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Heat Exchange

stiffness to the tubes. Avoid providingtoo small or too large baffle cuts, as theyresult in poor velocity distribution andhigh localized velocities.

Entrance/exit areas. Entrance andexit areas are generally more susceptible

to damage from vibration. To avoid dam-age due to erosion and vibration at theseareas, it may be required to also providean impingement plate. Impingementplates are typically provided when:

There is two-phase flow at theshell inlet

In the case of a single-phase fluidat the shell inlet, when Rho-V 2 exceeds 2,232 kg/ms2

In the case of single-phase fluidat boiling point, when Rho-V 2 exceeds 744 kg/ms2.

Impingement devices can be of threetypes: circular plate, rectangular plateand rods.

Circular or rectangular plates are thefirst choice due to simplicity and lowcost. Studies have shown that adding aplate-type impingement device acceler-ated flow around the edges of the plate,

while a rod-grid device provided a moreuniform velocity profile across the en-trance of the exchanger, resulting in alower pressure drop and reducing veloc-ity transients that can lead to vibration. Yet, the benefits must be weighed against

the higher cost of these devices and theloss of the number of working tubes with-in the heat exchanger diameter.

Use of support plates. It may be use-ful to provide partial support plates at theentrance/exit areas to reduce unsupport-ed tube spans and vibration.

Limiting the bundle entrance andexit momentum. Limiting the bundle en-trance/exit momentum helps in minimiz-ing vibration issues. This can be achieved by removing tube rows under the nozzlesthat are most susceptible to vibration.

Tube pitch/layout pattern. The tubepitch is a very important parameter thatcan help in reducing vibration. As a start-ing point, tube pitch is usually selected as1.25 times (in the case of 1-in. tubes) or1.33 times (in the case of 0.75-in. tubes)of tube outside diameter. Larger tubepitches result in a reduced cross-flow ve-

locity for an unsupported tube span. Asa consequence, flow-induced vibrationsare lower. However, an exchanger de-signer has to be cognizant of the fact thatincreasing tube pitch leads to the accom-modation of fewer tubes within a speci-

fied shell diameter, which could lead toa larger, more expensive exchanger. Thetube layout pattern is also one of the pa-rameters that could help in reducing vi- bration. The 45 tube layout pattern can be used in cases where vibration occursdue to fluid-elastic instability. However,a 45 layout pattern is to be avoided incases where the exchanger is susceptibleto acoustic vibration.

U-bend regions. The U-bend regioncan be particularly vulnerable to vibra-tion issues. Vibration at the U-bend re-gion can be avoided by providing a fullsupport plate at the U-bend. Further, itmay be useful to provide strip baffles atthe U-bend to ensure that the U-bend re-gion is vibration-free.Note: Providing afull support plate at the U-bend will re-duce the heat transfer area and must beaccounted for in thermal design.

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