The shell n tube heat exchanger has two major components-round tubes mounted inside a cylindrical shell. The tubes are thin-walled tubing produced specifically for use in heat exchangers. The shell cylinder can be fabricated from rolled plate or from piping (up to 24 inch diameters). One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be composed of several types of tubes: plain, longitudinally finned, etc.