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

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Process Equipment and Plant Design
For a cylindrical shell, (subject to the thickness of uppermost course being more than the minimum for diaofthe tank in question), the thickness of the courses of shell

Remains same throughout
Increases upwards
Decreases upwards
May decrease or increase upwards depending upon whether vacuum or positive pressure would be maintained inside the shell

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