Swenson Technology, Inc.

Energy Conservation

     Energy conservation is a primary consideration in the design of every Swenson evaporator.  In order to conserve energy, Swenson evaporators are utilized in any one, or a combination, of the following configurations: multiple-effect, mechanical recompression, or thermal recompression.  They are also frequently used in conjunction with preheaters and/or condensate flash systems for additional energy savings.  These evaporator configurations will be described in greater detail in the following pages.

Multiple Effect

     The multiple-effect configuration combines two or more evaporator bodies to conserve steam, which is condensed in the first-effect heat exchanger only.  Water evaporated in the first-effect vapor body is condensed in the second-effect heat exchanger, which provides energy for evaporation in the second-effect vapor body (and so on for additional effects).  Vapor from the last effect flows to a condenser.
     The last-effect vapor body is maintained at a high vacuum with a steam-jet air ejector or mechanical vacuum pump.  The driving force is the pressure drop from the first to the last effect.
     For the same overall pressure differential and process conditions, the evaporator with more effects will require less steam.  A rough "rule of thumb" for comparison purposes is that the evaporation in each effect will be approximately 0.7 to 0.9 pounds for each pound of steam condensed in the first-effect heat exchanger.
    Thus, for the same evaporation rate, a quintuple-effect evaporator will require about 20% of the steam required for a single-effect evaporator.  In actual practice, the steam economy can vary widely because of differences in feed temperature and other energy requirements such as heats of dilution and crystallization.  Figs. 1, 4, 14 and 15, plus the cover photo (included in the download), show multiple-effect evaporator configurations.
     For some applications, single-effect evaporators are used because multiple-effect evaporators cannot be economically justified.  In particular, single-effect evaporators are utilized for small evaporation rates, or for liquids that boil at high temperatures (high boiling-point elevation liquors).

Preheaters and Condensate Flash Systems

Preheaters and condensate flash systems are often incorporated into Swenson process designs to reduce steam consumption.  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.
     A Swenson preheater is designed either as 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.
     Condensate flash systems are sometimes employed in place of, or to complement, preheaters.  Condensate from a heat exchanger is flashed into a heat exchanger that is operated at a lower pressure to provide supplemental energy.

Fig. 11. Swenson Double-Effect, Falling Film Evaporator

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