The coefficient of thermal conductivity (λ) expresses the amount or flow of heat that passes through the surface unit of a sample of the material, of infinite extension, parallel flat faces and unit thickness, when a difference of temperatures equal to the unit, in stationary conditions.
Determination of the coefficient of thermal conductivity
An empirical formula found by statistical means is used since deducing it by the laws of Physics presents great difficulties since it is necessary to consider the cellular dimensions, the thicknesses of the layers of water molecules on the inner surface, the cosines in the direction with respect to the fibers and the heat conductivity coefficients of each of the component parts.
- λ = 0.168 ρ0 + 0.022 [Kcal / mh ºC]
- In the International System of Units (SI) it is measured in watt / meter × kelvin (W / (m · K)). The coefficient varies with the conditions of the material mainly the humidity it contains and the temperature at which the measurement is made, so the conditions have to be created to do it, generally for dry material and 15 ° C and on other occasions, 26 , 84 ° C.
Thermal conductivity of different materials
Thermal conductivity values of some materials | ||
materials | Thermal conductivity (W · m-1 · K-1) | |
Steel | 68-98 | |
Water | 0.98 | |
Air | 0.01 | |
Aluminum | 209.3 | |
Asbestos | 0.04 | |
Zinc | 106-140 | |
Cork | 0.04-0.30 | |
Tin | 64.0 | |
Fiberglass | 0.03-0.07 | |
Common brick | 0.80 | |
Firebrick | 0.47-1.05 | |
Brass | 1.14 | |
Wood | 0.13 | |
Lead | 35.0 | |
Polyurethane | 0.018-0.025 | |
Glass | 1.0 |