AHU Fan motor power reduction due to air capacity reduction

[slynk]

Hi everyone,

Wonder if you guys can provide a good method of calculating the estimated fan motor power after I reduce the rpm of the fan? The info available are the current fan power, current flow capacity and new flow capacity.

I know there are the fan laws where Q1/Q2 = rpm1/rpm2 and power1/power2 = (rpm1/rpm2)^3. So I took power1/power2 = (Q1/Q2)^3. Is this correct?

Thanks in advance!

 

[quark]

Correct.

 

[slynk]

Okay...

So, if I have an original Q of 76000CMH running at 70kW, and an estimated reduction in flow to 56000CMH, the new power of the fan would be 28kW?

Is that really possible? What is a good safety factor I can use for this? I need it for cost savings calculation. Thanks!

 

[quark]

Good question. This is not really possible and I don't have a universal safety factor for this.

I do the following exercise.

First check what is the actual power drawn by the motor. If you are not operating the fan at BEP and have damper control then the savings will be higher than those you get by the formula.

Check at what efficiency your fan will be running in the new condition. You can use extrapolation if you have isoefficiency curves of the fan.

Check what is the efficiency of the motor at lower load conditions.

Check what is the efficiency of VFD at new condition (many threads in electrical fora discussed this topic)

Most importantly check whether the static pressure corresponding to 56000 cmh will suit the system.

 

[ChasBean1]

M'man Quark gives the 4.0 answer as always. But for esimation purposes you're right... the 70 KW becomes 28 KW.

 

[Zesti]

Here is one I find very handy:

flow x pressurediffernce = power

!!! when used in metric !!!

[m3/s] x [Pa] = [W]

This works for fans pushing air as well as pumps for water etc.

Put in your fan's efficiency and you will have required engine power.

 

[Zesti]

Air resistances in your system might not all be "quadratic" with flow.

For instance, HEPA-filters have a relation between flow and pressure that is nearly linear.

 

[mauricestoker]

You may want to look at the power factor at lower rating as well.

ASHRAE has a standard method for calculating energy difference between VFD and inlet guide vane. I've used it before (it includes changes in efficiency) and can dig it up.