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

[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.