Natural evaporation: refers to the evaporation of a solution at a temperature lower than the boiling point, such as when salt is exposed to seawater. In this case, due to the solvent only vaporizing on the surface of the solution, the solvent vaporization rate is low.

The indirect heat source heats the walls of the container and transfers it to the evaporated solution. The heat transfer process carried out in a wall type heat exchanger.