Answer: The exchangers are always designed with the implicit assumption that the thermophysical properties of the fluid remain constant throughout the exchanger, whereas this assumption is not made during the calculation of the processes. The result is that when you recalculate a temperature based on the exchanger equations, slight differences may exist between the value of the exchange module and the value of the corresponding process. If you want extremely good accuracy, you must iterate several times. Generally two or three times is enough.