Types of Evaporators:

1. Falling Film Evaporators

In falling film evaporators, liquid and vapors flow downward in parallel flow. The concentrated liquid is preheated to boiling temperature. Through a distribution device in the evaporator’s head, an even thinfilm enters the heatingtubes, flows downward at boiling temperature, and is partially evaporated. The co-current vapor flow augments the gravitationallyinduced downward movement. Falling film evaporators can operate with very small temperature differencesbetweentheheatingmediaandtheboiling liquid. They have very short product contact times, typicallyonlyafewsecondsperpass.Theseproperties makethefallingfilmevaporatoridealforheat-sensitiveproducts,anditisnowthemostcommonly usedtypeofevaporator.

However, falling film evaporators must be carefully designed for each operating condition. Adequate wetting of the heating surface by liquid is critical for the plant’s trouble-free operation. Drypatchesandincrustationswillformiftheheatingsurfacesarenotsufficientlywetted;atworst, the heating tubes will become completely clogged. In critical cases, extending or dividing evaporatoreffectswhilemaintainingthebenefitsofsingle-passoperationcanincreasethewetting rate.Toachievefullandevenproductwettingofthetubes,theliquiddistributionsystemmustbe properlydesigned.

2. Rising Film Evaporators

These work on the basis of the “thermo-siphon” principle. As the feed enters the bottom of the heating tubes, steam begins to form. Because of the ascending force of the steam produced during boiling, liquid and vapors flow upwards in parallel flow. Simultaneously, vapor production increases and the product is pressed as a thin film on the walls of the tubes. As a result, the liquid rises upwards. This co-current upward movement has the advantageous effect of increasing turbulence in the liquid. This is beneficial when evaporating highly viscous products or products thattendtofoultheheatingsurfaces.

Typically, the temperature difference between the heating and boiling sides of this type of evaporator must be quite large. Otherwise, the vapor flow’s energy is insufficient to convey the liquidandproducetherisingfilm.Typically,thelengthoftheboilingtubeswillnotexceed23feet. Thistypeofevaporatorisfrequentlyusedinconjunctionwithproductrecirculation,inwhichsome of the formed concentrates are reintroduced back into the feed inlet. This is to produce sufficient liquidloadinginsidetheboilingtubes.

3. Forced Circulation Evaporator

Forced circulation evaporators are used to avoid boiling the product on the heating surfaces due to the product’s foulingcharacteristics.Thecirculatingliquidisheatedasit flows through the heat exchanger and then partially evaporated as the pressure in the separator is reduced, allowing the liquid to cool to the boiling temperature corresponding to this pressure. Because the liquid is typically only heated a few degrees for each pass through the heat exchanger, the recirculation flow rate must be high.

Asthereisnoevaporationandthusnoconcentrationincreaseontheheattransfersurface,thistype of evaporator is also used in crystallizing applications. When the liquid is flash-evaporated in the separator vessel,itevaporates.Thisis wherecrystalsformincrystallizer applications,andspecial separator designs are used to separate crystals from the recirculatedcrystalslurry. Dependingon thespecificrequirements,theheatexchangercanbearrangedhorizontallyorvertically.

4. Plate Evaporators

Plate evaporators are small in size. Short interconnecting pipework connects the separators to the plate packages. As a result, space requirementsareminimal,andheightsaretypicallylimitedto20to25feet (6-8 meters). As a result,plate evaporators canbe installed in the majority ofbuildings.Framedplatescanbeusedasaheatingsurfaceinsteadoftube and shell heat exchangers. These plate assemblies are similar to plate heat exchangers but have larger vapor flow passages. A product plate and a steamplateareconnectedalternatelyintheseunits.

Theproductpassageisdesignedtoensureevenliquiddistributionontheplatesurfacesandalowpressure drop in the vapor phase. Because the plate package is easily opened, surfaces can be inspected,individualplatescanbechangedasneeded,andtheevaporationratecanbeadjustedby adding or removing individual plates. The units can be designed to meet USDA Dairy sanitation standards.

Read More: What is the Working Principle of Evaporators?