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Basic Fan Curve Question - TWO CFM's, same RPM ?

Basic Fan Curve Question - TWO CFM's, same RPM ?

Basic Fan Curve Question - TWO CFM's, same RPM ?

I have blower curves and programs from two manufacturers who say that it is acceptable to go to "the left of the peak" of a blower curve.  They claim that the performance is stable, within the limitations they set out. (These are for Forward Curved, Double Inlet, Centrifugal Fans. )

Unfortunately, I don't understand how this can be true:
For the same static pressure, the same rpm, you can have TWO different horsepower and delivery CFM ratings!

I finally pulled an equipment catalog for a Direct-Fired Heater manufacturer and found similar tabular information.

I just can't get my mind wrapped around the idea that a blower will "decide" to run at 6,000 cfm and 6.02 hp at 1208 rpm when it can pump out 12,000 cfm at 1212 rpm, jump up to 14.67 hp and burn up the motor.  Never mind the fact that the blower speed must actually slow down in between these cfm's - all at 3.5" static pressure.

If anyone can explain this, I would be grateful.

RE: Basic Fan Curve Question - TWO CFM's, same RPM ?

The fan will not run without giving you problems if your operating point falls in the unstable region. The fan will hunt between the two flowrates. It is always better to choose the fan to the right of the peak point. Otherwise, you have to have a good control system.

RE: Basic Fan Curve Question - TWO CFM's, same RPM ?


Thanks quark -

I found an article on "Surge, Stall, and Instabilities in Fans" on the internet - referenced back to a fan supplier's literature and documentation:


Pretty good reference materials.  It recommends "Select fans so that the operating static pressure is lower than the lowest point of the dip in the curve." i.e. to the right and so far down that it never operates at any speed that could possibly be in the stall region.  

This would make my selection procedure kind of complicated...the selection software specifically says that the fan will perform through the peak and a little way into the "stall" region.  I don't know what criteria they use to cut it off.

Next, the guys we compete against apparently use this region of the curve, based on rpm/cfm/hp data. I wonder how many problems they have> And how they set cfm when the rpm's are the same?

Thanks again.

RE: Basic Fan Curve Question - TWO CFM's, same RPM ?

Are the systems supplemented with VFDs or automatic damper control?

Almost all the FCblade fans I operate are with VFDs and the flowrate is monitored by a flow sensor and controlled by VFD. This will give you good flexibility to select the fan at the peak point(and where you have the best efficiency).

If you go with an automatic damper control, you should select the fan for a bit higher size and reduce the flowrate by damper control. When the resistance increases in future, due to filter clogging etc., the damper can be opened and you can bring the fan operating point to the right.

Your competitor may be playing it bad to reduce the initial cost by reducing the fan size. you may have to go for a higher fan size if you design it in such a way that your operating point is below the lowest dip in the pressure head curve. You should better speak to the end user about this problem clearly.

The link you referred is an excellent resource about fans and I recommended it in these forums many times in the past. I would suggest you to put your further queries in HVAC Forum where you can get better response.

Good luck,

RE: Basic Fan Curve Question - TWO CFM's, same RPM ?

You can operate a fan in the  unstable area of the curve provided you don't mind the weird performance that you sometime get. Typically the application would be a fan operating infrequently (i.e. a smoke fan which would be used during a fire and nothing else)

Its not generally recommended to do this but on the odd occasion, it can be done.(with some words of caution)

I used to work for a fan manufacturer who did this on a few jobs. You have to know what you are doing though.If the operation is too unstable, the fan bearings can go 'Kaput'.

On one system that was installed in this way (In this case by accident rather than design), the fan was tested and a smoke bomb was used to see how fast the smoke cleared.

The fan operated between the two curves and it was clear that it 'hunted' between the higher and lower volumes. You could see the smoke disappear quickly for a few seconds, then the fan noise would change and the smoke clearance would reduce in rate.

In this instance the operation was too unstable and the fan was replaced. (Actually the designer messed up on his pressure drop calculations and the fan couldn't cope)

Friar Tuck of Sherwood

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