Process Equipment and Plant Design
For turbulent flow (NRe > 2100) of low viscosity fluid (μ > 20cp) in steel pipes, the optimum inside pipe diameter is given by(where, Q = fluid flow rate, ft³/sec.ρ = fluid density, lb/ft³ μ = fluid viscosity, centipoise Di = optimum inside pipe diameter, inches)

Di, opt = 3.9 Q0.45 . μ0.95
Di, opt = 4.7 Q0.36 . μ3.2ρ0.13
Di, opt = 3.9 Q0.45 ρ0.13
Di, opt = 3 Q0.36 . μ0.88

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

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
Perferably flow outside the tube
Preferably flow inside the tube for its easier internal cleaning

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