Many people use vacuum flasks on a daily basis but not everyone is interested in the question: “How does a vacuum flask work?”. Most of us are not surprised that a tea that has been just cooked will soon get cold. It happens because of the temperature of the surrounding that is much lower than one of boiling water. Some of this heat flows in the air, the other part is lost because of conduction, convection and radiation. All these physical processes take part in cooling the water and in an hour or less you will have a cold tea.
But to preserve a high or low temperature of the liquid or food for a longer period of time people have invented a vacuum flask, insulator or thermos. Its work is also based on the same physical processes but they function in a different way, so let’s get a closer look.
Heat transfer in a vacuum
To understand the work of the vacuum flask it is important to know the basics of three ways how heat moves. Heat is an energy that can move whis help:
Convection is the process when heat rises upwards. Its density is lower than one of cold air, so it moves up and cooler air gets out of its way.
Conduction can happen in case heat is touched by someone or something as a hot object gives heat away.
Radiation is the way of heat moving when hot things give off light due to the increased speed of atoms. Because of instability when they come back to their normal state they start giving off energy in the form of light of different colors.
The consideration and analysis of these ways allowed people to invent a thermos. Every vacuum flask consists of the following parts and layers:
- A screw-down stopper.
- External case made from plastic or stainless steel.
- External glass layer accompanied by a reflective material adjusted to it or stainless steel.
- Inner layer of glass or stainless steel.
- Supports to prevent inner vacuum container from moving.
- Another insulation to reduce heat loss and prevent flask from impact.
So how does vacuum insulation work?
Vacuum makes conduction impossible so hot liquid or food will not lose its temperature. A tight screw-down stopper with a rubber usually does not allow air to get into and out of the flask, so convection does not take place as well. The same happens with radiation. Trying to give off energy through light the last one can not leave the flask thanks to the reflective material contained inside.
To sum up
That means that heat does not have any ways for escaping and it remains in the flask for a longer period of time. However, the more often you will open the thermos, the quicker temperature of the liquid or food contained in it will go down.
Modern thermoses are made of different materials that allow buyers to choose the best food thermos with the most suitable characteristics here.