design lecture heat exchanger reboiler type

Reboiler Design
Dr. M. Subramanian
Associate Professor
Department of Chemical Engineering
Sri Sivasubramaniya Nadar College of Engineering
Kalavakkam – 603 110, Kanchipuram (Dist)
Tamil Nadu, India
msubbu.in[AT]gmail.com
12-Sep-2011
CH2407 Process Equipment Design II
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Contents
• Reboiler types, diagrams, and photos
• Kettle type reboiler design considerations
• Example design problem
12-Sep-2011 M Subramanian
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Thermosyphon Reboiler
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Kettle Type Reboilers
• Kettle reboilers, and other submerged bundle equipment,
are essentially pool boiling devices, and their design is
based on data for nucleate boiling.
• The tube arrangement, triangular or square pitch, will not
have a significant effect on the heat-transfer coefficient. A
tube pitch of between 1.5 to 2.0 times the tube outside
diameter should be used to avoid vapour blanketing.
• Long thin bundles will be more efficient than short fat
bundles
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Disengagement of Vapor and Liquid
The shell should be sized to give adequate space for the
disengagement of the vapour and liquid. The shell diameter
required will depend on the heat flux. The following values
can be used as a guide:
The freeboard between the liquid level and shell should be at
least 0.25 m.
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Check for Maximum Vapor Velocity
• To avoid excessive entrainment, the maximum vapour
velocity (m/s) at the liquid surface should be less than
that given by the expression:
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Boiling Heat Transfer Coefficients
• In the design of vaporisers and reboilers the designer will
be concerned with two types of boiling: pool boiling and
convective boiling.
• Pool boiling is the name given to nucleate boiling in a pool
of liquid; such as in a kettle-type reboiler or a jacketed
vessel.
• Convective boiling occurs where the vaporising fluid is
flowing over the heated surface, and heat transfer takes
place both by forced convection and nucleate boiling; as in
forced circulation or thermosyphon reboilers.
• Boiling is a complex phenomenon, and boiling heat-transfer
coefficients are difficult to predict with any certainty.
Whenever possible experimental values obtained for the
system being considered should be used, or values for a
closely related system.
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Critical Heat Flux
• It is important to check that the design, and operating,
heat flux is well below the critical flux.
• The maximum heat flux achievable with nucleate boiling is
known as the critical heat flux.
• In a system where the surface temperature is not self-
limiting, such as a nuclear reactor fuel element, operation
above the critical flux will result in a rapid increase in the
surface temperature, and in the extreme situation the
surface will melt. This phenomenon is known as “burn-out”.
• The heating media used for process plant are normally self-
limiting; for example, with steam the surface temperature
can never exceed the saturation temperature.
• Care must be taken in the design of electrically heated
vaporisers to ensure that the critical flux can never be
exceeded.
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The critical flux is reached at surprisingly low temperature differences;
around 20 to 30oC for water, and 20 to 50oC for light organics
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Boiling Heat Transfer Coefficient
Estimation
• The correlation given by Forster and Zuber (1955) can be
used to estimate pool boiling coefficients:
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Boiling Heat Transfer Coefficient
Estimation (contd.)
• The reduced pressure correlation given by Mostinski
(1963) is simple to use and gives values that are as reliable
as those given by more complex equations.
• Mostinski’s equation is convenient to use when data on the
fluid physical properties are not available.
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Check for Critical Heat Flux
• In SI units, Zuber’s (1961) equation can be written as:
• Mostinski also gives a reduced pressure equation for
predicting the maximum critical heat flux:
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Check for Critical Heat Flux (contd.)
• The modified Zuber equation can be written as:
• Palen and Small (1964) suggest that a factor of safety of
0.7 be applied to the maximum flux estimated from
equation
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Heat Transfer Coefficient of
Condensing Steam
• Steam is frequently used as a heating medium.
• The film coefficient for condensing steam can be calculated
using the methods given in the previous sections; but, as
the coefficient will be high and will rarely be the limiting
coefficient, it is customary to assume a typical,
conservative, value for design purposes. For air-free steam
a coefficient of 8000 W/m2 oC can be used.
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design lecture heat exchanger reboiler type

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