A forced evaporator is a industrial device to concentrate solutions, especially with heat sensitive components.
In forced evaporators the liquid product usually enters the evaporator at the head of the evaporator. The product is evenly distributed into the heating tubes. A thin film enters the heating tube and it flows downwards at boiling temperature and is partially evaporated. In most cases steam is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product from its vapor is undergoing in the lower part of the heat exchanger and the separator.
Forced evaporators can be operated with very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These characteristics make the forced evaporator particularly suitable for heat-sensitive products, and it is today the most frequently used type of evaporator.
However, forced evaporators must be designed very carefully for each operating condition; sufficient wetting (product film thickness) of the heating surface by liquid is extremely important for trouble-free operation of the plant. If the heating surfaces are not wetted sufficiently, dry patches and incrustations will occur; at worst, the heating tubes will be completely clogged. In critical cases the wetting rate can be increased by extending or dividing the evaporator effects, keeping the advantages of single pass (no recirculation of product) operation.
The proper design of the product distribution system in the head of the evaporator is critical to achieve full and even product wetting of the tubes.
Because of the low liquid holding volume in this type of unit, the forced evaporator can be started up quickly and changed to cleaning mode or another product easily.
Forced evaporators are highly responsive to alterations of parameters such as energy supply, vacuum, feed rate, concentrations, etc. When equipped with a well designed automatic control system they can produce a very consistent concentrated product.
The fact that forced evaporators can be operated with small temperature differences makes it possible to use them in multiple effect configurations or with mechanical vapor compression systems in modern plants with very low energy consumption.
Features of SAF Engineers:
- They result in vapor with very little entrained liquid
- They provide high rates of heat transfer
- They require lower liquid circulation rates (smaller pumps)
- They are suitable for operation at low temperature differences.
The process fluid to be vaporized is feed to the evaporator at the top of the tube sheet. A suitable distribution unit is necessary in order to achieve an even liquid distribution. It is paramount that for this process minimal wetting rate is achieved.
Forced evaporators are used extensively in chemical process industry, food and paper industry. Due to the absence of static head effect caused by liquid column as in other types of evaporators, evaporation can take place at very small effective mean temperature differences. The temperature difference are typically between 3 – 8oC. This is significantly less than in other devices used for evaporation, e.g. forced reboilers or kettle evaporators, here the effective mean temperature difference is between 15
and 300C. The film heat transfer coefficients are in general high, and characterised by surface boiling.
The absence of hydrostatic head allows this type of evaporator to operate at very low absolute pressures.
Product residence time can be very short, especially in one through operation. These characteristic of short retention time low operation pressure and small required effective mean temperature differences makes this type of evaporator particularly suitable for concentration of heat sensitive liquids. The absence of nucleate boiling under normal operation conditions, and low temperature differences also reduces possible fouling tendencies.