Figuring CFM

[Larry Blighton]

Ok so I was able to get ahold of an anemometer over the weekend and did some readings.  However the problem I have now is when I select the CFM mode on the anemometer it gave me the readings in thousands not the hundreds.  

 

So what do I need to do with this number to get my true CFM reading?

[PeteM]

Let me preface:  when you're done with all this, the figure for CFM you come up with will be somewhere between 50% and 200% of reality.  IOW, field testing without a lot of instruments is pretty inaccurate, and I wouldn't use an anemometer anyway.

 

     Anemometers generally measure VELOCITY not volume, if what you have is actually just an anemometer.  Some meters are programmable to calc/display CFM (cubic feet per minute).  DC systems typically operate in the 2000--5000 FPM (feet per minute) range, so if it's reading in "thousands", I would think you're seeing velocity (heh, read the manual to make sure).  Anemometers are deceptive instruments because you really have to understand the dynamics of air flow to interpret the results.  I assume you're measuring across an opening of some sort?  Fan inlet, duct inlet? Outlet?  If you could see the air, you would find very irregular patterns of flow, including pulsing (air is compressible, so it does).  Along the edges of an opening, the air velocity is much lower than in the center.  If you are near a bend in the duct, the highest velocity isn't necessarily in the center.  The smaller the opening, the less accurate your measurement because the anemometer tends to read the high end of a mixed flow pattern.  Are you trying to read a 4" or 6" duct opening?  Typically, anemometers work better on large openings such as a 24x24 air grille.  You also have to apply a "k-factor" for the type of opening:  the factor varies with the geometry of the opening, and the type of probe* used on the anemometer.  

     The test method for rating a fan sets up a straight duct, smooth sides, no bends, duct 10D (10 times diameter), smooth, (long) transition to a large rectangular opening, where the anemometer is carefully moved across the opening to get a fully representative average velocity.  The reading usually takes a minute or more of back and forth movement to get consistent measure.  There is a post you screw into the anemometer to hold it, because if you hold it in your hand, your hand/arm interfere with the reading.  Of course, when you're done you realize the air flow you calculate is only good for that configuration of duct, not the actual duct you have (irregular surfaces, bends, holes).  

     But most mfrs don't use anemometers for such measurements.  They use a calibrated orifice in the duct and take static pressures (carefully) on either side of the orifice to determine the pressure loss of the orifice, then use that to calculate air flow/CFM.  In field air flow measurement of exhaust systems, measuring the outlet of the fan is usually more reliable than reading the inlet duct.  Often, measuring pressures and power draw at the fan, and using the mfr fan curve/table, is actually more reliable than direct air measurement.  [And I have tried to get fan curves from DC mfrs, but they don't even know what that is.]

 

I went to work in HVAC (mfrs sales engineer) in 1972, still dabble in forensics.  What do I do?  Straight smooth duct, long smooth elbows, flex duct limited to 5'.  If I'm not happy with the performance, I buy a bigger DC.  I accept "close enough good enough" as a mode.  Fluid measurement in the real world isn't science or art.  It's darts.

 

*Some anemometers are just impellers that you stick in the air stream.  Others use remote tubing and probes.  Many anemometers are simply propellers that read rotor flux; some models use "hot wire" tech that measures the cooling effect on a heated probe.  Quality and reliability, uh, vary.

[Larry Blighton]

20 minutes ago, PeteM said:

Let me preface:  when you're done with all this, the figure for CFM you come up with will be somewhere between 50% and 200% of reality.  IOW, field testing without a lot of instruments is pretty inaccurate, and I wouldn't use an anemometer anyway.

 

     Anemometers generally measure VELOCITY not volume, if what you have is actually just an anemometer.  Some meters are programmable to calc/display CFM (cubic feet per minute).  DC systems typically operate in the 2000--5000 FPM (feet per minute) range, so if it's reading in "thousands", I would think you're seeing velocity (heh, read the manual to make sure).  Anemometers are deceptive instruments because you really have to understand the dynamics of air flow to interpret the results.  I assume you're measuring across an opening of some sort?  Fan inlet, duct inlet? Outlet?  If you could see the air, you would find very irregular patterns of flow, including pulsing (air is compressible, so it does).  Along the edges of an opening, the air velocity is much lower than in the center.  If you are near a bend in the duct, the highest velocity isn't necessarily in the center.  The smaller the opening, the less accurate your measurement because the anemometer tends to read the high end of a mixed flow pattern.  Are you trying to read a 4" or 6" duct opening?  Typically, anemometers work better on large openings such as a 24x24 air grille.  You also have to apply a "k-factor" for the type of opening:  the factor varies with the geometry of the opening, and the type of probe* used on the anemometer.  

     The test method for rating a fan sets up a straight duct, smooth sides, no bends, duct 10D (10 times diameter), smooth, (long) transition to a large rectangular opening, where the anemometer is carefully moved across the opening to get a fully representative average velocity.  The reading usually takes a minute or more of back and forth movement to get consistent measure.  There is a post you screw into the anemometer to hold it, because if you hold it in your hand, your hand/arm interfere with the reading.  Of course, when you're done you realize the air flow you calculate is only good for that configuration of duct, not the actual duct you have (irregular surfaces, bends, holes).  

     But most mfrs don't use anemometers for such measurements.  They use a calibrated orifice in the duct and take static pressures (carefully) on either side of the orifice to determine the pressure loss of the orifice, then use that to calculate air flow/CFM.  In field air flow measurement of exhaust systems, measuring the outlet of the fan is usually more reliable than reading the inlet duct.  Often, measuring pressures and power draw at the fan, and using the mfr fan curve/table, is actually more reliable than direct air measurement.  [And I have tried to get fan curves from DC mfrs, but they don't even know what that is.]

 

I went to work in HVAC (mfrs sales engineer) in 1972, still dabble in forensics.  What do I do?  Straight smooth duct, long smooth elbows, flex duct limited to 5'.  If I'm not happy with the performance, I buy a bigger DC.  I accept "close enough good enough" as a mode.  Fluid measurement in the real world isn't science or art.  It's darts.

 

*Some anemometers are just impellers that you stick in the air stream.  Others use remote tubing and probes.  Many anemometers are simply propellers that read rotor flux; some models use "hot wire" tech that measures the cooling effect on a heated probe.  Quality and reliability, uh, vary.

Pete, the Anemometer that I used was a Proster MS6252A.  I dont know how good this one is as I found the same one on the internet for $35 so I am assuming that is where my buddy got it from.  The instruction manual says to measure CFM to select CFM mode However when I select CFM mode it is giving me readings in the thousands.  As far as to what I measuring, yes I am at the 4" blast gate right before each tool.  My system is made up completely of 4" PVC. 

 

I understand that I do not know how accurate this meter is but I am trying to at least get some kind of ballpark numbers.   

[Fred W. Hargis Jr]

Use this internet form, switch the meter to velocity (LFM) and plug the numbers into the worksheet. Should give you the answer. I looked that meter up and I think it reads everything but feet per second...so use one of the measurements that the worksheet accepts.

[PeteM]

A typical ballpark measures about 300 feet on a side, right?  Something like that.  So, yeah, you're in the ballpark.  But to continue the analogy, that device placed over a duct opening might indicate whether you're in the infield or outfield.  I guess it would reliably indicate whether the gate is open.