Curious Facts / Everyday Life
Where is the vacuum in an insulated flask?
Beside the drink chamber lies a gap designed to make heat transfer harder.
The evacuated space is between the walls
The vacuum gap lies between the inner and outer walls, not in the chamber holding the drink. Removing air leaves fewer particles to transfer heat. It also reduces convection, in which moving air carries heat.
Separate the routes heat can take
A vacuum greatly reduces conduction and convection across the gap, but it does not block all thermal radiation. Reflective surfaces can reduce heat transfer by infrared radiation.
| Part | Heat-transfer route it reduces |
|---|---|
| Vacuum gap between walls | Conduction and convection across that gap |
| Reflective surface | Thermal radiation |
Heat can still pass through parts such as the lid and wall connections, so a real flask cannot maintain the same temperature forever. It slows heat leaving hot contents or heat entering cold contents from warmer surroundings.
What did you discover?
3 questions · Read the explanation and evidence, then continue to the next question.
Sources and scope
- University of Hong Kong Physics — How Things Work: Kitchen Science
The Vacuum flask: double walls, vacuum, reflective surface, insulating cap and supports
Scope of this fact
- This is a basic account of vacuum insulation, not product duration, food-safety guidance or a buying comparison.
- Reduced transfer across the gap is not zero transfer through the entire bottle. A real flask is not a perfect insulator.
Original writing and English translation by Rantap