Hot tea and iced water look like two separate abilities, yet one mechanism works in the flask. Heat always flows from the warm side to the cold side: tea gives heat to its surroundings, iced water takes heat from them. The flask slows that flow whichever way it runs; it neither heats nor cools the contents, it only delays their approach to room temperature.
| Heat route | What happens in a flask |
|---|---|
| Conduction | The vacuum between the walls leaves no air to carry heat |
| Convection | Does not form, as there is no moving air |
| Radiation | Bright surfaces facing the gap reflect most of it back |
| Neck and lid | No vacuum there, so most of the loss happens here |
That is why the lid decides most of the time. A flask that stays closed lasts far longer than one opened often. Rinsing the inner wall with water at the same temperature before filling stops the drink from losing its first heat or cold to the wall. A flask that sweats on the outside with a cold drink, or warms on the outside with a hot one, has lost its vacuum and is now an ordinary double-walled vessel.
Transport containers of several hundred litres do not use a vacuum. A very large inner vessel cannot hang from the neck alone, and every spacer placed between the walls becomes a thermal bridge. At this size thick foam insulation takes the place of the vacuum; reefer containers are insulated with polyurethane foam. Insulation produces no cold, so a cold source is added to the container: GEBHARDT Isotec containers hold the cold chain for up to 24 hours with eutectic plates or dry ice.
