In its simplest form, a heat exchanger consists of a bundle of tubes arranged between two thick perforated tube plates and enclosed by an outer shell. Covers over the tube plates allow one fluid to pass inside the tubes while a second hotter or cooler fluid is introduced via openings in the shell. Since they contain no moving parts it might be expected that heat exchangers would be highly reliable pieces of equipment. However, everyday experience with small domestic and automotive heaters and coolers suggests this is not always the case. In the case of mass-produced items such as these, replacement of a defective component is usually relatively straightforward and inexpensive. However, for the much larger heat exchangers used in the process chemical and power industries, a single failure may result in significant financial losses.