HEAT TRANSFER ANALYSIS OF A FALLING-FILM HORIZONTAL TUBE EVAPORATOR

The heat transfer performance of a falling-film horizontal tube evaporator was theoretically investigated. A theoretical model was developed to predict the heat transfer coefficients of condensation inside and evaporation outside the tubes of horizontal evaporators. Analytical results, correlations and a practical method for predicting the heat transfer performance of horizontal evaporators were presented. Experiments on pilot evaporators were conducted. The test results provided reference conditions and a comparison basis for the predictive method.;Differential equations governing the kinematic, dynamic and thermal aspects for vapor condensation inside and film evaporation outside the tubes, which were derived from a thin film flow modeling, were simplified by carrying out the order of magnitude analysis.;A theoretical method, which takes into account the effects of vapor shear at the film surface for condensing stratified flow, was developed to obtain the condensation heat transfer coefficient along with the pressure gradients and stratified angle along the tube length.;For liquid flowing over the outside of the tube bundle, the hydrodynamic and heat transfer solutions were obtained by a finite difference method. The values of laminar heat transfer coefficients were modified to account for waviness. Furthermore, the effects of in-between tubes evaporation and the heat transfer variation between tubes in a bank were analyzed.;Results of these condensation and evaporation predictions were found to agree well with the reported data. In view of this good agreement, heat transfer correlations for vapor condensation inside and film evaporation outside the tubes of horizontal evaporators were developed from the numerical solutions by means of regression analysis. Furthermore, a practical method for predicting the overall heat transfer performance for a horizontal evaporator was developed by using the proposed correlations.;To test the validity of the proposed method for predicting the heat transfer performance of horizontal evaporators, an experimental investigation was conducted on pilot evaporators. The experimental results agree with the predicted values within a discrepancy of 6%. Therefore, the predictive method is recommended for use in the thermal design of horizontal tube evaporators.