Acceleration affects on fan 1

[gordonl]

I've been doing the electrical investigations on the use of a VFD on a 3500HP induced draft fan. We would be cycling the fan between full speed and base speed (approx 20%) several times an hour. Currently the fan is dampered back several times an hour, but with the VFD we would ramp the speed up and down with the process. The existing practise is to shut down the fan only around six times a year when the process is down for 8-16 hours, to minimize the starts on the motors.

The fan supplier is telling us that it is hard to design the fan for this cycling duty and that it would very signifigantly shorten the fan's expected life. (1-3 years) Can anyone enlighten me as to why the controlled accel/decel of this fan would have such a detrimental affect.

 

[electricpete]

A. I think possibly your manufacturer is worried about repeated high vibrations from passing through resonance speed on a routine operating basis, rather than only during start (rapid pass-through) and stopping.

You should clarify the concern with him.

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[electricpete]

Much less likely imho are torsional vibrations induced by vfd. I mention only because it was subject of a recent thread.

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[aolalde]

Cyclic loads produce fatigue, and hence, lifetime capability can be reduced.

 

[jbartos]

Comment: A cummulative temperature elevation effect might also appear. It can reduce the fan motor life expectancy. However, the thermal sensors should be installed at this size of the motor and indicate the motor temperature. According to the temperature elevation, an engineering calculation or judgement can be made on the motor temperature caused accelerated aging.

 

[electricpete]

aolalde - why would we consider that the cyclic loading is more severe when cycled by vfd than when cycled by adjusting a damper?

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[sreid]

Electricpete,

I think the answer to your question is that the load on the fan motor is reduced when you decrease the air flow by throttling the fan. Less air pumped, less power required. Accels of the motor clearly add heat due to I^2R losses of the accel current.

Gordonl,

I think it would be fair for you to ask the fan supplier the reason for the reduction in life expectancy. If they have a good reason they should be willing to share it with you.

 

[electricpete]

If I were not concerned about radial or torsional resonances, my bet is that varying speed by vfd (with reasonable ramp rate) is much easier on the fan than throttling the damper and pushing off of bep.

I also ASSUMED the comment applied to the fan and not the motor. Can the original poster clarify that?






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[rmw]

Fans are run variable speed all the time, using turbines, and/or liquid drives all the time in boiler applications. However, the turndown ratio is not normally as great as posted above.

The only way the mfg'r's concern makes any sense to me has already been mentioned above, and it says to me that the mfg'r must know that the fan has to pass through a critical in the turndown range that is stipulated.

Can the original poster state the fan wheel diameter, and/or the WR^2 of the fan??

rmw

 

[jbartos]

Comment: The standard motor design for frequent motor starts and wide speed variation (high turndown ratio) may need to be reconsidered in favor of a heavy-duty motor design or custom built motor. There is a great deal of leeway in the motor design. There are motors designs available for frequent jogging and plugging.

 

[Farkel]

I agree with Electricpete's comments! Most likely fatique of fan when going through resonance, which you can have your drive skip over quickly.

 

[gordonl]

Thank you for the input to date.

The concern stated was for the fan and not the motor.

The fan inertia is 63500 lb-ft^2, diameter approx 9.5 ft.

Thanks Again,
Gord

 

[rmw]

Check with your fan OEM, and see if their concern was resonance, or passing through a critical, as some would say.

With a wheel that diameter, undoubtedly you have at least one critical speed, if not more to go through to get to operating speed. Constantly passing back and forth through that speed would someday "shake" the fan apart, especially if your control did not know where it was, and let it linger for a time in that speed range. Then the wheel would fatigue rapidly, and (gulp) someday fly apart.

I have been to some very spectacular fan wrecks. They are not a pretty sight, but great business for fan rebuilders.

It may be that you can determine what the critical speed(s) are, and operate away from those speeds, and avoid the problem altogether.

rmw

 

[quark]

One reason will be the dynamic balancing issue. Generally fans are dynamically balanced (i.e balanced in two planes) at one speed and if you have to operate your fan other than this speed for longer duration you will have problems.

This is not a problem with the fans we generally use in HVAC because they are comparitively very small.

Regards,


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[rmw]

Most of the fans I have much experience with are on solid fuel boilers, bark, coal, black liquor, operated variable speed, via turbines, liquid couplings or vfd's. Balance was almost a constant variable as material built up, and then sloughed off. Maybe they just wore out before they fatigued out. (or wrecked first)

rmw

 

[renasis]

I would strongly suggest you talk to the mtr manufacturer before u used a vfd.