Fouling in pre-heat trains of atmospheric crude distillation units is one of the major issues hindering the efficiency of oil refineries. This paper illustrates the benefits of using dynamic simulation, combined with plant measurements, to evaluate heat exchanger retrofit opportunities that provide the largest savings to the refinery. Once the fouling behavior of the network is well characterized and the most critical heat exchanger is identified, the performance of a retrofit design for that exchanger is assessed by re-running history using plant measurements for an extended period of operation. The comparison of the original design versus alternative ones, based on the potential reduction of fouling, provides an overall assessment of the impact on energy/economic savings for the whole network given by the proposed retrofits options. This enables the design engineer to use available plant data to evaluate and select the most beneficial design option, not simply based on expected operating conditions and fixed fouling factors, but on time-varying conditions and dynamic fouling build-up as it happens in the plant. The method is demonstrated with an industrial case study. The estimated savings in furnace duty were of 1.5-2.7 MW, resulting in expected economic benefits of $270-700 k per year.