Another Dreaded DUST COLLECTOR Question !! - But Ive added a "nice" diagram..LOL

[Brad Wardman]

Officially new to posting, but been following on SMC a while...so hello !!

Now, I will make this as painless as possible. Im a long time woodworker, have dust collection but cannot afford a new unit at this time. Have enlarged my shop ansd am looking to setup for improved velocity and maybe a little cfm, mostly to eliminate more fine dust.

I have a 2HP HF and a 1HP Shop Fox and have been using 2.5 micron bags on both. 6" duct in the shop (28x24) with 6" to most all the tools and a blast gate on everything....only 1 (maybe 2) tools used at one time.

New setup idea will have the DCs in another room, so concern is added velocity and pressure, not necessarily some fine particles escaping the bags and contaminating the work area. Improvement to venting outside for my purpose ?? Yes, I can vent outside here and I realize I need an air intake for the vacuum loss in the shop as well.

Looking to run both DCs through a wye, into a trash can thein baffle separator, then into the ductwork. Remove the filter bags and vent outdoors on both the DCsSo, the question is....Will my setup diagram "work" ?? And what will, if anything, be improved ?? CFM, velocity, etc ??

[ian maybury]

It doesn't sound like a bad set up Brad - it's aiming for decent airflows, and by exhausting outside side steps the filtration issues and reduces the resistance to flow a bit more.

The theory (they say) goes to the effect that if you run blowers in parallel that you should get basically the same pressure capability as a single blower, but something close to the CFM of the two added together. (in series the pressures come close to adding, but the smaller fan more or less determines the CFM) It's better that the two be of similar pressure capability, as differences bring some (unpredictable) risk of the fans interfering in an unstable way with the load each sees. (hunting effects and so on)

If one fan was creating significantly more suction than the other, then it would probably 'steal' more than its nominal share of the airflow - which would reduce the performance of the combination.

So the likelihood is that barring an imbalance/hunting issue you should get roughly similar pressure performance to the fans alone, but the CFM of a roughly 3HP or a bit less blower. (or whatever the combined HP of your fans really is - some of the Eastern stuff is a bit over rated that way) Which provided your 6 in ducting runs are short and with not many bends etc (bends very quickly add resistance) and it's a true total 3HP should give decent airflow on 6in ducts - especially since you are exhausting outside. (a saving of several inches WG versus a slightly dirty cartridge filter - and a lot more compared to a typical bag)

You would need to check with Phil regarding how well the baffle and trash can set up will work. Stock garbage can separators it seems blow pretty much all the dust and chips straight through at the sort of 800 -1000CFM you might hope to achieve. (which despite what you said is great, as it opens up the possibility of good fine dust collection) I think that with a baffle fitted it may do better, but I don't know for sure.

You would need to take care that the two blowers only ever ran together. This because there's a fairly high likelihood that with the fairly low resistance of a 6in duct that whichever blower was running alone could draw more current than it is rated for and burn out. (reducing resistance increases the load seen by the blower motor)

Please don't take my word for the above, as I'm simply guesstimating/passing on stuff I've seen but never tested, and there are some unpredictable risks in coupling the blowers - you need to satisfy yourself as to the feasibility of the scheme.

My instinct would be to mock the system up, and give it a trial run. I'd be looking to ensure that each blower was drawing the correct amps (getting towards full load, but with a little cushion) with the correct length and size of duct and bends attached to whatever you plan to use it on, and ditto the cyclone, blower inlet and exhaust connections.

Each motor would at least need it's own properly sized overload protection. Might be wise too if it was wired so that one motor tripping would take out the other too.

No doubt there's others that'll have views on this too...

ian

[Dan Hintz]

If I'm reading you right, you intend to run two DCs at the same time? IF so, you're going to starve one for air and make the other work a lot harder than it should... one will eventually give up the ghost, and in the end your collection won't be any better.

[ian maybury]

Pardon my coming in again Dan. For sure there is a possibility of problems, and it's definitely unorthodox. Success would depend on the fans and the overall set up delivering a correct and stable balance I think.

Paired fans in HVAC are as best I know (which may not be much) normally mounted on a common plenum - so that both have at least got more or less guaranteed similar intake conditions. Then it becomes a case of matching the fans so that both are experiencing pressure drops that place them at a stable point on both fan curves - where small changes in flow or pressure won't result in big changes in output as this would increase the risk of the hunting and interactions I mentioned. Better too if the ducting has enough restriction that neither fan is running too close to max amps.

Chances are it'd make sense to ensure for this reason that the ducting runs and after the 'Y' to the fan inlets were as short as they possibly can be.

