In natural circulation evaporators, the liquid enters with velocity 0.3 to 1 m/s and generally, the heat transfer coefficients are very low and particularly with viscous liquids. So, whenever we are dealing with a concentration of highly viscous and scale forming solutions there is no alternative but to use forced circulation evaporators. By increasing the velocity of the liquid flow (generally 2 to 6 m/s) through the tubes heat transfer coefficients increases enormously and high liquid velocity which is resulting from the pumps prevents the scale formation on heating surfaces.
Two types forced circulation evaporators.
(i) horizontal heating type
(ii) vertical heating type
Construction:
They consist of a centrifugal pump, a 1-2 shell & tube heat exchanger on the case of the horizontal heating element and 1-1 shell & tube heat exchanger in the case of vertical heating element, an evaporator body with vapor outlet at the top, deflector plate, an outlet for product discharge at the bottom.
Working:
Centrifugal pump forces the liquid through the tubes at high velocity and gets heated by condensing steam on the shell side. The solution becomes superheated and flashes into a mixture of vapour and liquid in the evaporator body. Deflector plate separates the vapour and liquid in the vapour space, and vapours are removed at the top, concentrated liquid comes out at the bottom.
Advantages:
1. High transfer coefficients obtained even with viscous solutions.
2. Whenever we are dealing with the concentration of highly viscous and scale forming solutions forced circulation evaporators prevents the scale formation on heating surfaces
3. Residence times are low so that heat sensitive viscous materials also can be used.
Disadvantage:
The main disadvantage of forced circulation evaporators is high pumping cost.
Applications:
To handle high viscous solution in continuous operations external pumping system is provided by forced circulation evaporators. Salt, plastic, polymer, and pharmaceutical industries use it for crystallizing, concentrating and thickening of the products.