The flow of material or energy through a system on more than one occasion. Multipass operations are used to improve process efficiencies. A multipass heat exchanger is a heat exchanger device in which one or both fluids pass more than once. They are used when the length of tube for heat transfer is too great to make a single pass operation practical. The heat transfer medium passes through either the shell side or tube side transferring heat, and is then returned back for further heat exchange. The temperature profiles are more complex than for single pass operations since some parts of the equipment will be cocurrent and other parts will be countercurrent. For the number of tube passes used, correction factor charts are used to adjust the log mean temperature difference calculations.