How Does a Thermos Flask Work?

Thermos Flask

​Thermos flasks were invented to help prolong the time of liquids and food to maintain a certain temperature.

This was the greatest invention to people who are often away from a heat source or a cooling source. Often, we find ourselves caught up with things that take up most of the time.

Having a thermos flask to have a hot meal or beverage or an icy cold drink comes in handy in these situations.Vacuum flasks are used by many people on a daily basis to help retain the food or drink just made at the same temperature for hours. We have created a guide of all the best models on the market here

This brings us to the next question, how does a thermos flask work. 

​Let’s Find Out

​Ever wondered about the magic that happens when you put your food or drink in a thermos flask for hours and then when you remove it, the food is still hot and ready to be consumed? I know this might not interest many people as long as they have the flask with them, life goes on. But for the few that really take keen interest in knowing the inner workings of a thermos flask, then you are in the right place to acquire the knowledge.

To understand the inner mechanisms of a thermos flask, first you need to know the number of ways heat flows.

Conduction Convection Radiation

​Heat energy is the prime reason for the thermos flask is able to keep the inner temperature of the contents at the same time for a long period of time. Learning how heat travels is essential in knowing what exactly happens in the thermos flask when put to work. Flowing of heat happens in three ways namely, conduction, convection and radiation.


​This is the process of heat transfer from an object to another object through direct contact. Whenever the objects interact, there is transfer of heat, usually from the hotter one to the less hot.


​Convection is kind of different from conduction because heat transfer doesn’t necessarily depend on the objects. It is the process that eliminates cold air in the way of hot air as it rises. As the air gets heated, it becomes less dense so it rises up and stays on top of the colder air that has a higher density. This explains why when heating hot water in a pot, the water at the top is usually hotter than that at the bottom. At the end of the heating, the water will have achieved even heat distribution through convection.


​When there is an increase in the speed at which atoms move, they get unstable as they acquire more energy. This leads to the hot item letting out light. The light produced is just a way of the atoms stabilizing to return to their normal state. The whole process is known as radiation.

​How a Thermos Flask Work

A thermos flask consists of the following:

  • Screw-on stopper
  • External stainless steel or plastic casing
  • Outer layer with a silver coated finish (to reduce infrared radiation)
  • Vacuum
  • Inner layer of glass
  • Supports to keep the inner vacuum in place
  • Extra insulation to reduce heat loss and guards the flask from external impact

To retain the drinks at the same temperature, a thermos flask minimizes heat transfer through conduction and radiation. Convection keeps the temperature inside even. The vacuum insulation is very useful at eliminating heat transfer by conduction and convection. A vacuum contains no atoms whatsoever, and without atoms, not heat transfer will happen. Then there is a glass envelope that holds the vacuum. Around the glass is a vacuum space. 

The glass, obviously, is a very delicate object so there is a plastic or stainless steel casing to protect it. This encased glass is made of a reflective material (like a mirror) which reduces heat transfer by radiation. There is a stopper that encloses the contents of the flask that is made tight to prevent air from coming in or going out of the thermos flask.

In simpler terms, a thermos flask doesn’t any of the three ways that heat can travel in. The glass is made like a mirror to prevent radiation and the vacuum created by the double walls prevents convection. The glass is made so thin to prevent heat coming in or getting out through the flask by conduction. Outside is an extra casing that provides additional insulation. And the stopper seals in the heat and prevents heat getting in by convection.


​This knowledge of the inner workings of a flask help in maintaining the flasks. You get to know things you need to be doing to continue receiving the great service you always get when using one. Cleaning the flask will be done with extra care, tightly closing the stopper to prevent heat from getting in and other ways to handle your flask.

About the Author Harry Thompson

Gaining a BSc Honours in building/ property maintenance at the University of Portsmouth Harry went into the field of renovating properties. Along the journey, he has developed a wide variety of home & garden DIY skills such as installing new kitchens and landscaping gardens.