The evaporators which are used to evaporate the volatile and non volatile materials are called as falling film evaporators.
In the falling film evaporator the liquid film falls by gravity in the vertical evaporation surface that is inside the tube or plate, while the heat is transferred through the walls by the condensing steam.
The mixture of the liquid vapor from the bottom of the tube or plates enters a vapor liquid separators
From which the concentrated liquid is pumped out and water vapors are directed in to the condenser.
In a falling film evaporator feed enters from the top and flows the walls of the tubes. The liquid gets heated rapidly due to heat transfer steam. The liquid boils and becomes vapor which forms the small bubbles. They tends to fuse to form layers of bubbles which travels down the tubes. Concentration takes places during this down wards journey. Vapor and liquids are separated at the bottom.
The construction of the falling film evaporator is similar to that of the climbing film evaporator, but it is inverted. In this evaporator the heating unit consists of steam jacketed tubes. The feed inlet is from the top of the steam compartment. The other provision are steam inlet, vent and condensate outlet remains same. The outlet for the product is provided at the bottom and is connected to a cyclone separator.
Steam is supplied into the steam compartment. Feed enters the top of the tube. The temperature of the boiling liquid is same as that of the vapor head.
The feed flows down the walls of the tubes. The liquid gets heated rapidly. The liquid boils and becomes vapor which forms smaller bubbles.
These tends to fuse to form layers of bubbles, which travels down the tubes. Concentration takes place during this down wards journey.
Vapor and liquids are separated in the cyclone separator.
- Falling film evaporator is used to separate volatile and non volatile materials when the feed is low viscosity.
- It is used for the concentration of yeast extract, manufacturing of gelatin, extracts of tea and coffee.
- It is also useful for concentrating the heat sensitive materials such as fruits juices.
- Falling film evaporator is suitable for high viscous liquids because the flow of vapor film assisted by gravity.
- The liquid is not over heated during passage and heat transfer coefficents are high even at low boiling temperature.
- Highly acidic and corrosive feeds can be concentrated using impervious graphite tubes rubber lined vapor heads.
- Easy distribution of the feed to the individual tubes may be accomplished using the perforated plate above the tubes or using spray nozzles. Hence it is not suitable for the suspensions as the solids clog the perforated plate.
- It is not suitable for salting or scaling liquids. The feed distribution in the tubes is poor. For continuous supply, the liquid may be re circulated or the ratio of feed to evaporation is kept high.
- k. sambamurthy, pharmaceutical engineering, 1999, ISBN: 81-224-1169-X.
- Wareem c.mccabe, unit operations of chemical engineering, 2005.