SMS combines more than 50 years of experience in manufacturing and application of wiped film evaporators of the companies Luwa, Samesreuther and Canzler. Wiped film evaporators are available in an unchallanged variety of different rotor designs, horizontal, vertical orientation as well as cylindrical and conical shape for direct current and counter-current vapor flow. Therefore they are perfectly adapted to a multitude of separation tasks.
Wiped Film Evaporator
Special characteristics of wiped film evaporators compared to other evaporator types are:
- the reliable continuous processing of viscous, fouling and soiled liquids
- gentle treatment due to short residence time and narrow residence time distribution
- the evaporation performance and
- the low hold-up with minimal losses in case of product changes
in a wide range of pressures, temperatures, viscosities and evaporation performances. These advantages expound why thin film evaporators are the best choice for a large range of applications, especially for the treatment of mixtures that are difficult to handle.
For the thermal separation of a mixture a thin film is produced at the heated wall. A distribution ring on the rotor distributes the liquid evenly across the periphery. Then, the blades fitted at the rotor spread the liquid as a thin film of min. 0.5 mm over the heat transfer surface.
The model concept for flow in the wiped film evaporator assumes that prior to each rotor blade a bow wave is formed. In the gap between the rotor blade and the heating surface, fluid is supplied from the bow wave of a highly turbulent area with intense heat and mass transport. This results in a good heat transfer performance even with viscous products. In addition, the formation of deposits is avoided and the intensive mixing also protects temperature-sensitive products from overheating.
Another important task of the rotor is to stabilize the liquid film on the heating surface at high evaporation rates. On the one hand evaporation in the area of nucleate boiling is possible without ruptures of the film. On the other hand the liquid film is pressed against the heating surface by the centrifugal force. This avoids the adverse evaporation mode, in which a vapour layer with insulating effect is formed under the liquid film. Therefore, due to the functional principle extremely high specific evaporation rates are achievable in wiped film evaporators.
Types of wiped film evaporators
Wiped film evaporators are designed in vertical orientation with cylindrical heating jackets. The product which has to be processed is fed at the upper end of the apparatus. Depending on the application the volatile components evaporate and are taken out at the upper or the lower end of the evaporator. Entrained droplets or foam are removed from the vapour before the vapour is fed to further process steps.
A typical wiped film evaporator design is the cylindrical body design DVB („SAMBAY“). In the wiped film evaporators type DVB, rotors are equipped with hinged wiper blades that can be optimally adapted to the task. The centrifugal force presses the wiper blades to the heating surface, so that high evaporation ratios can be achieved at low flow rates. The hinged wiper blade types of evaporators are particularly suitable for products with fouling tendency.
For other applications such as the separation of monomers from pre-polymers and polymers, vertical thin film evaporators type DVF with spring wiper elements and DVS with Smith-Rotor are available. The specially designed wiper elements out of PTFE or graphite are partially spring loaded and support the liquid in the film.