Process Equipment and Plant Design
In the case of a shell and tube heat exchanger, the logarithmic mean temperature difference

Is always more than arithmetic mean value and the geometric mean value
Is always less than arithmetic mean value, but more than geometric mean value
May be either more or less than geometric mean and arithmetic mean value depending upon whether the flow of stream is co-current or counter-current
Is always less than arithmetic average value

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Process Equipment and Plant Design
Vertical condenser is advantageous to the horizontal condenser from operation point of view, when

Both the functions of condensation & sub cooling are carried out in a single unit
Sub cooling of condensate is desired
Only the function of condensation is to be carried out
Hydrostatic head is required for refluxing the condensate

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Process Equipment and Plant Design
The function of manholes provided in the shell of a distillation column is to

Guard against foaming & entrainment by dumping antifoaming agent through it
All of these
Give access to the individual trays for cleaning, maintenance and installation
Keep a check on the liquid gradient over the plate by direct visual observation

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