For engineering students.

2. Evaporator & Design

3. Presented By
ROSHAN ALI
TALHA IBRAHIM
HAFIZ MUHAMMAD AWAIS
MUHAMMAD ARSALAN IJAZ
MUHAMMAD ISMAIL
NAUMAN ALI

4. CONTENT
 Introduction Evaporator
 Evaporator Diagram
 Five Common Evaporator
o Horizontal tube Evaporator
o Short Vertical Tube Evaporator
o Long Vertical Evaporator
o Forced Circulation Evaporator
o Falling Film Evaporator

5. CONTENT
 Boiling Point Elevation
 Duhring-Line Chart
 Evaporator Model
 Key assumptions of mathematical model
 Mathematical Model
 Evaporator Multiple Effects
 Boiling point elevation summary

6. Definition of Evaporation
Evaporation is the process of a substance in
a liquid state changing to a gaseous state
due to an increase in temperature and or
pressure. Evaporation is a fundamental part
of the water cycle and is constantly
occurring throughout nature.

7. Continued….
Before crystallization of a solute from an aqueous
solution takes place, it is customary to bring the
concentration of the solute close to the solubility
curve. This is accomplished by evaporation of the
solvent in an evaporator.
Such a device is also used to concentrate
solutions even when the solute is not subsequently
crystallized, e.g., solutions of sodium hydroxide.

8. Example Diagram of Falling-Film Type
Evaporator

9. Types of Evaporator

10. Horizontal Tube Evaporator
 The horizontal-tube evaporator is a
development of the open pan, in which the pan
is closed in, generally in a vertical cylinder. The
heating tubes are arranged in a horizontal
bundle immersed in the liquid at the bottom of
the cylinder.
 Liquid circulation is rather poor in this type of
evaporator.

11. Short Vertical-Tube Evaporator
 The evaporators in which
the movement of the liquid
takes places as a result of
convection current set up by
the heating process are called
vertical tube evaporator or
short tube evaporator.
 This type of evaporator is
not suitable for very viscous
solutions.

12. Long Vertical-Tube Evaporator
 A rising film or vertical long tube
evaporator is a type of evaporator that is
essentially a vertical shell and tube heat
exchanger.
 The liquid being evaporated is fed from
the bottom into long tubes and heated
with steam condensing on the outside of
the tube from the shell side. This is to
produce steam and vapour within the
tube bringing the liquid inside to a boil.
The vapour produced then presses the
liquid against the walls of the tubes and
causes the ascending force of this liquid.
As more vapour is formed.

13. Forced-Circulation Evaporator
We use pump in film type evaporator.
 Increase heat transfer coefficient.
Useful for viscous fluid.
 External heating provide more ease to
cleaning to tubes more complicated
piping is used.
 For viscous we use positive pumps and
for colloidal we use low power pumps.
 Heating element is placed at the lower
level to avoid boiling on heating surface,
this reduces the rate of deposition of
solids.

14. Forced-Circulation Evaporator
 We use pump in film type evaporator.
 Increase heat transfer coefficient.
 Useful for viscous fluid.
 External heating provide more ease to
cleaning to tubes more complicated
piping is used.
 For viscous we use positive pumps and
for colloidal we use low power pumps.
 Heating element is placed at the lower
level to avoid boiling on heating surface,
this reduces the rate of deposition of
solids.

15. Falling Film Evaporator
Liquid is fed from top and flow
down as a thin film.
Vapour liquid separation usually
takes place at the bottom.
 Used widely for concentrating
heat sensitive materials such as
orange juice and other fruit juice.
Holdup time is very small (5-10).
 High heat transfer coefficient
(due to high velocities )

16. Boiling-Point Elevation
For a given pressure in the vapor space of an
evaporator, the boiling temperature of an aqueous
solution will be equal to that of pure water if the
solute is not dissolved in the water, but rather
consists of small, insoluble, colloidal material. If the
solute is soluble, the boiling temperature will be
greater than that of pure water by an amount known
as the boiling-point elevation of the solution.

17. Duhring-Line Chart
THE BOILING TEMPERATURE OF THE
SOLUTION CAN BE ESTIMATED BY USING A
DUHRING-LINE CHART IF IT IS AVAILABLE FOR
THE PARTICULAR SOLUTE. THE STRAIGHT
LINES ON THIS CHART FOR DIFFERENT MASS
FRACTIONS OF NAOH OBEY DUHRING S RULE,
WHICH STATES THAT AS THE PRESSURE IS
INCREASED, THE BOILING TEMPERATURE OF
THE SOLUTION INCREASES LINEARLY WITH
THE BOILING TEMPERATURE OF THE PURE
SOLVENT.

18. Evaporator Model
The following mathematical
model is widely used to
make material balance,
energy balance, and heat-
transfer rate calculations to
size evaporators operating
under continuous flow,
steady-state conditions.

19. Key assumptions in Formulating the
Mathematical Model
1. The thin-liquor feed has only one volatile component,
e.g., water.
2. Only the latent heat of the heating steam at T, is
available for heating and vaporizing the solution in the
evaporator.
3. Therefore, the temperature of the solution in the
evaporator equals the exiting temperature of the thick-
liquor concentrate. Thus, Te = Tp. Also, Te = Tp.
4. The overall temperature driving force for heat transfer
= AT = Ts - Tp.

20. Continued…
5. The AT is high enough to achieve nucleate boiling
and not so high as to cause film boiling.
6. The exiting vapor temperature, tv = tp = te
correspond to evaporator vapor-space pressure, p,
taking into account the boiling-point elevation of
the solution unless the solute is small, insoluble
particles, such as colloidal matter.
7. No heat loss from the evaporator.

