dust collection duct sealing

[Lorus]

Howdy:

I am looking for some advice as to what I should spec to seal the high-pressure spiral ducting used for dust collection systems.

Max negative operating pressure will be 10"

So far, I believe the best way is to use a slip coupling, riveted 3 or so times or ether side of center, with a silicon sealer. No Duct tape allowed, as it will likely dry rot.

Any thoughts?

Anybody have a good experience with a company’s sealant for this type of application?

 

[krb]

Do a search for duct sealants on google. Then contact one of the manufacturers and see what they recommend.

 

[KenRad]

What kind of fittings are you using? There are several manufacturers who make pre-gasketed fittings for this kind of system (Lindab is a good one). When I use these fittings in dust collection or HVAC, I don't use any sealants at all. The fittings have an elastomer gasket that compresses on assembly, and does not leak. The added benefit of this kind of joint for dust collection is easy disassembly for cleaning or duct modifications.

---KenRad

 

[Lorus]

Thank you for the response KenRad.

After checking them out, I dont think they will work for me, as they are rated as seal class c, which SMACNA indicates is not acceptable for any ducting operating at a positive or negative pressure greater than 2".

 

[KenRad]

Lorus,

My Lindab catalog shows that it meets SMACNA leakage class 3 from -20 in WG to +12 in WG.

---KenRad

 

[CountOlaf]

Speaking of SMACNA, one of their publications is "Round Industrial Duct Construction Standards" (I've also got ones in my collection for rectangular as well as "Accepted Industry Practice for Industrial Duct Construction"). Anyhow (enough bragging about resources, right!), in section 5 on "connections" they state that "the most popular method of connecting industrial duct is by means of matched angle or bar flanges" and they offer a few tables based on duct diameter, weight of particulate, etc. and show you steel or aluminum flanges that will work. They also show details of welded bars/plates or angles with gaskets or sealant.

Regarding the specific sealant, they state that the gasket or sealant materials must be selected based on temperature and material being handled, so it sounds like doing a search on sealants and/or contacting reps is a good idea for the sealant portion (as krb stated). That being said, MASTERSpec calls out for flanged joint mastics: One-part, acid-curing, silicone, elastomeric joint sealant complying with ASTM C 920, Type S, Grade NS, Class 25, Use O; and for flanged gaskets: Butyl rubber or EPDM polymer with polyisobutylene plasticizer. This could be a good place to start.

SMACNA also has another typical connections detail for light gauge duct (I'm not sure what qualifies) which shows 4 types of joints in addition to the flanged methods:

1. Large and small end riveted joint ("slipped joint") where they call for rivets 3" center to center in addition to soldering (for steel duct)
2. Butt welded joint
3. Draw band joint where the axial length of the banded part is 4" min.
4. Beaded sleeve joint, also 4" min length; This last one is similar to the “slip joint” but it has an additional “fitting” with male & female ends, conceivably to allow the two same size duct ends to be mated. I suppose this could be riveted and soldered as well, although there would be twice as many required at each connection. It also may be one of those proprietary-gasketed types that KenRad was referring to.

By the way (KenRad or Lorus), is Selkirk Metalbestos one of those pre-fabricated manufacturers for this type of system, or are they just for the flue gas type jobs?

Olaf

 

[cme]

you want to contact united mcgill, i don't spec those gasketed fittings

 

[KenRad]

cme,

Is there a reason that you "don't spec those gasketed fittings" that we should know about? I have not only specked them, but have actually installed them myself. And in the ductwork world, they are the best thing since cordless drills and sawzalls. Anyone out there who has installation experience with these types of fittings AND the more traditional ones will agree.

You cut the spiral duct to length (most sizes come in 10 foot sections), push the pipe onto the fitting, install 3 or 4 self-tapping screws, and you're done. No sealants needed. Even an Engineer can handle that.

---KenRad

 

[cme]

Most of my spiral experience is in labs, max 60" dia. For that, I spec ductmate joints with exceptions on welding and slip couplings for different materials and locations.

Ductmate proseal is good to 15" wg.

 

[rogzog]

Something to consider in a dust collection system is the type of material being carried in the air flow. Depending on the density and abrasive quality of the material, failure to lap the connections in the air flow direction properly can cause the system to clog or erode away prematurely.

Hardcast or it's equal is a good sealing option.

rogzog

 

[dpoppeli]

Tape sucks since they all use adhesive - duct tape sucks the most since it doesn't even hold form like the aluminum stuff. Read the recent ASHRAE article.

Gasketed duct is great with Lindab ok for fine particles where likelyhood of cleaning out is low. If plugs are expected and you need to get into the duct use either siliconed spiral with vanstoned bolted angle-ring connections (12" diameter and larger) or even better and more expensive, one of the quick-clamp products which feature rubber o-rings such as Nordfab or the Kirk and Blum equivalent.

I suppose on the cheap, riveted slip coupling would work but watch that the edges are turned the right direction and use plenty of silicone. May want to install some cast-alumninum cleanouts too. Also, if using formed elbows these often need to be siliconed as well and watch the direction of installation, i.e. ensure that the folds are going the right way with the airstream.

Not sure how typical HVAC mastic holds up in material handling applications. Silicone does well.