HEAT TRANSTER AT EVAPORATIVE CONCENTRATION OF DOWN FLOWING IN VERTICAL PIPES SOLUTIONS IN ANNULAR REGIMES

摘要

In many branches of food processing, particularly in sugar production, the solution concentration takes place within industrial multi effect evaporator; here, the solution moves down as a liquid film forming annular descending flow in vertical heated pipes. It should be noticed that within the evaporation station the liquid which is being evaporated significantly changes its physical properties due to the fact that the concentration of the solution increases by the range of 14…65% of dry matter. The operational process parameters (pressure, temperature, etc.) also change to a noticeable degree. These factors determine a wide range of liquid films flow patterns, namely film thickness, surface wavy structures, velocity distribution across the films, interphase shear stress and, as a result of these, the making of a certain heat transfer mechanism including bubble boiling or its suppression. Unfortunately, the reliable methods of prediction and calculation of heat transfer coefficients are unavailable, which seriously hampers the designing and calculation of efficient evaporators and other heat transfer equipment. The work presents the results of modeling heat transfer to saturated liquid films of solutions flowing down inside the vertical pipes as in the regime of evaporation from the interphase surface and in that of surface boiling. The correlations given allow calculating the heat transfer coefficients for liquid films being concentrated in the vertical pipes of industrial evaporators.