CFC tube sizing

I am appealing to the vast knowledge database here in order to figure out this problem. I have noticed that many of the commercial CFCs (even the non convoluted one) are 1/2" on the inside and 7/8" on the outside. It seems that when people build their CFCs they always go 3/8" inside and 5/8" outside. I get the idea of reducing to 3/8" for the inside tube in order to increase surface area and reduce the size of the center flow, but wouldn't using 7/8" for the outside make more sense?

I am not an engineer or a welder and this is going to be one of my first projects and I want to do it right. My thought is that 7/8" copper for the exterior would allow more cold water and allot more heat to be exchanged. If you consider that most CFCs have the initial water coming out steaming hot isn't there a call to maybe allot for more water flow?

Thoughts please!

Also, has anyone seen square copper tube? I have looked with the thought of trying to build a convoluted CFC, but can't find any.

I like that idea. I just went the in the opposite direction on accident. I just stuffed a 1/2" OD copper in a 5/8" hose! I am not sure if I have enough surface area for the coolant.I may have to pump...

That square tubing would be slick too. Expensive I bet.

I spend some time working out the math on various combinations of inner tubes and outer jackets. It's about midway down the page: Reference Materials

Area is in square inches:

CFC Configuration #1 3/8" OD Copper in 5/8" Hose
Wort Flow Cross Sectional Area 0.078
Coolant Flow Cross Sectional Area 0.196
Coolant to Wort Ratio 2.5

1/2" copper which is a typical supply to a hose bib is only 0.254 sq in of cross section so the coolant path in that CFC isn't much of a restriction.

On the other hand...

CFC Configuration #2 1/2" OD Copper in 5/8" Hose
Wort Flow Cross Sectional Area 0.149
Coolant Flow Cross Sectional Area 0.110
Coolant to Wort Ratio 0.7

I wouldn't be too happy with that design. It's going to flow the wort too fast in relationship to the coolant flow.

But what about using 3/8 inside of 7/8?

Edit - Sorry, just looked at your page and it looks like it comes out at a 2.0. So the 3/8 inside 5/8 is really the best option eh?