vacuum flask by conduction. The cover prevents convection and radiation is reflected.
With information from: Explain that Stuff ¹, Explain that Stuff ², Science ABC e Brazil School