Conductive Heat Transfer

Heat transfer will take place as conduction when there exists a temperature gradient in a solid or fluid

Conduction take place when a temperature gradient exists in a solid (or stationary fluid) medium.

Energy is transferred from the more energetic to the less energetic molecules when neighboring molecules collide. Conductive heat flow occur in the direction of decreasing temperature because higher temperature are associated with higher molecular energy.

The equation used to express heat transfer by conduction is known as Fourier's Law and is expressed as:

q = k A dT / s (1)

where

q = heat transferred per unit time (W, Btu/hr)

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

The heat transfer can be calculated as:

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

= 98,000 W or 98 kW

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)

• Convert between thermal conductivity units

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