DC Question

[Jeff Cord]

I'm starting to shop for a DC for my shop and have some confusion about the "ratings" (CFM, FPM, etc).
I've got the FWW review from March in front of me but something isn't making sense. I've also read Bill Pentz's web site a couple of times and am still having a hard time following along.
My question relates to static pressure. SP is measured in "inches" (inches of water?)
I understand (I think) that static pressure is like friction in that as the pressure rises the CFM and FPM drop. At a certain point there is so much pressure that the CFM and FPM drop to a point where the DC is no longer doing its job.
Where does the pressure come from? Connectors, hoses, etc. I think I'm OK so far.
So, in order for the DC to work for me I need to be sure I keep the pressure down so that the air flow is high enough.
Now the confusion. Tonight I'm looking in a catalog that lists a DC (cyclone) as having 1900 CFM AND 11.2 (inches I assume) of static pressure. Why does the DC itself have static pressure?
Is this a characteristic of cyclones that non-cyclones don't have?
I'm confused.
jeff

[Kent Fitzgerald]

Yes, SP is measured in inches of water.

If you've read Bill Pentz's pages, you've read about DC performance curves, which show the amount of air (CFM) that a DC can move at a given SP.

When a manufacturer just specs two numbers, they usually represent the extremes of the DC's performance curve: in this case, the DC can move 1900 CFM (at 0" SP), or pull 11.2" of SP (at 0 CFM).

[tod evans]

jeff, you can read and injest information `till you`re blue in the face but all of the advertising gibberish is just that. if you`re wanting a hobby-shop type unit that`ll suck both the clearvue and onida get good reviews here from folks who use `em......02 tod

[Dino Drosas]

Jeff, After going through much of what you describe, I made my decision and bought the ClearVue cyclone. I have just finished installing it using sriral ducting from Penn State and could not be happier with my choice. I have emptied the 30 gal dust container twice and have yet to see even a teaspoon of fine dust at the final filters (this is what really tells the story). I still don't understand all the lingo but after reading Bill Pentz's work I know I made the right choice.

[Jim Becker]

To your original question, "where does the pressure come from": The duct work can only hold so much air at a time and the fan creates a pressure differential which moves said air. As the size of the duct decreases or other "restrictive" features are added, like turns and flex hose, you are making it harder to move air through the system. This increases the pressure differential. At some point, you don't have enough air or can't move enough air at a given velocity to effectively transport chips and dust. This point is generally shown on a graph called a "fan curve" and is the point where the CFM drops below a number, such as 300-350 CFM. The SP listed at that point is the maximum SP for that particular blower/DC combination. CFM below that number just can't move the dust and chips effectively.

A practical example of this is when you hook up a DC to a tool with a very small dust collection port. While it creates a lot of noise, you can't move much air; dust collection is inadequate in most cases. In fact, a shop vac will work better on tools with very small dust ports than a DC system. Shop vacs are designed to operate at very high SP and that is the situation with small ports and small hoses. If your tool has a 4"-6" dust collection port, you can and want to move a lot of air through it to transport the dust and chips. The SP is potentially very low in this case and the big fan of the dust collector can do its job since there is enough air available with little restriction on its movement.

[Bill Pentz]

I believe most already understand much of the basics of dust collection simply from a lifetime of experience with water. At typical dust collection pressures air becomes nearly incompressible, just like water. The amount of water you can move through a garden hose depends upon the hose and the available pressure. At the same pressure a big fire hose will move lots more water than a small garden hose unless the valve is mostly closed. A single point of constriction, in this case a valve will control the flow through the entire hose, so to move maximum air or water we need the hose to not have any small openings which we call ports, no smaller hose meaning duct or flex hose, and no sharp bends or interior pipe roughness. Engineers have long studied all the different ducting components measuring how much resistance each adds. We use a static pressure calculator to total up the resistance of the parts to get a fairly good idea of how much total overhead our system will create.

You then need to use a blower that has ample power to overcome this resistance to be able to move the desired volume of air at the needed duct speed. Blowers sling the air off their blades creating a vacuum. The strength of this vacuum depends upon impeller type, size, and speed. We typically use material handling impellers that are flat plates with blades. The blades are made with heavy metal able to handle hits from small blocks and such. The blades on material handling impellers are also designed to be self cleaning so they do not trap dust or strings that could throw the impeller out of balance and quickly ruin our motor bearings. With our direct drive 3450 RPM motors we typically get blowers that will provide a maximum pressure about six times the motor horsepower. Vacuum cleaners simply put a cap on top of the blades to turn it into what is known as a caged impeller then spin the impeller six to eight times faster creating pressures that are typically about 30 times the motor horsepower.

