## Calculated Volume Flow Through ID Fan?

## Calculated Volume Flow Through ID Fan?

(OP)

Hi,

I'm trying to get some help doing a rough estimate calculation of the volumetric flow through and induced draft fan on a balanced draft unit without doing a performance test. Here is the data that I have available to me in our historian:

- fan inlet & outlet pressures

- fan inlet temperature

- fan speed

- fan motor (VFD driven) operating amps

- fan motor (VFD driven) operating kW

I have used a motor sizing equation to give me a rough estimate, but am not sure if this formula can be used to calculate the CFM?

Horsepower = {CFM*Pressure(lbs/sq ft)}/{33,000*efficiency}

I have a few questions regarding this method:

- is this a valid equation to use for a rough estimate?

- is the efficiency the efficiency of the motor or the fan?

- solving for CFM, will it give me ACFM or (S)CFM?

- I am using pressure rise across the fan for the pressure

Any help would be great.

Is there a different equation I could use to determine a rough estimate of the vol. flow?

I'm trying to get some help doing a rough estimate calculation of the volumetric flow through and induced draft fan on a balanced draft unit without doing a performance test. Here is the data that I have available to me in our historian:

- fan inlet & outlet pressures

- fan inlet temperature

- fan speed

- fan motor (VFD driven) operating amps

- fan motor (VFD driven) operating kW

I have used a motor sizing equation to give me a rough estimate, but am not sure if this formula can be used to calculate the CFM?

Horsepower = {CFM*Pressure(lbs/sq ft)}/{33,000*efficiency}

I have a few questions regarding this method:

- is this a valid equation to use for a rough estimate?

- is the efficiency the efficiency of the motor or the fan?

- solving for CFM, will it give me ACFM or (S)CFM?

- I am using pressure rise across the fan for the pressure

Any help would be great.

Is there a different equation I could use to determine a rough estimate of the vol. flow?

## RE: Calculated Volume Flow Through ID Fan?

Do you have the design calc for the fan? That should give you a good estimate for what the fan is rated for and what you should be seeing.

If you are suspecting that the fan is not doing what it is supposed to, do a traverse across the outlet of the fan and see what is being pushed.

As for ACFM or SCFM, I can't remember right now. If I can find a copy of MERM I will find out.

## RE: Calculated Volume Flow Through ID Fan?

rmw

## RE: Calculated Volume Flow Through ID Fan?

The true problem is we suspect the fan is surging or in a rotating stall at certain operating points because the ductwork and structural steel is vibrating severely. If I can plot the operating point live, I can determine how much it is surging.

## RE: Calculated Volume Flow Through ID Fan?

rmw

## RE: Calculated Volume Flow Through ID Fan?

The severe structural vibration occurs predominately at intermediate loads.

Initially we thought it was flow induced vibration, but then someone suggested the rotating stall.

## RE: Calculated Volume Flow Through ID Fan?

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If we learn from our mistakes I'm getting a great education!

## RE: Calculated Volume Flow Through ID Fan?

We have considered the mechanical resonance in the ductwork and steel and have gone far enough as to find the natural frequencies of some of the more severe vibrating members. This yielded results frequencies that were not at the operating speeds. Originally we suspected a flow induced vibration that was at the resonant frequency of one of the internal turning vanes. No luck their.

When the vibration is occurring the frequency of the members is about 2/3 running speed.