DC Duct Decision

[Dan Case LR]

Background: I've got a 1.5HP DC with a Thien baffle and Wynn canister filter, connected to a 6" PVC duct system. I'm looking at an upgrade to a cyclone in the 3 or 3.5 HP range. The cyclone will be in a different location than the current DC, which will require a complete rework of the duct system--but I'm rearranging tools at the same time, so I'd need to do that anyway.

The Decision: I originally planned on rebuilding with 7" steel mains and 6" drops. Then I started pricing that out and it doubled the cost of the upgrade-a real budget killer in this case. I've mitigated that a little with some careful shopping and splitting the order between multiple sources to get the better overall pricing, but it's still more than I have right now. My alternative is to re-use the PVC pipe and fittings I already have and add some extra pipe and a few more fittings. This can get me into a comfortable range financially, but I don't want to put money into something I know won't be a long-term solution..

I know there are lots of installations with 6" PVC pipe throughout, and I don't want to revisit the dreaded "PVC or Metal" debates--so let's ignore the material and talk just in terms of duct size. What am I sacrificing (in terms of performance) by using all 6" pipe vs 7" with 6" drops? Am I better off to place this on hold until I can do the metal ductwork?

My biggest air demand is a table saw with 6" port below and 4" on top.

What do you think?

Thanks!

D.

[Tom Hartranft]

If you only run one woodworking machine at a time, then 6 inch main coupled with 6 inch drops appears to be fine for a 3 to 3-1/2 hp cyclone. However, if you generally run 2 machines at once that need dust collection, then 7 inch mains coupled with 6 inch drops may offer noticeably better performance.

Since you already have your shop set up with 6 inch mains and drops, you might just use this setup with your new 3 to 3-1/2 hp cyclone for awhile to seat-of-the-pants judge performance. Might even install a pitot tube in your main line to measure and map velocity profile in order to calculate system CFM ... plus install static pressure tap manifold at entrance to cyclone. This'll get you a system performance curve that shows CFM vs static inlet pressure for each machine on your system. Compare to manufacturer's performance curve to judge the effectiveness of your 6 inch mains' performance.

I have 6 inch mains and 6 inch drops along with 6 inch openings at my machines with longest main ~25 ft from cyclone to 6 inch drop. Sucks well at the machine (tablesaw) and my pitot tube velocity measurements coupled with inlet static pressure show a good CFM in the 6 inch main for keeping the dust suspended in the main duct.


Tom

[Anthony Whitesell]

Any chance of a duct layout picture?

You are already limited at the TS because of the 6" main and the 6" and the 4" ports of the saw. Any time the square inches of openings (41 sq in) exceed the trunk line cross section (28 sq in), you can't pull full flow from both and something lags somewhere.

The request for the layout is two fold. First more information for all. Second, if by chance the TS was first inline I would suggest increasing the trunk to 7" (38.5 sq in cross section) until you reach the TS then either split to a 6" and 4" drop or drop at 7" and split closer to the saw. The point being to merge the 6" and 4" ports into a 7" line and skipping the restriction of a 6" duct/gate, etc. If you are lucky that the TS is first in line then you could split the cost and only partially upgrade the duct work.

Otherwise to know the effects of the 6" versus 7", it would depend on the fan curve and the length of your ducts.

[Dan Case LR]

Unfortunately, all the layout files are at my house, and I'm not.

I can't do the classic diagonal trunk with drops that I'd do with a large open area. I'm in a 2-car garage (sans cars), so I have to work around that blasted garage door and those pesky ceiling-mounted 8 foot fluorescent lights. Add to that a stepped ceiling profile--the ceiling in the back 1/3 of the shop is about 18" lower than the front.

The cyclone will be in a storage room off to the side at the front (closest to garage door). I split the main immediately on entrance to the garage, one that runs under the garage door to serve the table saw and 6" jointer, located across from the cyclone with just enough room for a clear walking path (it takes advantage of a wide opening into the adjacent storage room for extra outfeed room when needed). The other half of the split runs above the garage door and splits again, one to serve machines on the same wall asthe Cyclone and the other to the wall on the other side with a drop at the lathe along the way.

I'll try to post the layout later on when I'm at the house.


Thanks!

D.

[Anthony Whitesell]

...I split the main immediately on entrance to the garage, one that runs under the garage door to serve the table saw and 6" jointer, located across from the cyclone with just enough room for a clear walking path...



I did miss the part where you mentioned you were reworking all the duct work anyway. But you also mentioned a full duct work upgrade may be over budget. It sounds like you may have upsized the ports on your tools. Unfortunately PVC doesn't come 7", so providing the trunk is upgraded all to 7" you will have a transition from 7" metal to 6" PVC somewhere. The trick becomes to find the best spot to get the most bang for your buck. Under the premises of spend-some save-some and maintaining as much 7" trunk as possible, here are my thoughts.