Exhausting the fans through separate filters would probably be looking for trouble, in that if one blocked before the other it would disturb the balance - but that's not the plan here.

There's no guarantees, but a mock up and trial to check amps on both motors would tell a lot. It'd need careful monitoring for a while too..

ian

[jerry nazard]

Brad,

I am running two identical DC's in parallel. They connect to the duct work via a plywood plenum, not a wye. My first setup with these two units utilized an elongated sort of wye system and was not entirely successful as the two DCs fought each other, resulting in one collection bag filling up faster than the other. Connecting the units by using the plenum solved this problem.

Even with a plenum connecting your machines, they may still want to fight each other. I wish you good luck and will be most interested in learning how your setup works!

-Jerry

[Alan Schaffter]

Well, let me weigh in. The answer is it is hard or next to impossible to be able tell exactly what will happen. On one hand the CFM in your system "might" be better or it "might not" be. On the "not" side, here is what might happen- if the static pressure (SP) resistance of your Thein and ducting is high enough, the 2 HP blower may actually fight against the 1 HP blower, trying to pull air back through it. It is possible that the 1 HP blower will see an inlet SP that is high enough (per the fan curve) that the CFM will be close to zero in that blower. The only way to see what will happen is to test it using a manometer and pitot tube.

One thing, you won't have the issue that can result in a burned up motor since you are not attempting to connect the blowers in series.

Also the SP of each blower will not be additive- the max SP possible is the SP of the largest blower.

[Dennis Ford]

If the two dust collectors have matching performance curves, this should work. If you don't have the performance curves, at least check that they have the same diameter impellers before spending time trying this.

[Dan Hintz]

...............

[ian maybury]

To Dennis' point on impeller size. For sure a larger diameter impeller running at the same RPM as a smaller one is going to do better at higher pressure drops - basically because of the higher tip speed. There's likely a greater risk of problems in circumstances where the blast gates in the ducting are closed/largely closed, or on a machine with restrictive hood - the small impeller will find itself more or less off the high pressure end of its curve (trying to pull air against a level of suction generated by the larger fan which is higher than it can manage). The result may be hunting or just a sharp fall off in CFM.

Against that if the pressure drop/suction level in the system won't be so low as to cause a problem like this there is the possibility that it'll work OK.

One difficulty with a plenum is that for it to work it has to drop the air speed in the chamber quite a bit. Probably not a problem if the separator can be relied upon to not carry over chips and that only fine dust passes through the blowers, but potentially an issue otherwise.

It's all a bit unpredictable, and certainly begs the question (as posed by Dan and Dennis) as to whether it's worth all the faffing about for the sake of buying a decent sized motor and blower combo - which is probably easily enough sourced used too....

ian

[Brad Wardman]

I certainly appreciate everyones input...and I figured this would bring as much debate as many similiar threads in the past. I will likely "play" with a couple ideas as time allows, though I realize this is ultimately another "holy grail" type scenario.

As an aside, does anyone have additional comments or experiance on pressure, velocity or machine breakdown issues when going to an external venting as opposed to the filter bag ???

Thanks again everyone.
Brad

[Alan Schaffter]

There is just no comparison- the very best you can get is from a system that vents the blower directly outside with no separator or filter. Remember, SP resistance which affects the fan curve and hence the CFM of the blower and therefore collection at the source can be upstream and downstream from the blower and is cumulative. Without a separator and filter the only SP resistance you have is the from the machines, ducting from the machine to blower, and ducting from the blower to the outside (same ducting rules apply). Plus, there is no filter to monitor and clean and no SP resistance from a clean, partial or heavily clogged filter.

If you can't do that because of neighborhood issues, the next best is a separator on the input or output of the blower that removes a majority of the chips and dust before the air is vented outside.

A relatively distant third is a system with a filter. But even with a clean filter you are adding SP resistance, and that resistance increases as the filter begins to capture fine dust and eventually clog. In some cases, especially with cartridge filters, even with cleaning, it may be very difficult, and nearly impossible to return a filter to its initial SP specs. You can see the effect of a filter to some degree by looking at the fan curves on Oneida's website, some of them show separate curves for filtered and unfiltered systems- I believe the number is for a clean filter also. Remember the point on the fan curve where your system operates depends on the SP resistance of your entire system and a filter, especially a relatively clogged one, may drive the CFM operating point from barely marginal to totally unsatisfactory.

FYI, though it is not the proper term, I use SP "resistance" instead of just SP, so no-one will confuse it with sucking ability. The higher the SP in a DC system the poorer it will operate- unlike shopvacs which can pull CFM against high SP created by small hoses, DC's can't and don't.