Difference btn flows in a circle vs. rect.

[jrnewma]

I am looking to bypass an existing sewer pipe with another pipe. The existing pipe is rectangular, approximately 9'x5'. I am looking to use a circular bypass pipe of 6' diameter. Since the velocity of "water" flowing thru the pipe will be the same then I need to compare the surface areas, correct? The surface area of the 6' diameter pipe is 28.3 SF. Normally to find the surface area of the rectangle I would take l*w to get 45 SF BUT since I am comparing flows do I need to use Hydraulic Diameter of the rectangular pipe and then find the surface area using pi*r^2? Doing it that way I get a surface area of 32.2 SF.
Can I do that? Is that because the flows thru a circular pipe has less turbulence? thanks.

 

[maxx_payne]

the majority of piping systems specially in liquids using round pipe to make the flow is smoother and reducing eddy losses also because of the hydraulic diameter and also case of pipes fittings it will be easier to work on a circle than a rectang