15-1 Basic Principles of Heat Transmission

Problems of greatest importance and most frequently occurring in the entire field of engineering are unquestionably those that involve the strength of materials. Second in importance to these only are those problems that involve the flow of heat. Most engineers readily recognize the significance of the first group, and are rather well equipped to work out methods for their solution.

On the other hand, when confronted with a problem involving the flow of heat, the average engineer will look up the formulas in his elementary physics text, and a table of thermal conductivities in a handbook, and considers that he has the complete data for solving the problem. Just to clarify the point, in the published proceedings of a technical society some years ago it was stated that since copper had seven times the thermal conductivity of iron, substituting copper tubes for iron tubes in a piece of heat-transfer equipment should increase the capacity to seven times the original figure using iron. This, of course, is not true. It is doubtful if the substitution would actually increase the capacity 10 per cent.

There are many such similar contradictions that only add to the confusion. Much has been published on the subject of heat transmission, ^[*] but we shall attempt to help clarify the situation somewhat in this chapter. Our treatment, because of the lack of space, will be systematic and in compact form.