Forced circulation evaporator are used if boiling of the product on the heating surfaces is to be avoided due to the fouling characteristics of the product, or to avoid crystallization on the heat transfer surface. The flow velocity in the tubes must be high, and high-capacity pumps are required.
The circulating product is heated when it flows through the heat exchanger and then partially evaporated when the pressure is reduced in the vapour separator. The liquid product is typically heated only a few degrees for each pass through the heat exchanger. To maintain good heat transfer within the heat exchanger it is necessary to have a high recirculation-flow rate.
Forced Circulation Evaporator are recommended for viscous and slurry type liquids. As the liquid is uniformly heated in the steam jacket with flashing taking place in the separator and no boiling taking place within the tubes, fouling on hot tube walls is reduced.