A shell and tube heat exchanger consists of tubes surrounded by a shell. One fluid flows through the tubes while another fluid flows over the tubes in the shell. Heat is transferred between the two fluids through the walls of the tubes. The shell and tube design allows fluids at different pressures to exchange heat, with the higher pressure fluid typically flowing within the tubes. Common applications include cooling engine fluids like oil and hydraulic fluid, as well as heating or cooling liquids like pool water. The cylindrical shell design makes it suitable for a wide range of pressure applications.