Vacuum thermal insulation
Vacuum thermal insulation describes a highly efficient system for thermal insulation in which the heat transport caused by the gas molecules in the air is reduced. A distinction is made between insulation systems in which the vacuum is only used to prevent convection ( steel jacket pipes , vacuum collectors ) and real vacuum heat insulation systems in which the heat conduction of the gas molecules is prevented (e.g. in Dewar vessels or vacuum insulation panels ).
principle
Vacuum to prevent convection
In free gases, convection , i.e. heat transport via a particle flow , is the main heat transport mechanism. In order for convection to develop in a gas layer, the temperature difference, gas density (the gas pressure) and geometric factors (layer thickness) must match. By applying a rough vacuum, the formation of convection in a gas layer can be prevented, which means that the heat is transported through the pure heat conduction of the gas. The thermal conductivity of an immobile gas is quite low. B. for air at 0.026 W m −1 K −1 .
"Real" vacuum thermal insulation
In thermal insulation materials , heat is transported via thermal conduction of the solid structure, thermal conduction of the gas contained and thermal radiation . Heat transport via convection is prevented in insulation materials. The contributions of the thermal conduction of the solid structure and the thermal radiation to the heat transfer are relatively small, the thermal conduction of the gas contained in the insulating material, usually air, makes the largest share. As an example: The thermal conductivity of conventional insulation materials such as Styrofoam and mineral wool are in the range of 0.040 W · m −1 · K −1 , the thermal conductivity of non-moving air is 0.026 W · m −1 · K −1 . If the air is removed from the insulation material by evacuation , the thermal conductivity of the insulation material drops. With an initial pressure reduction there is no change over a wide area, since the gas thermal conductivity in this area is independent of pressure. This only changes when the residual pressure is very low, when the continuum flow collapses and the free molecular flow finally occurs. The necessary residual pressure to achieve the free molecular flow can be calculated for the specific individual case with the help of the Knudsen number . In order for a free molecular flow to occur, the pressure must be reduced to such an extent that the mean free path of the gas particles ( atoms , molecules ) is greater than the free path in the surrounding solid. By using microporous filling materials, the free path in the surrounding solid is greatly reduced, so that the free molecular flow occurs even at a correspondingly higher residual pressure. The residual pressure is usually reduced to such an extent that there is a reliable free molecular flow and not a Knudsen flow (transition flow or even sliding flow). It should be noted that, even in vacuum thermal insulation, heat transfer continues to take place through thermal conduction of the solid structure and thermal radiation, and the gas thermal conduction is greater than zero even with free molecular movement.
application
Vacuum to prevent convection
In district heating , steel jacket pipes are used, especially at higher temperatures (> 144 ° C) of the medium to be transferred , in which the space between the pipe in contact with the medium and the outer support pipe is evacuated.
Vacuum insulation without a support core
In thermos flasks or Dewar flasks , vacuum thermal insulation is achieved by evacuating the cavity of a double-walled container, i.e. placing it under vacuum. Since the walls of the container are a few millimeters apart, the pressure in the cavity must be in the range of 10 −3 mbar , i.e. one millionth of the atmospheric air pressure . The double-walled container is made of glass or stainless steel so that such a low pressure can be maintained over a longer period of time. Furthermore, vacuum insulation without a support core can only be implemented in rotationally symmetrical containers (e.g. cylinders, ellipses or spheres), since the walls of the container must withstand the air pressure. In such vacuum-insulated vessels without a supporting core, the edge losses through the contact points between the two parts of the double jacket account for the largest part of the heat loss.
Vacuum insulation with support core
In vacuum insulation panels, a porous support core is used, which is provided with a gas-impermeable envelope. The support core serves to absorb the air pressure, so that in principle any shapes are possible with vacuum insulation panels. On the other hand, the pore walls in the support core serve to limit the free path of the gas particles. This means that the vacuum requirements are lower. In the case of vacuum insulation boards with a support core made of microporous silica , the pores of which are only a few 100 nm in size, a pressure in the insulation board of 10 mbar , i.e. one hundredth of the atmospheric air pressure, is sufficient to make the heat conduction through air negligibly small. With vacuum insulation panels, thermal conductivities of less than 0.004 W · m −1 · K −1 can be achieved. Only the heat conduction of the support body and heat radiation remain as mechanisms of heat transport.
literature
- Thomas M. Flynn: Cryogenic engineering. Marcel Dekker Ltd, New York NY et al. 1997, ISBN 0-8247-9724-8 .