We measure the “maximum static pressure” a blower can generate by sealing up our duct system tight so there is no air movement. With all sealed and the blower running we connect a tube stuck in water. The “maximum static pressure” is the number of inches that blower pulls the water column up the tube, often abbreviated as W.C. inches This is the maximum possible suction that a blower can produce and defines the available pressure we have to move air. In terms of water this is about the same as how high and big is our storage tank. The taller that tank and more water it holds the higher the available pressure.

This maximum static pressure then needs to be combined with an open duct to measure how much air actually gets moved which we generally measure as cubic feet per minute (CFM). Sadly, “maximum CFM” is a serious problem area with small shop vendors because it is too easy to create test conditions that move a “maximum CFM” that is ridiculous and meaningless except for advertising hype.

To understand why advertised “maximum CFM” becomes pretty much meaningless we need to go back to looking at how our blower is actually sized to meet a specific need. Almost all dust collectors and cyclones use material handling impellers that are near identical except for impeller size. Each size impeller will move a maximum amount of air when there is the least amount of resistance for the air going into the blower and will move less air until hitting “maximum static pressure” when the blower inlet is closed. It takes a certain amount of work measured as horsepower to move any given volume of air, plus the additional horsepower to overcome the resistance that is slowing the air from entering the blower. We plot each blower impeller performance as a “fan curve” that goes from maximum airflow at lowest resistance to minimum airflow at “maximum static pressure”. In addition to needing more horsepower we also need a bigger impeller to generate the additional vacuum to overcome the resistance. Because even a small 1-car garage sized shop with ducting will have a worst case resistance of about 6”, almost all dust collector and cyclone blowers are sized with oversized impellers to address this additional resistance. If you add a typical hobbyist cyclone which takes a lot of power to turn the air in its tight separation spiral, you add up to 3.5” more resistance resulting in needing just over 1 hp larger motor and an impeller that is roughly 2” larger in diameter than a dust collector that would move the same amount of air. So with that long winded explanation, the problem is simple. If a vendor tests their blower without the expected resistance they get huge impressive CFM airflow numbers and will quickly burn out blower motors. Because our heavy duty induction motors can handle a large startup load for a short while, they often can support testing that runs the horsepower to well over double what the motor can sustain without burning out. As both advisor and critic of some of the more recent magazine dust collector and blower testing, I know a lot of blower motors “let the smoke out”, meaning burned up from pulling many times their rated amperage. This is why any “maximum CFM” without knowing the amperage being used at that CFM and the motor’s rated amperage is pretty much meaningless and dangerous. In real use you can figure on only getting about half this “maximum CFM”.

To put this in perspective let’s look at the typical overhead in our shops then what sized motor and blower is needed to overcome this resistance. The ducting and hoods on a typical large stationary tool generates about 2.32” of overhead. Now we add our ducting overhead. Wheeling a portable between machines adds about 1.29” for a typical 6” flex hose and 2.12” for a 4” hose. If we instead use all 6" ducting with smoother walls, etc. our typical 1-car garage sized shop will add a minimum of about 1.69” of resistance, 2-car sized shop about 2.64”, and 3-car garage sized shop about 3.58”. We also need to add the resistance for our filters. Filters start off with minimal resistance when clean and new and that resistance climbs as they “season” meaning build up a thick cake of dust in the pores that does not all come out with normal shaking and cleaning. This caking results in a normal fine bag type filter that comes with most dust collectors and cyclones to have a resistance level that ranges from around 1” to a maximum of about 5”. Although big cartridges can drop this to as low as 0.25”, I choose to use 2.5” because that is average for most of the units we buy. Adding the pieces gives us a total resistance of 6.11” for a small dust collector wheeled between machines, a 1-car garage sized shop with ducting at 6.51”, a the typical average 2-car garage sized shop at 7.46”, and a 3-car garage sized shop at 8.40”. If we use a cyclone or trashcan separator we also need to add that resistance. Figure 3.5” for a typical cyclone and 3” for a cyclone with neutral vane, or about 2.25" for my cyclone design.

