Page 170 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

168 Chapter 6 Evaporators

Vapour

Steam
Vapour head

Deflector
Gas vent

Feed
Annular
downtake

Basket type
Steam heating element
condensate
Thick liquor outlet

Salt filters
where required

FIGURE 6.7
Basket-type evaporator.
From Richardson, J.F., Harker, J.H., & Backhurst, J.R. Particle technology & separation processes (5th ed.). Coulson and
Richardson’s chemical engineering (Vol. 2). Butterworth-Heinemann.

Forced circulation evaporators ensure high heat transfer rates even with increasing viscosity of the
liquid, thus enabling the formation of a more concentrated product.
Since pumping costs increase roughly as the cube of velocity, the
added cost of operation may make it uneconomical. Nevertheless,
Advantages and Limitations
many forced circulation evaporators operate with a liquor velocity
of 2e5 m/s through the tubes, which is significantly higher than
that obtained in natural circulation. These units can be made
smaller and cheaper than those relying on natural circulation and are preferred where stainless steel or
expensive alloys, such as Monel are used. Forced circulation evaporators are suitable for the widest
variety of evaporator applications. The main applications are in (A) crystallizing evaporators,
(B) concentration of solutions where solubility decreases with temperature and (C) for thermally
degradable materials, which result in a solid deposition. It is suitable for concentrating meat extracts,
salt, caustic soda, alum, and other crystallizing materials and also with glues, alcohols, and foaming
materials. In all cases, recirculation ratio as high as 100 to 150 kg of liquor per kilogram of water