Geometrical Details of Baffles & Shell Side Fluid Dynamics

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

Ideas to Achieve Optimal Combination of Axial & Cross

flow……

Segmental Baffle Cut Geometry

Segmental baffle cut height :Lbch

Assuming that the segmental baffle is centered within the shell inside diameter .

The small difference between the shell and baffle diameter is called the clearance Lsb and it is important for leakage corrections.

Selection of Baffle Cut

• Baffle cut can vary between 15% and 45% of the shell inside diameter.

• 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.

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.

Non-Optimal Baffle Cut & Fouling Zones

Too small Baffle Cut Too Large Baffle Cut

LBCH

LBCH/Ds

Ds

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.

Equalize cross-flow and 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.

Recommended segmental baffle cut values

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.

Vertical Vs Horizontal Cut

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 a means 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

Helical Baffles

Superiority of Helical Baffling

Closing thoughts

• Baffling is the most crucial shellside consideration in shell-and-tube heat exchanger design, because baffles regulate shellside 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 their advantages and disadvantages is essential to choosing an effective baffle configuration.

Basic baffle geometry relations

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.

ctl: The angle intersecting Dctl due

to baffle cut.

ds: The angle intersecting Ds due

to extended baffle cut.

Historical Development of the Delaware Method • The Department of Chemical Engineering at the University of

Delaware started a comprehensive research program on shell-side design of shell-and-tube heat exchangers in 1947.

• This project is called Delaware Project and it finished in 1963.

• In 1947, the project started under ASME sponsorship using funds from:– the Tubular Exchanger Manufacturers Association,

– the American Petroleum Institute,

– Standard Oil Development Co.,

– Andale Company, Downingtown Iron Works,

– Davis Engineering Co., E.I. du Pont de Nemours and Company, and

– York Corporation.

• The principal investigators were Professors Olaf Bergelin and Allan Colburn of the University of Delaware.

Methodology of Development

• In 1947, the experimental program started with measurements of heat transfer and pressure drop during flow across ideal tube banks.

• Then several baffle cut and spacing configurations were studied inside a cylindrical shell with no baffle leakage first.

• Baffle leakages between baffles and the shell and between the tubes and baffles were added afterwards.

• Finally, the bypass flow around the bundle between the outer tube limit and the shell inner diameter was investigated.

• The first report was published in 1950 and the second report, in 1958.

• In 1960, a preliminary design method for E shell heat exchangers was issued.

• In 1963, the final report was published.

Shell-side stream analysis• On the shell side, there is not just one stream.• There are essentially two models that address the

flow on the shell side. • The ideal flow and real flow models.

Ideal Shell side flow

A nearly ideal flow can only exist in a heat exchanger if it is manufactured with the special mechanical features.

Realization of Ideal Flow : Condition - 1

• Each baffle is welded to the shell inside diameter at the contact line so that there is no possibility of leakage between the shell and the baffle.

Realization of Ideal Flow : Condition - 2

• The annular space between the tube and the baffle hole is either mechanically closed or a bushing is inserted to eliminate any fluid leak across the clearance between the baffle hole and the tube.

Realization of Ideal Flow : Condition - 3

• The tube bundle layout is such that there are no lands and extra spaces for ribs and impingement plates.

Realization of Ideal Flow : Condition - 4

• The outer tube limit (OTL) almost touches the inner diameter of the shell.

Shell side Real Flow

• When the tube bundle employs baffles, the velocity of fluid fluctuates because of the constricted area between adjacent tubes across the bundle.

• Only part of the fluid takes the desired path through the tube bundle, whereas a potentially substantial portion flows through the ‘leakage’ areas.

• However, these clearances are inherent to the manufacturing and assembly process of shell-and-tube exchangers, and the flow distribution within the exchanger must be taken into account.

Flow Path lines

Temperatures of Path-lines

Velocity Contours in Mid-plane

The Role of Fluid Viscosity• The shell side fluid viscosity also affects stream analysis

profoundly.

• In addition to influencing the shell side heat transfer and pressure drop performance, the stream analysis also affects the mean temperature difference (MTD) of the exchanger.

• It is important to realize that the

• LMTD and F factor concept assumes that there is no significant variation in the overall heat-transfer coefficient along the length of the shell.

• In the case of cooling of a viscous liquid — as the liquid is cooled, its viscosity increases, and this results in a progressive reduction in the shellside heat-transfer coefficient.

• In this case, the simplistic overall MTD approach will be inaccurate, and the exchanger must be broken into several sections and the calculations performed zone-wise.