MECHANISM OF CRYSTALLIZATION FOULING DURING POOL BOILING OF FINNED TUBES

The formation of deposits on heat transfer surfaces is the main hurdle in optimum operation of thermal desalination units. It is, therefore, common practice to operate these units i.e. MSF under sub-atmospheric pressure to boil saline water below saturation temperature of which depositions occurs. However this comes with high price-tag for excessive use of energy for pumping. Alternative and environmentallyfriendly approach is to modify the surface at such that it could produce lager number of bubbles which cause high scale of turbulence. The present study aims at investigating experimentally the mechanism of crystallization fouling of finned tubes during nucleate pool boiling of CaSO 4 solutions. The investigated finned tubes are low finned tube type with fins density of 19 fins per inch, 1.35 fins pitch, and Cu-Ni as substrate. Fouling runs during pool boiling of CaSO4 were carried out at atmospheric pressure for different heat fluxes. A set of video recordings and pictures during clean and fouling runs were also used to interpret the fouling results. Experimental results showed that for the finned tubes: 1) substantial reduction of deposition occurred in comparison to those of stainless steel smooth tubes and 2) if any deposition builds-up on the finned tubes, then it is only a thin layer of crystals which can be easily removed. The superiority of finned tubes to stainless steel smooth tubes is due to the mechanism of bubble formation and detachment from the finned tubes that play a key role in reduction of CaSO4 scale formation on the surface.