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
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

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Process Equipment and Plant Design
In a shell and tube heat exchanger, the clearance of the tube is generally

More than the tube diameter
Not less than one-fourth of the tube diameter or 3/16"
Equal to the tube diameter
More in case of triangular pitch as compared to the square pitch tube layout

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