How Do Thermoses Keep Things Hot For So Long?

As a mechanical engineer, heat transfer was at the heart of much of my education. However, it’s a little harder to define what “heat transfer” means, and the term is somewhat redundant. Heat refers to the movement of energy from one place or substance to another, similar to how rain refers to the movement of water, but isn’t really water in a manner of speaking. Perhaps “energy transfer” would be a better term for this concept.

Whether you call it heat transfer or energy transfer, the point of the insulation between your walls or on the inside of a Thermos is to keep whatever is inside either hotter or colder than the outside environment. Conveniently, insulation is able to make the “choice” between hot and cold with no interaction from the outside world. That brings up two questions: how does insulation know at what temperature to keep something, and why does it work so well?

Energy Transfer Basics

If you’re not in the mood for four to five years of engineering education, here is a short rundown on the subject. Heat (or heat transfer) acts upon something in three ways:

  1. Advection, a process where a mass is taken from one place to another. For example, your house gets ever-so-slightly warmer when a hot pizza is delivered.
  2. Conduction, the process where two objects touching each other will eventually become the same temperature.
  3. Radiation, which is wave energy that can act through a vacuum. This is how the sun can heat Earth from millions of miles away.

There is also convection, which is sort of a combination of advection and conduction, but if you eliminate the other three, it’s not going to be a factor.

Those are the only ways that energy is transferred. In theory, if you stop these three methods, you could keep something hot or cold forever. In reality, you can keep your house at a comfortable temperature in the winter without excessive heating. You can also keep coffee warm for many hours, which would have seemed like magic to someone before vacuum flasks were invented by James Dewar in the late 1800s.

Stopping the Transfer

Those vacuum flasks, commonly referred to as Thermoses, use a partial vacuum between two cylinders to stop nearly all energy transfer by conduction. There are no (or very few) molecules to conduct heat from the outside wall exposed to the environment to the inside wall holding your liquid of choice. This means that advection is also stopped, because a lack of molecules means a lack of moving molecules, as well as convection, which is just a combination of the two.

Of course, conduction can occur at the seams where a bottle is opened, or through the non-vacuum top in the case of Yeti cups, but they still do an excellent job of maintaining a certain temperature compared to many other types of insulation.

In addition to the vacuum’s insulation ability, the inside of these bottles can be silvered, meaning they’re covered with a reflective surface to eliminate heat transfer through radiation. Like something reflecting the sun’s rays with a mirror, energy-transferring electromagnetic waves can be reflected with the right type of coating.

How Does It Know?

Now the question is, how does a Thermos know whether to keep something hot or cold? I didn’t have a good answer to this immediately, but after thinking about it for a while, the answer is simple: Insulation, whether it’s in a Thermos or in your home, doesn’t keep things hot or cold. It always keeps something warm by preventing the transfer of thermal energy. The question is simply what’s warmer: the inside of the insulated structure, or the outside world?

If you have a hot drink or a warm house, energy tends to escape to bring things to the same temperature as the environment outside. If it’s cold inside your home or Thermos, energy wants to get in. Whether hot or cold, insulation is always doing the same job—impeding the flow of energy. This means that your coffee will be hot (or your home warm) for many hours to come!

 

Jeremy Cook is an engineer and writer in the southeastern US. He holds a BSME from Clemson University and has over 10 years of factory automation experience. He’s an avid maker and experimenter, building anything that comes into his mind. To learn more about insulation, you can go to the Home Depot website.

 

43 comments

Jim Ven
Jim V2 years ago

thanks

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Jerome S
Jerome S2 years ago

thanks for sharing.

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Lisa M
Lisa M2 years ago

Noted.

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Lisa M
Lisa M2 years ago

Noted.

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Elizabeth W
Elizabeth W2 years ago

Interesting; thanks.

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Carl R
Carl R2 years ago

Thanks!!!

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Carl R
Carl R2 years ago

Thanks!!!

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Margie F
Margie FOURIE2 years ago

And I just thought they worked. How many other things do we take for granted?

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Shailja Mukhtyar
Shailja Mukhtyar2 years ago

i use my thermos full of tea, every day, w/o worrying bout the mechanics, just reap the rewards,,,,,

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Clare O
Clare O'Beara2 years ago

Straw bale houses are becoming increasingly popular for keeping out sun and being cosy in winter.

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