21. Mathematical Model
Total mass balance:mf =mp+mv
Mass balance on the solute: wfmf= wpmp
Energy (enthalpy) balance on the solution:- Q =mvHu+mpHp –
mfHf
Energy (enthalpy) balance on the heating steam:Q = msAHVaP
Heat-transfer rate:- Q=UA(Ts-Tp)

22. Multiple Effect of Evaporator System
For a given pressure in the vapor space of an
evaporator, the boiling temperature of an aqueous
solution will be equal to that of pure water if the
solute is not dissolved in the water, but rather
consists of small, insoluble, colloidal material. If the
solute is soluble, the boiling temperature will be
greater than that of pure water by an amount known
as the boiling-point elevation of the solution.

23. Multiple Effect Evaporator System:

24. Multiple Effect Evaporator System
When condensing steam is used to evaporate water from an aqueous solution, the
heat of condensation of the higher temperature condensing steam is less than the
heat of vaporization of the lower-temperature boiling water. Consequently,
less than 1 kilogram of vapor is produced per kilogram condensation of heating
steam.
This ratio is called the economy. In Example 17.17, the economy is 13,3601
22,200 = 0.602 or 60.2%. To reduce the amount of steam required and, thereby,
increase the economy, a series of evaporators called effects, can be used as
shown in Figure 17.36.
The increased economy is achieved by operating the effects.

25. Multiple Effect Evaporator System

26. Multiple effect evaporator system:
In Figure 17.36a, referred to as a forward-feed, triple effect- evaporator system, approximately one-third of the
total evaporation occurs in each effect. The fresh feed solution and steam both enter the first effect, which
operates at pressure PI. The concentrate from the first effect is sent to he second effect. The vapor produced in
the first effect is also sent to the second effect where it condenses, giving up its heat of condensation to cause
additional evaporation of the solution. To achieve a temperature-driving force for heat transfer in the second
effect, the pressure of the second effect, P2, is lower than that of the first effect. This procedure is repeated in the
third effect. For three effects, the flow rate of steam entering the first is only about one-third of the amount of
steam that would be required if only one effect were used. However, the temperature-driving force in each of the
three effects is only about one-third of that in a single effect. Therefore, the heat transfer area of each of the three
evaporators in a triple-effect system is approximately the same as for the one evaporator in a single-effect unit.
Therefore, the savings in annual heating-steam cost must offset the additional capital investment for equipment.
When the temperature of the fresh feed is significantly below its saturation temperature corresponding to the
pressure in the first effect, backward-feed operation is desirable, as shown in Figure 17.36b. The cold fresh feed
is sent to the third effect, which operates at the lowest pressure and, therefore, the lowest temperature. Unlike
the forward-feed system, pumps are required to move the concentrate from one effect to the next because PI >
P;, > P3. However, unlike gas compressors, liquid pumps are not high-cost items.

27. Multiple Effect Evaporator System

28. Boiling-point elevation
When the temperature of the fresh feed is significantly below its
saturation temperature corresponding to the pressure in the first
effect, backward-feed operation is desirable, as shown in Figure
17.36b. The cold fresh feed is sent to the third effect, which operates
at the lowest pressure and, therefore, the lowest temperature. Unlike
the forward-feed system, pumps are required to move the
concentrate from one effect to the next because PI > P;, > P3.
However, unlike gas compressors, liquid pumps are not high-cost
items.

29. Boiling-point Elevation
For a given pressure in the vapor space of an
evaporator, the boiling temperature of an aqueous
solution will be equal to that of pure water if the
solute is not dissolved in the water, but rather
consists of small, insoluble, colloidal material. If the
solute is soluble, the boiling temperature will be
greater than that of pure water by an amount known
as the boiling-point elevation of the solution.

30. Boiling-Point Elevation
For a given pressure in the vapor space of an
evaporator, the boiling temperature of an aqueous
solution will be equal to that of pure water if the
solute is not dissolved in the water, but rather
consists of small, insoluble, colloidal material. If the
solute is soluble, the boiling temperature will be
greater than that of pure water by an amount known
as the boiling-point elevation of the solution.

31. Over all Heat Transfer Coefficient in
Evaporation
In an evaporator, the overall heat-transfer coefficient, U, depends mainly on the
steam side, condensening coefficient, the solution-side coefficient, and a scale or
fouling resistance on
The solution side. The conduction resistance of the metal wall of the heat-
exchanger tubes is usually negligible. Steam condensation is generally of the film,
rather than drop wise, type. When boiling occurs on the surfaces of the heat-
exchanger tubes, it is of the nucleate-boiling, rather than film-boiling, regime. In the
absence of boiling on the tube surfaces, heat transfer is by forced convection to the
solution. Local coefficients for film condensation, nucleate boiling, and forced
convection of aqueous solutions are all relatively large, of the order of 1,000 ~tu/h-
ft~(-5",7~0 0 w/m2-k). Thus, the overall coefficient would be about one-half of this.
However, when
Fouling occurs, the overall coefficient can be substantially less.

32. Over all Heat Transfer Coefficient in
Evaporators

33. Over all Heat Transfer Coefficient in
Evaporators
Lists ranges of overall heat-transfer coefficients for
different types of evaporators.
The higher coefficients in forced-circulation
evaporators
Are mainly a consequence of the greatly reduced
fouling
Due to the high liquid velocity in the tubes.