> tubing and short hose connectors. Not sure of what size yet but
> will be
> larger than 5/8" but smaller than 2". I am leaning toward 1.25" or
> 1.5".
>     I am considering running the alum tube inside a duct that I
> can put
> cooling air through.
>

Kelly,

I ran some numbers through the spreadsheet I created.  This assumes that we need to maintain a 20cfm flowrate.  I got the data and equations off the net.  Do a google search with "flow resistance weldbend". The first link takes you to a page that explains 'bend equivalence' and the next page (link at bottom) explains how to calculate flow rates and back pressure.  I used that to work up the following chart:


14ft 6ft 2ft
dia wt psi wt psi wt psi

.5 4.76 2574 2.04 1103 .68 368
.625 7.44 843 3.19 361 1.06 120
.75 10.71 339 4.59 145 1.53 48
.875 14.58 157 6.25 67 2.08 22
1 19.04 80 8.16 34.4 2.72 11
1.125 24.1 44.6 10.33 19.1 3.44 6.4
1.25 29.75 26.4 12.75 11.3 4.25 3.8

I would appreciate any sanity check that anyone has to offer, but based on these numbers I don't think my original plan would be optimum.  From the equations on the above referenced page, the biggest contributor to the backpressure is the flowrate as its value is squared.  

Questions for the list.
  How does the cooling performance drop as the flowrate goes down?
  Is 20cfm the very minimum flow necessary to provide adequate cooling?

I've attached the gnumeric spreadsheet file, just in case anyone wants to check over it.