Overall heat transfer through walls with extended surfaces

3 Overall heat transfer through walls with extended surfaces 

The overall resistance to heat transfer (1/kA) is found by adding all the resistances to convective heat transfer and conduction, as shown in (1.72). Therefore the largest single resistance determines the value for ( /kA). This is especially distinct when the other resistances are very small. It is therefore possible to significantly improve the insulation of a wall by adding a layer which has high resistance to conduction 4/AmTln, or in other words a thick layer of a material with low thermal conductivity. 

The overall heat transfer resistance is not reduced by the same degree. The increased heat transfer area brings with it an additional conduction resistance. The heat which is absorbed by the fluid from an area near the fin tip, has to be transported to the vicinity of the tip by conduction. This requires a temperature 

The heat flow Qg removed from the surface Ag of the finless base material (ground material) is 

If the fin was at the same temperature all over as at its base d0, then the heat flow would be given as 

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