As a kind of direct contact heating method, submerged combustion evaporator (SCE) has many advantages and is widely used in many engineering processes. The entrainment of water droplets in vapor significantly affects the separation efficiency of SCE with sparkling of hot gases. In order to ensure high evaporation efficiency, it is necessary to minimize water droplets containing high concentration of pollutants. Vapor separation space provides place for water droplets separating from vapor by gravity. Therefore, it is very important to study the gravity separation behavior of water droplets from vapor. Gravity separation model (GSM) for water droplets separation from vapor is elaborated in this paper, and its analytical solution for this model is described. Based on this model and its analytical solution, the effects of the gravity separation height and velocity of the vapor flow on the separation efficiency are discussed in details. It corresponds well with the existing numerical results. The results show that, all the droplets decelerate after entering the vapor flow; and it is not necessary to increase the height of gravity separation space after getting maximum separation efficiency. For the same height of gravity separation space, separation efficiency decreases with vapor velocity increasing. At constant vapor velocity, the maximum efficiency of separation for different initial velocities of droplet is the same. And the faster the initial velocity, the higher space for gravity separation is needed to get the maximum efficiency. This model could be used for the improvement of separation efficiency of SCE and optimization of its design. (c) 2006 Elsevier B.V. All rights reserved.