Page 195 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.6 Evaporator design 193

Steam pressure
Although steam at a higher pressure leads to a higher DTand a consequent decrease in the heat transfer
area (and cost) of an evaporator, it is rarely used as heating medium for evaporation because
• High pressure steam is much more valuable as a source of power than a source of heat. It is much
cheaper to generate power using high pressure steam and use the exhaust steam in evaporator than
to use boiler steam directly.
• Steam at higher pressure has a higher enthalpy but a lower latent heat of vaporization. So low
pressure steam can deliver more latent heat per kg steam.
• The construction of an evaporator to hold high pressure steam would be more expensive.
The disadvantage of low pressure steam is that it exists at a lower temperature and so provides
lower DT between steam and evaporating liquid. Usually the steam pressure is between 1 and
0.7 atm(g) and in some case can be up to 2.5e3 atm(g).
Pressure in the vapor space
It is important to note that evaporators do not necessarily have to work under vacuum. A vacuum is
necessary solely for the purpose of obtaining a larger DT ov . Vacuum also ensures that heat-sensitive
products and feeds do not decompose or alter during evaporation and entails cheaper construction
(lower capital cost).
2
2
One may use steam at 10 e 15 kg/cm (g) and take off vapor at 3.5e5.5 kg/cm (g) if there is a use
for such vapor in the plant but such cases are not common.
Influence of feed, steam and condensate temperature
The feed temperature usually has no effect on evaporator calculation. This arises because the volume
of liquid in evaporator is very large compared to the amount/rate of feed addition and the liquid is
always at the final concentration at the boiling point of the final solution. Moderate amounts of su-
perheat in the steam used for heating and subcooling of the condensate also has negligible effect on the
mean temperature of steam. Therefore, temperature of feed, temperature of condensate and (in most
practical cases) any possible superheating of steam are neglected in evaporator design calculations and
a practical working temperature difference is the temperature difference between temperature of
saturated steam and boiling liquid.
However, if the feed is at a temperature much below its boiling point and the amount of evaporation
is small, there would be areas where the liquid temperature is below the temperature of the thick liquor.
Since it is impossible to estimate the extent of these areas and their mean temperature in advance, the
only practical temperature that can be used in calculations is the boiling point of thick liquor.
Therefore, it is desirable to use a feed preheater to heat the feed to nearly its boiling point in the
evaporator.

6.6.4 Design algorithm for multiple-effect evaporator
The evaporator schematic is depicted in Fig. 6.23 with key variables shown. Steam is supplied to the
first effect on the left and the vapor boil up in effects are used to heat the subsequent effect. The final
vapor stream is condensed. To compute the energy balance for the units, information on enthalpy and
boiling point elevation are required. Heat transfer coefficient data or correlation are required for each
effect. The pressure, at which the last effect is operated, often at vacuum, must be known.