Heat exchangers are manufactured with various flow arrangements and in different designs. Perhaps the simplest is the concentric tube or double-pipe heat exchanger shown in Figure 1, in which one pipe is placed inside another. Inlet and exit ducts are provided for the two fluids. In the diagram the cold fluid flows through the inner tube and the warm fluid in the same direction through the annular space between the outer and the inner tube. This flow arrangement is called parallel flow. Heat is transferred from the warm fluid through the wall of the inner tube (the so-called heating surface) to the cold fluid. A heat exchanger can also be operated in counterflow, in which the two fluids flow in parallel but opposite directions. Concentric tube heat exchangers are built in several ways, such as a coil or in straight sections placed side by side and connected in series.