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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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Measuring the CFM of a fan
Maybe OT, but not as much as many.
I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of the air is what I need. I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great G. Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula? Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste. Advice anyone? Thanks, Bob |
#2
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Measuring the CFM of a fan
"Bob Engelhardt" wrote in message ... Maybe OT, but not as much as many. I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of the air is what I need. I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great G. Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula? Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste. Advice anyone? Thanks, Bob Talk to a local sailor. Borrow his wind guage. They fairly cheap for the plastic ball ones. |
#3
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
"Bob Engelhardt" wrote in message ... Maybe OT, but not as much as many. I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of the air is what I need. I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great G. Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula? Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste. Advice anyone? Thanks, Bob Check out this guy's air velocity measurements here http://billpentz.com/woodworking/cyc...easurement.cfm Art |
#4
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
"Bob Engelhardt" wrote in message
... Maybe OT, but not as much as many. I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of the air is what I need. I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great G. Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula? Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste. Advice anyone? I think your idea is great, altho the anemometer is good too. The formula is just like the one that uses CdA for cars -- no biggie. The problem is what assumption for uniformity of air flow do you use? Suppose the blower opening is 1 sq ft. The anemometer should "sample" flows at mebbe 6 or so points. Your spring calculation could either 1. measure the whole square foot with a sq. ft paddle on a spring, or 2. test 6 or so points with a smaller sprung square (or circle), say 2" sq. If doing 1, I would fashion an aluminum sq bigger than the opening, attached to a tube, which would ride in a second tube with a long soft spring it (Century Spring, Lee Spring). Linear bearing comes to mind, as well, but proly $$. There will be some static friction, so estimate that with the spring as well, and just add it in. The formula is simple enough, solve for v. Just make sure to use the drag equation (v^2) and not the energy equation (v^3). Altho once you have v, the energy equation could be used as a sort of check, if you amprobed the motor... Ackshooly a perty neat speriment!!! Your "wind energy" should certainly be less than the motor energy, giving you a kind of net efficiency. You'll know if you screwed up if the wind energy calc's out to be more.... I have this neat round food weigher, cheap, that seems pretty accurate. Around 6" in diameter. I wonder if THAT would do?? -- EA Thanks, Bob |
#5
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Measuring the CFM of a fan
I've done lots of air flow tests. A pitot tube hooked to a simple water
manometer is easy to do. One thing you need to know is that velocity drops to near zero at the walls. The velocity profile across a duct is a bell curve. The url that Artemus posted has an equation that seems to take the velocity profile into account. -- DT |
#6
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Measuring the CFM of a fan
JR North wrote:
Don't knock youself out. Compare it's size and motor specs to any number of fans at Grainger, MSC, et all. Ah ... good idea. I don't have to be that close - 25% is probably good enough (it's to calculate air change rate & if I get 4 when I calculated 5 (25% low), no big deal). Thanks, Bob |
#7
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Measuring the CFM of a fan
Bill McKee wrote:
Talk to a local sailor. Borrow his wind guage. ... Yes - and your reply prompts my memory of a neighbor's weather station, with an _anemometer_! I like a challenge, but not when a solution is right at hand. Thanks, Bob |
#8
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Measuring the CFM of a fan
On 04/11/2010 03:03 PM, Bob Engelhardt wrote:
