Baffles

Baffles serve two functions:

 Support the tubes for structural rigidity, preventing tube vibration and sagging

 Divert the flow across the bundle to obtain a higher heat transfer coefficient.

Type of baffles. Baffles are used to support tubes, enable a desirable velocity to be maintained for the shell side fluid, and prevent failure of tubes due to flow-induced vibration.  There are two types of baffles: plate and rod.Plate baffles may be single-segmental,double-segmental, or triple-segmental, as shown in Figure Baffle spacing. Baffle spacing is the centerline-to-centerline distancebetween adjacent baffles. It is the most vital parameter in STHE design. The TEMA standards specify the minimum baffle spacing as one-fifth of the shell inside diameter or 2 in., whichever is greater. Closer spacing will result in poor bundle penetration by the shellside fluid and difficulty in mechanically cleaning the outsides of the tubes.

The maximum baffle spacing is the shell inside diameter. Higher baffle spacing will lead to predominantly longitudinal flow, which is less efficient than cross-flow, and large unsupported tube spans, which will make the exchanger prone to tube failure due to flow-induced vibration.

Baffle cut.

 

As shown in Figure baffle cut is the height of the segment that is cut in each baffle to permit the shellside 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 pro-found than that of baffle spacing.

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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, as illustrated in Figure. 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.

 

For single-phase fluids on the shellside, a horizontal baffle cut (Figure) is recommended, because this minimizes accumulation of deposits at the bottom of the shell and also prevents stratification. However, in the case of a two-pass shell (TEMAF), a vertical cut is preferred for easemof fabrication and bundle assembly. Baffling is discussed in greater detail in (2) and (3).

Disc & Ring Baffles:

Disc and ring baffles are composed of alternating outer rings and inner discs, which direct the flow radially across the tube field.The potential bundle-to-shell bypass stream is eliminated.This baffle type is very effective in pressure drop to heat transfer conversion

Orifice Baffle:

In an orifice baffle shell-side-fluid flows through the clearance between tube outside diameter and baffle-hole diameter.