Introduction to evaporation
Food1040 module
Malcolm Povey
University of Leeds
Evaporator basics
The liquid feed moves on the "hot" inner walls in an
evaporator. Heat is continuously transferred from the
outside of the wall to the inside, thereby heating up the
liquid to the boiling point where water evaporates from
the liquid feed.
http://www.apv.com/anhydro/evapo/anhevap.htm
Liquid distribution
One of the most important issues influencing
the effectiveness of the evaporation is the
distribution of the liquid on the inner walls.
To make the evaporation effective the liquid
film on the inner walls should be as thin as
possible. However, if the film is too thin, the
product will burn on the walls, therefore it is
very important to be able to control the
thickness of the film also known as "the
wetting rate".
http://www.apv.com/anhydro/evapo/anhevap.htm
Typical applications
Milk and milk products
Sweetened condensed milk
WPC35, WPC60 and WPC80
Whey and whey permeate
Fruit juice (tropical, citrus, temperate)
Coffee, tea
Sugar (different types)
Vitamins, pharmaceuticals
Various teas and herb extracts
More applications
Various proteins e.g. blood or gelatine
Various broths e.g. chicken beef
Bulk chemicals
Waste water:
Spent waste water (distilleries)
•General waste water
The principles of evaporation
The product to be concentrated is separated from
the heating medium by a tube wall, or a flat curved
plate.
The typical heating medium is steam or vapour.
The system can be evacuated in order to lower
the boiling temperature.
The evaporated vapour can be reused as heating
medium often in combination with either
mechanical or thermal recompression.
(Regeneration)
The laboratory CFE

Heat and
mass
transfer are
connected
Heat transfer in a CFE

q ~(θs - θp)
More specifically :Equation 1
q = U.A. (θs - θp)
= U.A. Δθ
Where
A
U
θs
θp
= heat exchange area = 0.251 m2
steaminlet  steamoutlet
= overall heat transfer coefficient
2
= average steam temperature in the calandria jacket
= product temperature at boiling point (boiling set point)
Energy consumption
As evaporation is a very energy consuming
process, the availability and relative cost of
energy determine the design of the
evaporation plant.
Normally an evaporation plant is designed
to use the energy as efficiently as possible
by using more than one effect.
Therefore the following technical solutions
are used in order to keep the temperature
of the steam high enough to run the
process:
Optimising energy use
•Thermal Vapour Recompression (TVR)
•Mechanical Vapour Recompression (MVR) or
•combination of both
Shown below is a figure illustrating the relation between
the number of effects and energy consumption under
standard conditions for a typical product.
Furthermore, the figure also shows a rough correlation
between steam and electrical energy consumption per kg
water evaporated, allowing the customer to estimate,
whether the price of electric power in his local area
justifies the use of MVR instead of TVR.
Comparison
http://www.apv.com/anhydro/evapo/anhevap.htm
Evaporator pictures and
applications
Single effect recompression
•Corn Syrup
•Gelatin
•Salt
•Soybean Oil
•Steepwater
•Sugar
•Whey
Falling Film Plate & Tubular
Evaporators
The unique liquid distribution
•Short residence time
•Low temperature difference
with
•Single pass operation
•Even shorter residence time
•Larger capacities
Typical applications are:
•Milk, whey and milk products
•Fruit juices , Various vitamins , Pharmaceuticals
•Essence recovery
Falling film evaporator
http://www.swensonequip.com/vrml.html
Rising/Falling Film Plate
Evaporators
•The unique liquid distribution
•Short residence time
•Low temperature difference
Typical applications are:
•Milk, whey and milk products
•Fruit juices
•Various vitamins
•Pharmaceuticals
•Essence recovery
Rising Film Plate Evaporators
APV´s rising film plate evaporators
(Paravap) are particularly effective in
processing high viscous liquids.
Typical applications are:
•High viscous sugar
•Fruit puree and pulp
•High viscous pharmaceuticals
•Polymers
Other evaporators
http://www.swenson-equip.com/index.html
Forced circulation evaporator
Evaporator Diagrams
Falling film evaporator
Forced Convection with steam jet
recompression
Multiple effect evaporator
Wiped Film
Evaporator