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How does it work

How does it work?

A vacuum flask is a remarkable and intelligent tool. It keeps hot stuff warm and cold stuff cold. How is it possible that this object can remember when to keep cool or when to keep hot?

Let's find the answer.


A vacuum flask is a very useful tool but it is not so sophisticated that it can sense the temperature. It only tries to prevent heat-transfer. Let's execute the following experiment: 

1.   Take a glass of ice water and place it on the kitchen table. After a while, the water will not be as fresh anymore as it was in the beginning. Even later, the water will get as warm as the room temperature.

2.   A nice hot soup is at its best when it's still hot. When you wait too long the soup will cool down to room temperature and the taste is gone.


With this simple experiment, the purpose of the vacuum flask becomes clear. The ice water will stay very fresh and the hot soup will stay very warm. All this by slowing down the processes of natural heat transfer.


There are 3 processes that cause heat transfer: Conduction, radiation and convection.




What is heat? Heat is atomic motion. An atom represents its "heat" by its speed. At the temperature absolute zero, there is no atomic motion. But as atoms get warmer they move. Heat is transferred when one atom runs into another. When this happens, it is a little bit like billiard balls  colliding, the second atom picks up some of the motion of the first atom. Heat is transferred by these collisions.


Example: Let's take a metal bar and heat one end of it. The other end will get warm and then hot through conduction. When you put a metal pan on the stove, the inside of the pan gets hot through conduction of the heat through the metal in the bottom of the pan. Some materials (namely metals) are better heat conductors than others (for example, plastics).


Another side effect of atomic motion is vibration, and vibration leads to the unexpected phenomenon of infrared radiation. According to the Encyclopedia Britannica, "Infrared radiation is absorbed and emitted by the rotations and vibrations of chemically bonded atoms or groups of atoms and thus by many kinds of materials." Infrared radiation is a form of light.


Our eyes are unable to see infrared, but our skin can feel it. About half of all of the sun's energy that reaches us comes as invisible infrared radiation, with the rest of it visible to us as light. Infrared, like visible light, is reflected by mirrors and absorbed better by black objects. When infrared is absorbed, it results in atomic motion, and therefore, in a rise in temperature. Some common examples of infrared are the heat you feel radiating from an electric heater or a red-hot piece of metal, the heat you feel radiating from the bricks in a fireplace even if the fire has gone out and the heat you feel radiating from a concrete wall after the sun has gone down.



Convection is a property of liquids and gasses. It occurs because when a liquid or gas gets hot, it tends to rise above the rest of the body of liquid or gas. So, if you have a hot bowl of soup on the table, it heats a layer of air surrounding the bowl. That layer then rises because it is hotter than the surrounding air. Cold air fills in the space left by the rising hot air. This new cold air then heats up and rises, and the cycle is repeated. It is possible to speed up convection, that is why you blow on hot soup to cool it down. Without convection your soup would stay hot a lot longer, because it turns out that air is a pretty poor heat conductor


 A bonfire is a perfect example of all three phenomena. You probably need to stand at least 20 feet away from a big bonfire like this one.

The heat radiating keeps you away from the fire through infrared radiation. Red faces are the evidence of that. The flames and smoke are carried upward by convection: Air around the fire heats up and rises. The ground 3 feet away from the fire will get hot, heated by conduction. The top layer of soil is directly heated (by radiation), and then the heat is conducted deep into the ground.


Construction of a vacuumflask


To build a good vacuum flask, we need to reduce these three heat transfer phenomena as much as possible.

Let's concentrate first on conduction.  We have learned that conduction is heat transfer by atoms 'bumping into each other'. The higher the temperature, the higher the atomic motion/speed. We reduce conduction by using a double walled flask. The walls are not touching each other except at the neck where the two walls are welded. The conduction is minimised. 

Secondly, we have to minimize the heat transfer caused by convection. We have learnbed that convection is in fact air flow. So let't take the air out between the two walls so convection is no longer possible. In fact we form a vacuum between the walls by sucking the air out. The only convection left is that at the top of the flask at the opening. The convection is now minimised.

Finally we have to try to minimis the third possible method of heat tranfer, radiation. Again we go back to what we have learned. Radiation is  heat transfer by UV light / infrared light. It is not only the sun that radiates but for instance also the hot liquid we keep in the flask. What we 
want to do is keeping the radiation inside the flask and to keep radiation from the outside out. By using a silver coating between the wall we reverse the radiation and with that we minimise the radiation. Again the only radiation we find is at the top, where the glass bottle is open.

Can we reduce the heat transfer by 100% and keep the heat or the cold in the flask till eternity? Unfortunately, because of imperfections, this is not the case. The biggest loss on a vacuum flask is the cap ( the stopper). Of course, the stopper can be insulated but the heat transfer of the used insulation product like foam is always higher than vacuum. Another loss, by conduction, is the area where the outer- and inner bottle come together. As mentioned before, a perfect vacuum does not exist so through conduction a slight heat transfer will take place.

With all the prior mentioned imperfections, you would think it?s almost impossible to make a quality vacuum flask. Thanks to its experience, Hovac Belgium has developed a line of products with a high standard in insulation quality. A well studied shape of the flask, an insulating stopper, a silver coating in the vacuum space and a high level of vacuum are just some of the aspects which has made that Hovac and its products are at the top of its profession.




Hot. Cold. Any time. Any place.