Oil refinery preheat trains can exhibit unwanted two-phase flow behavior. An example is boiling of crude oil inside heat exchangers, when the local pressure is not high enough to keep crude in a liquid state. This often arises when the pump is undersized. Understanding the two-phase behavior and assessing the boiling heat transfer coefficients would result in a better prediction and estimation of exchanger fouling. Where single-phase modeling is used under boiling conditions, the anomalous behavior leads to unrealistic estimates of fouling resistance, and can severely underpredict the increased pressure drop and consequent loss of crude throughput. There is little public information on fouling in two-phase flows as laboratory experiments are very costly, despite the importance of this in refinery heat exchangers and furnaces. Indeed, the importance of crude boiling is likely to increase as lighter crudes such as shale oils are processed. These lighter crudes are often blended with heavier crudes to maintain an appropriate refining average density. This paper consists of two sections. The first section uses industrial monitoring data to illustrate fouling behavior for a heat exchanger that undergo both boiling and fouling. The second section discusses simulations to evaluate thermohydraulic behavior when the crude undergoes boiling. The analysis requires coupled heat transfer, and hydraulic and surface fouling aspects; a commercial preheat train network simulator, SmartPM, was used for this study.