First, run 7" from the cyclone to the trunk split. If this stays at 6" there isn't any sense in changing anything else.
Second, increase the trunk from the split to the TS, reworking the TS connection so the 4" and 6" lines join into a 7" drop or the trunk itself. Then reuse the old duct, reducing to the jointer.
Third and on, would be to increase the other split to the next junction up to 7", and repeat until you have spent too much.

This is still all predicated on having the correct CFM, SP, and fan curve to be able to make the 7" duct work.

[Dan Case LR]

Ok, here are a few simplified exports of the layout.304905304902304903304904

[Ole Anderson]

I am really surprised that going from 6" to 7" doubled the price. Where are you sourcing your pipe and fittings? No good reason for that amount of price differential. 7" is a fairly standard HVAC size. In looking at your layout, I would go 7" at least for the long ceiling run to your row of inlets on the opposite side of the shop. And I would look at two separate wyes instead of the triple/single you now have just outside the closet unless you can find that triple fitting without breaking the bank. Nice layout by the way.

[Jesse Busenitz]

Dan, I just put my system together last spring, and I'm a one man shop, so I used a 3hp cyclone, ran 6" PVC across and down,(my longest run is about 35 feet) and even if another gate is open I still get good collection. Could I get better? Probably... I haven't even worked at sealing up all the joints (one of these days....)so I'm sure I'm loosing some there, but I designed the whole system and ran it by Oneida since I bought their cyclone and they said the duct size should be fine.

[Anthony Whitesell]

That is a nice and simple layout. Long runs but pretty lose-less. As mentioned by Ole the triple wye may be hard to find or break the bank. I would suggest changing the triple wye to a true Y (intersection that actually looks like a Y). Then extend the run to the lumber rack to intersect the run over the doorway over the doorway with a regular branch Y (one that is straight with a 45 take off). This may not only reduce cost (removing the triple Y) but also removes the back to back 90* bends (very high SP losses).

[James Baker SD]

I also had a multi-level ceiling as you do (heating ducts for the house above). I did not want two 90 degree elbows close together when I had to change heights so I used two long radius 45s and a straight pipe between them. Not a neat and close to the wall as it could have been, but it hasn't interfered with anything I do yet. I also had 8' tube lights in the way. Some of them I managed to dodge, others I took down and moved them (a pain, but worth it in reduced complexity of the layout).

EDIT: I see you have used 45s for one height change, so I apologize for not looking before speaking.

[Dan Case LR]

I am really surprised that going from 6" to 7" doubled the price.

I never said that--what doubled the price was replacing the existing 6" PVC with 7" metal mains and 6" drops.

D.

[Ole Anderson]

I never said that--what doubled the price was replacing the existing 6" PVC with 7" metal mains and 6" drops.

D.
Now it makes sense. :o

[David Kumm]

IMO 7" is a big deal. Have you checked local HVAC installers? Some have bone piles of left over spiral. 7" for some distance and around a corner can really improve flow, even when only using one machine with a smaller port. If you can convert to pvc you can add and switch over time. Some of my best quality fittings came used from installations that were modified or removed. You can always rebuild over time. Dave

[Rick Potter]

Just curious, Dan. What size is the inlet on your 3HP cyclone? That should determine the size of the main line. My 3HP Onieda has an 8" inlet, and that is my main size, dropping to 6" feeders with 4" reducers at the end, when applicable.

I got my spiral duct at a local HVAC wholesaler, who sold to the public also. The spiral came in 10' lengths with crimps on one end. It was cheaper than plastic.

[Dan Case LR]

Thanks everyone for your input--very valuable!

I had already been playing with removing the cross-lateral (3-way) when the suggestion came up. Here's a stripped-down look at the updated plan:
304925304926

Cutting the $$$ for the cross-lateral will save a few bucks, and the BOM is a little lighter as well. I'm going to shoot for low-end metal (26 ga or heavier) and try to make the metal work with the budget. I hate building stuff I know I'm going to tear down later. Seems like false economy.


Thanks again, all!

D.

[Dan Case LR]

Just curious, Dan. What size is the inlet on your 3HP cyclone?

Both of the options I'm considering have 7"inlets.

D.

[Anthony Whitesell]

I keep trying to rotate (orbit) the pictures for a different view. LOL. I see you moved the duct out from the stepped wall. In that case the other end has some curves that may be able to be reworked. Keep in mind curves are bad on the SP and bad on the budget. The fewer the better for both. I was hoping the two branches toward the back of the shop could intersect along the wall to the right (DC side). Curve-curve or curve-curve-curve setups are also bad on the SP. The first drawing look as though they may be able to intersect near the back of the door way on the right side.

I agree on building stuff to just tear down later. But it isn't as bad if you already have the stuff on hand and aren't spending additional money to just throw away later. My phrase of late is "I don't have much time nor money. But I have a lot more time than I have money."

[Dan Case LR]

The duct is moved out from the stepped wall to avoid an obstruction (Garage door opener). I could (and probably would) straighten it out if I use an adjustable elbow, but for design purposes I'm staying with 45's and 90's.

D.