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
Are easier to clean
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
Liquid gradient over a tray can be minimised by

All of these
Using split flow, radial flow or cascade flow for column diameter large than 4 ft
Decreasing the number of rows of caps through which the liquid flows or by decreasing the velocity of liquid flow past the caps or by reducing the distance along the tray through which the liquid must
Providing a higher skirt clearance or a higher weir

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