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

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

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

Flow at a very slow velocity
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

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