DC Specifications/Reality

[Matt Schroeder]

I purchased a new 700 CFM rated 1 stage DC. Measuring the air speed at the end of the stock 6' of 4" flex hose (with a Kestrel 1000 anemometer) I calculate about 450 CFM for the machine. This is brand new (clean bags) and is how the manufacturer says they rate their unit (brand new, at the end of the stock hose). Is this reasonable? Has anybody else tested a new DC and found similar departure from manufacturer's claim?

Do you think I have reason to ping on the manufacturer and ask for a refund? I purchased this unit counting on the 700 CFM to get me at least 400 CFM by the time I got to the point of work. Starting at 450 only gets me about 275 at the point of work.

[Jeffrey Makiel]

That's an interesting test. I had a 1hp dc many years ago and approximated its CFM at about 250-300 with about 5" static pressure at the inlet.

I doubt any claimed rating could be achieved on these type of units even with no duct and no bags attached. Needless to say, manufacturer ratings on these units (1, 2 and 3hp) are grossly over inflated like horsepower ratings on shop vac, routers, air compressors, etc.

Check to see if the retailer has a satisfaction policy.

-Jeff :)

[Art Mann]

Remove the hose and the filter and you might get the stated cfm at the inlet. The only way to get a good idea of how a DC will work in a system is to get a fan curve showing cfm as a function of static pressure and then use one of several calculators available to estimate the SP of your particular system. The better manufacturers will often publish these curves for individual machines.

[Steven Wilson]

400 cfm is actually pretty good for that type of machine. Unless you have a good fan curve to use I take the published CFM numbers and cut them in half.

[Bob Malone]

Matt, my new unit was supposed to deliver 850 CFM - actual was 420 CFM. Most manufactures over estimate their figures dramatically.

Bob in Calgary

[Ken Fitzgerald]

Any type of test.....this one or any other type of test would have to be performed EXACTLY as the manufacturer performed it.....same, absolute exact conditions...to the "t" and dotted "i"....... If not performed (1) under the exact same, and I mean exact, test conditions and (2) with known calibrated test equipment, the results can not be compared accurately or fairly.

This is also why some manufacturer's post independent test results performed by an organization outside their company.

[David Giles]

Remove the hose and the filter and you might get the stated cfm at the inlet. And the dual 4" connection inlet plate.

Product manufacturers rarely flatout lie, but they have considerable leeway in how they write specifications on the product. Their intent is to sell product. And misleading DC curves sell product because so few folks ever check the advertised claims. Fewer still exact any sort of penalty on the supplier. So there is a very limited downside and lots of upside sales for bogus claims.

Buyer beware.

[Greg Funk]

Normally, to determine the airflow accurately you would need to make the measurements in a straight section of pipe using a Pitot tube. If you're just holding the Kestrel meter in front of the pipe I would be surprised if you were getting an accurate reading.

Greg

[Jim Becker]

The manufacturer's rating for typical mass produced dust collection systems is a "free air" measurement...no duct; no restrictions...pretty much at the point the motor might burn out. '0" of water SP (static pressure)', in other words. Typical real world performance is about half that rating. I'm actually surprised you were able to measure 450 CFM in that 4" hose given it's hard to move more than 300-350 CFM through such a small space.

[Michael Weber]

Doesn't an anemometer just measure velocity? Don't you have to apply a formula using the pipes effective area?

[Matt Schroeder]

Yes, the anemometer measures linear velocity. Taking that number with the cross sectional area of the pipe, minus the area blocked by the anemometer, allows calculation of the volume flow rate.

I understand that my method of testing is not exactly what the manufacturer does. However, the difference between 700 and 450 is quite large. I did ask the manufacturer about their spec, and they replied via email that it is with bags in place, new, at the end of the stock 6' hose (not the stripped motor and impeller only). They did not say how they measure that velocity. I sent a reply explaining what I had done and questioning the operation of my machine, but they have not replied in a week (to three emails), so I thought I would pick the collective brain of SMC and see what we could figure out.

It seems that the consensus is I got what I deserved by believing their spec and I have to decide if this is going to work for me or if I need to move to a larger machine to get the flow I want. You can be sure I will be very leery of trusting specs on machines in the future, especially from this company, but I guess from any manufacturer.

[Steven Wilson]

Matt, your manufacturer probably measured CFM with no bags on the machine and the 750 CFM is the free air CFM of the machine. Half of that is closer to what you get at the machines inlet with the bags installed. Your numbers are a bit on the high side of what I would expect. If you want 800 CFM then get a 1600CFM machine or buy one from a manufacturer that posts their air curves of actual performance with given restrictions (SP).

[Jeffrey Makiel]

It's difficult for most folks to accurately measure CFM (flow) on thier home system. One reason is that there needs to be a section of straight duct for a smooth velocity profile to develop as shown in the first pic below. And, you need to have the proper (and expensive) test apparatus that has been calibrated recently.

One common mistake is to take the velocity measurement at the middle of the duct and assume that's the average velocity of air thru the duct. However, the velocity distribution across the cross sectional diameter varies, with the center being the fastest. See first pic below. If you measure at the center point, and multiply by the sectional area, you are getting an unrealistic high result of air flow (CFM). Therefore, you must apply a factor which corrects this situation, or take multiple samples by 'traversing' the section. A small probe, like a pitot tube, is needed for traversing.

Further, if you try to measure air flow at the inlet of a duct with a vane axial anemometer, you will also get an incorrect value because the velocity is also not uniform. A velocity 'choke diameter' occurs (called "vena contracta") that will throw off your velocity measurements as shown in the second pic below. It's best to perform a traverse somewhere in the middle of the duct (by drilling a hole for a pitot tube) if you can find a section of straight duct long enough. 10 diameters is best. (4" duct = 40" of straight duct).

Nothings easy. :(


-Jeff :)

http://i3.photobucket.com/albums/y84/Beff2/Slide1-5.jpg

http://i3.photobucket.com/albums/y84/Beff2/Slide2-5.jpg