Heat Transfer
Every material will transfer some heat if a temperature gradient exists across its thickness. According to the known laws of thermodynamics, heat will always flow from a region of high temperature to one of lower temperature. The effectiveness of a material as a thermal insulator can be expressed in terms of its thermal conductivity. The energy transfer rate through a body is proportional to the temperature gradient across the body and its cross sectional area. In the limit of infinitesimal thickness and temperature difference, the fundamental law of heat transfer is:
Q= λA (dT/dx)
Q is the heat transfer (W)
A is the cross-sectional area (m2)
dT/dx is the temperature/thickness gradient (K/m)
λ is defined as the thermal conductivity value (W/m.K)
Microporous Insulation
A microporous insulation is defined in ASTM C168 as - "Material in the form of compacted powder or fibres with an average interconnecting pore size comparable to or below the mean free path of air molecules at standard atmospheric pressure. Microporous insulation may contain opacifiers to reduce the amount of radiant heat transmitted."
Microporous Principles
Heat conduction, Convection and Heat radiation
Heat transfer can occur through conduction (solid & gaseous), convection and radiation. Usually the overall heat transfer comes from a combined effect of all of them. The driving force in this process is the temperature difference. These mentioned types of existing heat transfers are responsible for the total thermal conductivity value of an insulation material. Lambda describes the insulation material effectiveness. A material with a low thermal conductivity value (Lambda) has a good insulation performance; one with a high value fulfils the precondition for good heat transfer.
Heat Conduction: Heat conduction describes the heat transfer in solids. Because of the molecule structure of the microporous material and the fact that all particles are spherical, the points of contact between particles are infinitely small. The outcome is a very low solid phase conduction.
Convection: When molecules collide with each other, heat moves through gases via exchange of energy. The cell structure of the microporous material with an average pore size of roughly 20 nm, is smaller than the mean free length of path of the gas molecules. Due to collision of molecules, the exchange of energy between the molecules is reduced to a minimum.
Heat radiation: Heat radiation takes place by electromagnetic waves and gains in importance with increasing temperatures (>400 °„C). By adding infrared absorbing materials to the microporous silica mixture, the fraction of this heat transferring phenomenon is significantly reduced.
Hence , the end result is a reliable product with an extremely low thermal conductivity or λ value, close to the lowest theoretically possible minimum according to the laws of physics.