Because blower technology is pretty mature, most commercial blowers that turn the same sized impellers at the same speed have near identical performance. This is not true for hobbyists as in all my testing only the Jet and Delta blowers tested with close to commercial airflows, all others tested less, some as little as half due to major design and blower construction errors. Regardless, if we round down to 7” for average sized shops and skip the cyclone or trashcan separator we check a good blower fan table and find our 1.5 hp motor will only move 680 CFM without overloading the motor. Our 2 hp can only move 890 CFM, and our 3 hp can only move 1150 CFM. Adding 3” for a cyclone leaves us with a 1.5 hp motor moving only 570 CFM, yet testing with 1” resistance results in a whopping 1504 CFM pulling 3.56 hp. Likewise, our 2 hp motor at this same 10” resistance level with cyclone added can only move a maximum of about 760 CFM, but when tested with just 1” resistance this same blower moves 1830 CFM drawing a whopping 5.2 hp. Likewise, our 3 hp motor can only move 1005 CFM without over stressing the motor, but when tested with 1” resistance it comes up with a maximum 1984 CFM pulling 6.32 hp. Adding the additional overhead for an average sized shop and maximum filter "seasoning", leaves nothing less than a 3.15 hp motor turning a 14” impeller to move ample air to provide 800 CFM at our larger tools. Since standard motor sizes are 1.5, 2, 3 and 5 hp we can either add a little resistance to make a 3 hp work, use a more efficient cyclone, or step up to a 5 hp motor. I chose to use both a bigger motor and more efficient cyclone because fifteen years of professional air engineering testing shows to get really good fine dust collection we really should move about 1000 CFM at our larger tools and dustier operations. The 800 CFM needed to meet OSHA requirements has already been abandoned due to too many getting ill. ACGIH and the European community recommend far better collection which requires either tools engineered from the ground up to contain and protect the fine dust, or moving more air.

[Jeff Cord]

I am starting to see how I can use the curve in the FWW mag.
I guess the next question is how can I find the curves for other collectors?
I'm trying to be sure I get the collector that gives me enough real performance to capture the dust.

Based on the information in the post above (using the estimates of the SP in small shops) the Delta 760 may be barely sufficient.

Also I'm curious about any opinions regarding the bag used on the Delta 760 (the 1-mil bag) versus the Wynn cartridge?
Is one better than the other at capturing the finer dust?

[Stu Ablett in Tokyo Japan]

I'll not even try to add to what Bill has said, except to the question of bags vs filters.

I just checked Delta's site, and that 760 DC has a total bag area of 20.5 square feet of bag filter area.

There are two filters that Wynn seems to recommend for doing the bag to cartridge conversion, the 35A100SBOL and the 35A274SBOL. the first has 100 sqft of cartridge filter area, and the second has 274 sqft.

The Torit filters I use on my cyclone have 226 sqft each (I have two filters).

With the smaller of the Wynn filters you are still getting nearly 5 times the filter area, thus your possible airflow will be much, better, with the 274 sqft, it is 13 times the filter area.

Now once the bags get dirty, the actual sqft drops a lot.

With a good cyclone, your filters don't see hardly any dust, so you get a constant high potential air flow.

If that makes sense.........

Cheers!

[Jim Becker]

I guess the next question is how can I find the curves for other collectors?

Very difficult, if not impossible, unless the unit you are considering was reviewed by a magazine that actually did testing. The fan curves would be almost "anti-marketing" for the manufacturers! Most of this has been with cyclones, not the single stage systems. Most of the single stage systems can move about half of their manufacturer "specification" relative to CFM with appropriately sized duct work, but there is no guarantee of that.

[Bill Pentz]

Jeff, the FWW curves are probably the most accurate comparisons I have seen to date, but you need to remember that this testing leveled the playing field by testing whole dust collectors instead of just the blowers, plus all were tested with the same lower resistance high airflow fine Wynn filter instead of stock filters. I remind all that you need to put a heavy piece of window screen in these units to protect that filter (see my pages for more info).

