Evaporator Types
1) Forced Circulation Evaporator
Solution is pumped from bottom cone of the evaporator vessel through the heat exchanger where heat is supplied. Heat exchanger is not used in flash cooling design. So, problems with scaling in tubes are avoided. Sufficient solution height should be maintained above the heat exchanger to suppress boiling on the tube surfaces. Optimal velocity in the tubes should be maintained in order to get high heat transfer.
To minimize liquid droplet entrainment into the vapor space above the boiling surface, evaporation rate must be controlled to obtain optimum upward vapor velocity.
Fig. 1. Principle of Forced
Circulation Evaporator
2) Falling Film Evaporator.
Solution is supplied to the top of the heat exchanger bundle and distributed inside of the tube wall as a thin film. It flows down on the tube wall. The liquid film partially evaporates due to the external heat source out of the tubes. Because of the gravity and flow of vapor formed, liquid flows downward. Liquid and vapor are separated at the vessel located after the heat exchager.
Main problem with falling film evaporators is to distribute the liquid evenly to all tubes. All tubes must be wetted uniformly. So, recirculation of the liquid may be required. Distributer design has great importance in the design of falling film evaporators.
Falling film evaporators are mainly used for heat sensitive products because of the low residence time. It has high energy efficiency because of the high heat transfer coefficients. It also operates at low temperature differences.
Fig. 2. Principle of Falling Film Evaporator
3) Agitated Film / Wiped Film Evaporator.
High heat and mass transfer rates are the benefits of a thin film produced by mechanical agitation. The unique feature of this equipment is the mechanical agitator for producing and agitating the film. Mechanical agitation permits the processing of high-viscosity liquids. It is advantages to use it for the following materials;
-
Heat sensitive products
-
Materials that tend to foul
-
Viscous liquids