1.3 Fouling and Cleanability -Fouling and cleanability are among the most important design considerations for Fouling should or change exchangers and for some gas-to-fluid exchangers be evaluated for both design and off-design points. Periodic cleaning and or replacement of some changer components depend on the fouling propensity of the fluids employed In applications involving moderate to severe fouling. either a shell-and-tube or a gasketed plate heat exchanger is used, depending on the other operating parameters. In a shell-and-tube exchanger, the tube fluid is generally selected as the heavily fouling fluid since the tube side may be cleaned more easily A plate heat exchanger is highly desirable in those relatively low temperature appli- cations 300 C (575 F) where severe fouling occurs on one or both sides, as plate disassembly, cleaning, and reassembly is a relatively easy task. For highly corrosive fluid heating or cooling applications, shell-and-tube exchangers are used exclusively regardless of operating pressure and temperature conditions. Plate-fin exchangers usually have small hydraulic diameter passages and hence are more susceptible to fouling. They are also relatively difficult to clean and are not employed in even moderate fouling applications unless they can be cleaned chemically or thermally by baking (see Section 13.4)