Thermal insulation materials. Choice of thermal insulation materials. Big encyclopedia of oil and gas

Each object can serve as a "bridge" through which heat will pass from a more heated body to a less heated one.

Such a bridge is, for example, a teaspoon dipped into a glass of hot tea. Metal objects conduct heat very well. The end of the spoon in the glass becomes warm in just a second.

If you need to mix any hot mixture, then the mixer handle must be made of wood or plastic. These solids conduct heat 1000 times worse than metals. We say "conduct heat," but you might as well say "conduct cold." Of course, the properties of the body do not change depending on which direction the heat flow goes through it. On frosty days, we are careful not to touch metal with our bare hands, but without fear we take up a wooden handle.

Poor heat conductors - they are also called heat insulators - include wood, brick, glass, and plastics. The walls of houses, stoves and refrigerators are made from these materials.

All metals are good conductors. The best conductors are copper and silver - they conduct heat twice as well as iron.

Of course, not only a solid body can serve as a "bridge" for the transfer of heat. Liquids also conduct heat, but. much worse than metals. In terms of thermal conductivity, metals are hundreds of times superior to solid and liquid non-metallic bodies.

To show the poor thermal conductivity of water, they make such an experiment. In a test tube with water, a piece of ice is fixed at the bottom, and the top of the test tube is heated on a gas burner - the water begins to boil, and the ice still does not think to melt. If the test tube were without water and made of metal, then a piece of ice would begin to melt almost immediately. Water conducts heat about two hundred times worse than copper.

Gases conduct heat ten times worse than condensed non-metallic bodies. The thermal conductivity of air is 20,000 times less than that of copper.

The poor thermal conductivity of gases allows you to take a piece of dry ice in your hand, the temperature of which is -78 ° C, and even hold a drop of liquid nitrogen in your palm, which has a temperature of -196 ° C. If you do not squeeze these cold bodies with your fingers, then there will be no "burn". The point is that with a very vigorous boil, a drop of liquid or a piece solid body covered with a "steam jacket" and the resulting layer of gas serves as a heat insulator.

The spheroidal state of the liquid - the so-called state in which the drops are enveloped in steam - is formed if water falls on a very hot pan. A drop of boiling water that falls on the palm burns the hand severely, although the temperature difference between the boiling water and human body less than the temperature difference between the hand and liquid air. The hand is colder than a drop of boiling water, heat escapes from the drop, boiling stops and no steam jacket is formed.

It is easy to figure out that the best heat insulator is vacuum - emptiness. There are no heat carriers in the void, and the thermal conductivity will be the lowest.

So, if we want to create thermal protection; hide warm from cold or cold from warm, it is best to build a shell with double walled and pump out air from the space between the walls. In this case, we are faced with the following curious circumstance. If, as the gas is rarefied, we follow the change in its thermal conductivity, then we will find that up to the moment when the pressure reaches several millimeters of mercury, the thermal conductivity practically does not change, and only when moving to a higher vacuum our expectations are justified - the thermal conductivity drops sharply.

What's the matter?

In order to understand this phenomenon, one must try to visualize what the phenomenon of heat transfer in a gas consists of.

The transfer of heat from a hot place to a cold place occurs by transferring energy from one molecule to the next. It is clear that collisions of fast molecules with slow ones usually lead to acceleration of slow molecules and deceleration of fast ones. And this means that a hot place will become colder, and a cold one will heat up.

How does pressure reduction affect heat transfer? Since a decrease in pressure reduces the density, the number of meetings of fast molecules with slow ones, at which energy is transferred, will also decrease. This would reduce thermal conductivity. However; on the other hand, a decrease in pressure leads to an increase in the mean free path of molecules, transfer heat over long distances, and this contributes to an increase in thermal conductivity.The calculation shows that both effects are balanced, and the ability to transfer heat does not change for some time when air is pumped out.

This will be the case until the vacuum becomes so significant that the length of the run is equal to the distance between the walls of the vessel. Now, a further decrease in pressure can no longer change the path length of the molecules "dangling" between the walls, the density drop is not "balanced" and the thermal conductivity rapidly drops in proportion to the pressure, reaching negligible values ​​when a high vacuum is reached. The device of thermoses is based on the use of vacuum. Thermoses are very common, they are used not only for storing hot and cold food, but also in science and technology. In this case, they are called, after the name of the inventor, Dewar vessels. In such vessels (sometimes they are simply called dewars), liquid air, nitrogen, and oxygen are transported. Later we will explain how these gases are obtained in the liquid state * .

* (Anyone who has seen thermos bottles has noticed that they always have silver-plated sides. And why? The fact is that thermal conductivity, which we talked about, is not the only way to transfer heat. There is another method of transmission, which we will discuss in another book - the so-called radiation. Under normal conditions, it is much weaker than thermal conductivity, but still quite noticeable. To attenuate the radiation, the walls of the thermos are silver-plated.)

 

The burner with a flame must be kept in motion at all times for uniform heating. The degree of heating of the product is best judged by the beginning of the melting of the solder; it is necessary to draw conclusions about the degree of heating from the color of the heated parts with great care, since the visual perception of these colors largely depends on the lighting conditions of the workplace. When heating dissimilar metals or alloys, the flame must be directed to one of them, which is the best conductor of heat.

