Conductive Heat Transfer

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

Conduction as heat transfer takes place if there is a temperature gradient in a solid or stationary fluid medium.

Energy transfers from more energetic to less energetic molecules when neighboring molecules collide. Heat flows in direction of decreasing temperatures since higher temperatures are associated with higher molecular energy.

Fourier's Law are used to express conductive heat transfer

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 is constructed of solid iron with thermal conductivity 70 W/moC, thickness 50 mm and surface area 1 m by 1 m. The temperature is 150 oC on one side and 80 oC on the other.

The conductive heat transfer can be calculated

q = (70 W/m oC) (1 m) (1 m) ((150 oC) - (80 oC)) / (0.05 m)

    = 98000 (W)

    = 98 (kW)

Heat Transfer through Plane Walls or Layers in Series

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

q = (T1 - Tn) / ((s1 / k1 A) + (s2 / k2 A) + ... + (sn / kn A))     (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 1.2 cm thick stainless steel inner layer covered with 5 cm outside insulation layer of asbestos board. The inside surface temperature of the steel is 800 K and the outside surface temperature of the asbestos board is 350 K. The thermal conductivity of the stainless steel is 19 W/(m K) and the thermal conductivity of the asbestos board is 0.7 W/(m K)

The conductive heat transport through the layered wall can be calculated as

q = ((800 K) - (350 K)) / (((0.012 m) / (19 W/(m K)) (1 m2)) + ((0.05 m) / (0.7 W/(m K)) (1 m2)))

          = 6245 (W)

Thermal Conductivity 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)

  • 1 W/(m K) = 1 W/(m oC) = 0.85984 kcal/(h m oC) = 0.5779 Btu/(ft h oF) = 0.048 Btu/(in h oF)

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

Related Documents

Tag Search

  • en: conduction heat transfer temperature
  • es: conducción temperatura de transferencia de calor
  • de: Leitungswärmeübertemperatur

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