Acceleration affects on fan
Acceleration affects on fan
(OP)
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.
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.





RE: Acceleration affects on fan
You should clarify the concern with him.
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RE: Acceleration affects on fan
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Acceleration affects on fan
RE: Acceleration affects on fan
RE: Acceleration affects on fan
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Acceleration affects on fan
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.
RE: Acceleration affects on fan
I also ASSUMED the comment applied to the fan and not the motor. Can the original poster clarify that?
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RE: Acceleration affects on fan
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
RE: Acceleration affects on fan
RE: Acceleration affects on fan
RE: Acceleration affects on fan
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
RE: Acceleration affects on fan
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
RE: Acceleration affects on fan
This is not a problem with the fans we generally use in HVAC because they are comparitively very small.
Regards,
Eng-Tips.com : Solving your problems before you get them.
RE: Acceleration affects on fan
rmw
RE: Acceleration affects on fan