Fan problem

[Oxidation]

A centrigugal fan used as a blower for combustion has been causing problems. It was vibrating, but still running & producing less duct pressure than previously. It has an inlet butterfly valve for flow control.
I hadn't noted the motor amps, but there are 3 other machines identical in set up & each MCC shows circa 60amps/phase on the 'good' three units (all three at similar load levels)
The fan was shut down to look in to the vibration, not duct blockages could be seen, not water/ice in the casing could be seen. So we thought that fan blade has some dirt build up & so is unabalnced. Management decided they wanted the unit running again, no matter what. When I came to restart it the motor driving the fan kept tripping out after 2 seconds on 'low run current' (circa 20 amp) (600 hp motor at 4160volt).
My question is:
Is this typical of fan failure, no compression of air=low load to motor? What is typical blade to casing clearance for blower (26000cfm @75inH2O discharge pressure)

 

[25362]

One good source of information on balancing fans to avoid vibration and noise is Fan Engineering An Engineer's Handbook on Fans and Their Applications, published by the Buffalo Forge Company - Buffalo, NY-.
I hope you can find it in a technical library.

Besides, I believe the HVAC/R forum engineers dealing with air conditioning may submit more and better tips. Good luck.

 

[abeltio]

that is a pretty hefty fan you are dealing with...
low discharge pressure can be caused by:
1. wrong rotation (CW vs. CCW) - it will develop pressure but much lower...
2. wrong clearances (or wrong impeller size)
3. no back pressure from system (i.e. a big leak)
4. blocked suction

high vibrations:
1. misalignment
2. damaged bearings
3. damaged impeller (needs balancing)
4. wrong rotation
5. flow induced vibration

for a centrifugal compressor the motor load with no compression could be very high (0verload) if the discharge is open (i.e. no back pressure) to have very low load on the motor it could be due to a blocked suction.
HTH

saludos.
a.

 

[RNDguy]

I wonder if this could be due to some problem in the fan-to-motor attachment, meaning in the coupling or driving belts? Has this been checked?

 

[Combustor1]

Oxidation,

You might need to get a fan rep in this is a big fan - I have seen the result of some big fans coming apart - very dangerous. You should be seeing vibration below 10 mils.

If you are tripping when you start check to make sure that your damper (inlet or outlet) is closed when starting a fan this size until it reaches full speed and check the amps.

Pressure is directly related to speed - you can put a tachometer on the fan wheel itself and also on the motor to see if they are operating per spec. Another item to check is the system resistance - see if something has changed/come apart - if you have an exhaust fan damper take a look at it or if you are using a pressure transmitter to control pressure make sure it is properly calibrated.

Good Luck & Be safe!

 

[Haugen]

Couple of things I would check. Ultimately, the fan is not doing the work, but I don't know what the control system is for the fan, nor the process...so here are a few thoughts.

Have you checked for discharge piping/duct leaks? A large leak would exhaust the air and the fan may be doing less work as a result, it would depend on your control system and what the demand signal is driven from.

Check that the motor and fan are turning at the design rpm. Not sure if you can check the fan itself, but strobe what you can to satisfy yourself that everything is coupled properly and spinning as fast as it should be.

Check the voltage supply to the fan - all three phases. Amps are only one part of the equation - power moves air, a function of V, I and Power Factor.

Check bearing temperatures.

I don't believe that these fans simply wear out overnight, I think you have a failure of some kind other than clearances.

thinking out loud
m