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Evaporator by om


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Evaporator by om

  2. 2. Evaporation: -Factors affecting evaporation -Evaporators -Film evaporators -Single effect and multiple effect evaporators -Mathematical problems on evaporation Principal reference: Chapter 8 in C.J. Geankoplis, Transport Processes and Unit Operations, 3rd Edition, Prentice-Hall of India Content of Lectures
  3. 3. Evaporation:- Evaporation is an important unit operation commonly employed to remove water from dilute liquid foods to obtain concentrated liquid products. Evaporator :- an evaporator consists of a heat exchanger enclosed in a large chamber; a non contact heat exchanger provides the means to transfer heat from low-pressure steam to the product. Methods of Evaporation:- Single effect evaporator Multiple (triple) -effect evaporator
  4. 4. Single effect evaporator:-
  5. 5. Multiple (triple) -effect evaporator
  6. 6. Types of evaporators Batch-Type Pan Evaporator Natural Circulation Evaporators Forced-Circulation Evaporator Plate evaporators Rising-Film Evaporator Falling-Film Evaporator Rising/Falling-Film Evaporator Horizontal-tube evaporator Vertical-type short-tube evaporator
  7. 7. Batch-Type Pan Evaporator :- The product is heated in a steam jacketed spherical vessel. The heating vessel may be open to the atmosphere or connected to a condenser and vacuum.
  8. 8. Natural Circulation Evaporators :- In natural circulation evaporators, short vertical tubes, typically 1- 2 m long and 50- 100 mm in diameter, are arranged inside the steam chest. The whole calandria (tubes and steam chest) is located in the bottom of the vessel.
  9. 9. Forced-Circulation Evaporator The forced-circulation evaporator involves a non- contact heat exchanger where liquid food is circulated at high rates. Axial flow pumps are generally used to maintain high circulation rates with linear velocities of 2- 6 m/s, compared with a linear velocity of 0.3-1 m/s in natural-circulation evaporators
  10. 10. Plate evaporators Plate evaporators use the principles of rising falling-film, falling-film, wiped-film, and forced-circulation evaporators.
  11. 11. Rising-Film Evaporator In a rising-film evaporator a low-viscosity liquid food is allowed to boil inside 10-15 m long vertical tubes. A temperature differential of at least 14°C between the product and the heating medium is necessary to obtain a well developed film.
  12. 12. Falling-Film Evaporator In contrast to the rising-film evaporator, the falling-film evaporator has a thin liquid film moving downward under gravity on the inside of the vertical tubes. Typical residence time in a falling- film evaporator is 20-30 seconds, compared with a residence time of 3-4 minutes in a rising-film evaporator.
  13. 13. Rising/Falling-Film Evaporator In the rising/falling-film evaporator the product is concentrated by circulation through a rising-film section followed by a falling-film section of the evaporator.
  14. 14. Types of evaporators Horizontal-tube evaporator: Dilute feed Condensate Concentrated product Vapour Steam inlet
  15. 15. Horizontal-tube evaporator: - relatively cheap  used for non-viscous liquids having high heat- transfer coefficients and liquids that do not deposit scales  poor liquid circulation (and therefore unsuitable for viscous liquids)
  16. 16. Types of evaporators Vertical-type short- tube evaporator:  - Liquid is inside the tubes  - Steam condenses outside the tubes - used for non-viscous liquids having high heat- transfer coefficients and liquids that do not deposit scales
  17. 17. Types of evaporators Vertical-type short-tube evaporator: Dilute feed Condensate Concentrated product Vapour Steam inlet