Introduction to How Thermoses (Vacuum Flasks) Work

Most people have or are familiar with the "Thermos" (also known as a vacuum flask or a dewar). I can remember as a kid having one that came with my lunch box. One day my mother might put grape juice in it and at lunch I would have nice, cold grape juice. The next day she would put hot soup in it and I would have hot soup for lunch. And I can remember asking, "How does it know whether to keep stuff hot or cold?" Where's the switch, in other words...

Or, similarly, "You heat things up in an oven and cool them down in a refrigerator -- how come this thing can do both?" In this edition of HowStuffWorks, we will learn how a Thermos "knows" what to do.

Heat Transfer

Let's say you take a glass of ice water or a bowl of hot soup and let them sit out on the kitchen table. You know what will happen: The bowl of soup will cool down to room temperature, and the glass of ice water will warm up to room temperature. This is a thermodynamic fact of life -- if you put any two objects with different temperatures together, then heat transfer will cause them to reach the same temperature. So a "room" and a "hot bowl of soup" reach the same temperature by the heat transfer process -- the room gets slightly warmer and the bowl of soup gets a lot colder.

If you want to keep a bowl of soup hot as long as possible -- that is, if you want to slow down the natural heat transfer process as much as you can -- you have to slow down the three processes that cause heat transfer. The processes are:

  • Conduction - Let's start with a simple question: What is heat? Heat is atomic motion. An atom represents its "heat" by its speed. At the temperature absolute zero, there is no atomic motion. But as atoms get warmer they move. Heat is transferred when one atom runs into another. When this happens, it is a little bit like billiard balls colliding -- the second atom picks up some of the motion of the first atom. Heat is transferred by these collisions.

The best example of this phenomenon would be to take a metal bar and heat one end of it. The other end will get warm and then hot through conduction. When you put a metal pan on the stove, the inside of the pan gets hot through conduction of the heat through the metal in the bottom of the pan. Some materials (namely metals) are better heat conductors than others (for example, plastics).

  • Radiation - Another side effect of atomic motion is vibration, and vibration leads to the unexpected phenomenon of infrared radiation. According to the Encyclopedia Britannica, "Infrared radiation is absorbed and emitted by the rotations and vibrations of chemically bonded atoms or groups of atoms and thus by many kinds of materials." Infrared radiation is a form of light.

Our eyes are unable to see infrared, but our skin can feel it. About half of all of the sun's energy that reaches us comes as invisible infrared radiation, with the rest of it visible to us as light. Infrared, like visible light, is reflected by mirrors and absorbed better by black objects. When infrared is absorbed, it results in atomic motion, and therefore, in a rise in temperature. Some common examples of infrared are the heat you feel radiating from an electric heater or a red-hot piece of metal, the heat you feel radiating from the bricks in a fireplace even if the fire has gone out and the heat you feel radiating from a concrete wall after the sun has gone down.

  • Convection - Convection is a property of liquids and gases. It occurs because when a liquid or gas gets hot, it tends to rise above the rest of the body of liquid or gas. So, if you have a hot bowl of soup on the table, it heats a layer of air surrounding the bowl. That layer then rises because it is hotter than the surrounding air. Cold air fills in the space left by the rising hot air. This new cold air then heats up and rises, and the cycle repeats. It is possible to speed up convection -- that is why you blow on hot soup to cool it down. If it weren't for convection your soup would stay hot a lot longer, because it turns out that air is a pretty poor heat conductor.

You can see all three of these heat transfer processes occurring when you stand next to a bonfire:

You probably need to stand at least 20 feet away from a big bonfire like this one. What keeps you away is heat radiating from the fire through infrared radiation. The flames and smoke are carried upward by convection: Air around the fire heats up and rises. The ground 3 feet beneath the fire will get hot, heated by conduction. The top layer of soil is directly heated (by radiation), and then the heat is conducted through layers of dirt deep into the ground.

