The vacuum flask keeps your food or drink hot or cold. It was designed in such a way to keep heat from escaping from the hot contents to its surroundings or to prevent heat transferring from the surroundings heating up the cold contents. Heat is transferred by conduction, convection and radiation so the flask was designed to reduce heat transfer by these three methods.
Explore the flask
Click on the blobs for more information about the different parts of the flask.
Conduction is reduced by
- the vacuum between the glass walls. Remember that conduction can occur in a gas such as air (although at a much slower rate than in solids). Having a vacuum prevents conduction from occurring completely because there are no molecules to transfer the heat.
- trapped air in the foam sponge and hollow plastic stopper. Since air is a poor conductor of heat, having trapped air in the sponge and hollow plastic stopper help to prevent heat loss by conduction.
Convection is reduced by
- the vacuum between the glass walls. Convection can occur in a gas such as air, removing all of the air and creating a vacuum helps to prevent heat loss by convection.
- trapped air in the foam sponge. The air is trapped in small pockets of the foam sponge. The volume of each pocket of air is too small for convection currents to set up so heat loss by convection is reduced.
Heat loss occurs by convection when you remove the cover.
Radiation is reduced by
- the two silvered walls that form the vacuum. Radiation cannot be reduced by the vacuum but the first silvered wall serves as a poor emitter of any radiation that falls on it. If there is any radiation that’s emitted, the second wall reflects it in the same direction that it came from. This way, heat is kept in the flask if the contents are hot or heat is kept out of the flask if the contents are cold.