Manipulating velocity to reduce scrubbing

[windmill]

I was discussing the baffle design today with another woodworker, and there are some interesting things to try.

One problem with Dust Collection in general is the tradeoff between velocity and volume. For chips, you want velocity. For fine dust, you want volume. With the smaller DCs like the one from Harbor Freight, 4" is about optimal. A 6" pipe would be better for the fine dust, but the chips would get stuck. This could be used to an advantage on the trash can separator, by making the inlet 4" but the outlet 5" (or even 6" - whatever fits on your DC). Since we have a sealed system, increasing the pipe size will cause a drop in velocity, which is exactly what we want to further reduce scrubbing.

Again, this is a tradeoff. Even though the fine dust is very light, you still need some velocity. So don't reduce your trash can lid to just the rim  :)

An outlet of 5" precludes the use of PVC, but 5" metal fittings are readily available at the local home center. 5" hose is not as common as 4", but not too difficult to find on Amazon or Grizzly.

Increasing the outlet size may not make too much of a difference on the smaller systems. And for the average woodworker, it's irrelevant whether the baffle efficiency is 97.2% or 98.5%. But, with larger impellers and motors, this could be important.

Bas.

[memilanuk]

Interesting idea... I'm looking at getting a 1.5-2hp dust collector this spring (leaning heavily towards the Delta 50-760, but waffling between that and an upgraded HF 2hp unit) and putting a large trash can or drum separator in line with it.  I am worried about scrubbing, particularly if I went with the 50-760 (more cfm than the HF unit) as I'd be sorely tempted to mount the can directly under the blower so it could be on the same cart/footprint, all nice and neat.  Using a 6" hose for that short run (1 foot?) between the can and the blower might help some.

[windmill]

On my original trashcan separator/ HF unit combo, scrubbing was terrible. With the baffle, it's practically zero. The Delta 50-760, while no doubt a little better than the HF unit, probably won't give you any problems (as long as you add the baffle). The next level up would be a true 2HP unit (running on 220V). Based on the results so far (see some of Phil's posts), anything that runs on 110V shouldn't have scrubbing issues

On the HF unit, the bags clogged severely in a very short amount of time. These bags were rated at 30 micron, which falls in the category "sieve". The bag on the Delta is much better, but the surface area is only 20sq.ft. By comparison the Wynn cartridge has 100sq.ft. (poly) or 275sq.ft. (paper). You may want to look at the Delta canister model. JET, Grizzly and JDS also have some affordable canister DCs. I noticed a tremendous difference in the suction when I replaced the bags. Even now that the filter has been "seasoned" (i.e. plugged up), I still get adequate airflow. That wasn't the case with the bags. Again, it's not an apples-to-apples comparison with the Delta bag, but I'd keep that in mind if you're trying to maximize airflow.

It would definitely be nice to mount the trashcan on the DC platform. I have a small shop myself, can't argue with that. I've seen people put a structure over the motor instead, and mounted the trashcan to that. It looked awkward getting it down to empty it, but it did save some space.

[Ken45140]

You said:  "And for the average woodworker, it's irrelevant whether the baffle efficiency is 97.2% or 98.5%. But, with larger impellers and motors, this could be important."

I am interested in what you base this statement on.  I thought that much less than the 98.5% and higher efficiencies would still lead to filter plugup, especially in high volume generation of small particles such as sanding tables, exhaust capture from orbitals, etc.

I have just put a separator together (using ShopVac and 2 1/2 hose) and did detailed testing and got 97.2%.  Phil has pointed out that I have one design flaw (my center outlet port is flush with the surface of the top plate rather than extending down in a short distance), so maybe I can achieve higher.  Based on your comment, I shouldn't worry, though....which is why I am asking.

Thanks,
Ken

[KC7CN]

Interesting idea... I'm looking at getting a 1.5-2hp dust collector this spring (leaning heavily towards the Delta 50-760, but waffling between that and an upgraded HF 2hp unit) and putting a large trash can or drum separator in line with it.  I am worried about scrubbing, particularly if I went with the 50-760 (more cfm than the HF unit) as I'd be sorely tempted to mount the can directly under the blower so it could be on the same cart/footprint, all nice and neat.  Using a 6" hose for that short run (1 foot?) between the can and the blower might help some.

I have the Delta 50-760, and attempted to put my trashcan separator under the blower.  It can be done, but it's a real tight fit!  The problem is, the 20+'' diameter trashcan can not be centered directly under the DC input port -- the offset is about 2 inches.  I decided to give up on the idea for the time being, to much hassle to get the can in/out!

This is the shop-made adapter I made to connect to the separator.


The best approach would be to modify the cross brace on the collector stand, in order to move the can in line with the port.

You can see the brace in this picture.


Good Luck!

-Don

[windmill]

Ken, you ask a good question. My statement was somewhat presumptuous. I wanted a trash can separator for several reasons:

- It is a big pain in rump attaching plastic bags to the DC. It involves duct tape as temporary holders and a metal strap that never wants to stay in place. A separator means I won't have to change bags but a few times per year. The plastic bags are not expensive, but not exactly cheap either.
- A separator protects the impeller from any large, foreign objects (screws, nails, gold doubloons)
- The DC filter will last much longer without large particles (shavings, chips) wearing away the inside

My primary concern is volume. I built a simple workbench and ended up with three trashcans full of shavings. Without a separator, I would have had to hang three new bags.

My DC is lousy at capturing fine dust. It only has 1.5HP, and with 4" ducting it will never move enough air for the fine stuff. That's why I use my ShopVac with a HEPA filter (and a stocking to slow the clogging) with my sander. Of course, you're using your separator with a ShopVac! So, in your case I can see why you want the better separation. If I built a separator for my ShopVac, I'd want it to filter as much as possible too.

Let me rephrase my statement: "And for the average woodworker trying to collect the larger shavings and chips, it's irrelevant whether the baffle efficiency is 97.2% or 98.5%.".