In recent times, thermoses have become more popular than ever before and many businesses are looking to design thermoses as a corporate gift for their customers and clients. But, how does a thermos actually work? A Thermos is specially designed to retain the heat of a liquid over a long period time. An everyday type of thermos is capable of keeping hot drinks warm for at least 8 hours while larger and higher quality thermoses can retain heat for 12-18 hours. That leaves us questioning, how is it possible for a thermos to be able to keep drinks hot for a long time aswell as keeping cold drinks cool.
History
First, let's look at the history of how a thermo originated. A thermos is a vacuum flask which insulates a liquid in a container over long periods of time. It was first invented in 1892 by Scottish Scientist Sir James Dewar, he created a vacuum flask which was composed of two flasks which one flask was placed inside the other where they joined at the neck which created a vacuum to keep the temperature of the contents inside the flask stable. In 1904, his creation was later transformed into a commercial item by two german scientists, Albert Aschenbrenner and Reinhold Burger who realised that this idea could be utilised to keep hot drinks hot and cold drinks cool. Therefore, they created the first thermos for everyday use.
Thermos Flask Structure
The structure of a stainless steel thermos flask is composed of an inner flask, an outer flask and a flask cover. The gap between the two flasks is partially absent of air which creates a vacuum that results in reducing heat transfer by conduction and convection. There is a layer of silvered glass that is placed on the inner flask wall to provide insulation to reduce heat transfer. This vacuum structure is effective in maintaining the contents of the flask for a long period of time.
How a Thermos Retains Heat
To understand how a thermos retains heat its first important to understand the different forms of heat transfer. According to the second law of thermodynamics there are three types of heat transfer: Conduction, convection and radiation. A thermos flask that can reduce these three modes of heat transfer can keep temperatures stable for a long period of time. Let's first look at the three modes of heat transfer.
Heat Conduction
Heat (thermal) conduction is the process by which heat is transferred from a hotter object to a colder object. In relation to thermoses, the vacuum flask design reduces the effectiveness of heat conduction. This is because the vacuum is inside another flask which is separated by a vacuum. The lack of contact between the flasks surfaces in the thermos greatly prevent heat loss.
Heat Radiation
Heat radiation is the transfer of heat from an object of high temperature to an object with lower temperature when the two objects are not in direct physical contact with each other. The best way this is prevented in a thermos is during the manufacturing stage when silver is plated between the two layers of flasks in the thermos. Silver can reflect heat radiation therefore, the heat loss can be effectively reduced. This structure makes it difficult for internal heat to dissipate.
Heat Convection
Heat convection is the transfer of heat from one place to another due to the movement of fluid. However, vacuum convection does not take place inside a thermos flask. This is because the vacuum between the walls does not allow convection to take place as there is no air movement between the inner and outer walls due to the absence of air. Thus, heat loss from heat convection is prevented in thermoses.
Where Heat Is Lost
The flask lid is composed of plastic and rubber. Most of the thermos is surrounded by a vacuum layer up to the neck of the water bottle. The only section of the thermos that can emit heat is the mouth of the flask. Therefore, during production the thermos bottleneck is shortened as much as possible to reduce heat loss. Therefore, the best thermos to consider purchasing as a corporate gift or for personal use, is a thermos that has a large capacity but a small bottle opening which will greatly reduce heat transfer.
How Cold Temperatures Are Retained
Not only are thermoses designed to retain the temperatures of hot liquids but they can also keep cold drinks cool for long periods of time. Due to the structure of the thermos flasks, the gap between the two flasks being absent of air, this vacuum effect virtually eliminates condensation on the outside of the flask when holding cold liquids. Thus, resulting in cold beverages staying cold a long time.
FAQ
The mouth of the flask does not have a vacuum therefore heat lost occurs above the bottle neck.
A thermos reduces heat loss by reducing the effects of conduction, convection and radiation in the thermos flask.
Graduated with a Masters degree from Ireland with a Masters degree in Business Information system. English native speaker specialising in business and data analyst. Currently enrolled in Tunghai University's language centre for studying Mandarin Chinese and undertaking a student marketing internship at SourceEC. My role involves content writing, SEO analysis and web page optimisation After my studies and completion of this internship, I am hoping to pursue a career in data analytics in the digital marketing field in Asia.
