Evaporators

An evaporator is a device that turns the liquid form of a chemical into gaseous form. This liquid gets vaporized in this process. In evaporation technology it requires to maintain the quality of the liquid in the process of evaporation in order to avoid the damage to the product.

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This may require lowest possible time of liquid exposure to the least possible boiling point. There are many other requirements that are necessary through many limitations that result in variety of designs that are available today. In every evaporator the heating medium is steam that is used to heat the product on the other side of the surface in the process of heat transfer.

Types:

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  1. Falling film evaporators:

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In a falling film evaporator a product of liquid enters the evaporator. This input is eventually distributed in the heating tubes. Now a thin film enters the heating tube which flows   in the downward direction and gets evaporated partially. In most of the cases this steam is used to heat up the evaporator. All this products flow in the downward direction due to the gravity induction that gets increased by a co-current flow of vapour. The separation of the product from the vapour is undergone in the heat exchanger and separator.

These evaporators can be operated with low temperature differences in between the heating media and boiling media . These have short period of contact times. These are best suitable for the heat sensitive products and it is the most frequent type used now a days. These are to be designed carefully under required operating conditions.

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2.  Rising film evaporators:

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These rising film evaporators operate on “thermo-siphon” principle. When a feed product is entered from the bottom of the tubes that are heating, stem starts to form. The increasing force of the steam that is produced while boiling will cause the upward parallel flow of liquid and vapour. The vapour production increases and the thin film product is pressed to the tubes which raises the liquid level. This co-current movement against gravity will benefit the high degree of liquid turbulence. This provides an advantage in the evaporation of high viscous liquids that generally have fouling effect on the heating surface. The temperature difference between the surfaces of heating and boiling must be high. With out this the vapour energy flow will  not be adequate to convey the fluid in the production of rising film.

3.Forced circulation evaporators:

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Forced circulation evaporators are used when the boiling is reached on the heating surfaces to avoid fouling effect or crystallization. High capacity pumps are required and the velocity of the tubes are high. The main circulating product is heated when the heat exchangers flow gets partially evaporated this is observed when the flash separator reduces its pressure. High circulation flow rate is required for the maintenance of good heat transfer. This type of evaporators are mostly used in crystallization applications where there is no concentration increment, and this takes place on a heat transfer surface. The heat exchanger can be arranged either horizontally or vertically depending on the requirement.

4.  Natural circulation evaporators:

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Natural circulations of evaporators are based on the density difference that is raised while heating. In the process of tubes usage the water begins to boil and the bubbles raise which in-turn cause circulation along with separation of the liquid and vapour at the top of the heating tubes. This evaporation amount depends on the difference of temperature between the steam and solution. If the solution is not well immersed then several problems may arise. In order to avoid this problem a pump is inserted to increase the pressure as well as circulation in it.

5.  Plate evaporators:

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In this type of evaporators framed plates are used as the surfaces of heating instead of tube and shell heat exchangers. These assemblies of plate are very much similar to plate heat exchangers which are equipped with large vapour flow passages. In this arrangement the product plate and steam plate are connected alternatively. The product passage is designed with even (systematic) distribution on the plate surfaces along with a vapour phase in the low pressure drop. These are compact in their design that are flagged directly to the plate with inter connections in the pipe work. The space requirement is very low for the equipment arrangement.      

6.  Compact evaporators:

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These evaporators operate with a simple falling film principle. Here the fluid is pumped to the top of caldaria with the help of liquid distributor and in-turn flows down by the side of each tube in the form of liquid film. Hot vapours condensate on the outer tube walls that releases heat and gets transferred to the fluid. Boiling takes place in the falling vertical tubes at turbulent conditions. Fluid may be concentrated in one or more series that flows parallel to or in counter flow with a vapour flow. All the vapours are removed from the shells of caldaria bottom in the form of liquid condensate. Waste heat gets absorbed in the surface condenser. The design functionalities are identical and time based, that are accepted in long tube falling film evaporators.

Applications:

  • These are mainly used in the product concentration applications.
  • Applied with dryer provide pre concentration.
  • Volume reduction, water or solvent recovery applications.
  • Mostly used in crystallization applications (forced circulation evaporators).

click here to know more about: Centrifugal pumps

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