Rate of Air Flow vs Volume of Air Flow

[DLB]

I am seeking confirmation.

Am I correct with the assumption that to achieve 200cfm through a 2 1/4" hose will require at least 7250 ft/min airflow?

If so, the outlook is bleak.

An additional note. I looked at several anemometers yesterday while they were on sale. For many, 7250 fpm (82 MPH) is outside of their range. Several budget friendly models had a max of 67.1 MPH.

- David

[RFGuy]

An additional note. I looked at several anemometers yesterday while they were on sale. For many, 7250 fpm (82 MPH) is outside of their range. Several budget friendly models had a max of 67.1 MPH.

- David

David,

Yeah, thanks for mentioning that and you raise a good point. Almost all of the anemometers (consumer level) that I have looked at max out at like 5900 fpm, so it isn't possible to measure 200 CFM or 7250 fpm at the 2-1/4" port on the DC-3300. This is also why ANY measurement of the larger 1000+ CFM dust collectors is often performed using a manometer or pitot tube anemometer, I believe. You do have to be careful when making measurements with these off the shelf anemometers, but as long as you keep the max airspeed under their rated max spec then it should be possible to get some data and make some comparisons. All test equipment and instrumentation have limitations and anemometers are no exception. In the manual that Dusty posted, it shows a range of 59-5905 fpm as a spec for his anemometer.

[dusty]

I am seeking confirmation.

Am I correct with the assumption that to achieve 200cfm through a 2 1/4" hose will require at least 7250 ft/min airflow?

If so, the outlook is bleak.

Dusty,

Yes, this is correct. Also, why I have not understood the guidance from Jim on what the DC-3300, upgraded DC-3300, or DC-6000 are capable of. That 3 port manifold is VERY restrictive and then the limitation of a 2-1/4" port is also very limiting. I can get almost 5000 ft/min out of 1 port on mine, but that is it. Again, I am not trying to knock the new DC-3300 upgrade kit or even the DC-6000, but I just want honesty of what to expect from them. I do think the upgrades probably give some improvement, but we need to quantify what that improvement is and whether you can even see it with the 3 port manifold installed.

I do not regret doing the upgrade BUT since I am, for the most part, a single port user the upgrade bought me little if any real improvement. Yes, I could also connect to the belt sander at the same time and still have reasonable performance BUT that very rarely happens.

With only one anemometer, how do I perform a valid test on multiple ports. It seems to me that the best I can do/say is that the air flow/cfm decreases with each additional port opening. No, I am not buying two more anemometers EXPECIALLY if the ones that I might buy are not even capable of measuring what might be the maximum airflow produced by the DC.

At this point it does appear that I could be maxing the anemometer without reaching max CFM from the DC.

Using my anemometer (and current interface which has not been finalized) the best measurement that I get is 59.6MPH. The highest airflow that my anemometer can handle is 5905 ft/min (per spec sheet).

[RFGuy]

I do not regret doing the upgrade BUT since I am, for the most part, a single port user the upgrade bought me little if any real improvement. Yes, I could also connect to the belt sander at the same time and still have reasonable performance BUT that very rarely happens.

With only one anemometer, how do I perform a valid test on multiple ports. It seems to me that the best I can do/say is that the air flow/cfm decreases with each additional port opening. No, I am not buying two more anemometers EXPECIALLY if the ones that I might buy are not even capable of measuring what might be the maximum airflow produced by the DC.

At this point it does appear that I could be maxing the anemometer without reaching max CFM from the DC.

Using my anemometer (and current interface which has not been finalized) the best measurement that I get is 59.6MPH. The highest airflow that my anemometer can handle is 5905 ft/min (per spec sheet).

Dusty,

Thanks. So, you are getting 59.6 mph out of 1 port now, which is 5244.8 fpm, correct? If I can ask, what changes did you make to get from 2990 fpm to 5244.8 fpm?

Yeah, measuring more than 1 port is problematic. That is why when I reported my numbers before, I did 1 port open, 2 ports open and 3 ports open. For the latter 2 cases I could only measure a single port at a time so I left the non-measured ports open and drawing in shop air at the same time that I measured a single port. Not a perfect measurement, but not much of a choice. However, with the upgrade (based on Jim's feedback), I wouldn't expect you to lose any airflow with more than 1 port open, right? Is this what you are seeing? My airflow dropped significantly with more than 1 port open, e.g. 142.7 CFM -> 98.4 CFM -> 74.6CFM. Can you try measuring similarly to how I did below for comparison? I would expect the upgrade to not drop as much with 2 or 3 ports open as my original DC-3300 does. You can just report airspeed (fpm) numbers. In the data below, "left" is the left side of the 3 port manifold, " right" is the right side, and "center" is the bottom port on the 3 port manifold.

