Rate of Air Flow vs Volume of Air Flow

[dusty]

Good catch. It looks like I used diameters in my calculations where there should have been the radii of 2 1/2" couplings and hoses. All of my DC parts and accessories are standard Shopsmith items.

The 2990 ft/min now calculates to 173.17 cfm while 2687 ft/min calculates to 155.62 cfm. Not what I wanted to see but these are certainly more real world numbers for this dust collector, hoses and couplings.

[Matanuska]

Perhaps I misunderstood the suggestion, but sealing the flow path between the anemometer and the suction inlet could erroneously change the measured velocity if the anemometer diameter is different than the suction inlet.

[Matanuska]

Unless of course you take the different flow areas into account when you convert velocity to flow rate, which is probably what you are doing anyway(!)

[RFGuy]

Good catch. It looks like I used diameters in my calculations where there should have been the radii of 2 1/2" couplings and hoses. All of my DC parts and accessories are standard Shopsmith items.

The 2990 ft/min now calculates to 173.17 cfm while 2687 ft/min calculates to 155.62 cfm. Not what I wanted to see but these are certainly more real world numbers for this dust collector, hoses and couplings.

Dusty,

Again, how are you getting this?


CFM = (fpm * area)

You measured 2990 fpm.

Assuming a 2-1/2" port (2-1/4" ID),

area=π * r² and d = 2 * r (for a circle)

Therefore area (circle) = π * (d/2)²

So, area = 3.14159 * (2.25/2/12)² = 0.027611631 = 0.028 ft² (rounding up)
(NOTE: I divided by 12 to put 2.25" into units of feet.)

Therefore,

CFM = 2990 fpm * 0.028 ft² = 83.72 CFM for 2-1/2" hose aperture.

Please go line by line through my example calculation above and tell me how you calculate 173.17 cfm from 2990 fpm for a 2-1/2" port???

[RFGuy]

Perhaps I misunderstood the suggestion, but sealing the flow path between the anemometer and the suction inlet could erroneously change the measured velocity if the anemometer diameter is different than the suction inlet.

Matanuska,

Dusty has measured 2990 fpm airspeed using an off the shelf anemometer and this reading was taken from a 2-1/4" round dust port from the DC-3300. So, I believe any airflow (CFM) calculation MUST be performed using the area of a circle of diameter 2.25", for this particular case with the DC-3300. Do you agree with this? Based on my calculations, he has an airflow of only 83.72 CFM through that 2-1/4" inlet which is quite low IMHO. A minimum airspeed of 4000 fpm (vertical) or 3000 fpm (horizontal) is required to not have dust settle out in dust collector ductwork or hoses. Dusty isn't even getting enough airspeed to prevent dust from settling in horizontal runs.

[dusty]

Good catch. It looks like I used diameters in my calculations where there should have been the radii of 2 1/2" couplings and hoses. All of my DC parts and accessories are standard Shopsmith items.

The 2990 ft/min now calculates to 173.17 cfm while 2687 ft/min calculates to 155.62 cfm. Not what I wanted to see but these are certainly more real world numbers for this dust collector, hoses and couplings.

Dusty,

Again, how are you getting this?


CFM = (fpm * area)

You measured 2990 fpm.

Assuming a 2-1/2" port (2-1/4" ID),

area=π * r² and d = 2 * r (for a circle)

Therefore area (circle) = π * (d/2)²

So, area = 3.14159 * (2.25/2/12)² = 0.027611631 = 0.028 ft² (rounding up)
(NOTE: I divided by 12 to put 2.25" into units of feet.)

Therefore,

CFM = 2990 fpm * 0.028 ft² = 83.72 CFM for 2-1/2" hose aperture.

Please go line by line through my example calculation above and tell me how you calculate 173.17 cfm from 2990 fpm for a 2-1/2" port???

Thanks for your inputs and your patience. Hang with me. I'll figure out what I am doing (with help of course). Obviously I am calculating cross section area differently (wrongly).

[RFGuy]

Thanks for your inputs and your patience. Hang with me. I'll figure out what I am doing (with help of course). Obviously I am calculating cross section area differently (wrongly).

Dusty,

No problem. I just want to make sure we are both on the same page. By the way, I am fairly certain I am calculating all of this correctly and I have validated my measurements/calculations with my other woodworking equipment. For example, my Festool dust extractor has spec'ed airflow numbers for it and I have measured airspeed and calculated airflow (27mm and 36mm hoses). I get very close to what is spec'ed for their dust extractor which gives me good confidence. Similarly for my ShopVac™ vacuum I match what is spec'ed from the manufacturer, so I am fairly sure my calculation is correct as a result.

[JPG]

An assumption.)??)

Does not the anemometer respond to the air velocity passing through it's chamber/bore/whatever? If so I would think the area of the fan bore is the relevant area to be used to convert to cfm.

[dusty]

An assumption.)??)

Does not the anemometer respond to the air velocity passing through it's chamber/bore/whatever? If so I would think the area of the fan bore is the relevant area to be used to convert to cfm.

I would agree. I would definitely agree if we were working in an industrial or military environment but here ion my shop - not so critical to be spot on.

However, that said the chamber that surrounds the anemometer fan has an opening that is very, very close to that of a Shopsmith hose coupling. I am seriously considering a physical modification to the anemometer that would permanently mate it to a hose coupling.

Hand held is not working - absolutely no repeatability.

[RFGuy]

An assumption.)??)

Does not the anemometer respond to the air velocity passing through it's chamber/bore/whatever? If so I would think the area of the fan bore is the relevant area to be used to convert to cfm.

JPG,

Once a restriction or limiting orifice is set, like the DC-3300 2-1/4" inlet, the airspeed should be constant, or at least that is my assumption. The airflow will vary throughout the system, e.g. if there is a change in duct/hose size up/down the airflow will vary up/down. For example, let's say someone has an expensive dust collector that is capable of 1000 CFM. Connect a 4" hose to it and you likely won't see that full 1000 CFM, but something much less than this by the ratio of the 4" hose size to the dust collector duct size. Choke it further down to a 2-1/2" hose and good luck getting much of that 1000 CFM to go down it. I am just pointing out that since Shopsmith's equipment has a 2-1/4" inlet (2-1/2" hose) for dust collection and since many of us use 2-1/2" hoses with our Shopsmith equipment that the airflow will be limited by this and this is the proper way to calculate airflow for comparison purposes IMHO. Again, I have actual measured data using my anemometer that matches very closely with published specs from Festool for airflow on my CT26 and also for my Shop Vac™ vacuum. So, I have repeatability with my measurements and have validated them to other manufacturer's reported specs.

To answer your question, sure we can calculate airflow assuming the chamber/bore/whatever of the anemometer but this is kinda pointless unless we are going to create a hose/ductwork for our dust collector to match it to the size of the anemometer. The true measurement is airspeed (fpm) and airflow (CFM) is a calculated construct. The fact that Dusty isn't even measuring 3,000 fpm is a concern because dust particles will start to settle out and clog hoses at such a low airspeed. 4,000 fpm is the bare minimum for vertical ducting and 3,000 fpm for horizontal ducting.