To build a good thermos, what you want to do is reduce these three heat transfer phenomena as much as possible.

Inner Workings of a Thermos

One way to build a thermos-like container would be to take a jar and wrap it in, for example, foam insulation. Insulation works by two principles. First, the plastic in the foam is not a very good heat conductor. Second, the air trapped in the foam is an even worse heat conductor. So conduction has been reduced. Because the air is broken into tiny bubbles, the other thing foam insulation does is largely eliminate convection inside the foam. Heat transfer through foam is therefore pretty small.

It turns out that there is an even better insulator than foam: a vacuum. A vacuum is a lack of atoms. A "perfect vacuum" contains zero atoms. It is nearly impossible to create a perfect vacuum, but you can get close. Without atoms you eliminate conduction and convection completely.

What you find in a thermos is a glass envelope holding a vacuum. Inside a thermos is glass, and around the glass is a vacuum. The glass envelope is fragile, so it is encased in a plastic or metal case. In many thermoses you can actually unscrew and remove this glass envelope.

A thermos then goes one step further. The glass is silvered (like a mirror) to reduce infrared radiation. The combination of a vacuum and the silvering greatly reduces heat transfer by convection, conduction and radiation.

So why do hot things in a thermos ever cool down? You can see in the figure two paths for heat transfer. The big one is the cap. The other one is the glass, which provides a conduction path at the top of the flask where the inner and outer walls meet. Although heat transfer through these paths is small, it is not zero.

Does the thermos know whether the fluid inside it is hot or cold? No. All the thermos is doing is limiting heat transfer through the walls of the thermos. That lets the fluid inside the thermos keep its temperature nearly constant for a long period of time (whether the temperature is hot or cold).

Experiments to Try

If you are the experimental sort, you might want to try some experiments to see how different forms of insulation compare to a thermos. Or you might want to try to improve the performance of a thermos. "Can you keep hot coffee hot all day?" is the ultimate question: If you can answer this question affirmatively it is likely you could base an entire business empire around it...

One avenue of investigation involves understanding your thermos better:

  1. Start with a thermos.
  2. Fill it with boiling water and cap it.
  3. Measure its temperature with a thermometer every hour or two and see what the temperature graph looks like.

If you put the thermos inside a small foam cooler, does that change anything? What if you store the thermos upside down during the day -- what effect, if any, does that have?

Another thing you can try is a set of experiments to find the insulation values of different materials. Find several jars that hold the same amount of liquid as your thermos and try insulating them in different materials. Try things you have around the house like foam, wool, aluminum foil, plastic, newspaper, etc. Also try combinations of these materials, and different thicknesses. You will learn a lot about the heat conductivity of different materials!

The Biggest Thermos

One question often asked at this point is, "If a vacuum is such a good insulator, then how do you cool a spacecraft?" Heat builds up in a spacecraft from its electronics, its fuel cells, its rocket engines and incoming solar radiation, among other things. All of this heat needs to go somewhere or the spacecraft will overheat. However, the spacecraft is floating in the world's biggest thermos -- the vacuum of outer space. So how does a spacecraft dump its excess heat?

It turns out that heat dissipation is a fairly significant part of the spacecraft design process. For example, if you look at this page you will see that Skylab had a gold coating to reject infrared radiation coming from the sun, and a large radiator to dissipate heat that built up. A space radiator can use nothing but infrared heat radiation to dissipate heat, so it must be much larger than a similar radiator on Earth, where convection plays a big part in the cooling process (almost all radiators on Earth use fans to improve the effects of convection). Similarly, the inside of the space shuttle's cargo bay doors are lined with radiators. Once the shuttle is in orbit, one of the first things the crew does is open these doors so that heat can radiate away, as this page explains.

So if space is a giant vacuum and a vacuum is an insulator, why do astronauts get cold fingers on space walks? The cold-finger problem is actually quite interesting. This article discusses some of the reasons.

For more information on thermoses and related topics, check out the links on the next page.