Horsepower or CFM?

[Aaron Rosenthal]

I'm retiring my old inefficient dust collector; another case of closing the barn door after the horse is gone - I'm diagnosed with COPD.

Point: I wear an Eclipse respirator with (finally available) new filter cartridges.

Due to the closed border, going down to Bellingham to pick up a 2 or 3 HP Grizzley is out of the question. Here is my plan;
The 6" jointer and the lunchbox planer, as well as the table saw all have 4" ports.
Now, I'm using the corrugated flex hosing, but as I change the system, I'm going to relocate the tools & collector and use PVC or heat ducting.
The chop saw & bandsaw have 2- 1/2" outlets. Those smaller duct units will be hooked to a dust deputy driven by a Largish Ridgid vac. My sandeers etc have the Rockler small tool dust attachments and they go into the Dust Deputy.

Local outlets have "indicated" 2 and 3 HP collection systems, both single bag and dual bag. They vary widely on CFM. For dust collection, what's more important, HP or CFM. Since I plan to put in a cyclone separator, it's a draw as well and I know there were some posts on the site, but I can't find them, and I've looked.
Budget is also an issue. Spending $1400. $USA is out of the question. Local prices range from $500. to $1100., but what is my best criteria?

Since I have lots of 220V outlets in the shop, current isn't an issue.

[james manutes]

CFM ratings are almost useless , can't really be trusted . HP is important , but in a single stage system , it's not the only thing to think about . Good design is very important , choosing the right size ducting , eliminate every bit of flex hose possible . I would certainly add a second stage cyclone , but it's not going to make your DC move more air . You should factor in impeller diameter , as well as it's height as well if that's possible . Grizzly for example , has 3 impellers of 12 3/4" on the 1.5HP thru 3HP baggers , but not all the same height . Throw in Shop Fox (same company) - a different impeller . The bigger impeller should move a little more air , generally speaking . Another upgrade is to lose the bag for a cannister filter , again you'll move more air . So I'd try to pair the strongest DC I can afford w/ a well designed duct system . You can't really know what that will be untill you have the DC . I personally would build a shop built cyclone if- you have a 2 or 3 HP blower, cyclone , and a cannister filter . That would be much more effective than the same pieces connected w/ flex hose . But either way , I wish you well.

[Andy D Jones]

Neither in isolation. Reputation of the manufacturer is a good guide, with a good reputation, you can trust more about what they claim.

Beware when comparing efficiencies that are near 100 percent. Consider how much the pass, rather than how much they capture. A 99.9% efficient filter passes (misses) 3.3 times more dust than a 99.97% efficient filter, at the given particle size.

Avoid short, stubby cones for cyclones. They are not good separators, and will clog your filter more frequently.

Beware of using HVAC duct for dust collection. It is likely not strong enough to avoid collapsing at the suction levels a good dust collector may be called to provide.

-- Andy - Arlington TX

[Michael W. Clark]

Andy is leading you in the correct direction. HP and CFM are not independent, the system SP (static pressure) or resistance to airflow is also part of the equation.

Locating your equipment closer to the collector and reducing flex hose will reduce the SP losses which means that a 2HP blower will deliver more CFM than if you had your tools located farther away and/or lots of flex hose.

With what you describe, I think you would be fine with a 1.5HP bagger unit or you could go to a 2HP unit and maybe add a cyclone or pre-separator later.

I went with a 3HP 2-bag unit (still need to hook it up). I wanted more HP to pull from a TS and router station that have 3" drops for the blade guard and router fence. I also wanted more flow for a miter saw station with 6" connection. My planer and jointer are located close to the unit so I can put a cyclone on that branch in the future if I want.

I also went with felted bags instead of the cartridges. The bags will be easier to clean, cartridges foul quickly as the dust is on the inside of the cartridge where the pleats are the narrowest.

[Jim Dwight]

I don't really know anything about COPD but I am sorry you have it. My guess is that you need to wear an effective filter to do woodworking. Better DC performance can certainly reduce dust in the air and a filter can also reduce it. But I still get a lot of dust when using a router (hand held) and in some sanding operations. I believe Trend makes a face mask that incorporates a filter and that could be a good option. Maybe I am wrong but I am thinking you may have to wear your last line of defense and use it at least when doing operations where you cannot control the dust as well.

Bill Pence also has health issues from dust and his guidance for DCs is based upon his health issues - and helping others avoid them. HP and CFM do not correlate one to one, design makes a difference but only within limits. I do not think you can get the CFM Bill recommends from a 2hp system, no matter how well it is designed. But his site is a lot better source than my memory of his site. I am pretty sure that Bill REALLY likes the 5 hp system he uses. It might be more than your budget, you would need to check. With a system that moves enough CFM, and with effective hookups to each machine, you may be able to avoid a mask with the bigger equipment.

But if you accept you will need a mask or filter of some kind on you all the time, then your requirements drop and a 1-5 to 2 hp system may be viable. I use a 2hp HF unit sucking through a super dust deputy cyclone (the middle one) and exhausting outside. It doesn't get everything partially because my hookups to the machines is not optimum. But it also does not move enough air, despite 5 inch metal hookups almost all the way to the key tools, to be reasonably expected to get everything. I am sure there are better 1.5 to 2hp units but I don't think any of them will equal even a decent 3 hp DC.

