Best Dust Collection if venting outside

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Forum topic by LSGss posted 05-01-2013 03:13 AM 7463 views 0 times favorited 8 replies Add to Favorites Watch
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69 posts in 2621 days

05-01-2013 03:13 AM

Hello all,

So I am moving into a new house soon and want to build my first official shop. The first thing I want to purchase is a good dust collection system. I have been doing a lot of research, including reading Bill Pentz’s website. I was thinking of purchasing a clear vue cyclone. However I was wondering since I plan to vent outside does it really matter to get a 2 stage cyclone dust collector. It seems single stage dust collectors are a lot cheaper and have a much higher CFM for a given HP. For example the clear vue cyclone CV1800 is a 5HP setup with 1442 CFM for approximatly 1500 depending if there is a sale. While a SHOP FOX W1687 3-Horsepower for $700 provides 2800 CFM. Can someone please elaborate on this.

Thank you,

8 replies so far

View crank49's profile


4032 posts in 3605 days

#1 posted 05-01-2013 03:59 AM

CFM has no relation to anything if it is not specified at what pressure.
Thats like saying I got a car that goes zero to 60 MPH in 3 seconds and my car gets 40 MPG.
Both might be true statements, but you can bet that they both don’t happen at the same time.

You want inexpensive, get two HF 2hp dust collectors and set them outside in an enclosure. With coupons they cost $139 each. So, for ~$280 you have about 1200 CFM at about 8” static pressure. Or if you want to play the same game as the ShopFox folks you will have a 2400CFM collector for only $279.98. Plus, if you are like most folks and only need to run one tool at the time, you can probably just run one collector and save a bunch on elcetricity. I’d hook this up with 7” pipe in case I did run both machines at the same time. And I’d put a drop out box before the fan to catch the big stuff.

View LSGss's profile


69 posts in 2621 days

#2 posted 05-01-2013 04:14 AM

The shop fox was just an example. I was looking at the May 2013 wood magazine and they had the powermatic dual canister producing the best cfm at the machine while the Oneida produced a little less cfm but filtered better. Since I will not filter and will just exhaust outside then filtering shouldn’t matter correct. I will say I am just a hobbies and would not run more than one machines at a time unless on rare occasion a friend is working with me at my shop. That being said dust collection is not some thing I want to be cheap about so I am open to the healthiest suggestions
Thank you

View gtbuzz's profile


427 posts in 3075 days

#3 posted 05-01-2013 04:22 AM

The world of dust collector specs can be very confusing so I’ll try to help out where I can. I’ve done quite a bit of reading on this topic over the past few years for my own purposes, and honestly there are times where I’m still left somewhat puzzled.

First, regarding single stage vs. dual stage performance – all things being equal with the motor, impeller, etc, a dual stage dust collector will pull fewer cfm than the it’s single stage counterpart. This is because the separator stage of a 2 stage system increases the static pressure in front of the impeller (static pressure goes up, cfm goes down). This is typically a very short-lived advantage though, because single stage DC’s do a very poor job of keeping debris out of the filter elements. As the filters get clogged with debris, the back pressure increases and the motor can’t breathe as easy (think about blowing through a straw vs blowing through a paper towel roll) and cfm drops. That’s where the advantage for the two-stage system comes in as the pre-separator stage generally keeps all but the smallest of particles from reaching the filter. The end result is that over time, the performance of a dual-stage system will remain relatively constant, while a single stage system will degrade significantly.

Regarding the pure CFM numbers from manufacturer to manufacturer? For the most part, single stage machines are lower end machines, while dual stage ones are higher end ones. I’ve always found lower end ones are more apt to exaggerate their numbers, so there’s probably a fair amount of that going on. Why does your shop vac claim to be 7hp and cost $50, but the 5hp Clearvue is $1500? Or even the 3hp Shopfox is $700? I feel a reliable measurement of performance is HP (but that’s sometimes overrated too) and impeller size.

So, what does this mean for you, if your plan is to vent outside? Honestly, I’d still go with some sort of pre-separator (cyclone, Thien baffle) because if you don’t that debris that would otherwise get sucked into the filter is going to be ejected outside and it’s just gonna be a mess to clean up. Conversely, if you only let the finer particles vent outside, most of them will just blow away harmlessly with the wind. In my own experience, there’s a pretty significant amount of debris that can get sucked into the filter when not using a pre-separator.

My previous dust collector was a Jet DC-1100 single stage unit that had a Dust Dog cartridge with paddles that I used for a while before upgrading to a Wynn nano cartridge. With no pre-separator there would always be a lot of debris on top of the ring as well as in the filter that would drop down into the bag when I rotated the paddle. This is all stuff that would have gone outside. After I added a device to separate the heaver stuff, it was a night and day difference w/rt the mount of debris in the filter and on the ring; it was almost non-existent.

In the previous issue of Fine Woodworking, they postulated that as the collector bag passed about the half-way point the vortex action pulls the debris from the bag into the filter. Honestly I’ve never tested that to be sure and it sounds plausible but I still say in my own experience there was a difference with and without the separator even when the bag wasn’t very full. I’m not sure of the physics as to why, it just was.

There’s a design consideration too. The design of all single stage collectors is pretty much the same. The exhaust from the motor/impeller goes into some sort of ring and the large stuff falls to the bottom and the lighter stuff goes up to the filter. I’m not entirely sure how you’d fashion something that exhausts from the top part of the ring outside. I suppose you could make a donut and attach and air-tight-take-off but I have a feeling you’re still going to be venting a good number of chips outside.

