Forced circulation evaporators 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. 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 flash vessel (separator). The liquid product is typically heated only a few degrees for each pass through the heat exchanger. To maintain a good heat transfer within the heat exchanger it is necessary to have a high recirculation flow rate.
This type of evaporator is also used in crystallizing applications because no evaporation, and therefore no concentration increase, takes place on the heat transfer surface. Evaporation occurs as the liquid is flash evaporated in the flash vessel/separator. In crystallizer applications this is then where the crystals form, and special separator designs are used to separate crystals from the recirculated crystal slurry. More information about crystallization is available in the crystallization section.
The heat exchanger (in evaporator parlance sometimes called the "calandria") can be arranged either horizontally or vertically depending on the specific requirements in each case.