We've discussed balance quality before, but I rarely balance a cooling tower fan, so I've never looked into what is "acceptable". Am looking for a quick answer on what is acceptable balance for cooling tower fans.

This is what I'd call a "medium-sized" fan, about 15 ft. in diameter, running at 235 rpm. I took readings horizontally, on the gearbox, at a right-angle to the support tube (which is the least stiff direction). I have no idea what the unit weighs, so can't calculate a residual imbalance (right?).

My final readings (filtered at 1x rpm) were 2.5 mils (pk-pk), or 0.04 ips (pk). Based on your experience, is that acceptable?

Hi Rusty,

2.5 MILs pk-pk at 535 rpm is in this vintage chart's "good" zone, and was likely < 0.07 ips pk. http://www.balancingservice.com/images/fig2bal.jpg


How did the structure (deck, stack, etc?) around the cooling tower feel now, and before balancing? What was the ~balance correction and the vibration reduction? Did the balance corrections produce fairly reasonable predictable results?

regards,

Dan T

This calculation could be misleading and I don't know the value. One can get the residual imbalance from the trim balance calculation.

Since this sounds like a single plane balance (on the hub?), one could try using the mass of the blades and hub. A manual, replacement part, weight one - can give an estimate for a blade (times number of blades). The hub mass shouldn't be too difficult to approximate (no exotic materials).

Then you can look at it as the mass eccentricity of the blades and hub.

The supposedly stiff direction does not always have the lowest vibration. Stiff is frequency dependent, i.e. resonance.

Rusty,

I would be very happy with 0.04 IPS (2.5 mils) at fan speed as long as you do not have high vibration at motor speed and blade pass. Most of the cooling towers that I get called on to balance have excessive vibration at motor speed and require balancing the drive shaft (hubs). According to an article in "Maintenance Technology Magazine" July 1991, on cooling tower vibration, 2.5 mils (at fan frequency) would be at the lower side of the "No Faults" range on steel towers and 3.5 mils (at fan frequency) would be at the lower side of the "No Faults" range on wood towers. The article was written by Dan Murphy, Marley Cooling Tower Co.

Regards,
John J

This message has been edited. Last edited by: John J,

Thanks for the input. This fan was running 35 mils at 183 rpm (70% speed) when I started (I was afraid to go to 100%.... extremely high amplitudes also usually make a unit "non-linear").

Originally, it had 22 lb-in. of weight on it; this was eventually moved about 180 degrees, and I added 10 oz. at 18" to bring the total to 33 lb-in. This six-bladed fan has 6 weight holes at a 4.5" radius, but the hub plates were 36" in diameter.... why do they put the weight holes so close to the center where you have to use such large weights?

I wonder which ISO standard covers the fan balancing jobs?

This message has been edited. Last edited by: Josh,

quote:

Stiff is frequency dependent

Do you mean for a "flexible" rotor? Otherwise, I think of "stiffness" as a physical property, dependent on structural aspects. For instance, an I-beam is stiffer in the direction of the long axis.

On a cooling tower fan, almost without exception, the lowest vibration will be inline with the support tube (i.e., the drive shaft).

Stiff relates to resistance to a force with regards to motion. For forces at different frequencies the resistance differs.

For that I-beam, it may be statically stiffer in the direction you mention, but at different frequencies for the force this can and will change.

It is possible for soft directions to provide isolation, too.

quote:

why do they put the weight holes so close to the center where you have to use such large weights?

They don't always! Sometimes they are on the end of the blades.

The soft direction may be more damped and not always the best 1X indicator. The coupling type may also have a play.