A new concept of design procedure for heat exchangers in a heat exchanger network is presented. The procedure makes it possible to take real operating conditions into account. The reduction of heat flux due to fouling deposits, changes in mass flows resulting from changing feedstock quality or product demand, changes in inlet temperatures as well as heat flux uncertainty due to uncertainty in heat transfer coefficient and fouling resistance, are considered. These four factors are related to minimum difference temperature (DeltaT(min)), and Biot fouling (phi). When selecting heat transfer areas in the individual heat exchangers, the allowable uncertainty margins in target temperatures are also accounted for.