Ans: The temperature difference between hot and cold fluids is not constant along the length of the heat exchanger.
For example, in parallel flow HX the temperature of the fluids is changing separately as well as the temperature difference between hot and cold fluids is changing continuously along the length. This makes it difficult to directly use dT in the heat transfer calculations.
Therefore, a log mean temperature difference (LMTD) is calculated which is an actual representation of the average temperature difference between hot and cold fluids.