It is shown by the detailed analysis of its heat mass transfer mechanism that the variation of the surface temperature of scaling layer has an important influence on the scaling process. A heat mass transfer model is constructed to take the variation of the scaling layer surface temperature into consideration. With the model

simplified analyses of the scaling processes are carried out under both the constant wall temperature and the constant wall heat flux conditions. The results show that the variation of the scaling layer surface temperature is one of the major factors that cause the asymptotic tendency of scaling process. Comparisons of the model with experimental data are given. The results show that the present model can give a very good prediction of the experimental results.