Measuring CFM/Efficiency of DC/Cyclone

[Floyd Cantrell]

In looking at many of the posts here it looks like there are quite a few people that know a lot about Dust Collection. I have a question regarding how you know how well your system is working. How do you measure CFM or some type of measure of different configurations? I installed a DC system about 8 years back before so much information was readily available. I now know that for any additional improvement I need to make changes. I don’t want to make changes just to say I have a Cyclone or some other system. I would like to be able to measure my existing system and change some ducting to see what effect that has on a system. When I added the oversize top filter bag from American Filter I could feel a change but how much I don’t know. I am looking for some way to measure change and is the change significant if CFM? Any input on how you folks measure the efficiency of your systems would be appreciated?

[Larry Reimer]

Floyd, to measure the cfm (cubic feet per minute) of your dust collection system you need to know the size of the duct and the speed at which air is traveling through the duct. An anemometer will tell you the speed of the moving air. Price of these meters can run from say $60 to many hundreds of dollars. Higher prices generally get you better accuracy and ability to read accurately over wider ranges of airflow. For a dust collection system I'm not sure it's worth spending that kind of money. I'd try a "deflection anemometer". That's a small plate suspended from the top and allowed to "swing". Then you put a scale at the side of the plate and see how much the plate is deflected by the airflow. Won't give you a reading of quantity but will let you know if you've changed for better or worse.

[J.R. Rutter]

I'm no expert, but if you have a fan curve for your blower, you can get an idea of CFM by measureing the static pressure just before the blower. You can easily measure pressure with a manometer made with a piece of clear flexible tube filled partway with water. One end of the tube gets inserted into the duct, then the tube runs down to form a tall, narrow "U" shape. the other end is open to the air. When you turn on the DC, the water will be drawn up the leg of the "U" connected to the duct. The difference in water level between this side and the open side is the static pressure in inches of water. Your goal is to get this number as low as possible with a blast gate open, indicating higher air flow. To get an idea of your base blower pressure capacity, see what the pressure is with all gates closed.

The best my old 2 HP blower could do was 10.5" in front of the cyclone with all gates closed (close to zero CFM). Since I had a fan curve, I knew that if the gauge read 4", then I was pulling about 750 - 800 CFM. I haven't finished installing the ductwork for my new blower, but when I tested a section of snap-lock, it got sucked flat before the blower got up to full speed...

[Allan Johanson]

Hi Floyd,

I've run dozens of airflow tests on different DCs. I use a Dwyer pitot tube, part # 167-6 and two different magnehelic gauges. A 0-10" and a 0-2". I've found the 0-2" is the best for most typical airflow measurements. I use the 0-10" when I'm measuring SP and cases where the airflow is very high and the velocity pressure reading would exceed 2".

Buying these parts from Dwyer would cost you $44.75 for the pitot tube and $61.50 for each gauge. Buying used or from a discount firm would save you some money.

Using a method like this is more accurate than an anemometer and it's easy to measure a completed ductwork section with the tool hooked up. Just insert the pitot tube and read the gauge. But of course this costs much more money. A big downside to what I do.

If you have a stock DC and a reliable system performance chart, trying to estimate airflow with a vinyl tube manometer is a reasonable way to do things. You can always ignore numbers and simply watch the water level for changes as you tune your system. But as soon as you change your filters from anything besides clean, stock ones then you will have a different resistance level on the blower outlet side and this will change the system performance curve from what the factory published. Bottom line, don't worry about "cfm", just check the SP level to see if your changes are lowering it.

When it comes to subtle changes in ductwork, you may have a hard time seeing the changes with a "U" tube manometer. In this case, I suggest making an inclined-tube manometer. I have a combo U-tube/inclined manometer. You can build whatever you want. It's just vinyl tubing stapled to a board. I'm using my pitot tube for accuracy since I have it, but you can even use one of these (or similar) from the Borg:

Part # A-339
http://www.dwyer-inst.com/htdocs/misc/fittings-filters.cfm

This type can thread into a metal pipe and project enough into the airstream to get a more accurate reading. Might be a little short for PVC. Find one with longer threads in that case. Just stuffing a hose into a hole in the pipe may not be right. You may end up reading some velocity pressure when you want the hole to be perpendicular to the airstream to measure static pressure.

Or you can buy a proper static pressure tip from Dwyer or Grainger, etc.

About that new filter bag of yours:

For a generic 2HP DC with a home-made 6" inlet and a short length of 6" pipe....it flowed 700cfm with the typical dust-caked factory top bag. This was after the "bag shaking" to help clean it.

When that top bag was replaced with a Lee Valley aftermarket top bag, when clean it flowed 930 cfm. After my friend uses his DC more then I'll go back and see how much that number will drop when a typical dust cake develops.

Some folks think a quick bag shake will restore all the original cfm....we'll see what data this case study will provide. I'm really curious.

Cheers,

Allan

[Floyd Cantrell]

Thanks for the input on measuring the air flow. I decided to go with a Dwyer pitot tube and Dwyer Magnehelic Gauge. I had a 1-10 in H2O but also have a bid in on a new 0-1 H2O Dwyer Magnehelic Gauge on EBay. In looking at the Air Velocity Flow Charts for the Pitot tube I think the 0-1 in H2O will serve my needs. May have some other questions once I receive the equipment and get some time to make measurements.<O:p</O:p

[Allan Johanson]

Hi Floyd,

That 0-1" gauge will be good for approx:

200 cfm in 3" pipe
350 cfm in 4" pipe
550 cfm in 5" pipe
785 cfm in 6" pipe

A 2" gauge will be good for almost 50% more airflow, that's why I like it.

When it comes to using the gauges and pitot tube, look at the pic on the first page of this link:

http://www.rd.com/americanwoodworker/toolguide/TT_DustCollection.pdf

You don't want to do that. I hope this was just some random pic for the article, because if they tested the DCs like that then I'd question all their results. You want to hold the pitot tube directly into the airstream. Not at some arbitrary angle.

I used a wood block, duct tape and zip straps to hold the pitot tube firmly in place, exactly 90 deg vertical to the pipe and directly into the airstream.

Also, if you take the magnehelic gauge you have now and hold it in your hand, you'll see the needle move as you hold it at various angles from dead vertical to flat horizontal. What I did was to clamp the two gauges I used to a piece of wood so they wouldn't move. Then I zeroed out the needles.

By doing it the way I did, I made sure I removed a couple of human error variables from my testing. Whatever you decide to do is fine, just be consistent from one day to the next for a higher level of accuracy.

Doing your own testing is cool and you will learn a lot. Especially when it comes to multiple branches. If you've ever seen someone give advice on these forums where they should run a 6x4x4 wye on a drop to a tool and run ducting to the tool on one 4" branch and leave the other 4" branch at the wye always open, "to feed the main duct"....well, you'll see firsthand why that's a really bad idea. :eek:

You'll have fun.

Cheers,

Allan