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Yet Another Dust Collection Question From Malaysia

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Forum topic by BenHo92 posted 07-26-2021 10:58 AM 431 views 0 times favorited 5 replies Add to Favorites Watch
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BenHo92

3 posts in 59 days


07-26-2021 10:58 AM

Topic tags/keywords: dust collection 2hp ducting ducting size help

Hi guys! Greetings from Malaysia.

I’ve been reading all I can about dust collection from my medium sized shop and I have one last question I have unanswered (for now. I’m sure more will come up). I have a 2hp single stage dust collector that I would like to upgrade to a 2 stage. From everything I’ve read, 6” ducting is the way to go for the mains, which will reduce to 4” right by each machine. That’s all well and good but the inlet to my dust collector is 5”. So the question is this: is there any use in running 6” ducting if the inlet is going to be reduced to a 5? Is the CFM going to be limited by the 5” inlet port, hence negating the need of the 6” ducting? 5” PVC is near impossible to find in my country. It jumps from 4” to 6”, and from everything I’ve read, 4” is too small. Unfortunately wood working is a scarce hobby in my country, which means tools and materials are often hard to come by. I have to build and rig things up with what’s available. (which I quite enjoy doing)

Also, this is my first post. If I’m doing anything wrong or posting in the wrong section, please let me know! Appreciate all the help!

Ben


5 replies so far

View Fred Hargis's profile

Fred Hargis

7185 posts in 3706 days


#1 posted 07-26-2021 11:49 AM

Most dust collectors have the inlet mounted on a plate that can be removed and replaced with a different size inlet if needed. I suspect that’s the case with yours. If it has that plate, remove it cut a copy from a piece of plywood (guessing you can get some) cut a 6” hole in the center and then fit a 6” HVAC “start collar” to it. But truthfully if you choke it down to 4” at the machine you’re still limiting the performance. So try to increase the machine ports to the diameter of the main line. One caution about all this, you may have so much air flow it will overload the motor, so be sure to check the amperage draw.

-- Our village hasn't lost it's idiot, he was elected to congress.

View tingaling's profile

tingaling

69 posts in 1533 days


#2 posted 07-26-2021 03:39 PM

I disagree that you need to increase the size of the ports at the machine. The reduction from 6 to 5 does cause a pressure drop, but that small loss is compensated by the increase in air flow speed which means you only have a slight decrease in CFM. Having more length of smaller ducting on the other hand usually causes a greater pressure drop and a greater loss of CFM. The inlet ports of machines are (hopefully) designed to match the desired airflow within the machines structure to gather up the dust caused by the cutting heads of that particular machine.

-- Nick, now in the heat of Sacramento

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BenHo92

3 posts in 59 days


#3 posted 07-27-2021 07:03 AM

Thanks for the feedback guys! Most of machines come with 4” dust ports. So What I’m hearing is I should modify them to at least 5”? Thanks for the tip Fred. Yes it does come with the mounted plate, and it shouldn’t be too difficult to have that fitted with a 6” fitting.

View tvrgeek's profile

tvrgeek

2259 posts in 2862 days


#4 posted 07-27-2021 10:47 AM

Not too sure about these observations.
A larger or heavier impellor will overload the motor. But even if free air, it should not. But Fred’s advice to check the current is always a good thing.

Reasoning behind larger trunk is to reduce the drop from the ductwork. Even with small at-machine ports, the larger ducts matter and increase airflow.

Much can be gained by the shape of the inlets. Most are strait with no radius. This can reduce the effective cross-section by quite a bit. Some even have grates over them which is even worse. Just a 1/2 inch radius to a flat plate does wonders. No need for a elliptical curve, 7 degree taper for that last half a %. If you do want to push the limits, you can get flairs for sub-woofer ports. I disagree with the OEM design for ports being correct. For some reason, woodworking machine designers seem to have zero clue about fluid dynamics. I have yet to see a machine well designed in this respect. Cheap dust collectors tend to have way too much clearance from inlet to impellor which can half your airflow.

A smaller port than the duct will increase velocity in that port, but reduce overall flow in the system by far more.

Best advice is to follow the Bill Pentz recommendations.

Too large a duct may reduce the velocity too much and cause dust drop-out which is very bad. There are calculators on the WEB to figure this out. BUT, ratings on DCs tent to be highly optimistic. You really only know your velocity buy measuring it. An anemometer is not expensive.


Most dust collectors have the inlet mounted on a plate that can be removed and replaced with a different size inlet if needed. I suspect that s the case with yours. If it has that plate, remove it cut a copy from a piece of plywood (guessing you can get some) cut a 6” hole in the center and then fit a 6” HVAC “start collar” to it. But truthfully if you choke it down to 4” at the machine you re still limiting the performance. So try to increase the machine ports to the diameter of the main line. One caution about all this, you may have so much air flow it will overload the motor, so be sure to check the amperage draw.

- Fred Hargis


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BenHo92

3 posts in 59 days


#5 posted 07-27-2021 12:58 PM

Thanks tvrgeek! I actually have never looked into inlet shapes before now. Thanks for the tip. I, like wood working machine manufacturers, don’t understand fluid dynamics. Thank you for helping me out!

About what you said regarding the clearance between the inlet plate and the impeller, would it be worth modifying the plate to close up that clearance?

About shaping the inlets, can this be achieved by extending the port with some pvc, heating it and molding it with something conical?

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