Trash Can Seperator Question

[Anthony Whitesell]

I am wondering why the 'trash can' seperators cause a loss in CFM if they are completely sealed. From what I have found on the 'effective' seperators (not the generic cheapo-s widely available) it appears that they wouldn't have any more loss that a 90 degree elbow.

I've read through the posts here and on Bull Pentz site and haven't figured it out on my own, so I thought I'd ask. Anyone know why they cause such a loss in the DC system?

[glenn bradley]

The seperator functions by slowing the air enough to allow the heavy material to drop out of the airstream as does a cyclone. CFM is a measurement of volume over time (like the speed past a known point of a given size). Air slows down, CFM drops. I used to leave mine in all the time as I loved only having to change the bag once for about 6 - 10 trashcan emptyings.

I removed it during some shop rearrangement and what a difference, . . . I had forgotten how big the hit was. I still use it but now move it in and out of the path based on what I am doing; planing and jointing = in the path, tablesaw and router table = out of the path.

[Anthony Whitesell]

Why does the airflow slow down? If there is a 4" inlet and a 4" outlet what goes in must go out and the can is sealed so as not to add any other flow paths to the system, what is the principal causing the drop? Would the same drop occur is the 4" line expands to 6" and runs 4' and reduces back to 4"?

[Jason King]

Any turbulence results in a loss of CFM. Friction (pipe walls, etc.) also cause a loss of CFM. A trashcan separator is designed to create a large amount of turbulence and friction that slows down the air stream enough for particles to drop out. The friction and turbulence takes more energy to move than a laminar air stream. If you have a 2 hp motor pulling laminar air, and a 2hp motor pulling air with friction and turbulence, the first one wins in CFM every time.

Without the separator, you rmotor doesn't have to fight all that resistance, and thus can pull more CFM's. With the separator, you trade some CFM for better separation.

[Jason Beam]

It's not just 4" in to 4" out ... it's more like ... 4" in, swirl around a little while, 4" out. That swirling is what's putting resistence in the line. ANY time you change direction, it adds to the static pressure - I have yet to see a means of separating without introducing directional changes or other static pressure increases. It'd be nice if someone would hurry up and invent it befor I go buy a cyclone :P

Now, if you just took a garbage can and poked two holes in it - one on each side - facing each other ... essentially creating a pass-through with no change in direction ... you'd see far less reduction in CFM there because the changes in direction are minimal. It's all about the orientation of the ports (which is why most of the trash can ones i've seen are useless). The idea is to set up a circular path on the input side and a clean path on the output side.

The ideal form is to have the inlet come in at an angle and the outlet be vertical straight from the center - the cleanest part of the circular flow of dust (since most of the dust is thrown to the outer edges, it stands to reason the center is the cleanest part). It drives me nuts to see some of those lids with two ports that both come in at an angle opposing one another - what a pointless means of separation.

[Mark Engel]

We should ask James Dyson to figure something out for us. :D

[Jim Myers]

The CFM should not change in either of the 4" hoses since you have a system that is moving the same ammount of air before and after the separator.

What happens in the can is the pressure will go down since it has a bigger space to move the air in.

I am not arguing that with out a separator your collector does not move more air I am just saying that in a sealed system the air flow (CFM) in both hoses would have to be equal or you do not have a sealed system.

Jason King has it correct that you are adding resistance to the motor so that is why it goes down.

[Anthony Whitesell]

To reduce some of the pressure drop, can a booster fan be added at either the inlet or outlet of the can? Or perhaps a SMALL impeller fan (cyclone style fan) to the top of the of the can? I know the second option would push the outlet from the center of the can if using the Bill Pentz design unless the motor was mounted above and the impeller driven from a long motor shaft.

[Bruce Wrenn]

First lesson is air has weight. Anytime you slow air down, and then speed it up again, you have to have more energy to get air back up to original speed. Also, inside separator there are friction losses due to air rubbing against the sides of the can. Then the air has to make a 90 degree turn to exit the can. The effect is the same as adding a couple lengths of pipe to the duct work.