Process Equipment and Plant Design
For laminar flow (NRe < 2100) in steel pipe, the optimum inside pipe diameter is given by

Di, opt = 3 Q0.36 . ρ1.86 μ0.08
Di, opt = 7.6 Q0.36 . ρ0.9
Di, opt = 3 Q0.36 ρ0.18
Di, opt = 3.9 Q0.45 . ρ0.23

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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

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

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