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HEAT TRANSFER 781

dharm

\M-therm\Th15-1.pm5

It follows from eqn. (15.1) that materials with high thermal conductivities are good
conductors of heat, whereas materials with low thermal conductives are good thermal insulator.
Conduction of heat occurs most readily in pure metals, less so in alloys, and much less readily in
non-metals. The very low thermal conductivities of certain thermal insulators e.g., cork is due to
their porosity, the air trapped within the material acting as an insulator.
Thermal conductivity (a property of material) depends essentially upon the following factors :
(i) Material structure (ii) Moisture content
(iii) Density of the material
(iv) Pressure and temperature (operating conditions)
Thermal conductivities (average values at normal pressure and temperature) of some com-
mon materials are as under :

Material Thermal conductivity (k) Material Thermal conductivity (k)

(W/mK) (W/mK)

1. Silver 410 8. Asbestos sheet 0.17


2. Copper 385 9. Ash 0.12


3. Aluminum 225 10. Cork, felt 0.05–0.10


4. Cast-iron 55–65 11. Saw dust 0.07


5. Steel 20–45 12. Glass wool 0.03


6. Concrete 1.20 13. Water 0.55–0.7


7. Glass (window) 0.75 14. Freon 0.0083
   Following points regarding thermal conductivity—its variation for different materials and
   under different conditions are worth noting :


8. Thermal conductivity of a material is due to flow of free electrons (in case of metals) and
   lattice vibrational waves (in case of fluids).


9. Thermal conductivity in case of pure metals is the highest (k = 10 to 400 W/m°C). It
   decreases with increase in impurity.
   The range of k for other materials is as follows :
   Alloys : = k = 12 to 120 W/m°C
   Heat insulating and building materials : k = 0.023 to 2.9 W/m°C
   Liquids : k = 0.2 to 0.5 W/m°C
   Gases and vapours : k = 0.006 to 0.05 W/m°C.


10. Thermal conductivity of a metal varies considerably when it (metal) is heat treated or
    mechanically processed/formed.


11. Thermal conductivity of most metals decreases with the increase in temperature (alu-
    minium and uranium being the exceptions).
    — In most of liquids the value of thermal conductivity tends to decrease with tempera-
    ture (water being an exception) due to decrease in density with increase in temperature.
    — In case of gases the value of thermal conductivity increases with temperature. Gases
    with higher molecular weights have smaller thermal conductivities than with lower
    molecular weights. This is because the mean molecular path of gas molecules decreases
    with increase in density and k is directly proportional to the mean free path of the
    molecule.


12. The dependence of thermal conductivity (k) on temperature, for most materials is almost
    linear ;
    k = k 0 (1 + βt) ...(15.2)