# Downgrading a 4 bag dust collector



## eayres (Jan 3, 2008)

Hey y'all. 
I've got a Shopfox W1687 dust collector. It's 3HP, 220v, 4 bags on a rolling platform. I got it on a screaming deal even tho I knew it would be too big for my shop. 
I've got 2 options that I can see to use this thing. 1. Build an overhang outside the shop and put it there and run the hoses into the shop. Or 2. Remove 2 of the bags and put it on a smaller rolling platform.
Can you remove bags from a dust collector? I would remove the left 2 bags and block the port with some plywood. Would that strain the motor? What else should I think of?
Thank you for the input!


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## Kudzupatch (Feb 3, 2015)

To strain the motor you have to put a bigger load on it. Lowering the air flow by blocking off two bags would do the opposite. It would lower the amount of air the motor is pushing (or pulling) so it would lower the load on the motor.

You would probably hear it speed up because of this. These seems to mean there is more load but it is right the opposite, Motors slow down with load, not get faster. They speed up because they are doing less work.

It will also lower the performance on the DC because you are moving less air. It may still move plenty for you use but no way for me to say.


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## clagwell (Dec 20, 2018)

You can. Not only will it not strain the motor, it will actually reduce the load on the motor. The downside of that is that it will also decrease the maximum CFM you can get. Whether or not that's noticeable depends on how much restriction is on the inlet, as in hose, duct, tool ports, etc. A long 4" hose won't show much difference. A short, straight 6" duct will see a big reduction in flow rate.


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## wapakfred (Jul 29, 2011)

What they said^^^^. But if you are planning on hooking it to 4" ductwork I suspect it will work as well as a smaller DC. 4" wouldn't (probably) allow the full air flow of that unit anyway.


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## sunnybob (Oct 12, 2021)

My brain is hurting trying to understand this.
If the fan is designed to pull a certain amount through a pipe, then surely reducing the amount that can be pulled through the pipe will speed up the air and increase the resistance on the motor?
Thats like trying to tow a heavy load with a truck thats in high gear, wear will occur.
If the fan is standing in free air with no pipe on either side, then there is less resistance and the motor will speed up?


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## ibewjon (Oct 2, 2010)

Sell it for a profit and buy what you need.


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## clagwell (Dec 20, 2018)

> My brain is hurting trying to understand this.
> If the fan is designed to pull a certain amount through a pipe, then surely reducing the amount that can be pulled through the pipe will speed up the air and increase the resistance on the motor?
> Thats like trying to tow a heavy load with a truck thats in high gear, wear will occur.
> If the fan is standing in free air with no pipe on either side, then there is less resistance and the motor will speed up?
> ...


Yes, it's easy to confuse a centrifugal fan with an air compressor, but they are fundamentally different machines. An air compressor approximates a constant flow machine, piston displacement is independent of pressure. A centrifugal fan however approximates a constant pressure machine, the centrifugal force generated by the rotating mass of air independent of flow.

Or, to put it another way, the compressor is flow limited and the fan is pressure limited.

Now, power equals pressure times flow, so for a compressor power increases directly with pressure since flow is near constant. A centrifugal fan has nearly constant internal pressure so power increases directly with flow.


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## Brawler (Nov 12, 2018)

I have a similar 3HP DC and a similar space issue. I just built a small 3×8 shed behind the shop and put it in there with open discharge directly outside and a separator in front of it. The shed also houses my generator and a small snowblower. The downside is your conditioned air (heat/ cooling) get sucked out of your shop. However I found I never have it on long enough for it to matter much and don't notice a temp change. Another downside is noise, I live in the x-urbs so my neighbors are far enough away the noise doesn't bother them. The nice thing about the 3 HP DC is it is powerful enough to handle an entirely plummed shop even though it is sitting ten feet away.


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## JAAune (Jan 22, 2012)

Whaever mods you do, test the amp draw before committing to use. Blocking the airflow on the output of the DC is not necessarily the same as blocking the inflow. It's always good to test amp draw before mods and after mods to ensure everything stays within the motor specs.


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## eayres (Jan 3, 2008)

Thanks for all of the input! I think for now I'm just going to leave it as is and move around it. I should have mentioned that I actually tried selling it locally and had zero bites. I'm going to plan on building a small shed like structure on the outside of the shop to eventually move this to.
Thanks!


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## dbhost (Jul 20, 2009)

Where is local?

3HP would make for a really nice cyclone build with stacked pleated filters.


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## eayres (Jan 3, 2008)

> Where is local?
> 
> 3HP would make for a really nice cyclone build with stacked pleated filters.
> 
> - dbhost


I am just outside Athens, OH. We're about 1 1/2 hours from Columbus. Good idea about the cyclone!


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