In a climbing film evaporator, tubes are heated externally by steam. The preheated feed enters from the bottom and flows up through the heated tubes. The liquid gets heated rapidly due to the enhanced overall coefficient of the preheated feed.) The liquid near the wall becomes vapor and forms small bubbles. These tend to fuse to larger bubbles, which travel up in the tubes along with entrapped slug. The liquid films are blown up from the top of the tubes and a strikes entrainment separator (deflector) kept above. This throws the liquid concentrate down into the lower part from where it is withdrawn.
Construction of Climbing Film Evaporator:
The construction of a climbing film evaporator is shown in Figure. In this evaporator, the heating unit consists of steam jacketed tubes. Here, the tubes (long and narrow) are held between two plates. An entrainment separator is placed at the top of the vapor head. The evaporator carries a steam inlet, vent outlet, and condensate outlet. The feed inlet is from the bottom of the steam compartment.
Working of Climbing Film Evaporator :
The preheated liquid feed (to be evaporated) is introduced from the bottom of the unit. The height of the liquid column is maintained low, i.e., 0.6 or 1.2 metres above the bottom tube sheet. Steam enters into the spaces outside the tubes through the inlet. Heat is transferred to the liquor through the walls of the tubes. The liquid becomes vapour and forms smaller bubbles, which tend to fuse to larger bubbles. These are of the width of the tubes, thereby the bubbles trap a part of the liquid (slug) on its way up in the tubes. As more vapour is formed, the slug of liquid is blown up in the tubes facilitating the liquid to spread like a film over the walls. This film of liquid continues to vaporise rapidly. Finally, the mixture of liquid concentrate and vapour eject at a high velocity from the top of the tubes.
The entrainment separator not only prevents entrainment but also acts as a foam breaker. The vapor leaves from the top, while the concentrate is collected from the bottom.
Using a climbing film evaporator, thermolabile substances such as insulin, liver extracts, and vitamins can be concentrated. Clear liquids. foaming liquids and corrosive solutions in large quantities can be operated. Deposit of scales can be removed quickly by increasing the feed rate or reducing the steam rate so that the product is unsaturated for a short time.
- In a climbing film evaporator, a large area for heat transfer is provided employing long and narrow tubes.
- Since liquid flows at a high velocity, the resistance for heat transfer at the boundary layers is reduced. As a result, the heat transfer is enhanced.
- The time of contact between the liquor and the heating surface is very short. The liquid is in the heater for one second, while its residence time is 20 seconds in the evaporator. Hence it is suitable for heat-sensitive materials.
- Unlike a short tube evaporator, the tubes are not submerged. So there is no elevation of the boiling point due to the hydrostatic head.
- It is suitable for foam-forming liquids because foam can be broken by an entrainment separator.
- It requires a low hold-up and small floor space.
- Climbing film evaporator is expensive, construction is quite complicated.
- It is difficult to clean and maintain.
- Large headspace is required.
- It is not advisable for very viscous liquids, salting liquids, and scaling liquids.
- If the feed rate is high, the liquor may be concentrated insufficiently. If the feed rate is low, a film cannot be maintained. Dry patches may form on the tube walls.
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