Shell and tube heat exchangers are used for the majority of Swenson installations. The heating medium is typically steam which is normally condensed on the outside of the tubes to heat the liquor or slurry which flows inside the tubes. Both vertical and horizontal tube heat exchangers are utilized. The horizontal exchanger is used for installations with limited headroom or where maximum liquor submergence is needed to prevent surface boiling and subsequent salt precipitation on the tubes.
Heat exchangers are typically one or two passes and are designed for relatively low temperature rises in the solution. This limits the supersaturation of scaling components when heating materials with inverted solubility. In most applications, the steam to liquid delta T is also limited to prevent mass boiling within the tubes or vaporization at the tube wall.
Most systems use vertical heat exchangers. Steam enters the enlarged shell section and flows around and up through the annulus formed between the inner and outer shells. The steam enters the tube bundle uniformly around the entire circumference beneath the top tubesheet. The Swenson steam inlet prevents condensate impingement on the tubes, reduces tube vibrations, provides for uniform steam distribution and, for some applications, acts as an entrainment separator. The non-condensible gases are swept downward and removed near the bottom tubesheet. Special vent baffles are provided for large diameter heat exchangers.
Internal tube supports are not normally required for vertical heat exchangers. The tubes are usually fastened mechanically to the top and bottom tubesheets by expansion of the tube into the tube holes.
Preheaters are often incorporated into Swenson process designs for heat recovery to reduce steam consumption and operating costs. Preheaters are used to heat the feed to an evaporator with a heat-transfer medium, preferably of less energy value than that of the steam or vapor condensed in the evaporator heat exchanger. Typical heat-transfer fluids used in preheaters include evaporator condensate, evaporator vapor and process liquor. Heat that is added in preheaters replaces the heat that would have been added in the evaporator heat exchangers; therefore, less heat-transfer surface area is required for the evaporator heat exchangers when preheaters are used.
Preheating can be done either in a separate exchanger or as an integral part of an evaporator heat exchanger or condenser. Preheaters can also improve heat-transfer coefficients for rising- and falling-film evaporators because the non-boiling zone is reduced or eliminated. For some scaling applications, separate preheaters, with bypass piping, provide an additional benefit because scale forms in the smaller, easier-to-clean preheater rather than on the tubes of the larger evaporator heat exchanger.