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MAPower (Mechanical) (OP)
9 Jan 09 13:20
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?
 
mizzoueng (Mechanical)
22 Jan 09 16:38
I think motor eff is what you can use.

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.
rmw (Mechanical)
22 Jan 09 19:56
The efficiency in the above formula is fan efficiency.  The equation doesn't know if you are driving the fan with motors, turbines or horses.

rmw
MAPower (Mechanical) (OP)
23 Jan 09 7:57
I do not have have the design calc for the fan. The fan is performing as it should as it relates to pressure rise. What I am trying to do is plot the operating point on the fan curve live with the instrumentation that I have.

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.
rmw (Mechanical)
23 Jan 09 8:27
How is this fan's flow controlled and what is the control device doing all this time?

rmw
MAPower (Mechanical) (OP)
23 Jan 09 9:19
The primary control is for furnace pressure on a boiler. We are controlling  fan speed with a variable frequency drive. Fans are in parallel. The drive is doing what the control system is demanding, which is trying to control the furnace pressure (-0.5"). Furnace pressure oscillates +- 0.5" and that equates to a speed swing of about +- 10-15 RPMs on a 900 RPM motor.

The severe structural vibration occurs predominately at intermediate loads.

Initially we thought it was flow induced vibration, but then someone suggested the rotating stall.  
ScottyUK (Electrical)
23 Jan 09 11:23
Have you ruled out plain old fashioned mechanical resonance? The fan will have a critical speed which, if it is a big fan, probably lies below the normal operating speed. The steel itself has a resonant frequency which may be excited by any slight imbalance in the rotating fan.
 
  

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

MAPower (Mechanical) (OP)
23 Jan 09 12:52
The fan has no imbalance, it run less that 0.1 in/sec. The critical speed of this fan system is 460 RPM and we normally do not run at that speed. We are currently running above 500 RPM.

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.  

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