Fluids are heated and cooled in a variety of industrial and comfort applications using tube heat exchangers. They are made up of a shell with either improved surface tubes for superior thermal performance or conventional plain tubes. One of the fluids, which flows inside the tubes and the other outside the tubes inside the shell, can be either a liquid or a gas. Heat exchangers can be single-phase or two-phase; the latter can be used to boil or condense fluids.