Vacuum flasks are a must in our lives. The principle of thermos insulation is to minimize the loss of heat, so as to achieve the effect of heat preservation. However, how does thermos reduce heat loss?
Today we will study:
1. How does a vacuum flask keep liquids hot or cold?
2. What does the length of the insulation bottle keep warm depending on?
The structure of the thermos bottle and the principle of heat preservation
The thermos bottle was developed from a thermos pot. The earliest thermos bottle was the inner tank of a small thermos. But for the convenience of drinking, the liner mouth became open. With the development of technology, such fragile glass liner thermos bottles are rarely seen, and more thermos bottles are made of stainless steel.
The structure of the vacuum flask is generally as shown below.
(The picture comes from the network)
So, why should the thermos bottle be like this?
It starts with the way the heat is transferred.
There are three methods of heat transfer: conduction, radiation, and convection:
Conduction refers to the direct contact between objects, and the heat energy is directly transmitted by the atomic vibration from the high temperature to the low temperature. For example: if one end of an iron rod is burned on a fire, the other end will quickly become hot.
The body of the vacuum flask will be vacuumed, and no heat will be transferred in the vacuum state, which is equivalent to cutting off the heat transfer medium, so it will isolate the heat transfer. The higher the vacuum degree, the better the thermal insulation effect. The vacuuming technology is divided into two types: tail vacuuming and tailless vacuuming. Now, most vacuum flask manufacturers use tailless vacuuming because this technology is more advanced.
Radiation means that objects can achieve temperature equilibrium by emitting and absorbing each other’s infrared rays without any medium. For example, in winter, even if there is a certain distance, you can feel the heat from the electric heater.
The inner tank of the insulated water bottle is plated with copper or silver, which can effectively form a layer of heat insulation net in the inner tank of the thermos bottle. In this way, copper plating can effectively reduce the heat lost through radiation by reflecting thermal radiation.
Convection refers to the use of fluid as a medium between objects, using the thermal expansion and contraction of the fluid and the characteristics of flow to transfer heat energy. If you blow cold air on a bowl of hot soup, you can make it cool down faster. That is because by blowing cold air, the low-temperature soup water above the bowl sinks, and the high-temperature soup water below the bowl rises and circulates back and forth to make the hot soup temperature reduce.
The bottle body of the vacuum flask adopts a double-wall structure, and the vacuum environment of the bottle inner and the bottle body can block heat transfer. And whether the sealing performance of the thermos bottle is good, it also plays a very important role in the insulation effect. The better the seal, the harder it is for heat to transfer, resulting in better insulation.
In short, conduction, radiation, and convection are the real reasons for the change between cold and heat.
After mastering these principles, we can prolong the cooling rate of hot water, as long as we use corresponding technical means and manufacture corresponding heat preservation appliances.
The length of the heat preservation time of the vacuum flask
- Influence factors
The length of the heat preservation time of the thermos bottle depends on the structure of the bottle body and the thickness of the bottle material.
The thinner the vacuum flask material, the longer the heat preservation time. But the bottle body is easily damaged and deformed, which affects the service life. The inner layer of the thermos is covered with metal film and copper plating, like a reflector, which can reflect the radiation of hot water. There is a vacuum interlayer in the middle of the thermos bottle. And there is no transfer medium, the air is not circulated, which can effectively inhibit the occurrence of heat conduction. The lid of the insulated water bottle is usually made of cork or plastic, and the material is not easy to conduct heat, which blocks the convection path of hot water.
- Not permanent
In addition, the thermal insulation effect of the vacuum flask is not permanent, and it will gradually deteriorate with time. Because general vacuuming cannot achieve an absolute vacuum. A getter will be added to the body of each vacuum flask for adsorbing residual air. The getter will have a “shelf life”. Over time, the getter will gradually fail, and the thermal insulation effect of the thermos will become worse.
It can be seen that an ordinary insulated water bottle has so much physical knowledge.
Have you sighed that the useful knowledge has increased again?