Duct geometry question, or where's Euclid when you need him

[JJohnston]

I'm running the drop to my drill press, and I have to make the 90° angle at the wall, but I also want to have some offset to one side (see picture). I tried over and over to put this together with 2 45s and a straight piece, turning the 45s so they were in two planes, but it just didn't seem to want to fit. When the length was right, the fittings weren't pointing in the right directions, and vice versa. I finally gave up, and I'm just going to use flex here.

Is there a way to figure out how to do this? Is it even possible to join two ducts like this with a 45? I don't recall ever having to take anything beyond plane geometry in school.

[bababrown]

J Johnston,
I think you can do it with two 45s only if you are willing to turn the fitting on the wall such that it points to the upper pipe. Otherwise I think you will need either 3, 45s or possibly 2, 45s plus a 22.5 elbow. I have used 1/2" copper or PVC fittings and pipe to model this same kind of problem. Good luck,
bababrown

[JJohnston]

Oh, I pointed both fittings every which way, and I never could get them both pointing in the right direction.

The more I think about this, the more convinced I am that it can't be done with 2 45s. I've been turning the 45s around in my head, and I think they will either be in the same plane, or non-parallel planes - no parallel plane possible.

[McRabbet]

You should be able to do it with three 45's. One connects to the end of your overhead and stays in the plane of the ceiling while pointing toward the drill press. The other pair make up a 90 that connect on a plane through the end of the new 45 and the connection at your drill press. You'll need to play with the lengths of the straight sections to get it right -- I just took three 45's and recreated the situation on my benchtop. Clear as mud?

[bababrown]

J Johnston,
I think I did not make myself clear. Make a 90 degree ell with two 45 fittings and put it on the upper pipe with it turned towards the fitting at the drill press. Remove that fitting from the wall and put it on the other end of the 90 degree ell. You will need some lengths of pipe to get it exactly where you want. Then again fasten the fitting on the wall. The fitting no longer will be vertical on the wall but at a slant and I'm not sure that will be acceptable to you.
bababrown

[JJohnston]

You're right - I want the duct to run vertically on the wall. My plan is to build a narrow shelf between it and the bench just visible on the left. It's already pretty narrow, and I don't want to compromise it any more. I also want the vertical branch of the Y to go on down to a floor sweep on the left side of the drill press.

[JJohnston]

McRabbet, now you're talking about a PVC version of one of those sheet metal HVAC adjustable elbows (sort of). I tried to use those in my miter saw ducting, and they were a pain.

Thanks for the toughts; I'm going to use flex. I can have it done in 20 minutes, vs. ???

[McRabbet]

I suspect I'd use flex, too! Sure there is some minor SP loss, but it is lots easier.

[holmes56]

I suspect I'd use flex, too! Sure there is some minor SP loss, but it is lots easier.
so there is loss when using flex hose?How much loss is there when using flex?

[McRabbet]

The losses in flexible duct vary by the smoothness on the interior and, of course, by the tightness of any bend. According to Bill Pentz, the SP loss is 2 - 3 times the equivalent straight section of hard pipe -- see FAQ 11 (http://billpentz.com//woodworking/cyclone/staticcalcfaqs.cfm) in his discussion on his Static Pressure Calculator.