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The evaporation duty is separated into stages operating at different temperatures. External heat drives the first effect of the evaporator, with subsequent effects being driven by vapor generated from the previous higher temperature effect. Product may be passed through the evaporator in forward, back or mixed flow configurations. Additional efficiency is achieved with the use of regenerative heaters, condensate heaters and vapor heaters. In general a Multi-Stage Flash Evaporator is a series of single stage evaporators, where the steam is used for direct heating in one effect only. And the feed flow in a linear way passes through all the single stage evaporators. The basic fundamental of the multi staging is utilization of the vapors generated in preceding effect heat exchanger or Calandria under negative pressure (vacuum) condition and lowering the boiling point of the feed in the subsequent stages. More the number of effects, lesser is the steam consumption. This helps economize the utility consumption.

The Falling Film Evaporators can be operated with a low temperature difference and since the product residence time is very less, they are best suited for liquids which are temperature sensitive, have a high rate of specific heat transfer, particularly for non-crystalline solutions. Having low liquid holding volumes, the falling film evaporators are very easy to operate, clean and are extremely sturdy, thus the most frequently used types. Caution must be used to ensure a continuous film wetting rate and to prevent nucleate boiling, otherwise the rate of heat transfer will fall off dramatically, and the rate of fouling on the heat transfer surface will increase

Rising Film Evaporators operate on a Thermo-Siphon principle. Feed product enters the bottom of the heating tubes and as it heats, steam begins to form. The upward force of this steam produced during the boiling causes liquid and vapors to flow upwards in parallel flow. At the same time the production of vapor increases and the product is pressed as a thin film on the walls of the tubes, and the liquid rises upwards. This co-current upward movement against gravity has the beneficial effect of creating a high degree of turbulence in the liquid. This is advantageous during evaporation of highly viscous products and products that have a tendency to foul the heating surfaces.

Forced Circulation Evaporators are normally used for liquids which are prone to fouling, scaling, crystallizing or for those which are inversely soluble or while concentrating thermally de-gradable materials. Here high re-circulation rate allows high liquor velocities through the tubes which help to minimize the buildup of deposits or crystals along the heating surface. The universal type, preferably used for concentration of solutions containing encrusting substances, hardening agents and un-dissolved solids, and for viscose solutions; also suitable for evaporative crystallization. Forced circulation is required when hydrostatic head prevents boiling at the heating surface. A pump can also be used to avoid fouling that is caused by the boiling of liquid on the tubes; the pump suppresses bubble formation. Other problems are that the residing time is undefined and the consumption of steam is very high, but at high temperatures, good circulation is easily achieved.

Natural Circulation Evaporators are very simple and are normally used where the effluent has a high viscosity, higher levels of hardening agents, un-dissolved solids, for products which are affected by their own high temperatures and longer residual times. The operation could be continuous, batch or semi-batch and do not require pumps for recirculation or intermediate product transfer. Natural circulation evaporators are based on the natural circulation of the product caused by the density differences that arise from heating.