Swenson Technology, Inc.

Heat Exchangers

     Shell and tube heat exchangers are used for the majority of Swenson evaporator installations.  The heating medium is almost always steam which is normally condensed on the outside of the tubes to heat the liquor or slurry which flows inside the tubes.  The energy transferred is simply the product of the steam flow rate and the difference in enthalpy between the steam and its condensate.  For saturated steam, the enthalpy difference is the latent heat of vaporization.
     The heat-transfer surface required for the energy exchange is calculated as follows:

A_i  =        Q         =   W_s (H_s - H_c)
         U_i Delta-T          U_i  Delta-T

Where A_i = inside tube area, sq ft
           Q = heat transferred, BTU/hr
           W_s = steam rate, lb/hr
           H_s = enthalpy of steam at a given pressure and temperature, BTU/lb
           H_c = enthalpy of condensate at the condensing temperature of the steam, BTU/lb
           U_i = overall heat-transfer coefficient, BTU/(hr) (sq ft) (^oF)
            Delta-T = temperature difference between condensing steam and the liquor or slurry (usually determined as log mean), ^oF

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.  A typical horizontal heat exchanger is shown in Fig. 9.
     The majority of Swenson installations use vertical-tube heat exchangers (as shown in Figs. 5, 7 and 8).  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.

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