Process Equipment and Plant Design
In a shell and tube heat exchanger, triangular pitch arrangement as compared to square pitch arrangement

Creates relatively lower turbulence on the shell side resulting in lower shell side heat transfer co-efficient
Facilitates easier shell side cleaning; hence is more suitable for handling high dirt factor shell side fluid
Results in higher shell side pressure drop
Can accomodate less number of tubes for a given shell diameter

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

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

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