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

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

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Process Equipment and Plant Design
The function of manholes provided in the shell of a distillation column is to

Keep a check on the liquid gradient over the plate by direct visual observation
All of these
Guard against foaming & entrainment by dumping antifoaming agent through it
Give access to the individual trays for cleaning, maintenance and installation

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