Design aspects of forced circulation evaporators of high salty water from the view point of scale formation

In this paper, dryout phenomenon was studied for vertical tubes working at the operating condition of forced circulation evaporator in zero liquid discharge systems to find the maximum allowable value of temperature difference between evaporator effects in which scale formation and fouling do not occur. Eulerian two-fluid approach coupled with Rensselaer Polytechnic Institute wall boiling model was used to simulate upward flow subcooled boiling. Critical near-wall void fraction was used as the criterion of dryout. The value of the critical temperature difference was investigated for 36 cases. Values varied from 4.8 to 10.7 °C for different amounts of tube length, tube diameter and mass flux. In addition, the effect of tube length, tube diameter and mass flux on critical temperature difference was analyzed. The results showed that critical temperature difference decreased approximately linearly with an increase in tube length. Furthermore, the value of critical temperature difference decreased with the increase in tube diameter and decrease in mass flux and the effect of tube diameter is more significant at higher mass fluxes.