Process Equipment and Plant Design
In a double pipe concentric heat exchanger, the equivalent diameter of annulus for heat transfer would be (where, D₁ and D₂ are I.D. of inner and outer pipes)

None of these
D₂ - D₁
(D₂² - D₁²)/D₂²
(D₂² - D₁²)/D₁²

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Process Equipment and Plant Design
Overall distillation column efficiency for most of the refinery columns can be given by (for μ = 0.07 - 1.4 cp and relative volatility < 4) (where η = overall column efficiency, % μ = molal average viscosity of feed at average column temperature and pressure, cp Δp = pressure drop per tray, psi)

Η = 1.66 - 0.25 log μ
Η = 0.25 log μ - 1.67
Η = 17 - 61.1 log μ
Η = 24.6 (Δp/μ)

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