Modelling of heat exchangers in desalination processe

A new concept of designing heat exchangers, evaporators, for seawater desalination process is introduced, which depends on using the surface temperatures of the fouling layers in the mathematical modelling calculations. This provides a better prediction of the value of the heat transfer area of the exchanger, which will help in decreasing the excess heat transfer area added by the designer or the purchaser to cover the effect of fouling. A precedence ordering technique is used to evaluate these two temperatures. These temperatures are used for the evaluation of the heat transfer coefficients inside and outside the tubes of various horizontal bundles in submerged; non-boiling spray, boiling spray and total spray models for desalination processes. The value of the condensation heat transfer coefficient inside the tubes is found to change according to the condition of the stream outside the tubes, even if the same correlation for condensation is used for all the models. The heat transfer coefficients of the dirty (fouled) exchanger are different in magnitude to these for a clean exchanger. Thus equalizing them to estimate the combined fouling factor of the plant will result in an error. Results revealed that the boiling spray model is the most underperforming model when the wall temperatures are used, while the submerged tubes model is the most underperforming when the steam and brine temperatures are used in comparison to the condition when the fouling temperatures are used. The controlling resistance for the boiling spray model is affected by the correlation used for the evaluation of the condensation heat transfer coefficient inside the tubes, which is not true for the other models in the study. For high values of the condensation heat transfer coefficient inside the tubes, the summation of resistances of the wall and the fouling layers will be controlling, which can be changed if the tube wall thickness is reduced. This will provide lower heat transfer area than increasing the thermal conductivity of the tube. To have a low heat transfer area, the submerged tubes model should be operated at the highest possible driving force temperature difference, while for the total spray model, the feed temperature should be as high as possible to decrease the non-boiling heat transfer area. The change of the design parameters for the low thermal conductivity tubes with cleaning is lower than tubes with higher thermal conductivity. The global parameters resulting from considering the saturated conditions inside the tubes leads to a shorter tube length than if a local calculation is considered only for the boiling spray model. The condensate condition at the exit of the tubes differ between the models, where saturated condensate is obtained for the non-boiling spray model, sub-cooled condensate for the submerged tubes model and condensate with a small amount of vapour for the spray boiling model, where extra heat transfer area is required.