Proposed Dust Collector Discussion

See BoD agenda item for details: https://dallasmakerspace.org/wiki/Board_of_Directors_Meeting_20180223#Purchase_Dust_Collection_System_for_the_Woodshop_.28Andrew_Zalaket.29

Felder info page: https://www.felder-group.com/us-us/products/dust-extractors/clean-air-dust-extractor-rl-350--350-mm.html

From the Felder page, the proposed unit has a nominal capacity of 4090 CFM at 2630 Pa (10.5" WC) with a 350 mm (13.7 inch) inlet.

This is a list from long ago, but will work for reference:
image

That’s 5800 cfm. That 5800 cfm is the total at the collection port on the tool. I also think the manufacturer recommended collection volume for the table saws is low. This is my first concern: the unit will not collect enough for the peak use periods.

My second concern is that the stated 4090 CFM in a 14" duct does not produce sufficient velocity in the duct to keep the particles suspended. 4090 CFM yields 3826 feet per minute (fpm) linear velocity. Guidelines for wood dust and chip collection vary between 4200 to 5200 fpm to maintain suspension. As dust settles in the duct, you decrease flow area – thereby decreasing flow rate. Dust settling in the duct also creates a fire hazard. The velocity required to resuspend the particles (called scour) is higher. This is why ducts should be accessible and regularly cleaned.

The flow rate of fans is a curve dependent on pressure. The harder is it to push (or pull in this case) air through the system the lower the flowrate is. As you close gates in the system, say at 2 AM when one guy is building a cabinet, the pressure increases because the system is trying to suck all 4090 cfm through the 1 dust collection port. Except it can’t suck that hard and ends up churning. This heats up the fan and motor dramatically.

I guess it comes down to what the goal is. If the goal is to appease the Fire Marshall, I hope we have in writing his concurrence that this system will fulfill his requirements. If the goal is to purchase a long term solution, I don’t think this fits. Commence discussion :slight_smile:

p.s. this is a great reference for duct data: http://www.spiralmfg.com/wp-content/uploads/2015/09/engineering_data-1.pdf

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I’ll also point out, that the nominal flows on the existing collectors are:

White Collector - 2150CFM
Yellow Collector - 1500 CFM

Or 3650 total, at a somewhat lower static pressure due to the much smaller ducting per collector (you can pull more air through a system with a lower static pressure).

The Fire Marshall flagged the dust in the shop, not the collector. He (correctly) attributed the dust to the undersized collection, because there simply wasn’t enough air flow to evacuate the tools, resulting in lots and lots of dust in the shop, clogged machines, etc etc.

So this proposal effectively spends $50,000 for only a very minor net gain, that is highly unlikely to change the problem in the woodshop in any way whatsoever, other than adding a ton of noise and heat to the shop…

Math is math. This is why we originally requested an engineer size the system - to avoid such obvious errors.

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Mat

Although my knowledge is less extensive than yours - the proposed solution is throwing good money after bad. Several times, I have posted links to the Bill Pentz site, which is one of the best sites I have found. Mat, you are spot on. The table that you included in your post is for chip collection, not dust collection, which is probably part of the confusion…If you don’t calculate dust and chip, you end up with a system like that being proposed. (Here’s a link to Pentz which puts a lot of it in plain english for non-engineers like myself. http://billpentz.com/woodworking/cyclone/dc_basics.cfm#CFMRequirementsTable

Thanks for speaking up

At any rate, I will be urging a no vote because it is a waste.

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