Conductive Heat Transfer

Heat transfer takes place as conduction if there is a temperature gradient in a solid or fluid

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)

where

q = heat transfer (W, J/s, Btu/s)

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)

Example - Heat Transfer by Conduction

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 Transfer through Plane Walls In Series

Heat conducted through several walls in good thermal contact can be expressed as

q = (T1 - Tn) / ((s1/k1A) + (s2/k2A) + ... + (sn/knA))     (2)

where 

T1 = temperature inside surface (K or oC, oF)

Tn = temperature outside surface (K or oC, oF)

Example - Heat Transfer through a Furnace Wall 

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)

Thermal Conductivity and Common Units

  • Btu/(h ft2 oF/ft)
  • Btu/(h ft2 oF/in)
  • Btu/(s ft2 oF/ft)
  • MW/(m2 K/m)
  • kW/(m2 K/m)
  • W/(m2 K/m)
  • W/(m2 K/cm)
  • W/(cm2 oC/cm)
  • W/(in2 oF/in)
  • kJ/(h m2 K/m)
  • J/(s m2 oC/m)
  • kcal/(h m2 oC/m)
  • cal/(s cm2 oC/cm)

Related Topics

  • Heat Loss and Insulation - Heat loss from pipes, tubes and tanks - with and without insulation - foam, fiberglass, rockwool and more
  • Thermodynamics - Thermodynamics of steam and condensate applications
  • Insulation - Heat transfer and heat loss from buildings and technical applications - insulation methods and coefficients to reduce energy consumption
  • Thermodynamics - The effects of work, heat and energy on a system

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