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

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Process Equipment and Plant Design
In a double pipe (concentric) heat exchanger, the hydraulic radius for heat transfer (for a fluid flowing through the annulus) would be

Same as that for fluid flow
D₂ - D₁ (D₁ and D₂ are I.D. of inner and outer pipes respectively)
Less than that for fluid flow
More than that for fluid flow

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