Falling film evaporator

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A falling film triple effect evaporator for caustic soda

A falling film evaporator is a industrial device to concentrate solutions, especially with heat sensitive components. The evaporator is a special type of heat exchanger, Evaporator.

In general evaporation takes place inside tubes. But there are also applications were the process fluid is located on the outside tube. The process fluid to be evaporated flows as a continuous film downwards controlled by gravity. The fluid will create a film along the inner tube walls, progressing downwards (falling) - hence the name. The Fluid distributor has to be designed carefully in order to maintain an even liquid distribution for all vertical in parallel alligned tubes through which the solution falls.

In the majority of applications the heating medium is placed on the outside of the tubes. High heat transfer coefficients are required in order to achieve equally balance heat transfer resistances. Therefore most commonly condensing steam is used as a heating medium.

Separation between the liquid phase (the solution) and the gaseous phase takes place in the tubes Increasing vapour velocities downwards to the bottom of the exchanger increases the shear force acting on the liquid film and therefore also the velocity of the solution. The result can be a high film velocity and a progressively thinner film resulting in increasing turbulent flow. The combination of these effects allows very high heat transfer coefficients.

The tube side heat transfer coefficient is determined by the hydrodynamic flow conditions of the film. For low massflows or high viscosities the film can be found of laminar type. Heat transfer is controlled purely by conduction through the film. Therefore in this condition heat transfer decreases with increased mass flow. With increased mass flow the film becomes wavy laminar and with further increased flow turbulent. Under this conditions the heat transfer increases with increased flow. Evaporation takes place at very low mean temperature differences between heating medium and process stream. Typically between 3 - 6K, therefore this devices are ideal for heat recovery in multi stage processes.(1),(2) A further advantage of the falling film evaporator is the very short residence time of the liquid and the absence of superheating of the same. Not considering the vapour separator, the residence time inside the tubes is measured in seconds, making it ideal for heat-sensitive product such as milk, fruit juice, pharmaceuticals and many others. Falling Film Evaporators are also characterised by very low pressure drops. Therefore they are often used in deep vacuum applications.