Falling film evaporators are equipment used extensively in sugar production for concentration of juice. Compared to other types of evaporator technologies they offer a number of operational advantages including increased heating surface area, effective heat transfer over wide temperature ranges, and reduced consumption of steam.
Falling Film Evaporator Operation
A falling film evaporator is, in effect, a vertical shell and tube heat exchanger. Steam is used as the heating medium on the shell side of the vessel, which is often called a calandria. Sugar juice is introduced to the top of the calandria from where it is distributed to the tubes. The juice flow rate is controlled so that it runs down the inside of the tube walls as a thin film and is heated by steam. The liquid starts to boil as it travels downward. The vapor formed flows in parallel to the liquid, enhancing its flow rate as both streams make their way to the bottom of the vessel.
Concentrated juice is pumped from the bottom of the vessel and transferred to the next stage of the process. The vapor flows into a cyclonic separator attached to the calandria where the juice droplets are separated. Separated juice is then sent back to the top of the evaporator for recycling.
The Importance of Wetting Rate
The juice distribution system in the head of the evaporator must be designed to allow for complete wetting of the tubes (from top to bottom). This is critical, as any dry areas will increase the likelihood of encrustation, which can result in the build-up of deposits and a marked reduction in evaporator efficiency. To ensure adequate wetting rates, large falling film evaporators can be divided into several compartments. Recycling of the juice also ensures sufficient wetting of the tubes.
Over time, scale and deposits that build up on the inside of the tubes will reduce heat transfer. Most falling film evaporators have a cleaning-in-place (CIP) system installed. Once emptied, a chemical solution can be circulated through the heated vessel to remove these deposits and ensure efficient operation.