Maybe OT, but not as much as many. I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of the air is what I need. I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great G. Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula? Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste. Another approach is to blow air through a room that contains a suitably large heater with known output. Calculate CFM from the temperature rise and the thermal mass of a cubic foot of air. Effects of the energy released by the blower itself and thermal leakage through the walls of the room should hopefully be minimal. -- Bob Nichols AT comcast.net I am "RNichols42" |
#9
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
Artemus wrote:
Check out this guy's air velocity measurements here http://billpentz.com/woodworking/cyc...easurement.cfm Great link - thanks. Complicated enough to make me glad that I remembered my neighbor's anemometer. Bob |
#10
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
"Bob Engelhardt" wrote in message
... JR North wrote: Don't knock youself out. Compare it's size and motor specs to any number of fans at Grainger, MSC, et all. Ah ... good idea. I don't have to be that close - 25% is probably good enough (it's to calculate air change rate & if I get 4 when I calculated 5 (25% low), no big deal). No..... NOO!!! Measure the force, Measure the force!!!!! Also, comparing specs may be easier said than done, as dimensions of the squirrel cage itself matter, including the pitch of the fins. That, and say 1/2" off nominal diameters could be a biggie. Don't know how standard all this is. -- EA Thanks, Bob |
#11
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Measuring the CFM of a fan
Existential Angst wrote:
The problem is what assumption for uniformity of air flow do you use? .... Well, my implicit, naive, assumption was total uniformity, but that can't be right. The link Artemus provided used 90% of theoretical - good enough for me. Bob |
#12
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
DT wrote:
... One thing you need to know is that velocity drops to near zero at the walls. The velocity profile across a duct is a bell curve. The url that Artemus posted has an equation that seems to take the velocity profile into account. A bell curve should mean that the average is calculate-able. Artemus' link uses 90% of peak (center). Bob |
#13
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
Robert Nichols wrote:
Another approach is to blow air through a room that contains a suitably large heater with known output. Calculate CFM from the temperature rise and the thermal mass of a cubic foot of air. Effects of the energy released by the blower itself and thermal leakage through the walls of the room should hopefully be minimal. Clever - I like it. Bob |
#14
Posted to rec.crafts.metalworking
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Measuring the CFM of a fan
"Bob Engelhardt" wrote in message
... Robert Nichols wrote: Another approach is to blow air through a room that contains a suitably large heater with known output. Calculate CFM from the temperature rise and the thermal mass of a cubic foot of air. Effects of the energy released by the blower itself and thermal leakage through the walls of the room should hopefully be minimal. Clever - I like it. Bob I dunno... calorimetry under the BEST of (adiabatic) circumstances is dicey. This is like freestyle mountian climbing.... vs. an elevator. -- EA |
#15
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Measuring the CFM of a fan
On Sun, 11 Apr 2010 16:03:12 -0400, Bob Engelhardt
wrote: Maybe OT, but not as much as many. I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of the air is what I need. I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great G. Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula? Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste. Advice anyone? Thanks, Bob Some benchrest shooters will have instruments like this: http://www.midwayusa.com/Search/#win..._1-2-4_8-16-32 For higher velocities than these meters can handle, consider an easily-made pitot tube, an easily-made water manometer, and pitot tube pressure vs velocity tables found online somewhere. |
#16
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Measuring the CFM of a fan
On Sun, 11 Apr 2010 19:52:15 -0400, Bob Engelhardt
wrote: DT wrote: ... One thing you need to know is that velocity drops to near zero at the walls. The velocity profile across a duct is a bell curve. The url that Artemus posted has an equation that seems to take the velocity profile into account. A bell curve should mean that the average is calculate-able. Artemus' link uses 90% of peak (center). Bob That velocity distribution depends a lot on degree of turbulence, somewhat predictable by Reynold's number and Prandtl number of flow in the region of interest. Laminar flow is simpler but only happens at low velocities in ducts. It's hard to argue with the fundamental truth of the large baggie, or of the heat-transfer experiment in a space of interest. |
#17