One other advantage of having a pre-separator that I haven’t yet mentioned, is that you protect your impeller. With a single stage system, everything (and I mean EVERYTHING) goes past the impeller. That includes small bowls that you’ve managed to knock out of a chuck or even a jar of Shellawax that you carelessly sucked in. Don’t ask me why I used those as examples, but trust me, those aren’t things you want to do more than once.

One last note about venting outside – you’ve got to replace the air that you expel. If you’re in a climate controlled area, that can get very expensive.

It’s interesting that you bring up the Shop Fox DC actually. I’m in the middle of building my own cyclone unit based around that motor. If you can wait another week or so, I’ll be posting info about my build.

Hope all this helps.

View Fred Hargis's profile

Fred Hargis

5979 posts in 3127 days

#4 posted 05-01-2013 11:22 AM

Venting a SS outside is a little trickier…either you blow everything outside which is probably unacceptable (you’ll have a hell of a mess somewhere) or you have a pre separator. Regardless, I would suggest you get the CV, and vent it outside…you save buying the filters that way.

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

View Charlie's profile


1101 posts in 2920 days

#5 posted 05-01-2013 11:42 AM

A friend of mine did this and I will probably eventually steal his idea in some form or another….

I live in the northeast (near Niagara Falls). My shop is not air conditioned or heated, but this idea could be adapted for those times when you need to heat or cool your shop.

His dust collector is walled into a “closet” in one corner of his shop. You could also do this with the DC outside in its own enclosure. The enclosure is key. He put a screened (for bugs) opening on the outside wall and an unscreened one on the inside. There’s a gasketed door on each opening (he used old cabinet doors).

Close the inside door and open the outside door and you’re venting outside. You’re also exhausting a lot of air from your shop so you need to have a window or door open to the outside to make up the air. When he heats his shop in winter (part time… it’s not always heated) he closes the outside door and opens the inside and he’s recirculating.

If your shop is in your house OR if your shop is in a garage ATTACHED to your house, be VERY careful about exhausting air to the outside if you have ANY gas or oil appliances (water heater, furnace, etc). Even at 600CFM you wil very likely start to backdraft the chimney for those appliances and suck carbon monoxide into your house. The newer your house, the less it takes to backdraft (generally tighter construction). Heck, when I’m cooking and turn my exhaust hood on anything other than its lowest setting, I have to open a window.

This would affect a detached shop with full time gas heat or hot water as well. Please be safe and pay particular attention to making sure you have enough make-up air to support venting outside.

View LSGss's profile


69 posts in 2621 days

#6 posted 05-01-2013 01:49 PM

Hey Fred, I actually ended up getting a Hammer K3 31×31 but I think you point still holds true.

Charlie and GtBuzz commented on the importance of having air come back into the shop when venting outside and I wanted to know if there are good principle for that. I live in Omaha, NE where there is hot summers and cold winters, so I can certainly see how it can become expensive to vent outside. However I am a resident and therefore only maybe get to work in my shop a few times a month so I don’t know how expensive it will truly be. I was more wondering if its worth spending the money on a cyclone separator if someone vents outside, because it seems like the dual cannister powermatic for around the same price draws more CFM at the machine which I would think is the more important thing if I planned to strictly vent outside.

Also in the recent May 2013 Wood magazine they reviewed many dust collectors and Onieda came up on top for filtration with adequate CFM, while the powermatic wasn’t great at filtration but provided a great CFM. It is a shame they didn’t include clearvue, I don’t really understand why they didn’t. Does anyone have a comment on the Oneida vs the Clearvue. I know the clearvue sticks to Bill Pentz’s original design better.

Thank you all

View Fred Hargis's profile

Fred Hargis

5979 posts in 3127 days

#7 posted 05-01-2013 02:37 PM

I can comment on the Oneida, since that’s what I have. On my system, almost all the finest dust gets to the filter. If I’m using my drum sander heavily, I can see the pressure in the filter build as it clogs. I put a Magnehelic on the outlet just to gauge when the filter needs cleaning. My DC is a little over 7 years old, and just a few months ago I replaced the OEM filter because it was so clogged I couldn’t get the pressure down any more by cleaning. OTOH, the CV has the squared sloped inlet, the air ramp, a proper neutral vane, and the body dimensions Pentz tested to be most effective at separation. My Oneida has none of that. Now, Oneida claims their unit gets 99% separation, and mine actually is very close to that (98.4%, and yep, I measured it by volume at Oneida’s request). But that’s not good enough for the finer particles. I don’t know what the CV gets (and you never see the magazines test separation) but in casual conversation with a couple of owners, it’s probably closer to 99.9% (?). That said, if you vent outside, it doesn’t matter. The fines would get dispersed in to air , and doing that would be the cat’s meow with my system. But like you said, the conditioned air is a problem. All this, of course, is my opinion and in no way reflects on all the Oneida owners who are happy with their purchase.

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

View MrRon's profile


5826 posts in 3877 days

#8 posted 05-01-2013 08:48 PM

What you don’t want to do is suck chips through the blower. You need a pre-separator to collect the large stuff and let the fine dust go through the blower. A single stage with pre-separator is all you need.

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