Evaporator

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Evaporator

• Equipment used in evaporation, the process of
  boiling a liquid in order to reduce its volume
• Need
• Reduces transportation cost
• Storage costs
• Prepare for the next Unit operation drying,
  crystallisation etc.
• Reduces deteriorative chemical reactions
• Better microbiological stability
• Recovery of solvent

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Evaporator

• Driving force
• Temperature difference in between steam chest
  temperature and product temperature.
• Result
• Volatile solvent is removed from the feed.
• Solution (volatile solvent non volatile solute)
• Concentrate (Higher solute Conc.)

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Examples

• Concentration of milk to produce condensed milk
• Concentration of juices
• Concentration of NaOH, NaCl from aqueous
  solutions to produce salt.
• Ether recovery from fat extraction

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Basic Parts of an Evaporator

• Heat-exchanger
• Vacuum
• Vapour separator
• Condenser

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Evaporator
Vaccum for non condensable
Coolant In
Vapor out
Coolant out
Condensor unit
Vapor Separator
Feed in
Steam in (Saturated vapor)
Heat Exchanger
Condensate out (Saturated Liquid)
Product out
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Evaporator Type

• Batch Pan
• Rising Film
• Falling Film
• Multiple Effect Evaporators

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Processing Factors

• Conc of solute in feed
• Conc of solute in product/concentrate
• Pressure and Temp. of the system
• Depends on temperature sensitivity of material.
• Boiling point elevation

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Processing Factors

• Steam pressure and temperature
• Material of construction
• Foaming
• Low heat transfer rate
• Entrainment loss

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Heat Transfer Coefficient, h

• A coefficient which indicates the amount of heat
  flow that is exchanged across a unit area of a
  medium or system in a unit amount of time with a
  unit of temperature difference between the
  boundary of the system.
• S.I. unit W m-2 K-1.

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Overall Heat Transfer Coefficient, U

• In cases of combined heat transfer for a heat
  exchanger, there are two values for h.
• Convective heat transfer coefficient for the
  fluid film inside the tubes
• Convective heat transfer coefficient for the
   fluid  film  outside  the  tubes.   
•  The  thermal  conductivity  (k)  and thickness
  (Dx) of the tube wall must also be accounted for.
   
• So an additional term (Uo), called the overall
  heat transfer coefficient, must be used instead.  

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Steam Trap

• Function
• Discharge condensate, air and other incondensable
  gases from a steam system while not permitting
  the escape of live steam

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Ball Float Steam Trap
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Reduces the ferocity of discharge and sound.
Sound levels can be reduced by up to 80.
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Vapor Out mvhv
Steam in mshs
QL
QE
Water in mwhw
Condensate out mchc
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Lab Calculation

• Calculate U, overall heat transfer coefficient
  for the evaporator
• Perform a mass balance and energy balance
• Calculate discrepancy

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Mass Balance

• Feed Side
• m (kg/s)
• m (feed in) m (Vapor)
• Steam Side
• m (steam) m (condensate collected) m (flash)

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Flash Steam Loss

• The amount of flash steam produced during the
  pressure reduction can be expressed as
• w (Hil - Hfl) / Hfe
• where
• w ratio of flash steam generated (kg flash
  steam / kg condensate)
• Hil initial liquid enthalpy (kJ/kg)
• Hfl final liquid enthalpy (kJ/kg)
• Hfe enthalpy of evaporation (kJ/kg)

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Energy Balance

• H (kJ/kg)
• Energy from the steam to the water QE
• The Heat Loss from the vessel to the room, QL
• Amount of heat in condensing steam QS
• QS QE QL

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Measurements

• QS and QE
• Mass Flow rate- condensate, feed water
• T,P will give H Specific Enthalpy
• Q mH
• QL
• Surface area
• Surface temperature
• QL hA ?T
• QE UA ?T

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• Further Reference
• Transport Processes and Separation process
  principle by
• Christie. J. Geankoplis
• Animations on evaporator visit
• www.rpaulsingh.com and click animations then
  evaporator

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Thank You
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Multiple-effect Evaporator

• Water is boiled in a sequence of vessels, each
  held at a lower pressure than the last.
• Because the boiling point of water decreases as
  pressure decreases, the vapor boiled off in one
  vessel can be used to heat the next
• Generally the first vessel (at the highest
  pressure) requires an external source of heat

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