Prior Wood Magazine and American Woodworker Magazine fan curves and recommendations had problems. They really should have had an engineer look at the fit, finish, design, and construction because most of the import blowers were just plain bad with major copying errors. There also were serious testing problems using inappropriately sized test pipes, failure to use an amp meter, some vendor funny business where motors and impellers were tested that did not match the products being sold, and some serious graphing problems. Careful examination of their graphs showed somehow the individual graphs, probably from MS Excel that automatically scales, were merged without converting each to the same scale. The results rewarded marginal performers and wiped out good performers.

Although the best testing to date, even the FWW tests have some problems. Again, our dust collector and cyclone blower impellers are sized to not burn up the motors in a minimal resistance configuration and still provide as much airflow as possible. This means that if a firm uses a larger impeller risking burning out motors if there is not enough resistance, they get to “win” the magazine rating wars. They will continue to win until the magazines test with an amp meter and throw out all CFM readings that exceed the motors’ rated amperage. FWW failed to use an amp meter so continued the not good practice of publishing portions of the fan curves that we know push the motors far over their maximum rated amperage. If you were to add the minimum resistance for a standard 10' long 6" flex hose connected to a large two port tool, that resistance leaves none of the portable dust collectors able to move the minimum 800 CFM needed for minimal good fine dust collection. It gets worse because that testing also did not adjust down the roughly 10% needed to compensate for only taking a centerline airflow. Still, FWW did a good job blowing through the confusion of just testing blowers. The FWW tests give real performance in a working system with a consistent filter. That saves having to add all of the various resistance overheads that make a huge difference in dust collector and cyclone airflows.

So you are correct, even that “best” Delta portable dust collector with the upgraded lower resistance fine cartridge filter is probably not going to give you the airflow in real use needed for good fine dust collection in spite of these being excellent "chip collectors". If you check, you would see the Grizzly tested has been a top scorer in those tests that looked at larger 2 hp dust collectors. This means most of the 2 hp dust collectors also have a problem moving the needed air if burdened with ducting or more filter resistance.

I published the fan curves for the 2 hp and larger blowers used on most hobbyist dust collectors and cyclones based upon my own testing. For what it is worth, the FWW testing confirmed my own with Jet and Delta getting top ratings because they are the only two firms of the many I have studied in depth that use really well engineered blowers and impellers. My worsening health left me unable to keep up this testing. I finally pulled down my testing curves after getting beat up pretty badly for not keeping that information current. I agree that many of the vendors that had “worst” performances on my curves fixed their problems, but none went the next step to actually move from “chip collection” into good fine dust collection airflows. A commercial blower fan curve will pretty much limit the high end of what you can expect from your dust collector. My below graph gives actual curves for real hobbyist dust collectors, but I pulled the names to save some embarassment. For cyclones you need to add as much as 3.5" for the cyclone overhead in some cases as much as 5" more for filter overhead. So far, I think FWW has the only curves you can trust to get a good idea of performance. I know Michael Standish has already completed his cyclone tests that should be pubhished this fall.

Although most probably do not want to hear me, I said you need a 3 hp dust collector turning a 13” impeller or 5 hp cyclone turning at least a 14” impeller to move the roughly 1000 CFM needed to meet the volumes of air needed to get the air quality standards already adopted in Europe that most medical experts recommend for good fine dust collection at your larger tools. I also said that dust collectors need to go outside and cyclone filters should either be tossed or put outside with fine open flow filter bags because fine bags and cartridges too quickly self-destruct when used indoors on most dust collectors and cyclones.

Hi Stu!

Changes made because I linked to the wrong graph.

[Bob Dodge]

Hi Jeff,

The required static pressure for "small shops", cannot be "generalized". When you look at a static pressure curve, it basically tells you what your dc placement and duct-run options are. In essence, you have control over how you use the available static pressure.

If you decide to place a marginal dc, in a "convenient" location which happens to be at a distance from a machine, you may be short-changing yourself. Part of the available static pressure, will have to deal with the added pipe length and fittings. It's really "in your hands" to decide where you place that dc in relation to your machines.

If you're going to go with a marginal dc, you have to be diligent in grouping specific machines and use short runs. If you do so, you will be no better off with a slightly more powerful dc, which is placed "at a distance".

Set your desired CFM target for the machine in question, look at your static pressure curve, then create the conditions for meeting that static pressure point. Allow some margin for pressure-drop at the filter as it loads. You create that margin by using a large diameter pipe, but not so large that velocity falls below specs. You want at least 3500 FPM through that pipe, or 4000 FPM in a vertical. The type of hood used, will have a profound impact.

