How does a thermos flask keep coffee hot and drinks cold?

To answer that question you need to understand heat transfer.

Let’s say you make some coffee and forget about it. Eventually it cools down to room temperature. Yech …cold coffee. This process can be explained by a branch of Physics known as thermodynamics. The first applications of thermodynamics were for making more efficient steam engines, but the same principles that applied there, are also at work in your kitchen.

If you put two objects of different temperatures together, transfer of heat energy will cause them to reach the same energy level (in this case measured by temperature). After all, when you heated your coffee you ADDED heat energy to the water. In the coffee example, the surrounding air has a lower heat energy level and so heat energy is lost from your coffee till the two are in equilibrium. The room actually gets slightly warmer because of this, but since it is huge in relation to a cup you don’t notice.

The reverse applies with a cold drink. To cool the drink, heat energy is removed from the drink by your refrigerator. Don’t believe me? Put your hand behind your refrigerator. There is radiator there that gives off the heat that was removed from inside the compartment.

To keep your coffee hot and your drink cold for as long as possible, you want to slow down the heat transfer process. In actual fact you have to control all THREE processes that cause heat transfer.


1. Conduction
Heat is actually atomic motion. An atom has motion as a result the amount of energy it has. This energy is transferred when one atom bounces into another one. The second atom picks up some of the motion energy of the first atom. Heat is transferred by these collisions.

When you put a metal pan on the stove, the pan gets hot through conduction of the heat through the bottom of the pan. The handle of the pan also gets hot even though it is not being directly heated. If the handle was not insulated you would burn. From this we can also see that some materials (mostly metals), are better heat conductors than others (such as wood and plastics).

2. Radiation
This one is a bit trickier, but another effect of atomic motion is thermal radiation. This is generated as infrared radiation “when energy from the movement of charged particles within atoms is converted to electromagnetic radiation.” (Wikipedia)

While we can’t see infrared radiation (also called infrared light) we can feel it on our skin when we stand in the sun. About half of all of the sun’s energy that reaches us comes as invisible infrared radiation while the rest of it is visible to us as light. Like visible light, infrared is reflected by mirrors and absorbed by black objects. When infrared is absorbed, it results in atomic motion and a rise in temperature. A common example would be the heat from the Sun, from an electric radiator, or from the embers of a fire.

3. Convection
Convection is a property of liquids and gases. It is the transfer of heat by the circulation or movement of the heated parts of a liquid or gas. “Convection describes the combined effects of conduction and fluid flow or mass exchange” (Wikipedia).

Heated fluid moves away from the source of heat, carrying energy with it. For instance air convection above a hot surface occurs because the hot air expands, becomes less dense, and rises. Convection also occurs as circulation in a liquid. For instance when you heat water in a saucepan. As the heated water expands it becomes less dense and rises. Cooler, more dense water near the surface descends and patterns of circulation are formed.

You can see all three of these heat transfer processes occurring when you stand around a fire. Heat radiates from the fire through infrared radiation. The flames and smoke are carried upward by convection. The ground under and near the fire will also get hot, heated by conduction.

So … to build something that keeps your coffee hot for longer, all you need to do is to reduce these three heat transfer phenomena.


How do you reduce heat transfer?

One way to to do this would be to make a container from something like plastic or wood, which are good insulators, but it turns out there is an ideal insulator – a vacuum. A vacuum is an absence of atoms and while it is near impossible to create a perfect vacuum, you can get close. Without atoms bouncing off one another, conduction and convection are eliminated completely.

The first person to systematically use this principle was Scottish chemist and physicist James Dewar (1842 – 1923). He gets credited for inventing “Dewar flasks” but the principles of vacuum as an insulator were well known to scientists of the time, and other versions of vacuum flasks had been proposed and constructed. In about 1892, as a result of his experimentation with liquid oxygen, the idea occurred to him to use vacuum vessels for the storage of liquid gases. He had a glassblower create a double walled glass container consisting of two jars placed one within the other but joined at the top. In the space between these jars he created a vacuum. Dewar tried various designs, and before long he first used foil and then silvered the glass (like a mirror) to reduce infrared radiation. The combination of vacuum and the silvering greatly reduced heat transfer by convection, conduction and radiation.

An Early Dewar Flask
A Dewar Flask

Dewar never patented his invention but by 1904 two German glassblowers (the guys who actually made these bottles for scientists) realized the commercial potential of the technology and formed the Thermos Gmbh company (see “Evolution of the Thermos Brand”)

The thermos flask must have seemed like magic to German factory workers for it’s ability to keep their coffee hot and water cold but the thermos doesn’t “know” whether the fluid inside it is hot or cold. All that it is really good at is limiting heat transfer through the walls of the flask. That lets the fluid inside the thermos keep its temperature nearly constant for a long period of time.


A final question. If a vacuum is a perfect insulator, why do hot things in a thermos flask still cool down (or cold things get warm)?

A vacuum is a perfect way to prevent heat transfer, but there are two flaws in the design of all flasks. One is the cap, and the other is the glass or metal itself which provides a conduction path at the top of the flask where the inner and outer walls meet. At both these points heat transfer takes place and as you can imagine, manufacturers spend a great deal of time designing better lids to minimize this. Low quality flasks often give themselves away as they become warm to the touch when filled with hot liquids, and condensation forms on them if filled with cold liquids (see 6 Things To Look For When Buying A Flask).

Simple Diagram of a Vaccum Flask
Simple Diagram of a Vacuum Flask