Multiple effect evaporator a type of vertical short tube evaporator, is a single effect evaporator. This evaporator air conditioning can be connected in many way for achieving large scale evaporation. Along with greater economy. Although multiple effect evaporator is not using in pharmaceutical industries, but its principle should be understood which has been explained using an example of triple effect evaporator with a parallel feed mechanism.
Principle
In a multiple effect evaporator, a number of vessels are arranged in a sequence each held at a lower pressure than the last. Water is boiled in this vessels. Since the boiling point of water decreases on decreasing pressure, the vapour obtained after efficiently using the heat boiling of water in one vessels is used for heating the water in the next vessel. This indicates that only the first vessel which is at the higher pressure uses an external heat source for boiling the water in it. Theoretically, evaporators may be built with an arbitrarily large number of stages; evaporators with more than four stages are rarely expansion valve used practically except in systems where the liquor is the desired products, such as in chemical recovery systems where up to seven effect are used.
Construction
Triple effect evaporator contains three evaporators. The other constructive features are similar to vertical tube evaporator. In the evaporator, vapour escaping from evaporator I is used as a heating source for evaporator II, while that for evaporator II is the absorbing heat medium for evaporator III. A vacuum pump is connected to the last evaporator.
Working
The evaporator utilizes parallel feed arrangement in which hot saturated solution of the feed is introduced to each of the three evaporators directly, instead of transferring the material from one evaporator to another. The feed is introduced to each evaporator up to the level of the upper tube sheets. This arrangement is used for concentrating Salt solution in which the solute crystallizes on concentration but the viscosity remains unaffected heat exchanger.
The steps involved for achieving the effects are specified below:
1) Only the vents (v1, v2, and v3) are kept open and all other valves are closed.
2) A high vacuum is created in the liquid chambers of the evaporators.
3) The steam and condensate valves (s1 and c1 , respectively) are opened, and steam economy is supplied which replaces the cold air in the steam compartment of evaporator I, After the complete removal of cold air, the valve v1 is closed.
4) steam to evaporate water is continuously supplied till desired pressure (p0) is created in the steam space of evaporator I. At this pressure air conditioner the temperature of the steam is t0.
5) The liquid feed in evaporator I consumes large volumes heat from the steam and undergoes condensation. The resultant of liquid refrigerant condensate exits through the valve c1.
6) Due to heat transfer, the boiling temperature of liquid increases evaporation rates up to the boiling point and it vaporises.
7) The vapour obtained displaces the air in the upper part of evaporator I and also the air in steam compartment of evaporator II.
8) When all the air in evaporator II has been displaced by vapour, valve v2 is closed.
9) The vapour of evaporator I transfer its heat to the liquid of evaporator II which refrigeration system condenses and the condensate is removed through the valve C2
10) The same steps are repeated in evaporator III.
Uses
Multiple effect evaporator has the following uses:
1) It is used to concentrate sugar in sugar cane juice.
2) It is also used for production of the salt and desalination of water.
3) It is used sensible heat in the condensate to preheat liquid to be flashed.
Merits
Multiple effect evaporator has the following merits:
1) It is suitable for large scale and continuous operation.
2) It is highly economical in comparision to a single effect.
3) About 5 evaporators evaporator coil can be attached in this type of set-up
Demerits
Multiple effect evaporator has the following demerits:
1) It is ability to produce and condense vapours in the succeeding sections when compared to the vapours produced in the previous section is limited.
2) The vapours produced in the first section of the evaporators apparatus for efficiently
may get contaminated with inert gases.
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Evaporation for steam jacketed kettle notes
Evaporation for Horizontal tube notes
Evaporators are classified as flooded type and dry type depending upon whether liquid refrigerant covers all heat transfer surfaces or some portion is having gas vapour being superheated.
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