University of Basrah Heat transfer / Third stage
College of engineering / chemical department Ahmed Al Rubaie
Evaporation
Evaporation is the type of vaporization that occur on the surface of liquid
as it changer in to gas phase.
Liquors which are to be evaporated may be classified as follows:
(a) Those which can be heated to high temperatures without
decomposition, and those that can be heated only to a temperature of
about(330)K.
(b) Those which yield solids on concentration, in which case crystal size
and shape may be important, and those which do not.
(c) Those which, at a given pressure, boil at about the same temperature
as water, and those which have a much higher boiling point.
Classification of th evaporation
They are two principles of vaporization equipment's used in the industry:
1- Boilers
2- Condensers
Heat transfer in evaporation and Duhring rule
For example, at atmospheric pressure (101.3 kN/m2), a 25 per cent
solution of sodium chloride boils at 381 K and shows a BPR of 8 K. If
steam at 389 K were used to concentrate the salt solution, the overall
temperature difference would not be (389 − 373) = 16 K, but (389 − 381)
= 8 K. Such solutions usually require more heat to vaporise unit mass of
water, so that the reduction in capacity of a unit may be considerable. The
value of the BPR cannot be calculated from physical data of the liquor,
though Duhring’s rule is often ¨ used to find the change in BPR with
pressure. If the boiling point of the solution is plotted against that of
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University of Basrah Heat transfer / Third stage
College of engineering / chemical department Ahmed Al Rubaie
water at the same pressure, then a straight line is obtained, as shown for
sodium chloride in Figure below. Thus, if the pressure is fixed, the
boiling point of water is found from steam tables, and the boiling point of
the solution from the figure. The boiling point rise is much greater with
strong electrolytes, such as salt and caustic soda.
Single effect evaporator
Single-effect evaporators are used when the throughput is low, when a
cheap supply of steam is available. Single effect units may be operated in
batch, semi-batch or continuous batch modes or continuously. The heat
requirements of single-effect continuous evaporators may be obtained
from mass and energy balances. If enthalpy data or heat capacity and heat
of solution data are not available, heat requirements may be taken as the
sum of the heat needed to raise the feed from feed to product temperature
and the heat required to evaporate the water. The latent heat of water is
taken at the vapour head pressure instead of the product temperature in
order to compensate, at least to some extent, for the heat of solution. If
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University of Basrah Heat transfer / Third stage
College of engineering / chemical department Ahmed Al Rubaie
sufficient vapour pressure data are available for the liquor, methods are
available for calculating the true latent heat from the slope of the Duhring
line. The heat requirements in batch operation are generally similar to
those in continuous evaporation. Whilst the temperature and sometimes
the pressure of the vapour will change during the course of the cycle
which results in changes in enthalpy, since the enthalpy of water vapour
changes only slightly with temperature, the differences between
continuous and batch heat requirements are almost negligible for all
practical purposes. The variation of the fluid properties, such as viscosity
and boiling point rise, have a much greater effect on heat transfer,
although these can only be estimated by a step-wise calculation. In
estimating the boiling temperature, the effect of temperature on the heat
transfer characteristics of the type of unit involved must be taken into
account. At low temperatures some evaporator types show a marked drop
in the heat transfer coefficient which is often more than enough to offset
any gain in available temperature difference.
Multiple effect evaporators
Multiple effect evaporators is enable the performance to be improved,
either by direct reduction in the steam consumption, or by improved
energy efficiency. For three evaporators arranged as shown in Figure
below in which the temperatures and pressure are T, T2, T3 and P1,P2,P3
Respectively, in each unit, if the liquor has no boiling point rise.
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University of Basrah Heat transfer / Third stage
College of engineering / chemical department Ahmed Al Rubaie
The economy of the system, measured by the kilograms of water
vaporised per kilogram of steam condensed, increases with the number of
effects. The water evaporated in each effect is proportional to Q, since the
latent heat is approximately constant. Thus the total capacity is
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University of Basrah Heat transfer / Third stage
College of engineering / chemical department Ahmed Al Rubaie
Comparison of forward and backward feeds
In the forward feed unit the weak liquor is fed to effect 1 and flows on to
2 and then to 3 . The steam is also fed to1 , since the feed is to the same
unit as the steam and travels down the unit in the same direction as the
steam or vapour. It is possible, however, to introduce the weak liquor to
effect 3 and cause it to travel from 3 to 2 to 1, whilst the steam and
vapour still travel in the direction of 1 to 2 to 3. This system, shown in
first figure is known as backward-feed. A further arrangement for the
feed is known as parallel-feed, which is shown in second figure. In this
case, the liquor is fed to each of the three effects in parallel although the
steam is fed only to the first effect. This arrangement is commonly used
in the concentration of salt solutions, where the deposition of crystals
makes it difficult to use the standard forward-feed arrangement. The
effect of backward-feed on the temperature distribution, the areas of
surface required, and the economy of the unit is of importance
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