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

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Process Equipment and Plant Design
When one of the fluids is highly corrosive and has fouling tendency, it should

Preferably flow inside the tube for its easier internal cleaning
Perferably flow outside the tube
Flow at a very slow velocity
Flow outside the tube, when the flow is counter-current and inside the tube when the flow is co-current

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