Process Equipment and Plant Design
A perforated plate has holes of diameter dh arranged in a pitch ph. Each hole has a tube of I.D. dt passing through it. The ligament efficiency is given by

(ph - dh)/ph
(ph - dt)/ph
(ph - dh)/dh
(ph - dh . dt/2)/ph

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

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