How Does Thermos Flask Work?

How Do Thermos Flasks Work

Have you ever wondered how a thermos flask or vacuum flask keeps your tea/coffee steaming hot during those chilly winter mornings? or how it keeps your iced tea refreshingly cold on a scorching summer day?

The answer lies in the clever design and scientific principles behind this incredible invention. In this blog, let us explore how does thermos flasks work; and learn about the science that allows it to maintain the temperature of liquids for a long period of time.

What is a Thermos Flask?

A thermos flask, also known as a vacuum flask or simply a thermos, is a container designed to keep beverages or food items hot or cold for extended periods of time.

Design of thermos flask

A thermos flask is designed to minimize heat transfer in order to maintain the temperature of its contents. It consists of an inner and outer wall, typically made of glass or stainless steel, with a vacuum-sealed space between them.

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Key components of a thermos flask

how does thermos flasks work

 A thermos flask consists of three main components:

1. Inner flask

The inner flask is typically made of glass or stainless steel and helps keep liquids retain their temperature for a long period of time. The inside of the inner flask is usually coated with a reflective material to reduce heat loss through radiation. It reflects thermal radiation into the flask, minimizing the exchange of heat with the surroundings.

2. Outer flask

The outer flask is made of strong material, like plastic or stainless steel and provides structural support and insulation. It prevents heat transfer through conduction by creating a barrier between the inner flask and the external environment.

3. Vacuum insulation

The space between the inner and outer flasks is a vacuum. The vacuum eliminates heat transfer through conduction and convection because there are no particles or fluids to transfer heat. It acts as an insulator, ensuring minimal heat loss or gain, thus ensuring that the temperature of the liquid inside remains relatively constant.

Additional components

Several additional factors contribute to the efficiency of a thermos flask in retaining temperature. One such factor is the thickness of the walls. Thicker walls offer better insulation, reducing the amount of heat transfer between the inside and outside of the flask.

The quality of the vacuum between the walls is also crucial. A high-quality vacuum, achieved through advanced manufacturing techniques, helps to maintain the integrity of the vacuum insulation and further minimizes heat transfer.

The materials used in the construction of the flask also impact its heat retention capabilities. Glass and stainless steel are popular thermal flask construction materials due to their low thermal conductivity. These materials do not readily conduct heat, thereby reducing heat transfer through the walls of the flask.

The lid of a thermos flask also plays a very important role in maintaining the temperature. It is typically made of plastic or stainless steel and is equipped with a gasket or sealing ring.

The insulation properties of the materials used in the lid and the sealing mechanism also contribute to heat retention. When the lid is tightly secured, it creates an airtight seal, further reducing the chances of heat exchange between the inside and outside of the flask. This seal helps to preserve the temperature of the liquid by minimising air infiltration.

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Physics Behind Thermos Flask

 The physics behind a thermos flask revolves around the principles of heat transfer and insulation. The primary purpose of a thermos flask is to minimize heat exchange between the contents of the flask and the surrounding environment, whether the goal is to keep the contents hot or cold.

Before we delve further into the science behind a thermos flask, let us first understand the basics of heat transfer.

Heat transfer happens in three ways: conduction, convection, and radiation.

  • Conduction involves the transfer of heat through direct contact between objects. For example, when you touch a hot pan, heat is transferred from the pan to your hand through conduction.
  • Convection is the transfer of heat through the movement of fluids or gases. When you boil water, the hot water moves up and the cooler water moves down, creating a continuous flow that transfers heat.
  • Radiation, on the other hand, is the transfer of heat in the form of electromagnetic waves, such as sunlight or the heat radiated by a fire.

How does Thermos Flask Work?

The design of a thermos flask effectively reduces heat transfer in all major forms – conduction, convection and radiation. This insulation enables the flask to maintain the temperature of its contents for an extended period.

Thermos flasks are not limited to keeping beverages hot. They are also great for preserving the coldness of drinks.

Since heat cannot escape from a vacuum flask, it logically follows that heat cannot penetrate the flask from the external environment either. The tightly sealed lid prevents heat from entering through convection, while the vacuum insulation effectively blocks heat transfer through conduction.

Additionally, the metal lining situated between the outer case and the inner chamber acts as a barrier, preventing heat from radiating into the flask.

While a thermos flask provides excellent insulation, it is not completely perfect. Over time, some heat exchange can still occur, albeit at a much slower rate compared to regular containers. Therefore, the temperature of the contents may gradually change over several hours, but the flask will still significantly outperform other containers in terms of heat retention.

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What are the different uses of Thermos Flask?

As it is commonly known, the primary use of a thermos flask is to keep beverages at the desired temperature.

It can keep hot beverages like coffee or tea hot for several hours, allowing you to enjoy a warm drink throughout the day. Similarly, it can keep cold beverages like chilled water, iced tea or cold juices cold for extended periods, providing a refreshing drink on the go.

This property of a thermos flask makes it convenient for you to carry and enjoy your home-made meals in liquid form, like stews, curries, and soups or beverages, retaining its temperature for a long time, when you are traveling or going on trips.

Thermos flasks are also used to store temperature-sensitive medications or medical samples that need to be kept at specific temperatures. The insulation provided by the flask helps maintain the required temperature for longer durations, ensuring the integrity and effectiveness of the medical contents.


 A thermos flask’s ability to maintain the temperature of its contents for extended periods is due to a combination of factors, including the vacuum insulation, reflective coating and airtight seal. By preventing heat transfer through conduction, convection and radiation, thermos flasks excel at keeping beverages hot or cold. It ensures that your drink stays at the desired temperature for an extended period of time.


1. What is the difference between normal flasks and thermos flasks?

The main difference between normal flasks and thermos flasks lies in their insulation capabilities and ability to maintain the temperature of the contents. Normal flasks typically have minimal insulation properties, providing a basic level of temperature retention for a short duration. Meanwhile, thermos flasks provide superior insulation, thus maintaining the temperature of their contents for a longer period of time.

2. Is the shape of a thermos flask important?

The shape of a thermos flask does have a role in its overall functionality, although it may not be as significant as other design elements, such as insulation. It can have an impact on the efficiency of heat transfer. For example, a slender and tall flask with a smaller opening reduces heat transfer better than a wide and short flask with a larger opening, as it has less surface area being exposed to the outside environment compared to the latter.

3. Who invented the thermos flask?

The thermos flask was invented by Sir James Dewar, a British chemist and physicist, in 1892.

4. How much does it cost to buy a thermos flask?

The cost of a thermos flask varies with its brand, design, capacity, material and other additional features. In India, you can find thermos flasks ranging from around ₹ 200 to 3000 or more.

5. Which brand is good for a thermos flask?

There are several reputable brands known for manufacturing high-quality thermos flasks. The choice of the brand ultimately depends on individual preferences, budget and other requirements. The best-known thermos flask brands in India are Milton, Cello and Borosil.

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