Process Equipment and Plant Design
In a rotary drier, the average retention time of solid is(where Z = length of the drier, metres ρ = apparent solid density, kg/m³ L = flow rate of dry solids, kg/see. m² drier cross-section H = hold up of solid.)

Z.H/L.ρ
Z.H.ρ/L
H.ρ/L
L/Z.H.ρ

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Process Equipment and Plant Design
For a cylindrical shell, (subject to the thickness of uppermost course being more than the minimum for diaofthe tank in question), the thickness of the courses of shell

Remains same throughout
May decrease or increase upwards depending upon whether vacuum or positive pressure would be maintained inside the shell
Increases upwards
Decreases upwards

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