The problem of calculation of a necessary evaporation surface area in a high-vacuum distillation apparatus with complicated geometry has not been fully resolved yet. Surface evaporation ranges (molecular distillation, the intermediate range, equilibrium distillation), in which different conditions of collisions between vapor particles exist, and their influences on effective evaporation still remain to be taken into account. An example of the apparatus, where the geometrical arrangement of the evaporator makes it difficult to estimate the evaporation rate, is a high-vacuum mechanical evaporator with rotating discs. A solution to this problem is proposed in this article.