mel709-24

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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 /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.

: The angle intersecting Dctl

to baffle cut.

qds: The angle intersecting Ds due

to extended baffle cut.

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