CHAPTER
5
The overall coefficient U for a composite system, consisting of surface film, composite wall, surface film, in series, can then be calculated as in eqn. (5.5) from:

EXAMPLE 5.11.
Effect of air movement on heat transfer in a cold store
In Example 5.2, the overall conductance of the materials in a coldstore wall was calculated. Now on the outside of such a wall a wind of 6.7 m s^{1} is blowing, and on the inside a cooling unit moves air over the wall surface at about 0.61 m s^{1}. The radiation coefficients can be taken as 6.25 and 1.7 J m^{2} s^{1} °C^{1 }on the outside and inside of the wall respectively. Calculate the overall heat transfer coefficient for the wall.
Now from Example 5.2 the overall conductance of the wall,
In eqn. (5.33) often one or two terms are much more important than other terms because of their numerical values. In such a case, the important terms, those signifying the low thermal conductances, are said to be the controlling terms. Thus, in Example 5.11 the introduction of values for the surface coefficients made only a small difference to the overall U value for the insulated wall. The reverse situation might be the case for other walls that were better heat conductors.
(a) For brick
(b) For aluminium
Comparing the calculations in Example 5.11 with those in Example 5.12,
it can be seen that the relative importance of the terms varies. In the
first case, with the insulated wall, the thermal conductivity of the insulation
is so low that the neglect of the surface terms makes little difference
to the calculated U value. In the second case, with a wall whose
conductance is of the same order as the surface coefficients, all terms
have to be considered to arrive at a reasonably accurate U value.
In the third case, with a wall of high conductivity, the wall conductance
is insignificant compared with the surface terms and it could be neglected
without any appreciable effect on U. The practical significance
of this observation is that if the controlling terms are known, then in
any overall heattransfer situation other factors may often be neglected
without introducing significant error. On the other hand, if all terms
are of the same magnitude, there are no controlling terms and all factors
have to be taken into account.
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