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

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
Is always more than arithmetic mean value and the geometric mean value

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

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

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