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

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Process Equipment and Plant Design
Triangular pitch tube layout as compared to square pitch in a shell and tube heat exchanger

Facilitates comparatively easier external cleaning because of large clearance
Both B and C
Permits the use of more tubes in a given shell diameter
Permits the use of less tubes in a given shell diameter

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