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Old March 22nd, 2004, 07:20 AM   #1
Andrew Evenson
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Power Monitoring

Hey guys,

Got another hopefully quick question. I posted a question earlier about measuring the opaquness of water. It had to do with a fume scrubber. This is related to the fume scrubber, not the opaquness measurment. The project manager/engineer want to measure the power consumed by the blower fan. She wants to correlate it to blower's HP vs. CFM curve to determine the amount of air flow going out the stack. I suggested a power monitor, but another person person said to just bring in one phase of the motor's current, and multiply it by a value of 120 to determine the power. He said the voltage should be pretty constant. I disagree. You have to take the pwoer factor into consideration dont you?? Thats what a power monitor will do?? Correct?? Thats at least what I remember from my power systems classes. If you dont take the power factor into consideration, you will have an error in the readings, because you arent measruing "useful" power, you would be measuring reactive power right?? I would love to hear your opinions and comments, am I out to lunch?? How would you guys do this??

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Andrew Evenson
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Old March 22nd, 2004, 07:43 AM   #2
jamesau
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See http://www.loadcontrols.com/

I used them some 15+ years ago.
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Old March 22nd, 2004, 07:47 AM   #3
Tom Jenkins
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You can go with a power monitor like Load Controls Inc or Rochester Instruments or such, and you will get an accurate power measurement. This can correlate to air flow based on the fan curve, but the accuracy of the curves is usually 4% for metal fans. If you are using a fabricated fiberglass fan the accuracy of the curve may be less than 6%.

You can get sufficient accuracy for most purposes by measuring current on one leg. The motor speed doesn't change much, and torque is a function of current for a given motor, and horsepower is a function of torque and speed, and horsepower is a function of the fan characteristics and air flow. Using motor amps to indicate air flow as SCFM is a standard of the industry, and I've used it succesfully on blowers up to 700 hp and at discharge pressures up to 10 psig. The correlation is more or less linear for most fans and blowers.

You are correct that there are a lot of inaccuracies. Certainly voltage varies, but not widely from moment to moment so a one time correction to your calculation is sufficient. Motor data sheet power factors and efficiencies are not precise, and as I said above even the fan curves are subject to error. However, for most purposes monitoring current provides sufficient accuracy. Correllataing actual power to the fan curve and using it will be more accurate, but for most purposes the incremental accuracy doesn't justify the incremental cost. If you use the motor data sheet for power factor and efficiency, use the nominal motor voltage, and the fan curve for SCFM vs. power, and do the calculations correctly you will be pretty close. For three phase: I(Amps) = (hp x 746) / ( V x 1.73 x eff. x PF) and conversely hp = (I x V x 1.73 x eff. x PF) / 746

The above only applies to constant speed centrifugal fans and blowers. If you are using a VFD on the fan then neither current or motor power are going to give you accurate flow readings.

If for some reason you really need to know air flow precisely then you should measure air flow directly, with a thermal mass flow meter or a pitot tube or such.

Last edited by Tom Jenkins; March 22nd, 2004 at 08:12 AM.
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Old March 22nd, 2004, 09:01 AM   #4
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Yes, Andrew, I bet that a cheap flowmeter will do the job and be closer than the motor curent-air flow correlation. Alway take a parameter measurement as close to the root as you can get. Why add extra complications? If you do the motor current, and it appears to be lower (or higher) than expected, then you will have to spend some more time explaining why it is thus-and-so. For $300 you can install a flowmeter and a digital readout. If it is not worth $300, then she probably doesn't need the air flow measurement anyway.
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Old March 22nd, 2004, 09:23 AM   #5
Andrew Evenson
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Thanks!!!

Gentlemen,

Thanks for your feedback and comments. It was greatly appriciated and I learnt alot. Load Controls Incorporated has some nice stuff. I will have to investigate them further.

Tom:

I agree with your formula, and it will work. You are assuming though that the blower runs at pretty much full load, where the PF is closest to 0.9. Am I correct?? If not, thats where the power monitor comes into play to corect for the P.F??

Again, thanks for all your help and comments. I always learn alot whrn I ask questions here.

Andrew Evenson
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Old March 22nd, 2004, 09:46 AM   #6
Tom Jenkins
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Quote:
Originally posted by Andrew Evenson
Tom:

I agree with your formula, and it will work. You are assuming though that the blower runs at pretty much full load, where the PF is closest to 0.9. Am I correct?? If not, thats where the power monitor comes into play to corect for the P.F??
Actually, you don't have to assume that the motor is near full load. Motor data sheets usually have motor power factor and efficiency at full, 3/4, 1/2, 1/4 and no load. Just read the nominal motor power from the curve and plug in the actual power factor at the actual load from the data sheet. You can linearly interpolate between values if you need to.

The difference between the two techniques (actual power or amps) is that amps uses a pre-determined power factor and voltage and so on at several points for the correlation, and actual power measures these parameters. As I said, though, there is extra cost associated with doing this, and you are spending money to get more accurate data based on a curve that may be off 5% in the first place. What's the point?

Using amps is accurate enough for data logging or if you want to compare performance for various time periods, or for controlling chemical feed pumps porportional to flow, and for most purposes. As I and Lancie said, if you have a need to know the air flow with precision then you should actually measure the air flow. If an approximate air flow is OK, then measure amps and correlate - that is how it is usually done. My main point is that using power instead of amps is more accurate, but it is still subject to a lot of errors and generally the additional cost to use measured power instead of measured amps isn't justified.
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Old March 24th, 2004, 03:59 AM   #7
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i would use the amp meter already installed and take some measurements of flow going out the stack. put the readings on your ampmeter and you will have the flow, the rest is all constant.

or indeed as suggested a flowmeter inside the stack itself.
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Old March 24th, 2004, 04:53 AM   #8
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I have tried to experiment with flow measurement from a large blower. It was an annubar type of sensor.
It didnt work well. The problem was there wasnt any good place to put the sensor. It requires a long straight section before and after the sensor. The problem increases with the diameter of the tubing.
We ended up with measuring the motorcurrent and implementing a calibration curve in the PLC. The customer accepted this solution.
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