A heat exchanger. (Image courtesy of the International Copper Development Association)

12.1 Introduction and Synopsis

Thermal properties quantify the response of materials to heat. Heat, until about 1800, was thought to be a physical substance called 'caloric' that somehow seeped into things when they were exposed to flame. It took the American Benjamin Thompson ^[1], backed up by none other than the formidable Carnot ^[2], to suggest what we now know to be true: that heat is atoms or molecules in motion. In gases, they are flying between occasional collisions with each other. In solids, by contrast, they vibrate about their mean positions; the higher the temperature, the greater the amplitude of vibrations. From this perception emerges all of our understanding of thermal properties of solids: their heat capacity, expansion coefficient, conductivity, even melting.

Heat affects mechanical and physical properties too. As temperature rises, materials expand, the elastic modulus decreases, the strength falls and the material starts to creep, deforming slowly with time at a rate that increases as the melting point is approached until, on melting, the solid loses all stiffness and strength. This we leave for Chapter 13.

Thermal design, the ultimate topic of this chapter, is design to cope properly with the effects of heat or, where possible, to exploit them. The chapter opening page shows an example: a copper heat exchanger, designed to transfer heat efficiently between two circulating fluids.