Fan amp draw vs CFM

[mingmongmang]

Is the amp draw of a given fan consistently proportional to the CFM being moved?

If I map amp draw vs CFM through a range of back pressures in the lab, can I then use that data to determine CFM in real-world applications by measuring amp draw? (using same fan of course)

Is this a commonly used technique and if so how reliable/accurate is it?
Thanks.

 

[MintJulep]

Mostly yes.

In actuality the power will be proportional to the mass flow.

If the air density in the lab is the same as in the field then you'll be good. If not it's a relatively simple correction factor to calculate.

 

[zekeman]

I think you're mistaken.

From Fan Laws

Power proportional to CFM^3

 

[MintJulep]

Right.

 

[TheTick]

Air balancers (the guys who adjust HVAC systems) use the amp-CFM linear relationship all the time when adjusting flow from an air handler. Very reliable.

 

[zekeman]

"If I map amp draw vs CFM through a range of back pressures in the lab, can I then use that data to determine CFM in real-world applications by measuring amp draw? (using same fan of course)"

I think it would be close

Given that fan power is proportional to CFM*static pressure and power is also proportional to current draw within a power factor, then you can write

Voltage *current*power factor* efficiency =K*CFM*static pressure.
So, if the efficiency and power factor remain constant

I=K1*CFM*Static pressure

 

[electricpete]

I doubt there is an easy answer to cover all situations but you have responses from more knowledgeable posters than me to work with.

Some finer points that might cause some problems:
1 - axial flow fans act nothing like radial flow fans. They draw lower BHP at higher flow. Hard to believe anyone can match that by assuming a simple linear relationship, unless you are allowing an offset and negative slope: y = mx+b where m is negative.
2 - As motor power gets lower the constant power factor efficiency assumption becomes a big error.

Also I have a question. Where does "fan power is proportional to CFM*static pressure" come from? Seems like dp is missing (fluid power would be proportional to product of volumetric flow rate times DP times density or pressure). Also seems like efficiency is missing. Maybe these two effects cancel out in general and this is some empirical approximation? If so under what conditions is it approximately true?

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(2B)+(2B)' ?