A heat exchanger is a device used to transfer heat from one fluid to another. The term tube sheet often applies to heat exchangers in nuclear power pressure water reactors, where ~10,000 pipes are bound together to form the large surface used for heat transfer. The term sheet comes from the small diameter of the tubes (~0.75 in), their great length (~50 ft total in a U-Tube SG), and tightness to which they're bound which gives the impressiong of a folded sheet of tubes.
