The evaporators in which the movement of the liquid takes places as a result of convection current set up by the heating process are called vertical tube evaporator or short tube evaporator.
Vertical tube evaporator or short tube evaporator comes under the classification of the natural circulation evaporators.
It consists of a number of tubes. About 1000 tubes are fitted in a vessel. The tubes are fitted with liquids and surrounded by steam. The level of the liquids is maintained slightly above the top if the tubes so that the space which is left in the evaporators can be used for the purpose of separation of vapor from the boiling liquids.
The liquid in the tube is heated by steam and it begins to boil. The mixture of the liquid and vapor will come out of the tube sets up a circulation with boiling liquid rising up in the smaller tubes of calandria and returning down the large central down take.
In standard vertical tube evaporator or short tube evaporator liquid passed through the vertical tubes and the steam is supplied from the out side the tubes.
Heat transfer takes place through the tubes and the liquid in side the tubes gets heated. The solvents evaporates and the vapor escapes from the top. The concentrated liquid is collected from the bottom.
- The construction of a standard Vertical tube evaporator or short tube evaporator consists of a large cylindrical body made up the cast iron with dome shaped top and bottom.
- In side the body calandria is fitted at the bottom. Calandria consists of a number of vertical tubes whose diameter ranges from the 0.05 to 0.075 m and length of 1-2 m.
- About 100 such tubes are fitted in a body measuring 2.5 m or more diameter. Inlets are provided for the steam and fed.
- Out lets are provided for vapor concentrated products, non condensed gases and condensate.
- Steam is introduced out side the tubes. The condensate is passed through the corresponding out let and non condensed gases escape through the vent.
- The feed is introduced in such a way as to maintain the liquids level slightly above the top of the tubes. The liquids in side the tubes is heated by the steam and begins to boils.
- As the liquid boils it spouts up through the tubes and returns through the central down take. It set up a circulation of hot liquid which enhances the rate of heat transfer.
- The vapor escapes through the top outlets. Steam is supplied until required concentration of the products is obtained. Finally the products can be withdrawn from the bottom out let.
- Vertical tube evaporator or short tube evaporator is used in the manufacture of the cascara extract.
- Vertical tube evaporator or short tube evaporator is used in the manufacture of salts and caustic soda.
- Vertical tube evaporator or short tube evaporator is used in the manufacture of salts.
- In Vertical tube evaporator or short tube evaporator tubes increases the heating surface nearly 10 to 15 times when compared with steam jacketed kettle.
- Vigorous circulation enhances the rate of heat transfer.
- It can be connected to a condenser and receiver, which further increases rate of evaporation. Such attachment is also suitable for the volatile solvents.
- A number of units can be joined to obtained more efficient effect.
- The liquid is maintained above the level of the calandria. Hence the upper layers of the liquid need a long time for getting heated. This problem can be minimized by removing concentrated liquid slowly at the bottom.
- The vertical tube evaporator or short tube evaporator is complicated hence installation cost increases.
- Cleaning and maintenance is difficult when compared with steam jacketed kettle.
- During operation the pressure in side the evaporator increases. In large evaporators where the liquid depth may be the order of 2 m the pressure increases to 25 kp, leading to elevation of the boiling point by 5 or 6 @c. this reduces the effective temperature gradients and may affects heat sensitive materials.
Vertical tube evaporator or short tube evaporator with propeller
In short vertical tube evaporator or short tube evaporator circulation depends completely on boiling . steam is supplied in to the calandria to induced boiling. When steam supply is stopped automatically boiling stops. As a result the particles settle down. These particles acts as nuclei which grow as crystals.
Therefore this evaporators is sometimes used as a crystallizing evaporators. If the crystallization is un desirable, the problem can be avoided by installing a propeller in the central portion close to the bottom. By increasing the revolving per min of the propeller the capacity of the evaporator can be doubled.
Since mild steel or cast iron is used, the evaporators suits well for clear liquids and crystallizing solutions. Non corrosive liquids and mild scaling solutions can be handled.
Heat transfer coefficients are high due to high temperature gradients values. It requires low head room. Cleaning and maintenance is easy. It is relatively in expensive.
Heat transfer coefficients are low due to low temperature gradients. It requires high floor space and is more in weight. Relatively more liquid is retained. Rate of heat transfer further decreases due to high viscosity liquids. Since body is large mild steel or cast iron are used forits construction to make it less expensive.
Basket type evaporators
The basket type vertical tube evaporator or short tube evaporator has a conical bottom and sometimes a flat bottom. In this type boiling is quite violent. Hence the spouting of the liquids leads to entrainment.
This problem can be avoided by placing a deflectors over the tubes. In this case the down take is annular in stead of being central. The advantages of this type is that entire heating elements is a single unit. The complete unit can be removed for repairs. In addition the deflectors prevents entrainment losses completely.
- Mehta RM, Pharmaceutics-I: heat processes - evaporation (1996), Page no: 165 - 166.
- C.v.s subrahmanyam et al., pharmaceutical engineering principles and practices - evaporation, 2001, page no. 345 - 347.
- K. Sambamurthy, pharmaceutical engineering, ISBN: 81-224-1169, 1999.
- Wareen C. mccabe, unit operation of chemical engineering, 7th edition, 2005, ISBN: 007-124710-6, 2005.