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
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
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 for a given heat transfer surface area, smaller diameter tubes are favoured as compared to larger diameter ones ; because the smaller diameter tubes

Are less prone to fouling
Can be fitted into a smaller shell diameter hence the cost of the heat exchanger would be less
Are easier to clean
None of these

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