Which way should I do this DC drop?

[Dan Case LR]

I'm adding an overhead DC drop for the Shark Guard I'll be waiting on for the next six weeks. I don't need to rush, but I want to get the PVC work in the shop done and get back to working with wood. :)

I have a conveniently located 6" PVC trunk that runs right over the place where I need to drop to, so all I need to do is add a wye and drop about 3 feet to a blast gate, then a 4" hose to the SGK1. No biggie, but here's the question: There are two ways I can do this.


I can insert a 6-6-6 wye and drop 6" to a 6" blast gate, then reduce to 4" for the hose, or
I can insert a 6-6-4 wye and drop 4" pvc to a 4" blast gate and the hose.

Does it make any difference in efficiency or performance to do this one way or the other?

Any reason to avoid one option or another?

Or am I just over-thinking this? :)

Thanks!

D.

[Dan Friedrichs]

It won't make a significant difference, so I'd go with the 4" drop, just because it's smaller and less obstruction.

The smallest diameter duct in the chain will essentially limit the airflow. Technically, you get more friction losses from the 4" duct than you'd get from 6", but when using PVC over such a short distance, these losses are extremely negligible.

[ian maybury]

I guess the theory says Dan that if you reduce to 4in anywhere in the line that the friction loss is not much influenced by how long the restriction is - a short restriction has a similar effect to a longer one. If as Dan (2) says you were running a long distance through the 4in then it would reduce the resistance a bit to use 6in for most of the way, and then drop down to 4in. That's not the case here though - so probably best to go with 4in for convenience.

ian

[Mike Cruz]

Dan C, yes there is a difference. From what I understand about dust collection, whenever you run a vertical drop, that drop should be consistent the entire way. The reason is that if you have a 4" port at the machine (whichever machine) and you run 4" pipe up for a certain distance, then make it larger (while still going vertically), the air speed will drop lower than what will be required to keep the dust particles air born, so they can accumulate or settle...likely at the blast gate (or wherever you "reduce"). My guess would be that you would get residuals falling down each time you turn off the saw if you did the first option. Your best bet is to keep it consistent.

[Dan Friedrichs]

Mike has a good point! If you suck up a small chunk of wood, there may be enough velocity in the 4" section to keep it moving upward, but once it hits the 6" section, the velocity drops, and it could just be suspended at that junction until you shut the blower off and it falls. I frequently have that problem with my 4" floor sweep hose that connects to a 6" vertical duct.

[ian maybury]

It wouldn't be the wisest to step up in the vertical Dan, that's for sure - the velocity would drop to less than half from 4 to 6in. On the other hand that's the step up you make in going from a 4in sweep up to a 6in horizontal header anyway - but it's easier on the horizontal.

In practice it probably depends a quite a bit on your fan and set up. My 4in sweep pulls through the longer of the Rockler hand set and slinky hose kits up a 4in vertical and into a 6in ceiling level horizontal header powered by a Clear Vue CV Max. With no other or one blast gate open it's pretty aggressive through the floor sweep - i doubt if it'd care much whether there was a step in the vertical or not. I've tested it with some small wood blocks set up this way, and it really hammers them up the hose and back through the 6in (actually 160mm) ducting.

Open a few more blast gates and it'll still do it, but it's not quite so fast through the 4in section (it belts off again once it's into the header though) and there's obviously less suction at the floor sweep - but it's not a situation the system was designed for anyway. I'm not sure, but i've a feeling the cyclone might not separate as well either at the resulting but practically irrelevant high CFMs.

It suggests though that a fan pulling a significantly lower level of suction might find that situation a bit more borderline???

ian

[Dan Case LR]

Sounds like the drop for the Shark Guard will be all 4" for sure. Somewhere in the back of my mind (along with a bunch of really scary stuff), I knew that the drop in velocity can present a problem on vertical runs. Now that I've been reminded of that, I need to reevaluate the (not yet installed) run to my SCMS, which will also have a vertical drop. I've been looking at a solution for collection at the SCMS that uses 4" hose/pipe and wishing it were available for 6" instead. Might need to get creative--the funnest part of all! :)

D.

[ray hampton]

how do you clean a blockage of the pipe, do they have a clean out plug ?

[Phil Maddox]

I would use the 6x6 wye and reduce as close to the port as you can. Reduces (slightly) the amount of friction in the pipe.

Good luck

[Mike Cruz]

Phil, I strongly and respectfully disagree with you. If you run a 6" vertical pipe and reduce to 4" AT the machine, you will not get enough air speed through the pipe to keep dust and chucks suspended in the 6" pipe.