Editing Heat exchanger (section)

===Fouling===
{{main|Fouling}}
[[File:Fouling02.jpg|thumb|A heat exchanger in a steam power station contaminated with macrofouling]]

[[Fouling]] occurs when impurities deposit on the heat exchange surface.
Deposition of these [[impurities]] can decrease heat transfer effectiveness significantly over time and are caused by:
* Low wall [[shear stress]]
* Low fluid velocities
* High fluid velocities
* Reaction product solid precipitation
* Precipitation of dissolved impurities due to elevated wall temperatures

The rate of heat exchanger fouling is determined by the rate of particle deposition less re-entrainment/suppression. This model was originally proposed in 1959 by Kern and Seaton.

'''Crude Oil Exchanger Fouling'''. In commercial crude oil refining, crude oil is heated from {{convert|21|C}} to {{convert|343|C}} prior to entering the distillation column. A series of shell and tube heat exchangers typically exchange heat between crude oil and other oil streams to heat the crude to {{Convert|260|C}} prior to heating in a furnace. Fouling occurs on the crude side of these exchangers due to asphaltene insolubility. The nature of asphaltene solubility in crude oil was successfully modeled by Wiehe and Kennedy.<ref>{{cite journal |last1=Wiehe |first1=Irwin A. |last2=Kennedy |first2=Raymond J. |title=The Oil Compatibility Model and Crude Oil Incompatibility |journal=Energy & Fuels |date=1 January 2000 |volume=14 |issue=1 |pages=56–59 |doi=10.1021/ef990133+}}</ref> The precipitation of insoluble asphaltenes in crude preheat trains has been successfully modeled as a first order reaction by Ebert and Panchal<ref>Panchal C;B; and Ebert W., Analysis of Exxon Crude-Oil-Slip-Stream Coking Data, Proc of Fouling Mitigation of Industrial Heat-Exchanger Equipment, San Luis Obispo, California, USA, p 451, June 1995</ref> who expanded on the work of Kern and Seaton.

'''Cooling Water Fouling'''.
Cooling water systems are susceptible to fouling. Cooling water typically has a high total dissolved solids content and suspended colloidal solids. Localized precipitation of dissolved solids occurs at the heat exchange surface due to wall temperatures higher than bulk fluid temperature. Low fluid velocities (less than 3&nbsp;ft/s) allow suspended solids to settle on the heat exchange surface. Cooling water is typically on the tube side of a shell and tube exchanger because it's easy to clean. To prevent fouling, designers typically ensure that cooling water velocity is greater than {{nowrap|0.9 m/s}} and bulk fluid temperature is maintained less than {{convert|60|C}}. Other approaches to control fouling control combine the "blind" application of [[biocide]]s and anti-scale chemicals with periodic lab testing.