Process Equipment and Plant Design
In a shell and tube heat exchanger having square pitch, the shell side equivalent diameter is given by (where, P = pitch, d = outside diameter of the tube)

Πd/4P²
4(P² - πd²/4)/πd
(P² - πd²/4)/πd
4P²/πd

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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

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

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