Conduction will take place if there exist a temperature gradient in a solid (or stationary fluid) medium.
Energy is transferred from more energetic to less energetic molecules when neighboring molecules collide. Conductive heat flow occur in direction of the decreasing temperature since higher temperature are associated with higher molecular energy.
Fourier's Law express conductive heat transfer as
q = k A dT / s (1)
A = heat transfer area (m2, ft2)
k = thermal conductivity of the material (W/m.K or W/m oC, Btu/(hr oF ft2/ft))
dT = temperature difference across the material (K or oC, oF)
s = material thickness (m, ft)
A plane wall constructed of solid iron with thermal conductivity 70 W/moC, thickness 50 mm and with surface area 1 m by 1 m, temperature 150 oC on one side and 80 oC on the other.
Conductive heat transfer can be calculated as:
q = 70 (W/moC) 1 (m) 1 (m) (150 (oC) - 80 (oC)) / 0.05 (m)
= 98,000 (W)
= 98 (kW)
Heat conducted through several walls in good thermal contact can be expressed as
q = (T1 - Tn) / ((s1/k1A) + (s2/k2A) + ... + (sn/knA)) (2)
T1 = temperature inside surface (K or oC, oF)
Tn = temperature outside surface (K or oC, oF)
A furnace wall of 1 m2 consist of a 1.2 cm thick stainless steel inner layer covered with a 5 cm this outside insulation layer of asbestos board insulation. The inside surface temperature of the steel is 800 K and the outside surface temperature of the asbestos is 350 K. The thermal conductivity for stainless steel is 19 W/m.K and for asbestos board 0.7 W/m.K.
The conductive heat transport through the wall can be calculated as
q =(800 (K) - 350 (K)) / ((0.012 (m) / 19 (W/mK) 1 (m2)) + (0.05 (m) / 0.7 (W/m.K) 1 (m2)))
= 6245 (W)