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Originally Posted by Carnut427
there is more wall area on the two 3/8" lines causing more restriciton, so I said they are roughly equivalent to one 3/4" line.
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The major factor for flow resistance is cross sectional area, which is a function of the square of the radius (diameter). Wall area is a minor factor, and doesn't really only come into play if you have two conduits with the same cross sectional area even though one has more wall circumference. The one with the greater wall circumference will have slightly higher resistance to flow due to higher boundry layer-flow interface, and therefore flow slightly less. The greater wall circumference effect would only be very significant if the cross sectional area is really small to begin with, or there is a very large discrepency between the two conduits minimum diameter or wall circumference.