Using a non-traditional approach to account for crude oil fouling in heat exchangers

Heat exchanger design has usually accounted for fouling by adding extra resistances, or fouling factors, to the overall heat transfer coefficient.

By Christopher A. Bennett

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Heat exchanger design has usually accounted for fouling by adding extra resistances, or fouling factors, to the overall heat transfer coefficient. An important question is which values the engineer should use for these fouling factors. Traditionally, these fouling factors are taken from tables, such as those published by TEMA (Tubular Exchanger Manufacturers Association) or company experience.

There have been decades of debate surrounding the use of fouling factors, and crude oils exemplify why. Crude oils are arguably the most problematic of all fouling fluids; they foul by many different mechanisms including sedimentation, crystallization/precipitation, corrosion, asphaltene flocculation, and coking. Crude oil will foul by one mechanism, or combination of mechanisms, at one point in a refinery preheat train but will foul via a different mechanism at another point. Many crude oils foul very heavily whereas others foul hardly at all. Furthermore, when crude oils do foul they usually do not reach the asymptote that many other fouling fluids attain; instead they foul perpetually.

Despite the inherent complexity of crude oil fouling behavior, TEMA provides crude oil fouling factors as functions of temperature, velocity, and desalting. Experience has repeatedly demonstrated, however, that the TEMA recommendations for fouling resistance can be either too high or too low, depending on the specific crude oil, wall temperature, shear stress, run time, exchanger design, and many other variables. Therefore, alternatives to fouling factors are needed.

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