As was discussed before, heat exchanger are equipment used for heat transfer between two fluid flows with different temperatures. In design of heat exchangers such as double pipe heat exchanger, shell & tube heat exchanger, etc., it must be noted that there are some factors that affect the heat transfer rate during the equipment operation period. One of the most important factors is called fouling.
In the 2nd part of this article, the calculation of convective heat transfer coefficient was explained. In a double pipe heat exchanger the internal and external heat transfer coefficients are different and it is required to use an overall heat transfer coefficient to perform the calculations.
As was mentioned before, UC, is defined as follows based on convection heat transfer coefficients: