Question

for a heat exchanger, why does increasing the number of tube passes increase the Re #?
why does increasing F factor increase number of shell passes?

Answer 1

Increasing the number of tube passes increases the overall length of the tube-side path and also increases the velocity at constant volumetric flowrate. The same thing happens on the shell side. Hence Reynolds number increases i.e

Re = d*V* /

The second statement should be in the form, increasing number of shell passes, increases F

LMTD correction factor (F) which is to be applied in cross-flow or counter flow heat exchanger increases with increase in the number of shell passes. On increasing number of shell passes, velocity of the flow increases which results in higher heat exchange in the system. Also higher temperature difference.