Natural Circulation Evaporators are very simple and are normally used where the effluent has a high viscosity, higher levels of hardening agents, un-dissolved solids, for products which are affected by their own high temperatures and longer residual times.
As thermal energy is used in evaporation and with its increasing cost, it is important to optimize upon the operating costs. Multistage designs help achieve reduced operational costs. Energy is applied to the first stage of the evaporator. Vapor from this stage is reused in the next stage as the heating media. Similarly, from the second stage, vapors are used as heating media in the third stage and so on. Evaporators up to seven stages are designed, based on the product properties.
Natural circulation evaporators are based on the natural circulation of the product caused by the density differences that arise from heating.