Learning from a Thermos – Designing a well insulated space

When I was a child I used to have a lot of curiosity while using a Thermos (or vacuum flask). It was always like magic that it used to keep hot things hot and cold things cold all the time. It neither had fire in it to keep things hot nor ice to keep things cool. Later in high school we learnt the basics of heat transfer, and that was when the theory behind the thermos became clear to me. After so many years, when I think of it, a lot of it can be applied to saving electricity as well. Heating or cooling of a room consumes so much electricity for most people that it feels as if the room was like a thermos which once filled with cold air remained cold for a long time or with hot air to remain hot without needing much of electricity.

Design of a Thermos

First thing first: it is not that a hot liquid kept in a thermos remains hot for an indefinite time. It does loose heat, but takes a much longer time than any other regular container. The basic science behind it is heat transfer that happens in 3 ways:

1.Conduction: Conduction happens when 2 objects with different temperatures come in contact with each other. The heat transfers from the hotter object to the cooler object. It is exactly how it happens when you touch a hot vessel: your hand is colder than the hot vessel and thus you feel the heat because heat is transferred from vessel to your hand.

2.Convection: Convection happens when heat transfer takes place due to movement of fluids (liquid or gas). Imagine sitting near a fire and the breeze blowing makes you feel hot. The breeze brings the heat from the fire to you.

3.Radiation: Radiation happens through electromagnetic waves. It can even move through vacuum (or without the presence of any matter). Radiation from sun is a perfect example. All objects radiate heat and lots of sensors (e.g. occupancy sensors) detect presence of objects by sensing the radiations.

A thermos is designed in such a way that it reduces heat transfer through all the above-mentioned 3 modes.

A thermos has double glass walls with vacuum between them. The vacuum between the two walls prevents heat transfer through conduction or convection from the inside to the outside and reverse.

Thermos also has a silver lining in the inner wall that prevents heat transfer through radiation as blocks any inner heat to go out through radiation.