4" inlet with a 6" outlet?

[ay8918]

Hello,
I just joined the forum and also just joined the world "dust collection" with my first real dust collector (a 2HP DustFX with HEPA filter).

I would like to add a simple in trash can baffle separator similar to Phil Thien's original design. My DC has a 6" inlet, but I will be using 4" hose (and eventually a few short runs of 4" PVC). Would I be better off running a 6" hose to the separator with a 4" inlet or should I stay 4" throughout?

If I stay with 4" between the separator and the DC, what is the best way to adapt the 6" inlet to 4" to minimize losses?

Thanks in advance!
Al

[cantfigureausername]

Normally, for a separator, the recommendation would be to keep both small because a smaller inlet and a smaller outlet improves the separator's efficiency. For the pipe/hose, it's not so clear what the "general" recommendation would be because there is a delicate play between pressure loss and volume/airflow.

However, luckily things are not that complicated here, figures seem to be pretty clear.

The DustFX already has a cyclone built-in, so the purpose of the trash can separator would not be being super efficient, but merely reducing the heavy particulate matter load. This enhances the cyclone's efficiency and shifts its cut-point downwards, which is a good thing (both for your HEPA filter and your lungs). But even the worst separator in the world will catch upwards of 95% of heavy particulate matter, the difference in that class is just not noticeable at all. It is the small and very small particules where you figuratively separate wheat from chaff, and the baffle separator isn't very good at dealing with these anyway (nor is that intended).

Insofar, it doesn't really matter whether the outlet is a bit wider (and thus the pre-separator is a bit worse). On the other hand, not changing the pipe diameter without need means to not needlessly introduce turbulence and pressure loss, which is a good thing. Plus, it's easier, cheaper, fewer parts, fewer leakages. So for the small bit between separator and DC, I'd just keep it at 6 inches.

Generally, having a thick pipe after a thinner pipe is not such a brilliant idea because that means that the airflow drops, and if you have a somewhat naive build with an insufficient blower, you may have heavier particulate matter drop out, clogging the pipe. That isn't a problem here, however. Unless the manufacturer is lying, the DC has almost twice as much volume per time unit than is needed to achieve 20m/s on a 150mm pipe (which is a "good" value). Also, after passing the pre-separator, the air doesn't contain a lot of heavy particulate matter any more, so there really isn't anything that could drop out in the first place. Therefore: don't worry.

About the 4 inch hose, I wouldn't worry, either.  The DC is supposed to provide "9 inch water lift" as per the technical data on their website, which is about 2230Pa, so a pretty common figure. Most DCs that have pipes anywhere from 4 to 7 inches are in that ballpark. Unless your hose is super, super, insanely long, it'll pull through those 4 inches just fine. Since you have that diameter on the hose already, you can as well make the inlet on the trashcan the same. Smaller is better anyway, so going from 4'' to a 6'' inlet, which is extra work, doesn't make a lot of sense. You would generate extra turbulences, entry velocity, and reduce the separator's efficiency. Not like efficiency really matters for the pre-separator, but why make a sacrifice when it's actually easier to just use the smaller diameter right away.

[ay8918]

Thanks for the reply. My DC does not have a cyclone (it is a single stage unit). So it sounds like having a larger outlet may impact efficiency of the separator, correct?

[cantfigureausername]

The image shown here: http://www.cwimachinery.com/dustfx-2hp-cyclone-dust-collector-review-item-cwi-dcp020h/ shows a kind of "trash can with funnel on the bottom" thing. Which is a cyclone.

If you subtract the impeller on top and go by the height of the inlet, I'd say it's about 1D1D, which is actually a pretty bad cyclone, comparatively. But still, it clearly is one. It will serve OK for shavings and large particles (eliminate 100% of shavings and ~99% of large particles), but it will be rather bad at separating the small particles (expect 30-40%, not 99%). That's going to be the HEPA filter's job. In order to properly separate small particles in the cyclone (i.e. with a ~95% rate), you would want a cyclone approx. twice the height.

The same will be true for the Thien separator, if you put one before this cyclone. The Thien separator is not super awesome because it separates so well, let alone small particles. Actually it's pretty bad at that, too. It is super awesome because it does not need to be 2 meters tall, and yet it can catches a very sizeable amount of matter. A larger outlet will impact its efficiency, yes. But that's not really something to worry about. What comes out is not high quality air anyway. Nor is the air that comes out of the cyclone high quality. This is why one needs a HEPA filter.

Less matter in the air overall means that the cyclone's efficiency will be increased. Fewer particles entering the cyclone means even fewer leaving it. Fewer particles in the air after the cyclone means even fewer particles after passing the filter, and a longer lifetime of the filter, too.

[ay8918]

Thanks again for the reply, this is the unit that I have: http://www.cwimachinery.com/product/dustfx-2-hp-hepa-dust-collector/

I bought it "second hand" for a really good price, but it had never been used (it was still in the original shipping boxes). My goal in adding an upstream separator, of course, is to keep the HEPA filter from getting quickly clogged with larger particles. I do not expect this to ever be up to the same standards as a Bill Pentz approved ClearVue Cyclone, but I am primarily a hand tool woodworker and only make occasional use of power tools (and my investment to date is less than 20% of even the cheapest ClearVue).