Refineries and chemical plants depend on clean heat exchangers to maintain efficient
plant operation. But over time, exchangers can build up fouling on both the tube side and
shell side of the bundle. In the case of crude unit exchangers, heavy asphaltenes from the
crude can accumulate on the walls of the exchanger tubes. Fouling is common in both
crude preheat trains and tower overhead condensers. Calcium and other minerals in the
plant cooling water system can plate out on overhead exchanger tubes. Designers
endeavor to size exchanger tubes such that the velocities through the tubes are
maximized in order to extend the time period before fouling begins to impede flow
(Chapter 10). But eventually, fouling is an unavoidable reality in the use of heat
exchangers.
As fouling increases, heat exchange is compromised and efficiency is diminished,
resulting in reduced throughput as well as increased energy requirements. For example,
as crude preheat exchangers foul, the crude temperature entering the furnace is reduced
and additional fuel is required to fire the furnace. In the case of tower overhead
exchangers, fouling can result in higher tower pressures, which impedes throughput
and/or fractionation quality. Chapter 12 describes a technique to determine optimum heat
exchanger cleaning cycle lengths in complex preheat trains, based on overall process
energy impacts. This chapter focuses on the methods used for cleaning.