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More Geometrical Features of Baffles
Mechanical Engineering Department I I T Delhi
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More Geometrical Details of Baffles
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Baffle Cut
• Baffle cut is the height of the
segment that is cut in each baffle
to permit the shell side fluid to
flow across the baffle.
• This is expressed as a percentage
of the shell inside diameter.
• Although this, too, is an
important parameter for STHE
design, its effect is less profound
than that of baffle spacing.
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Segmental Baffle Cut Geometry
Segmental baffle cut height :Lbch
Assuming that the segmental baffle is centered withinthe shell inside diameter .
The small difference between the shell and baffle diameter is
called the clearance Lsb and it is important for leakagecorrections.
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Selection of Baffle Cut
•
Baffle cut can vary between 15% and 45% of the shell insidediameter.
• Both very small and very large baffle cuts are detrimental to
efficient heat transfer on the shellside due to large deviation
from an ideal situation.
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Role of Baffle Cut on Flow Distribution
• If the baffle cut is too small, the flow will jet through the window
area and flow unevenly through the baffle compartment.
• If the baffle cut is too large, the flow will short-cut close to the
baffle edge and avoid cross-mixing within the baffle
compartment.
• A baffle cut that is either too large or too small can increase the
potential for fouling in the shell.
• In both cases, recirculation zones of poorly mixed flow cause
thermal maldistribution that reduces heat transfer.
• To divert as much heat-carrying flow across the tube bundle as
possible, adjacent baffles should overlap by at least one tube row.
• This requires a baffle cut that is less than one-half of the shell
inside diameter.
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Non-Optimal Baffle Cut & Fouling Zones
Too small Baffle Cut Too Large Baffle Cut
L BCH
L BCH /Ds
Ds
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Optimal Baffle Cut
• It is strongly recommended that only baffle cuts between 20%
and 35% be employed.
• Reducing baffle cut below 20% to increase the shellside heat-
transfer coefficient or increasing the baffle cut beyond 35% to
decrease the shellside pressure drop usually lead to poor
designs.
• Other aspects of tube bundle geometry should be changed
instead to achieve those goals.
• For example, double segmental baffles or a divided-flow shell,
or even a cross-flow shell, may be used to reduce the shellside
pressure drop.
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Equalize cross-flowand window velocities
• Flow across tubes is referred to as cross-flow, whereas flow
through the window area (that is, through the baffle cut area) is
referred to as window flow.
• The window velocity and the cross-flow velocity should be as
close as possible — preferably within 20% of each other.
• If they differ by more than that, repeated acceleration and
deceleration take place along the length of the tube bundle,
resulting in inefficient conversion of pressure drop to heat
transfer.
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Recommended segmental baffle cut values
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Orientation of Baffle Cut
• For single-phase fluids on the shellside,
a horizontal baffle cut is recommended.
• This minimizes accumulation of
deposits at the bottom of the shell and
also prevents stratification.
• In the case of a two-pass shell (TEMA
F), a vertical cut is preferred for ease of fabrication and bundle assembly.
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Vertical Vs Horizontal Cut
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Selection of Baffle Cut Orientation
• For single-phase service, single-segmental baffles with a
perpendicular (horizontal) baffle-cut orientation in an E- or J-shell are preferred to improve flow distribution in the inlet and
outlet regions.
• With vertical inlet or outlet nozzles, parallel-cut (vertical) baffles
are preferred if the shellside process fluid condenses and needs ameans of drainage.
• Parallel-cut baffles should also be used when the shellside fluid
has the potential for particulate fouling, and in multipass F-, G-,
or H-type shells to facilitate flow distribution.
• However, parallel-cut (vertical) baffles have the potential for
significant flow and temperature maldistribution in the end zones.
• This can induce local tube vibration and reduce the effective heat
transfer rate in the inlet and outlet baffle spaces
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Helical Baffles
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Superiority of Helical Baffling
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Closing thoughts
• Baffling is the most crucial shellside consideration in shell-
and-tube heat exchanger design, because baffles regulateshellside fluid flow and improve heat transfer while offering
significant tube support.
• Although TEMA baffles are easier to fabricate, they usually
have higher pressure drops than non-TEMA-type baffles.• It is equally important to consider how baffle selection affects
other shellside parameters, such as tube pitch ratio, tube layout
pattern, tube size, shell type, and shell diameter.
• A basic understanding of the various baffle types and theiradvantages and disadvantages is essential to choosing an
effective baffle configuration.
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Basic baffle geometry relations
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Basic baffle geometry relations
Dotl : Diameter of circle touching
the outer surface of outermost
tubes.
Dctl : Diameter of circle passing
through the centers of of
outermost tubes.
Lbb: Diametric clearance between
tube bundle and shell inside
diameter.
qctl
: The angle intersecting Dctl
due
to baffle cut.
qds: The angle intersecting Ds due
to extended baffle cut.