Process Equipment and Plant Design
When one of the fluids is highly corrosive and has fouling tendency, it should

Perferably flow outside the tube
Flow at a very slow velocity
Preferably flow inside the tube for its easier internal cleaning
Flow outside the tube, when the flow is counter-current and inside the tube when the flow is co-current

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Process Equipment and Plant Design
Triangular pitch tube layout as compared to square pitch in a shell and tube heat exchanger

Permits the use of less tubes in a given shell diameter
Both B and C
Facilitates comparatively easier external cleaning because of large clearance
Permits the use of more tubes in a given shell diameter

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