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

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

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Process Equipment and Plant Design
Weep holes provided in the plates of a distillation column

Facilitate draining out liquid from a tray when the unit is not in operation
Are normally located near the overflow weir so that any delivery of liquid during operation follows approximately the same path as the overflow fluid
All of these
Must be large enough (usually 1/4" to 5/8" dia) to prevent plugging but should not deliver excessive amount of fluid during operation

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