Help choose Ducting size - explain fan curves to me

[dbhost]

I went with HVAC ducting in 5" due to cost. I had to hammer out some crimps, and crimp some smooth ends, but got it to go. Connections will pull themselves apart if you do not have SOME kind of physical fastener. I used pop rivets, 2 in each joint 180 deg from each other and then tape to seal it. SO far so good...

[mpc]

Unless you have a really good hanger system, tape will not be enough to hold the joints. Screws or pop rivets, as dbhost just posted, will be necessary. I used HVAC foil tape on my setup and it worked well. Applying the tape around the wye fittings - the actual "Y" joint wasn't particularly air-tight on my piping - was the worst part. Several shorter tape pieces worked best. I did not try the sealant goop; that may work better on the wyes.

My hangers are "J" shaped metal straps that began as straight strapping from the Borg's plumbing section. They have holes every inch or so and are about 12 to 14 inches long... I don't remember the details now. I made a wood jig to bend them consistently. Screws attach them to wall studs and the ductwork. With my hangers there is still some motion in the ductwork - it is not perfectly rigid - so screws at the ductwork joints were essential. I ended up with 3 short hex-head screws at each joint, positioned so I can access them when the ductwork is still hanging in case I need to disassemble something or want to make changes. I've already changed my layout a bit as I swapped a couple tools around. Removing the old tape, rather than simply cutting it along the joint and leaving the old tape to be covered by new tape, is the biggest hassle. It just bugs me to think there is old/junk tape underneath any new tape applied to re-configured joints.

I found my metal blast gates (from Woodcraft) leaked quite a bit too - along the seams of the two main halves. The leaks were readily audible... so more tape.

mpc

[jussi]

Ok so I have another question. I have an overhead dust collection on my tablesaw which has a 4" port but tapers down to 2-1/2" Similar to the pic below. I'm going to have a wye around 6 feet before the tablesaw where one end will go to the tablesaw cabinet and the other to the overhead dc. My question is should I run 6" to the overhead dc as well or just reduce that down to a 4" immediately.

[mpc]

Where is the wye you are adding? Is it near floor level at the table saw itself or way up at the ceiling? I.e. are the two branches of the wye, going to the saw cabinet and the blade guard pickup, vertical runs or mostly horizontal runs with the output of the wye then going vertical?

In your guard pickup, you said the minimum diameter is 2.5 inches so that will be the flow bottleneck in that connection, limiting the total CFMs in that branch of the wye. I would not go larger than 4 inches from the guard to the wye for either a horizontal pipe run or a vertical run. Especially for a vertical run, an oversized pipe will have too low air velocity to reliably carry larger sawdust bits.

You may want to think about adding a blast gate or other means to control airflow in the two branches of the wye. Air is going to take the path of least resistance and you may find you get too much airflow in one branch and not enough in the other. A half-closed blast gate lets you restrict the too-good side, forcing more airflow into the other side, for better overall dust collection. I had to do this on my bandsaw setup: a wye branches to the lower wheelhouse dust port and to a flex hose just below the table to keep the lower guide bearings clean. I had to choke off the wheelhouse port a bit to get more suction/flow at the bearings. So even though the total CFMs got reduced, I got better dust collection overall and especially where I most needed it: at the lower guide bearing assembly. This is a common trick.

mpc

[jussi]

The wye is probably 6 feet away from the tablesaw. The wye will start at the ceiling. One branch will continue on the ceiling and then drop down to the overhead guard. The second branch will turn 90 degrees down and into the floor then come up and into the cabinet of the tablesaw.

The way my ducting traverses I would need 2 separate blast gates anyway. Is it ok to use SD piping for that 4" section. I still have tons of it from the old setup.

[capncarl]

If your blade overhead guard is like like mine and others I’ve seen upsizing your duct would be useless. My upgraded SawStop blade guard/dust collection uses something resembling 1 1/2” hose, while their “premium” overhead collection uses 3 inside, 4 outside,whatever that means. If that was the case I definitely wouldn’t go to the trouble of changing over to 6”