Vacuum insulated panel

A vacuum thermal insulation refers to a highly efficient system for the thermal insulation, wherein the gas caused by the molecules in the air heat transfer is reduced. A distinction insulation systems, in which the vacuum is used only for the Suppression of convection (steel casing pipes, vacuum collectors ) a real vacuum thermal insulation systems, where by the vacuum thermal conduction of the gas molecules is inhibited (eg, in Dewar flasks or vacuum insulation panels ).

• 2.1 vacuum to eliminate convection
• 2.2 Vacuum insulation without a core support
• 2.3 Vacuum insulation with core support

Principle

Vacuum for inhibiting convection

In free gases is convection, ie heat transport through a particle of the main heat transport mechanism. Thus, convection can develop in a layer of gas, temperature difference, the gas density ( the gas pressure ) and geometric factors (layer thickness) must match. By applying a low vacuum forming by convection can be prevented in a gas layer, characterized the heat transfer through the thermal conduction of the pure gas is limited. The thermal conductivity of a stationary gas is quite small, it is, for example, for air at 0.026 W · m-1 · K -1.

"Real" vacuum thermal insulation

In thermal insulation heat transfer via heat conduction of the solid skeleton, heat conduction of the gas and thermal radiation contained. Heat transfer by convection is suppressed in insulating materials. The contributions of the heat conduction of the solid skeleton and the thermal radiation heat transfer are relatively small, the heat conduction of the gas contained in the insulation material, usually air, providing the biggest share. As an example, the thermal conductivities of conventional insulating materials such as polystyrene 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. Removing the air from the insulating material by evacuation, the heat conductivity of the insulating material decreases. It is found at initial pressure reduction over a wide range, no change, since the gas thermal conductivity is independent of pressure in this area. This changes only at very low residual pressure when the continuum flow breaks down and eventually stops the free molecular flow. The necessary residual pressure to achieve the free molecular flow can be calculated for the specific case using the Knudsen number. Thus, setting free molecular flow, the pressure must be reduced so that the mean free path of the gas particles (atoms, molecules ) is greater than the mean free path in the surrounding solid. Through the use of microporous fillers mean free path is greatly reduced in the surrounding solid, which adjusts the free molecular flow even at a correspondingly higher residual pressure. The residual pressure is usually lowered so far that adjusts reliable free molecular flow and not a Knudsen flow ( transition flow or even Gleitströmung ). It should be noted that even in a vacuum insulation continues to heat transfer takes place by conduction of the solid skeleton and radiation, as well as the gas heat conduction is greater than zero even when free molecular motion.

Application

Vacuum prevention of convection

In the district heating ( > 144 ° C) of the medium to be transferred steel casing pipes are used, especially at higher temperatures where the space between the wetted tube and the outer support tube is evacuated.

Vacuum insulation without a core support

In thermos bottles or dewars vacuum thermal insulation is achieved by evacuating the cavity of a double-walled container, that is under vacuum. As the walls of the container are a few millimeters away from each other, the pressure in the cavity in the region of 10-3 mbar must occur, so a millionth of the atmospheric pressure. Hence such a low pressure for a long time can be maintained, the double-walled containers made of glass or stainless steel is produced. Vacuum insulation without a core support can also be realized only in rotationally symmetric containers ( eg, cylinders, ellipses or balls ), since the walls of the container must withstand the air pressure. In such a vacuum-insulated vessels without a core support the edge losses of the contact points of the two parts of the double shell make up the largest part of the heat loss.

Vacuum insulation with core support

Vacuum insulation panels, a porous support core is used which is provided with a gas-impermeable envelope. The support core is used once to receive the air pressure, so that vacuum insulation panels are in principle any forms available. Secondly, the pore walls are used in the core support as limiting the mean free path of the gas particles. Thus, the requirements for the vacuum are reduced. In vacuum insulation panels with a supporting core of microporous silica whose pores are only some 100 nm in size, a pressure in the insulation board of 10 mbar, so one-hundredth of atmospheric pressure is sufficient to allow the thermal conduction to be negligibly small due to air. With vacuum insulation panels to thermal conductivities of less than 0.004 W · m -1 · K-1 can be realized. This only left the heat conduction of the support body, and radiation as mechanisms of heat transport.