It is commonly known that the aim of the heat exchanger network (HEN) is maximization of heat recovery. Unfortunately, heat recovery is deteriorated by deposits mounting up on the heat transfer surface of the heat exchangers. Moreover, the process parameters (temperature, mass flow, etc.) are changing with time that also are influencing on heat recovery. Hence, to maximize heat recovery in real HEN, the authors proposed a mathematical model of on-line control of HEN. Unfortunately, the existing mathematical models of the heat exchanger are inaccurate because heat transfer coefficients are calculated with errors at values ranging from 10 to 40 %. To overcome this issue, the authors introduced a very accurate mathematical model of the heat exchanger. The model is based on industrial measurements of the process stream parameters. These parameters are used for on-line validation of the heat exchanger model. For the validated model, the results of calculation are very accurate, for the errors of calculations are less than 1 %. In this article, the proposed model is applied to HEN in Crude Distillation Unit (CDU), processing 800 t/h of crude oil. In proposed on-line model, the manipulated variables are mass flows of crude oil passing via several branches of the HEN. The results of the HEN simulation show that on-line control of the mass flow through several branches of the HEN gives an opportunity to 1.5 % increase of the total heat recovery.