How Vacuum Flask, Travel mug and Thermos Work?

Most of us have or know the vacuum flask. I can remember to be a child having one that came with my lunch box. One day my mother may put apple juice in it and at lunch I would have nice, cold apple juice. The another day she would put hot creamy corn soup in it and I would have hot tomato soup for lunch. And I could remember asking, "How does it know whether to keep stuff heat or cool?" Where's the transition switch, in other words... Or, similarly, "You heat stuff up in an oven and cool them down in a fridge -- how come this thing can do both?" In this edition of How It Works, you can learn how a vacuum flask knows" what to do. 

Heat Transfer

We take a glass of cold apple juice or a bowl of hot creamy corn soup and let them sit out on the kitchen table. Guess what will happen: The bowl of soup will cool down to kitchen temperature, and the glass of apple juice will warm up to kitchen temperature. It is a thermodynamic truth of life -- if we put any two things with different temperatures together, then heat transfer will let them to get to the same temperature. So a "kitchen" and a "hot bowl of creamy corn soup" get to the same temperature by the heat transfer process -- the kitchen has a bit warmer, the bowl of soup has a lot cooler.If we want to keep a bowl of creamy corn soup hot as long as possible -- that's if we want to reduce the natural heat transfer process as much as we can -- we have to slow down the three processes that cause heat transfer. The processes are:

• Conduction - Let's begin a simple question: What's heat? Heat is atomic motion.

The very best case for this phenomenon would be to take a metal stick and heat one end of it. The other end will get warmer and then hot through conduction. When you put a metal bar on the fireplace the inside of the bar gets hot through conduction of the heat through the metal in the bottom of the bar. Some thing (namely metals) are better heat conductors than others (for example, wood).

• Radiation - Our eyes can not see infrared, but our skin can feel it.

• 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.

We can see all three heat transfer processes occurring when we stand beside a bonfire:We possibly should stand at least 15 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 place 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 make a good vacuum flask or travel mug, what you want to do is reduce these three heat transfer as much as you can.

Workings of a Vacuum Flask Inside

One method to make a vacuum flask or travel mug - like container would be to take a beverage bottle and wrap it in, such as, foam. Insulation works by two basic principles. First, the plastic in the foam is not a good heat conductor. Second, the air trapped in the foam is an even worse heat conductor. Accordingly conduction has been slow down. Due to the air is broken into tiny bubbles, the other thing foam does is largely reduce convection inside the foam. Heat transfer through foam is as a result pretty small.Apparently, there is a much better insulator than foam: a vacuum. A vacuum has very few atoms. A "perfect vacuum" has zero atoms. It is almost impossible to make a perfect vacuum, but we can get close to it. Without atoms we reduce conduction and convection completely.What you find in a vacuum flask or travel mug is a glass envelope holding a vacuum. Inside is glass, and out of the glass is a vacuum. The glass envelope is easily fragile, so it's encased in a plastic or metal case. In many vacuum flasks or travel mugs we could actually unscrew and take this glass envelope out.So why do hot soup in a ever cold down? You can find in the figure two paths for heat transfer. The big one is the cap. The other is the glass, which provides a conduction path on top of the vacuum flask where the inner and outer walls meet. Even though heat transfer through these paths is small, it is not zero.Does the vacuum flask recognize whether the fluid inside it is hot or cold? No. All the thermos is doing is limiting heat transfer with the walls of the thermos. That lets the fluid inside keep its temperature nearly steady for a long time (whatever the temperature is hot or cold).