Here's an example using the Delta 50-760 that you mentioned, and a large planer that has a 6" hood and an 800 CFM requirement. Let's for the sake of this example, use the FWW Magazine figures. The test result stated 800 CFM @ 4.5" sp.

Well, if you tried pulling that 800 CFM through a 5" pipe, you'd have unnecessarily high velocity in that pipe, which also means a very high static pressure reading. An 11 foot length of reasonably smooth-walled flex, with a 60 degree taper hood, would have 4.5" sp at 800 CFM. (no elbows) Add a 90 degree elbow, and you'd have to shorten your flex-pipe to 7 feet to stay within that 4.5"sp range. That's allowing nothing for pressure-drop at the filter. You'd have to shorten that 5" flex-pipe, to 2 1/2 feet to allow for a 1" pressure-drop at the filter. The point is; your velocity would be at a ridiculously high level. 5870 FPM. That's totally inefficient and unnecessary. So, what do you do? You go to a 6" pipe of course. That'll bring that excess velocity and static pressure, wayyyy down, and you'll STILL have all the velocity you need.

Using the same figures of 800 CFM at 4.5" sp, you could run a 45 foot long 6" galvanized pipe, with the same tapered hood, and that same 90 degree turn, AND, that would be allowing for a 1" pressure-drop as the filter loads. The ducting I mentioned, would by itself produce only 3.5" sp.at 800 CFM. Velocity would be 4076 FPM.

What's important here, is to not allow yourself and your dc's capacity, to be controlled by components like cheap $3. plastic hoods with 4" ports, and inexpensive 4" flex-pipe. Plan those pipes properly, and you'll get a heck of a lot more out of your dc than you could have imagined. That convenient trip to the borg for 4" flex and fittings, costs you a lot more than you think.

Bob

[Bob Dodge]

Stu,

The filters have very little impact on the overall outlet resistance. That limitation occurs primarily at the blower outlet. Here's an example.

The Jet DC-1100, flowed roughly 900 CFM WITH the cartridge filter. Without the cartridge filter, and only a single 14 sq.ft. filter-bag, it flows 860 CFM. The difference, was only 40 CFM. Roughly a 4% improvement.

The Delta 50-760's needle-felt filter, is roughly 50% larger than the Jet's, at 20.5 sq.ft.

Bob

[Bob Dodge]

Very difficult, if not impossible, unless the unit you are considering was reviewed by a magazine that actually did testing. The fan curves would be almost "anti-marketing" for the manufacturers! Most of this has been with cyclones, not the single stage systems. Most of the single stage systems can move about half of their manufacturer "specification" relative to CFM with appropriately sized duct work, but there is no guarantee of that.

-----------------------------------------------------------------------
Jim,

As a general rule-of-thumb, you could generally state that CFM would be 1/2 of the "FREE-AIR" rating would be, but not 1/2 of a "test-result". You have to be careful though. "Claimed" free-air delivery ratings can vary widely with the "colour" and "manufacturer/re-sellers" brand label, even though both dc's are identical in every respect. Free-air delivery, is a useless, meaningless rating.

While there are few test results available out there for consumer-model single-stages, they are available with a little detective work. The June 2000 American Woodworker test-result, is one example. The recent Fine Woodworking Magazine test, is another.

While the test procedures themselves may have produced some higher-than-actual CFM results, they were accurate in the sense of the static pressure generated in the test-pipe, and, to illustrate the "difference" between one dc and another. All dc's were tested using the same method.

Bob

PS. The dc's were also tested with the same "seasoned" filter.

[Jim Becker]

-As a general rule-of-thumb, you could generally state that CFM would be 1/2 of the "FREE-AIR" rating would be, but not 1/2 of a "test-result".

Correct. I misspoke...and actually meant what you said. Sorry...

[Jeff Cord]

I'm already planning on keepig the DC portable and moving it to the machines.
So hopefully that will simplify things somewhat.
It also sounds like, since for the moment I'm looking for a more portable machine, I should stick with a Jet or Delta as those have better engineering.
I also plan on purchasing the hose from Wynn (smooth walled type).

I'm getting a feeling from some of the comments that having too little SP is also not ideal. That you want to get a balance from all elements of the system (hoods, ducts, connectors, DC) so that you don't get too much air velocity either.

I guess this type of confusion is typical for beginners?