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Measuring the CFM of a fan - Followup
Bob Engelhardt wrote:
Bill McKee wrote: Talk to a local sailor. Borrow his wind guage. ... Yes - and your reply prompts my memory of a neighbor's weather station, with an _anemometer_! ... I borrowed the anemometer & measured the blower's air speed - 64 mph (!) at the hot spot. Very uneven distribution, though. It would have been better to attach a piece of duct, to even out the flow, but I have a problem. I ran the blower for a 5 minutes, more or less, & afterward I noticed the smell of heat. Yes, the motor was getting hot. Damn. I checked the speed - it was only 630 rpm, and the rating is 1075. The cap was neither shorted nor open, but I tried another & still slow. I'm assuming that I'm SOL - that the winding is shorted - but what do I know. Am I right, or is there something else that could be wrong? And fixable? Thanks, Bob |
#18
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Measuring the CFM of a fan - Followup
On Wed, 14 Apr 2010 17:01:44 -0400, Bob Engelhardt
wrote: I ran the blower for a 5 minutes, more or less, & afterward I noticed the smell of heat. Yes, the motor was getting hot. Damn. I checked the speed - it was only 630 rpm, and the rating is 1075. The cap was neither shorted nor open, but I tried another & still slow. I'm assuming that I'm SOL - that the winding is shorted - but what do I know. Am I right, or is there something else that could be wrong? And fixable? Throttle the input or output and try again. Many blowers use maximum power when moving maximum air volume. Block the intake of a shop vac and note that the motor speed increases. -- Ned Simmons |
#19
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Measuring the CFM of a fan - Followup
"Bob Engelhardt" wrote in message ... Bob Engelhardt wrote: Bill McKee wrote: Talk to a local sailor. Borrow his wind guage. ... Yes - and your reply prompts my memory of a neighbor's weather station, with an _anemometer_! ... I borrowed the anemometer & measured the blower's air speed - 64 mph (!) at the hot spot. Very uneven distribution, though. It would have been better to attach a piece of duct, to even out the flow, but I have a problem. I ran the blower for a 5 minutes, more or less, & afterward I noticed the smell of heat. Yes, the motor was getting hot. Damn. I checked the speed - it was only 630 rpm, and the rating is 1075. The cap was neither shorted nor open, but I tried another & still slow. I'm assuming that I'm SOL - that the winding is shorted - but what do I know. Am I right, or is there something else that could be wrong? And fixable? Thanks, Bob You're overloading the motor. Go down to section 3 (about 1/2 way down the page) and read the Amp Meter Tests overview. http://billpentz.com/woodworking/cyc...easurement.cfm You'll need to add some ducting and/or baffles to get your blower back in it's design range. Art |
#20
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Measuring the CFM of a fan - Followup
In article ,
Bob Engelhardt wrote: I'm assuming that I'm SOL - that the winding is shorted - but what do I know. Am I right, or is there something else that could be wrong? And fixable? As others have already said, you are overloading it by running it wode open and you need some restriction. This is apparently a common way for homeowners to kill furnace fans, by "opening up" the ductwork so it can "do less work" - actually making it do more work, overloading it, and burning it out. If you have an ammeter, you can see the load go down as you restrict the airflow. -- Cats, coffee, chocolate...vices to live by |
#21
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Measuring the CFM of a fan - Followup
Ned Simmons wrote:
Throttle the input or output and try again. Many blowers use maximum power when moving maximum air volume. ... Artemus wrote: You're overloading the motor. Go down to section 3 (about 1/2 way down the page) and read the Amp Meter Tests overview. http://billpentz.com/woodworking/cyc...easurement.cfm You'll need to add some ducting and/or baffles to get your blower back in it's design range. You're right - this is a great NG, ain't it? I did the ammeter tests discussed in the ref'ed URL. The motor didn't have a FLA on the nameplate, but it is a 1/2 HP motor, so I'd expect FLA about 5 A (?). Measured, the "no load" (wide open duct) current was 13A(!), closed off (minimum) was 3+, and 60% blocked it was about 5A. The max air speed was about the same wide open & 60% blocked (60 mph). But the CFM would be 60% less. The air speed varied so much across the opening that I'm going to de-rate the max speed by 1/2 to get the average. That gives me 660 CFM. But that is so low compared to similar Grainger blowers that I'm going to experiment some more. Stay tuned, Bob |
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