Falling Film Evaporators
Â· Â· Â· High functionality
Falling FilmEvaporators, water is boiled in a sequence of vessels, each held at a lowerpressure than the last. Because the boiling point of water decreases aspressure decreases, the vapour boiled off in one vessel can be used to heat thenext, and only the first vessel (at the highest pressure) requires an externalsource of heat. This is called multiple effect and no stages/effects depend on thequantity of vessel / calendria in sequence. Theoretically, one kg of steamevaporates one kg of water. multiple-effect evaporator, one kg of steamevaporates up to 6kg of water by no of effects and vapour recirculation by Thermovapour recompression. It is called steam economy 1:6.
Feed is preheated to nearby boiling temperature in Preheater that is connectedwith all calendrias. All calendria are operated at the stage by stage vacuumwith reduced temperature. So feed is preheated stage wise from lowertemperature to higher. Then it is passed via first calendria tube bundles as a finefilm layer over the tube surface area. On the shell side, fresh Steam is givenvia Thermo Vapour Compressor. Feed is heated and evaporated in calendria.Evaporated Vapour is separated in Vapour Separator, the vapour is sent to the secondcalendria Shell side. Un evaporated feed is transferred to the second calendriatube side. Here further evaporation is done. So each calendria evaporates feedand concentrate coming out from the last calendria. In this system,concentration is achieved stage by stage and each calendria have a higherconcentration as the stage increases.
A part of evaporated vapour is recycled to the first calendria via thermovapour recompression. In Thermo Vapour Compressor, Fresh Steam with pressure ispassed via steam nozzle. Steam produce high velocity and produce a venturiEffect. This will recirculation a part of evaporated vapour. By using a ThermoVapour compressor and multiple stages, steam economy up to 1:6 is achieved.This result depends on feed water quality.
In the last vapour separator, evaporated vapour is condensed by direct contactcondenser or indirect contact condenser. In a direct contact condenser, coolingwater from the cooling tower is sprayed on top and absorbs, condense vapour intowater. This will heat cooling water and returned it to the cooling tower toreduce temperature. In an indirect contact condenser, vapour is condensed bycooling tower water by heat exchanger (shell & tube / Plate type). Non-condensablegas from the system is removed by a water ring vacuum pump.
We installed many falling film evaporator capacities up to 280KLD for variousindustries. Mainly falling film evaporators are suitable for handling reverseosmosis reject water and having more than 40,000mg/l TDS. This systemevaporates up to 90% of feed and gives concentrate 300gram per litre. Thisconcentrate is dried in drier / crystallized in crystallizer / or handled with theforced circulation with pusher centrifuge.