Question

What would be an acceptable assumption of the over all heat transfer coefficient (U) for a shell and tube heat exchanger with air as the hot fluid (90psi @250 deg F) and water as the cold fluid?

Answer 1

The heat transfer coefficient of air is very less

for this system, cold water should be fed from the tube side, and hot air should be fed from the shell side.

The overall heat transfer coefficient should depend on the individual heat transfer coefficient of water and air

1/U = 1/h_w + 1/h_A where. W = water, A = air, U = overall heat transfer coefficient, h= heat transfer coefficient.

Assumption: heat transfer coefficient of water is l very large in comparison to air

So 1/h_w is neglected in the above equation,  Resistance of Air 1/h_A is very high (dominating)

then above equation may be assumed that U = h_A ( Overall heat transfer coefficient is equivalent to heat transfer coefficient of air)