A characteristic feature of metals is a special metallic luster, due to their ability to reflect light well. There is a certain parallelism between the reflectivity of a metal, its electrical conductivity and thermal conductivity: the stronger the metal reflects light, the better the conductor of heat and electricity it is. So, copper, silver and gold are the most reflective, and they are also the best conductors of heat and electricity.

From the outside, metals are characterized primarily by a special, as they say, metallic luster. The reason for this shine is that the surface of the metal strongly reflects the rays of light. Another characteristic property of metals is their ability to test heat and electricity well, and, as a rule, the stronger the metal reflects the rays of light, the better it is a conductor of heat and electricity. Silver, copper and gold most strongly reflect rays of light; at the same time, they have the highest thermal and electrical conductivity.

Thermal conductivity is the property of metals to conduct heat when heated. The better the metal conducts heat, the faster and more uniformly it heats up. The thermal conductivity of metals is of great practical importance. If the metal has a low thermal conductivity, then for complete heating, oi needs prolonged heating; on rapid cooling, however, cracks form in it. The best conductors of heat are pure metals - silver, copper, aluminum. Steel has a much lower thermal conductivity.

Metal atoms form a crystal lattice, in the nodes of which, in addition to neutral atoms, there are also positively charged ions formed as a result of the loss of valence electrons by some of the atoms. The electrons torn off from the atoms move throughout the volume of the metal and do not belong to any particular atom. Due to the presence of easily moving electrons, metals are good conductors of electricity and heat. The best conductors of heat and electricity are silver, copper and aluminum.

Thermal conductivity is the property of metals to conduct heat when heated. The better the metal conducts heat, the faster and more evenly it heats up. The thermal conductivity of metals is of great practical importance. If the metal has a low thermal conductivity, then for complete heating it needs prolonged heating; with rapid cooling, cracks form in it, which leads to an irreparable marriage of products. The best conductors of heat are pure metals - silver, copper, aluminum. Steel due to complexity chemical composition has a much lower thermal conductivity.

All metals have a metallic luster, which is due to their ability to strongly reflect light rays. Most of them retain their brilliance only when they are in a continuous mass. When finely divided, most metals are black or gray, except for magnesium and aluminium. Metals are good conductors of heat and electricity, and the best conductors of heat are also the best conductors. electric current. Silver and copper conduct heat and electricity the best, and lead and mercury the least.

When the temperature drops beyond the transition point, liquid helium suddenly begins to conduct heat in a completely supernatural way for a liquid, Landau said in one of his popular lectures. You have probably heard that liquids in general conduct heat very poorly, in particular ordinary water does not conduct heat well. Other liquids do not have the best thermal conductivity, with the exception of mercury, which, like all metals, is a good conductor heat. Poorly conducts heat and helium I, ordinary liquid helium. And when the temperature drops to the transition point of liquid helium from helium I to helium II, it begins to conduct heat better than the best heat conductors - copper and silver, and the change occurs suddenly. The property of enormous heat transfer, of course, immediately attracted attention and showed that much more surprising is hidden in this incomprehensible liquid.

When the temperature drops beyond the transition point, liquid helium suddenly begins to conduct heat in a completely supernatural way for a liquid, Landau said in one of his popular lectures. You have probably heard that liquids in general are very poor conductors of heat, in particular, ordinary water is also a poor conductor of heat. Other liquids do not have the best thermal conductivity, with the exception of mercury, which, like all metals, is a good conductor of heat. Poorly conducts heat and helium I, ordinary liquid helium. And when the temperature drops to the point of transition of liquid helium from helium I to helium II, it begins to conduct heat better than the best heat conductors - copper and silver, and the change occurs suddenly. The property of enormous heat transfer, of course, immediately attracted attention and showed that many more amazing things were hidden in this incomprehensible liquid.

The location of metals in different places of the periodic system of chemical elements shows that many of their properties should vary greatly. Along with this, however, there are some properties that are inherent in all metals. Metals, with the exception of mercury, are solid substances. All metals have a characteristic metallic luster, which is due to their ability to strongly reflect light rays. Most of them retain their brilliance only when they are in a continuous mass. In finely divided form, most of the metals are black or gray. Metals conduct heat and electricity well, and the best conductors of heat are also the best conductors of electric current. Silver and copper conduct heat and electricity the best, and lead and mercury the least.

Engineers encounter the problem of heat supply and removal at every turn. A nuclear power plant is operating, which means that in nuclear reactor a huge amount of thermal energy is released, which must be brought out as quickly as possible to be converted into electricity. An electric motor is spinning, an internal combustion engine is puffing, a radio lamp is burning, a rocket crashes into the atmosphere - here we are already dealing with harmful heating, when heat must be quickly disposed of. It is not surprising that heat engineers have been racking their brains for many decades, trying to speed up the movement of slow heat flows. To pass only 10 kilowatts of thermal energy through a copper rod with a diameter of 2 - 3 centimeters and a length of less than half a meter, a huge thermal head is needed. One end of the rod would have to be heated three times hotter than the surface of the Sun, in fact turned into steam, while the other would have to keep room temperature. But copper is considered one of the the best guides heat. As for the heat pipe, with the same dimensions, it will pass such energy almost without resistance, and the temperature difference between its ends can hardly even be measured. Only a copper block with a diameter of three meters and a weight of 40 tons could have a similar thermal conductivity.