Evaporator Design for concentrating cane juice By, G Bharani Harsh Gautam M Jeevan M N Karthik

Evaporator types and applications Heating medium separated from evaporating liquid by tubular heating surface. Confined by coils, jackets, double walls flat plates Brought into direct contact with evaporating liquid Heating by solar radiation 1)Highest heat transfer coefficients are obtained in forced ciculation evaporators when the liquid is allowed to boil in the tubes ( most used in indus)

Forced circulation Evaporator Advantages: High heat transfer coefficients Positive circulation Relative freedom from salting scaling and fouling Disadvantages High cost Power for circulating pump Relatively high holdup or residence time

Forced circulation Evaporator Applications: Crystalline product Corrosive solutions Viscous solutions Frequent difficulties Plugging of tube inlets by salt deposits Poor circulation due to higher than expected head losses Salting due to boiling in tubes Corrosion and erosion Circulation is maintained regardless of the evaporation rate so this type of evaporator is well suited to crystallizing operation

Short tube vertical evaporators Advantages: High heat transfer coefficients at high temperature differences Low headroom Easy mechanical de-scaling Relatively inexpensive Disadvantages: Poor heat transfer at low temperature difference Relatively high hold up

Short tube vertical evaporators Applications: Clear liquids Crystalline product if propeller is used

Long tube evaporator Advantages: Low cost Large heating surface in one body Low holdup Small floor surface Good heat transfer coefficients at reasonable temperature differences (Rising film) Good heat transfer coefficients at all temperature differences (Falling film)

Long tube evaporator Disadvantages: High head room Generally unsuitable for salting and severely scaling liquids Poor heat transfer coefficients of rising film version at low temperature differences Circulation usually required for falling film

Long tube evaporator Applications: Frequent difficulties: Clear liquids Foaming liquids Corrosive solutions Large evaporation loads Low temperature operation- falling film Frequent difficulties: Poor feed distribution in falling film

Primary Design problems Heat Transfer Vapor Liquid separation Selection Product quality Heat surface represents largest part of evaporator cost. Selection based on highest heat transfer coeficeint Btu/hr/deg Usually to prevent entrainment, Fouling or corrosion on the surfaces where vapor is condensed evaporator performance is rated on the basis of steam economy (Kg of solvent evap/Kg of steam), The vapor produced can be reused to preheat the next evaporator, To inc efficiency add thermo compression evap. Crystalisation(The conc shd not reach beyond 78 Brix), salting( it’s a growth on walls which results in inc in solubility with inc in temp) , scaling ( growth on the heating surfaces) , product quality corrosion and foaming. Low hold up time, low temp operation to avoid thermal degradation dictates materials of construction to avoid metallic contamination or catalytic effect

Design Design parameters: Cross sectional area of tube Length of each tube Pitch in the tube bundles Total area of the vessel Height of head space Cross sectional area of down pour No. of tubes

Rising Film Evaporator

Heat transfer Coefficients Shell side: we use Dittus – Boelter equation Tube side: we use Zukauskas correlation: We take

Arrangements Staggered arrangement: It allows maximum possible density of the tubes Maximum heat transfer

Final Calculations Final Design parameters: Tube: ID = 32mm OD = 35mm Height = 3.5m Shell side: OD = 1.6m Area = 1.88m2 Down pour side : Diameter = 0.4m Evaporator: Area = 2.01m2 Total height of the evaporator = 7m Total no. of tubes in the evaporator = 1250