Originally Posted by
David L Morse
Excellent point about the "wetted periphery", it's often forgotten. You can quantify the effect of both by considering that pressure loss per unit length in a duct varies with the square of the flow rate and the inverse fifth power of diameter*. So, for two 4" ducts compared to one 6" duct the pressure drop for the two 4" is (1/2)^2 x (6/4)^5 = 1.9, or almost twice that of of the 6".
* Neither are exact because of the variation of friction factor with Reynolds number, but close enough.
I'll just pretend like I followed the math.
David Walser
Mesa, Arizona