AIRSPEED - <measured>
1 port open at a time: (units=ft/min)
Left: 4944 Center: 4494 Right: 4894

2 ports open at a time: (units=ft/min)
Left: 3411 Right: 3388

3 ports open at a time: (units=ft/min)
Left: 2584 Center: 2497 Right: 2479

AIRFLOW - <calculated>
1 port open at a time: (units=CFM)
Left: 142.7 Center: 129.7 Right: 141.2

2 ports open at a time: (units=CFM)
Left: 98.4 Right: 97.8

3 ports open at a time: (units=CFM)
Left: 74.6 Center: 72.0 Right: 71.5

[dusty]

I am using a hose coupler from one end of my dust collector hose. I had intentions of cutting it in a manner that would allow me to attach it permanently to the face of an anemometer. It was then suggested that I might find a gasket that would facilitate the connection without damaging the coupler.

With this in mind I chose to wrap a piece of self adhesive foam window/door seal around the coupler.. The foam would provide a better seal to make the contact with the meter. It worked some what.

Then Dave made his post and I better read the specs on the anemometer. Sure enough the anemometer is incapable of readings that would be high enough to indicate 200cfm. A likely show stopper because I am not going to spent what would likely be required to increase that range.

Foam collar not installed in image with anemometer.
.

[Matanuska]

Dusty,

At the risk of stirring the pot, does your coupler directly connect your anemometer to the 2.25” ID inlet port with no leaks? If so, if the anemometer has an ID larger than 2.25” the velocity it measures will be lower than the actual velocity at the 2.25” inlet port. Squinting at the anemometer and ruler picture I believe you posted earlier it looks like your anenometer ID is 2.5”. This would mean the velocity at the 2.25” inlet port is about 24% higher than the velocity measured at the anenometer (73.6 mph vs 59.6).

[RFGuy]

Dusty,

At the risk of stirring the pot, does your coupler directly connect your anemometer to the 2.25” ID inlet port with no leaks? If so, if the anemometer has an ID larger than 2.25” the velocity it measures will be lower than the actual velocity at the 2.25” inlet port. Squinting at the anemometer and ruler picture I believe you posted earlier it looks like your anenometer ID is 2.5”. This would mean the velocity at the 2.25” inlet port is about 24% higher than the velocity measured at the anenometer (73.6 mph vs 59.6).

Matanuska,

So, the 3 port manifold has a section that is 2.25" for the inlet. When I did my testing, I had the anemometer with a 2.5" fan placed right up against the 3 port manifold. There is maybe a 0.5" (inside the anemometer housing) that the air has to travel before contact with the fan of the anemometer. Do you really think that the airspeed slows down that much in only a half inch travel after leaving the confines of the 2.25" inlet of the manifold? I am no expert here, but it just seems like a stretch to me that the diffusion would happen that quickly. I guess we will never know because I don't have a 2.25" fan anemometer for comparison. I could see if we had gone down a few inches of duct or hose then I would agree with you, but I am placing the anemometer as close to being in the airflow as I can by placing it directly on the manifold so I was making the assumption that I am measuring the airspeed right at the inlet.

ALL,

Can we drop the mph units and restrict discussion to either airspeed in fpm or airflow in cfm on this thread please? I don't see too many dust collectors spec'ed in mph units.

[Matanuska]

I was referring to Dusty’s measurement with a 2.5” meter in an apparently closed system with no leaks, not yours. If I understand how you are making your measurements, your system leaks around the outer edge of your 2.5” meter so the air is not forced to flow over the entire x-sectional area of the meter and so I would agree the air velocity through your meter is probably very close to your actual 2.25” manifold inlet velocity.

Not trying to belabor this, just pointing out that Dusty’s upgraded system may be performing better than he thinks, especially if comparing his results with your non-upgraded(?) version. Apologies if I’ve made any incorrect assumptions about dimensions or how his and your measurements are being done.

