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

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.

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

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

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

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

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

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"

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

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

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

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

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

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

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.

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.

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

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

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

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

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