Process Equipment and Plant Design
In a shell and tube heat exchanger, triangular pitch arrangement as compared to square pitch arrangement

Results in higher shell side pressure drop
Creates relatively lower turbulence on the shell side resulting in lower shell side heat transfer co-efficient
Can accomodate less number of tubes for a given shell diameter
Facilitates easier shell side cleaning; hence is more suitable for handling high dirt factor shell side fluid

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

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