How close to DC fan can I put an elbow in ductwork

[Craig Behnke]

have to re-route my ductwork, new way calls for a two 90 degree elbows to raise the ducting shortly after the cyclone. the elbows are about 30 inches away from the cyclone body. the dc is a 3.5HP penn state industries tempest, the inlet main is 7 inch, reduces to 6" about 12 inches out of the cyclone, the elbows are 6" long radius premium steel elbows from penn state industries.


i know straight duct is better than curved, i know that if curves are necessary, further away from the fan is better..., with that said, what are some guidelines for performance impact in that 30 inch range? are we talking reduced performance of 10%, 30%, 50% with curves in that 30 inch range. I have no idea.

If the performance drop is too great, i'll look into moving the cyclone higher so I won't need the elbows...but that would involve cutting a hole in the ceiling and having the motor in the attic. that's not a deal breaker, but i'd rather avoid it

[Craig Behnke]

here is a sketch of the routing

DC Elbow Sketch.pdf

[David Kumm]

You would be better off to put two 7" 45s with a diagonal 7" pipe and reduce later or better yet after the first wye. You will gain more than enough with the 7" to offset the angles. Dave

[Craig Behnke]

You would be better off to put two 7" 45s with a diagonal 7" pipe and reduce later or better yet after the first wye. You will gain more than enough with the 7" to offset the angles. Dave

sorry, forgot to include this info in the OP...i tried that option already, but I decided not to use the two 45 degree connectors because they wind up interfering with an existing drop and another 6x6x6 lateral branch off that runs to my TS and Router. if I was to use the two 45's i would wind up having do a bunch more re-routing and yada yada. long story short, if the 90 degree elbows are a minor performance penalty i'm okay with it because the total run is only about 18 feet so the 3.5hp is enough to handle it as is, but i'd probably not be pleased if performance dropped something like 50%

[James Baker SD]

I'm interested in this also. I had similar problem with getting the ducting up to the top of the ceiling from the cyclone inlet. Raising the cyclone was not an option for me as it is in the basement below the master bedroom, don't think I want to sleep with the motor sticking up through the floor :-). I ended up with a pair of 7" 45's separated by about 4' of straight pipe. Hopefully I am not losing too much performance either. A long initial straight pipe from the cyclone just wasn't in the cards.

[Ole Anderson]

Straight is better, but it will still work fine with some curves. I used 7" alum flex from HD right after a 45 degree elbow out of my DC inlet.

[Steve Peterson]

Separation is better if you can avoid turbulance close to the cyclone. There are calculators on Bill Pentz's site that show how much pressure loss there will be for each elbow. If I remember correctly, each 90 degree elbow has the same resistance as 6-12 feet of straight pipe depending on the radius. Two 45's are no better than a 90 degree elbow according to the calculators except that they would use slightly less straight pipe.

Two elbows are not going to kill the performance. The worst case would probably be a bit more fine dust in your filters. It is probably easier to clean them occasionally rather than to re-design everything and cut holes in the ceiling.

My Clearvue has about 8' of straight pipe before the first bend. The dust shroud around my lathe ends up with about 40' of pipe and 8 45 degree bends. It still sucks like crazy.

Steve

[Michael W. Clark]

From a cyclone performance standpoint, if you have 6 diameters of straight, you are golden. If not, the main thing is that you do not put a horizontal elbow in that would turn the air counter-current to the rotation of the cyclone.

From a system loss standpoint, the two 45s (or some combination of 90 degrees) will be about half the loss of the two 90s.

[David Kumm]

If you need two 90s, I'd still use 7" to the first drop. I don't have the chart at hand but there is quite an advantage to increasing the ells. Dave

[Craig Behnke]

From a cyclone performance standpoint, if you have 6 diameters of straight, you are golden. If not, the main thing is that you do not put a horizontal elbow in that would turn the air counter-current to the rotation of the cyclone.

From a system loss standpoint, the two 45s (or some combination of 90 degrees) will be about half the loss of the two 90s.

so for this calculation, using the 7 inch inlet, or the 6 inch main, would mean 42 inches, or 36 inches, of straight before the first bend. I'm pretty darn close at about 30-32 inches.

If I were to use two 45's, it only gets me an additional 6 inches of straight off the cyclone but it would force me to add another 45 to get the ducts back to my TS and router.

sounds like it's a close call, so i'm going to go with the one that doesn't require additional spending and time and energy to further re-route.

I'll try to take pics and post them tomorrow

[Michael W. Clark]

so for this calculation, using the 7 inch inlet, or the 6 inch main, would mean 42 inches, or 36 inches, of straight before the first bend. I'm pretty darn close at about 30-32 inches.

If I were to use two 45's, it only gets me an additional 6 inches of straight off the cyclone but it would force me to add another 45 to get the ducts back to my TS and router.

sounds like it's a close call, so i'm going to go with the one that doesn't require additional spending and time and energy to further re-route.

I'll try to take pics and post them tomorrow

The cyclone performance should not be affected with your arrangement. The two 90s do add some additional loss for the fan to overcome. If you can do them in 7", the loss would be minimized.

Mike

[Craig Behnke]

I'll try to take pics and post them tomorrow

Here it is.100_4859.jpg

[Wade Lippman]

From a cyclone performance standpoint, if you have 6 diameters of straight, you are golden. If not, the main thing is that you do not put a horizontal elbow in that would turn the air counter-current to the rotation of the cyclone.

So if you take a particular DC and use 6" pipe you have to allow 36" for the first elbow. But if you are using 8" pipe you have to have 48". That doesn't make any sense; the 8" should require less, not more.

From a system loss standpoint, the two 45s (or some combination of 90 degrees) will be about half the loss of the two 90s.

Hard to argue with that; since two 45s are half the bend of two 90s, that should have half the loss.