What you really want to know is the CFM the DC will provide IN YOUR SYSTEM. It is possible to find performance curves of CFM versus restriction but you still need to know the restriction of your system. Bill's website also has information and tools to allow you to at least estimate the loss of various configuration options. It is quite involved, however.

To me the key question is what PPE you will use. With good PPE used all the time you are around the dust, your DC becomes much less important. If you are trying to avoid PPE, it seems you are taking a much more difficult path. You need both plenty of airflow and good hookups to all dust producers. With money enough you can buy the CFM but the hookups seem to involve ingenuity - not obvious to me that you can easily buy a great setup for a SCMS, for instance.

A key ingredient may also be a Dylos particle meter so you can see how effective your system really is.

[Jim Becker]

WIthout a fan curve...something difficult or impossible to obtain for mass market dust collectors...you're going to have to just try and do the best you can....higher horsepower/larger blower size (both required) is going to move more air, regardless of the actual potential CFM, than something with a smaller motor/impeller. Dust collection is all about air flow...the more air you move, the better the dust collection performance. Beyond that, most bag systems have bag filters that are not the best...you will very often be better service by immediately replacing them with something from a company like American Fabric Filter or similar.

[Bill Dufour]

Makers lie about horsepower and airflow all the time. I think the best idea is to compare amps. Most motors are similar efficiency so more amps means more power. If you have fan curves that will mean something if it is a honest maker. If the curve is from a unknown Harbor freight unit I would not believe the paper it is written on.
Bill D

[Curt Harms]

As others have said, horsepower is, in my opinion, about worthless as a measure of dust collector capacity. CFM is better but in most cases that is the absolutely best performance the system is capable of - probably no hoses, no filters or any other performance robbing necessities when measured. The other number I would look at would be blower diameter. If two blower wheels are of the same design, a larger diameter blower will move more air regardless of the motor turning it. A 10" blower with a 10 hp motor turning it will move less air than a 12" blower with a 2 hp motor turning it. This assumes the same RPM in both cases.

[Jim Becker]

I'm with Curt...hp and amps are not deciding factors...the impeller is where the chips meet the road. But obviously, a motor capable of spinning that impeller is important.

[james manutes]

I'm on my third shop built cyclone , only because I'm a "horse trader" . But one other thing to mention is the weight of your impeller if you change it . Newer ones could be cast aluminum , potentially lighter than steel , it varies . I would avoid putting a much heavier impeller on a 1.5 HP unit , compared to what came off it . 2 - 3 HP will be less risky , I'd say . My last build I upgraded from a small steel of 7.75 lbs. to a larger aluminum impeller a little less than 9 lbs . I was a little worried that may be too heavy , but its been fine . The DC I used was a 2HP Grizzly , no issues going on 3 years of almost daily use .

[roger wiegand]

I don't understand why CFM is not the right measure. Of course I would not believe the manufacturers claims (nor are they particularly relevant, vs all the variables in an installed system), but it doesn't seem as though it would be difficult to use an anemometer to measure fpm through the collector in the first place (if you know that and the duct size cfm is an easy calculation) and then see how it is affected by variables like different layouts and pipe types.

I use a much cruder measure myself, that is do the chips get picked up and dropped in the collector bin without clogging. I've reconfigured a couple times to make that happen as new machines are added or moved around.

[Jim Becker]

I don't understand why CFM is not the right measure.

It actually is the right measure. The problem is determining what it actually is for a given system because mass-market folks tend to publish misleading information and don't provide fan curves. So about the best way for someone to "generally" determine what a system can do is to look at impeller size. The majority of folks don't have measuring devices to determine air flow and even if they do, it means actually buying the system first so you can test it. It's kinda a shame that the industry is the way it is where you can only get "real information" from a few specialty manufacturers that can be trusted...because they do provide fan curves.

[Larry Frank]

CFM is a good measure but as mentioned difficult to get properly. Fan type anemometers are not a good way to measure and lead to wildly inaccurate results.

[Michael W. Clark]

If only looking at impeller size (diameter and width), you also need to look at impeller speed, which will be motor RPM since the fan is direct driven.

[Bill McNiel]

Neither in isolation. Reputation of the manufacturer is a good guide, with a good reputation, you can trust more about what they claim.

Beware when comparing efficiencies that are near 100 percent. Consider how much the pass, rather than how much they capture. A 99.9% efficient filter passes (misses) 3.3 times more dust than a 99.97% efficient filter, at the given particle size.

Avoid short, stubby cones for cyclones. They are not good separators, and will clog your filter more frequently.

Beware of using HVAC duct for dust collection. It is likely not strong enough to avoid collapsing at the suction levels a good dust collector may be called to provide.
-- Andy - Arlington TX

I disagree with this, my shop is 100% HVAC duct, collects virtually everything and has shown no signs of and collapsing or failure.