Turn distance, connections, etc. DC

[Craig Behnke]

A few questions setting up DC ducting. It's my first time installing. I searched the forum but didn't find direct answers to the following.

I have a 3.5HP tempest cyclone with 7" intake and 6" main lines of spiral steel duct. The way my preliminary layout is shaping up, the main line run will be straight off the cyclone, about 12 feet with 3 total drops. About 3 ft out from the cyclone I may place a wye to make a secondary perpendicular run of about 12 ft, straight, with 2 drops. The main and secondary lines are all 6”.

I see some pictures of DC systems with ducts that have major turns or wye's as close as 18 inches from the cyclone. What is a back of the envelope way to gauge the trade-off in performance as you move a wye closer to the cyclone? With the way I’d like to route my ductwork, it would require a wye about 3ft from the cyclone body. Is that getting too close to being a major performance killer?

How do you guys connect pieces together? Do you use foil tape, silicon sealer? screws? Any combination of those methods? Do you seal just the outside of the connection or do you also seal the surface portions of the joint (where the two pieces overlap)?

Also, it seems like having 6” ductwork hanging from the ceiling would not be very stable (side to side). Is it okay/good to have it be able to move a little? Should I try to get it stable (side to side).

Thanks for the input.

[Nick Lazz]

Craig,

If you already bought your steel duct, I guess you might as well use it. Very nice stuff...however you could save a significant amount of money by using plastic.
There are numerous outlets you could use to help design your system. With that said, and I'm no expert, you want to try to have your first few feet at least, going straight into cyclone. I was unable to do this based on my application and it hasn't seemed to hurt performance, however I'm not sure how I would really know. Obviously sweeping 90's and reducing the amount of transitions is key. Also, ensuring your connections to your machines are as large as possible. I have 6" lines going right up to each machine, and recently removed the 6 to 4 that went to my table saw and made it so I could connect the 6 directly to the cabinet...it made a difference!

Don't use anything to seal your pipes together. I doubt you will have enough loss to be noticeable. If you simply just have to do something, absolutely do not use silicone. If you ever want to make a change you just created a lot more work for yourself. On my system, all of my fittings are simply dry fit.
To hang my duct, I used sheetmetal straps to wire to anchor. There is no side to side support. You don't need it.

Have fun, and try not to over think it like I originally did (and tend to do with most of my projects) and I am sure your system will really suck! (pun intended)

Nick

[Paul Wunder]

Three feet should be OK. I have a ClearVue and they suggested at least two feet of straight duct before turns. Your two main runs are short and that is good. Although as the OP noted you may never know. Good luck!

[ray hampton]

the only reason to use screws or tape [duct tape of course ]is to keep the sections of tubing from falling apart

[Ole Anderson]

Sorry, Nick but I will have to disagree on a couple of points. That steel is way more expensive than plastic, sure if you use 24 gauge spiral pipe, but if you use 26 gage snap lock, you are at a buck a foot, plus or minus for the pipe, no big deal. Of course Craig already has the spiral so it is a moot point. Second you will suck air into all of your joints if you don't tape all of the straight sections and joints and use a sealant (silicone works very well) to seal the twisty joints on the bends. If you don't, you will hear a giant sucking noise at each joint.

I agree with your other points, including my having a flex aluminum sweep at the cyclone inlet didn't seem to kill my air flow.

Craig, if you use three 1/8" x 1/8" rivets on each joint at 12:00, 4:00 and 8:00 then wrap the joints with aluminum tape, your pipe runs will have almost no flex.

Check this out: http://www.sawmillcreek.org/showthread.php?158878-DC-Steel-Duct-Install&highlight=

[ian maybury]

Duct pricing seems fairly variable. Over here spiral from and industrial supplier is cheaper than PVC - i'd imagine that a manufacturing industrial supplier of spiral will be cheaper than a retailer in the US too.

You see various numbers listed for the length of straight duct run required before the cyclone inlet - sometimes several metres, although in practice it's probably better expressed in numbers of duct diameters. So far as i know the consideration is that turbulence can upset the dust retention performance of the cyclone. Trouble is that since nobody seems to publish good test data on the retention performance of their cyclones it's tough to even differentiate between models, never mind know what the reality of the effect of bends or connections might be. You certainly see installations with connections and bends very close in that seemingly work fine. So it's at this range a rule of thumb sort of deal - which may or may not hold water in all situations.

Whether or not you need screws and tape depends a lot on the type of ducting you use. PVC has seals built in, but spiral duct and fittings can have fairly loose fits, enough that you can lose substantial CFM to leakage. It's not impossible to end up with enough open area in your joints to account for say 25 - 30% of the cross sectional area of the duct in a long run.

A UL approved mastic backed foil tape seems to be a good choice, duct tape apparently doesn't last and i'd be wary of the lighter tapes.

Rivets allow you to pre-assemble sections on the ground (which is handy where alignment matters, and allows full access to the joints for riveting if the duct is going close to a ceiling), prevent any movement being concentrated into a single joint which is probably better for tape life, and are easily drilled out for modifications. Aluminium rivets are OK, but may shear when handling sections if they get loaded - so maybe there's an argument for stainless.

Chances are that the duct will be stabilised if there are branches at angles with hangers too. Drops attached to walls will stiffen things up too.

Not too sure what you guys use to hang ducts, but i found the purpose made steel split rings sold for use with spiral ducting worked well - they have a very convenient arrangement where a tab holds them closed even while the screws are loose. So you can open the ring, pop the duct in place and then swing and clip the ring shut - you only need to tighten the screws when you are ready.

ian

[Dan Hintz]

If you already bought your steel duct, I guess you might as well use it. Very nice stuff...however you could save a significant amount of money by using plastic.


Sorry, Nick but I will have to disagree on a couple of points. That steel is way more expensive than plastic, sure if you use 24 gauge spiral pipe, but if you use 26 gage snap lock, you are at a buck a foot, plus or minus for the pipe, no big deal.
I think you guys are on the same page... plastic is cheaper.

[Craig Behnke]

some clarification. i already have steel spiral duct, it came with the DC that I bought used off of CL. sad story, the bloke that originally bought it had the whole system, down to the last detail, designed by the DC company but he passed away within 3 months of buying it and never had a chance to open the boxes. Anyway, I have steel spiral duct now, so I have to run what i brung.