The vacuum panel.
The vacuum panel has a very low coefficient of thermal conductivity – 0,002 W/m·K. It allows to reduce the thickness of the insulating layer is 6 – 10 times compared to other insulation materials.
Vacuum panel consists of a porous material filler, which is placed in an impervious filmwrapper, from which the air is pumped up to a pressure of 1 mbar., then the shell is sealed.
The vacuum panel has a very low coefficient of thermal conductivity. Coefficient of thermal conductivity can reach values of 0.002 W/m·K.
The main role in the heat transfer process plays a gasin the pores. The smaller the pore size of the material and its branched structure, the better its thermal properties and, consequently, the lower the thermal conductivity. As the material of the filler used dispersed materials. For example, can be used nanoporous silicon dioxide SiO2, consisting of particles with a size of 5 – 20 nm, comprising a frame with a characteristic pore size of 20 – 150 nm.
Film-the shell – the material from which are formed the walls of the vacuum insulation panel. It consists of several layers, each layer is a very thin metal film (aluminium, stainless steel), which from both sides a layer of plastic. It has excellent barrier characteristics. To form a wrapper material, filler, membrane film welded at the edges.
– the use of vacuum insulation can reduce insulation thickness is 6 – 10 times compared to other materials
– the use of vacuum insulation can reduce the weight of the insulating layer 2 to 6 times,
– vacuum panel – environmentally friendly insulating material.
To understand high heat-insulating properties of vacuum insulation it is necessary to know the mechanisms of heat transfer.
The main mechanism of heat transfer in solids is the conductivity. When heated, one end of a metal rod, the heat flux moves to its other end.
By conduction heat can travel through gases. Fast molecules of the warm layer of gas collide with slower molecules adjacent cold layer. As a result, the flow of heat. Gases from light molecules (hydrogen) conduct heat better than heavy gases (nitrogen). By convection heat transfer is carried out only in gases and liquids and is based on the fact that when heated, the gas density decreases. With uneven heating lighter layers rise, hard fall. The vertical flux of heat associated with this movement, as a rule, considerably exceeds the flow associated with thermal conductivity.
Radiation is a mechanism of transfer of heat by electromagnetic waves. In this way is heated by the sun surface of the earth. The body’s ability to radiate and absorb electromagnetic wave is determined by its atomic structure.
Vacuum technology (vacuum panel) allows you to exclude all three mechanisms of heat transfer.
The Dewar vessel, or thermos, is a widely known example of vacuum insulation. In the space between the double walls of the Dewar creates a deep vacuum of about 10-2 PA. Because of this, the heat transfer due to convection and conduction, is almost completely eliminated, and the conductivity is extremely small— 10-3 — 10-4W/(m•K).
The need to create high vacuum greatly limits the choice of the form of the vessel and structural materials. Since the depressurization of the vessel may damage the insulation, the walls must be absolutely gas – and water resistant. To reduce heat transfer by electromagnetic waves between the walls of the Dewar in the list of materials limited to metal, film and glass with a metallic coating.
|Material||Vacuum panel||Flax insulation||Mineral wool||Concrete||Pushblock||Brick|
|Thermal conductivity, W/m•K||0,002 – 0,0046||0,037||0,046||0,14 – 0,66||0,3 – 0,5||0,52-0,81|