[dusty]

I was referring to Dusty’s measurement with a 2.5” meter in an apparently closed system with no leaks, not yours. If I understand how you are making your measurements, your system leaks around the outer edge of your 2.5” meter so the air is not forced to flow over the entire x-sectional area of the meter and so I would agree the air velocity through your meter is probably very close to your actual 2.25” manifold inlet velocity.

Not trying to belabor this, just pointing out that Dusty’s upgraded system may be performing better than he thinks, especially if comparing his results with your non-upgraded(?) version. Apologies if I’ve made any incorrect assumptions about dimensions or how his and your measurements are being done.

No apologies needed. I am stumbling my way through this and I appreciate all inputs.

I understand that a closed system is required to obtain an accurate evaluation and I am approaching that now. By wrapping the one end of a 2 1/2" hose coupler with foam I am able to create a soft pad that couples the meter to the DC. I saw the effects of that with my first preliminary test of this coupler. I achieved readings that were much closer to expectations than any test before.

However, I believe I will never get readings with the anemometer that I have simply because of meter limitations. Even with perfect coupling the meter is incapable of measuring airflow rates that translate to 200cfm. This anemometer is max'd at 5905 ft/minute. If my calculates are correct I need something like 7200 ft/min to measure 200cfm.

I have no way of judging the ability of the DC to produce that rate of air flow.

[RFGuy]

I was referring to Dusty’s measurement with a 2.5” meter in an apparently closed system with no leaks, not yours. If I understand how you are making your measurements, your system leaks around the outer edge of your 2.5” meter so the air is not forced to flow over the entire x-sectional area of the meter and so I would agree the air velocity through your meter is probably very close to your actual 2.25” manifold inlet velocity.

Not trying to belabor this, just pointing out that Dusty’s upgraded system may be performing better than he thinks, especially if comparing his results with your non-upgraded(?) version. Apologies if I’ve made any incorrect assumptions about dimensions or how his and your measurements are being done.

Matanuska,

Thanks. Yeah, I was just referring more to the reference plane of the measurement. Yes, I am holding the anemometer right onto the manifold so the plane of measurement is the center of the anemometer fan blade which is maybe 0.5" from the inlet exit. Also, I have tried just holding the anemometer up to the manifold versus taping it really well. I can tell by the sound difference when I have good coverage because leaking air sounds very different from the full speed of the anemometer fan. I was never going for lab quality measurements here, but just a guide. This guide helped me to determine that my Shop Vac™ vacuum has more airflow than my DC-3300. Dusty has the right idea with making a dust fitting and then sealing it against the anemometer. I can try to do the same, but my DC-3300 is in the attic so it is a pain to go up there and attempt another measurement. I wasn't talking about leakage though. My point is in both Dusty's modified dust fitting and in my case, we are measuring with a 2.5" anemometer fan that is perhaps 0.5" from the end of a 2.25" ID inlet/dust fitting. I was just debating if the air velocity diffuses and drops that much in a 0.5" distance away from that 2.25" inlet. You mention it could be as much as 24% higher velocity inside of the 2.25" inlet suggesting that we are measuring it in a 2.5" opening of the anemometer. You may be right, but I was just questioning if air velocity would change that quickly in a half inch distance that it has to diffuse in this case. Lastly, for sure I acknowledge that both Dusty and I could be measuring on the low side, i.e. I am taking the max reading that I can manage with my test equipment in a non-lab environment, but the real measurement could indeed be higher. In my case, I think any error in my measurement is small (<10%), but then I guess the question is whether my logic was flawed in thinking that the air velocity that I measure with the fan in a 2.5" anemometer the same as what is coming out of the 2.25" inlet near it?

P.S. One thing I don't get is the discussion here has turned into "Oh, we need a correction factor to adjust for inlet to anemometer size because of air diffusion". These anemometers are used extensively in the HVAC industry. I checked the area setting on my anemometer and it just uses the equation: CFM = (fpm * area). There is NO correction for the difference in the 2.5" aperture to the much larger aperture often found in HVAC supply registers. IF they did adjust then the actual airflow would be ridiculously small and nonsensical for the HVAC case where the supply register is significantly larger than the anemometer fan size. On this thread you are talking about going the other way, i.e. DC-3300 inlet is slightly smaller than the anemometer fan. Can someone explain to me why you don't convert in the HVAC case, but in Dusty's case we do??? Why doesn't the corollary hold?