EVAPORATORS
The process of evaporation is used widely in the chemical and process industry, and for a variety of purposes. These include the concentration of solutions (often as a precursor to the crystallization of the solute), re-vaporization of liquefied gases, refrigeration applications (cooling or chilling), and generation of pure and mixed vapors for process applications. The term evaporators are usually reserved vapors for the first of these applications, namely the evaporation of the solvent from a solution in order to concentrate the solution. Evaporators may be classified into falling film evaporators (in which evaporation takes place from the film interface with no nucleate boiling at the wall), nucleate boiling evaporators (in which wall nucleate boiling occurs over part or all of the heat transfer surface), flash evaporators and direct contact evaporators.
Falling Film Evaporators:
The falling film evaporator is a type of evaporator which is used as a heat exchanger. It consists of a vertical shell and a tube heat exchanger. The concentrated liquid is fed at the top of the tube and then flows down to the surface tube as a thin film. As it gets heated further, the vapors get generated. It increases the heat transfer and the vapor and liquid get separated further.
Rising Film, LTV (Long Tube Vertical) Evaporators:-
Nucleate Boiling Evaporators:
In evaporators with nucleate boiling, vapor generation is initiated by wall nucleation (though in much of the evaporator, nucleation may be suppressed). Such evaporators are really a subclass of boiling systems in general, and the heat transfer processes are those described in the article on Forced Conventional Boiling. The types of evaporators in this category include:
Climbing Film Evaporator. Here, there is a short region of nucleate boiling near the evaporator entrance but the main part of the evaporator has climbing film flow in the tubes with no nucleate boiling.
Short-tube Vertical Evaporator. Here, relatively short tubes may be used with an internal circulation device to aid circulation and minimize fouling.
Basket-type Evaporator. This is a variant of the short-tube type in which the whole tube bank may be easily removed for cleaning.
Long-tube Vertical Evaporator. In this form, there is no free surface in the evaporator (in contrast to the short-tube type the tubes are much longer and separation occurs in a separate vessel.
Plate Evaporators. Here, evaporation occurs in channels separated by plates of one form or another. The plate heat exchanger can be adapted to act as an evaporator and this also applies to the Plate Fin Heat Exchanger.
. Evaporators with spiral plates are also employed.
Horizontal Tube Shell-Side Evaporator. This is one of a number of types in which evaporation takes place outside surfaces in crossflow, with the heating on the inside of the tubes.
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Short Tube Vertical Evaporator
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Flash Evaporation:
In flash evaporation, the liquid is preheated at pressure and then flashed through a restriction into a vessel at lower pressure where the vapor is formed in the process of restoring the liquid to its saturation temperature. Perhaps the best-known case of flash evaporation is that used in seawater desalination plants, where saltwater is evaporated to produce distilled water in a series of stages at different temperatures as illustrated schematically
Forced Circulation Evaporators:-
A forced circulation evaporator is a type of evaporator which is used to separate mixture which is not possible by a normal evaporating unit. In a normal circular evaporator, the velocity of liquid flow is 0.3 to 1m/s but a forced circular evaporator increases the velocity of liquid flow to 2 to 6m/s. It contains a circulation pump and because of this, it is used for both heat exchanger and flash separation. It separates the mixture without using normal conventional boiling. It consists of a vertical shell and heat exchanger tube with a centrifugal pump.
Plate Evaporators:-
Plane evaporators consist of a plate and frame heat exchanger and are mainly used in the food and beverage industry. A plate evaporator is a type of evaporator in which a thin film of liquid is passed and flows between the plates for the process of evaporation. It is typically used for fruit juices, fruit puree, syrup, coffee, milk
Thermal Vapor Recompression(TVR):-
Thermal vapor recompression evaporator is a type of evaporator which is used as a thermal compressor to reduce the steam required for the process of evaporation. The vapor stream is compressed to a higher temperature and pressure by a steam jet pump.
Mechanical Vapor Recompression (MVR):-
Mechanical vapor recompression is a type of evaporator in which the water vapor is compressed which results in rising in temperature and pressure. The rise in temperature makes the temperature difference between the vapor and the fluid. The further heat transfer results in the heat exchanger. The compressed vapor is again fed back to generate more steam. This is an energy-efficient and energy recovery process.
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