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The choice of internal depends on the required duty.
Some of the options include:
vane pack (either for horizontal or vertical flow)
− axial flow (e.g. "swirltubes")
− reversed flow (tangential)
A combination can also be used, such as:
wiremesh + swirltubes (in SMS)
wiremesh + swirltubes + wiremesh (in SMSM)
vertical-flow vane + swirltubes (in SVS)
Selection depends on the required efficiency, capacity, turndown, maximum allowable pressure drop and fouling tolerance.