Page 167 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.3 Evaporator types 165

Heating may be either direct or indirect. Direct heating is provided by solar evaporation (pro-
duction of Glauber’s salt) and submerged combustion of a fuel. The advantages of this technique are
the ability to handle corrosive liquors, a large amount of heat release per unit volume (typically
3
70 MW/m ) in submerged combustion and the almost instantaneous transmission of heat to the liquid.
Generally, the most widely used mode of indirect heating is by condensing steam or vapor on the heat
transfer surface.
Evaporators can be batch or continuous. Batch evaporators, also called batch pans are the simplest
and one of the oldest designs. It consists of a steam jacketed vessel heated
with vapor or liquid heating medium as shown in Fig. 6.5. The batch is
heated to its boiling point, and the vapors are removed until the desired
Batch Evaporator
concentration is reached. The concentrate is drained or pumped out of the
tank through a nozzle. These are particularly suitable for products that are
clean, viscous, and not heat-sensitive, and are used for small-scale
applications like concentrating jam, jelly, syrup, and some pharmaceutical products. Its limitations
are (A) long residence time, (B) low heat transfer coefficient, (C) low heat transfer area per unit
volume occupied by the feed, (D) high tendency of fouling (as the product movement is by natural
circulation only) and (E) high boiling point of the product at the tank bottom (due to the liquid head).
Some designs incorporate agitator to improve the heat-transfer coefficient. Batch pans can be operated
under vacuum to reduce the boiling point.

Condenser
Water

Evaporator

Steam
to
jacket
To drain

Steam trap
for condensate
Product
FIGURE 6.5
Batch Pan evaporator.