AHU Bearing Failure

[krb]

Has anyone seen a bearing failure on a rooftop air handler supply fan that is related to airflow problems? We have a McQuay air handler with a return and supply fan and have had several failures of the drive bearing on the supply fan. About four months ago an energy recovery ventilator was added to this system to supply conditioned outside air to the McQuay unit. Since then there have been two bearing failures. The McQuay vendor blames the ERV and said there are flow imbalances causing the bearing failures. The system was not balanced after installing the ERV, but I have never seen a bearing completely destroyed like this as a result of flow imbalance. The McQuay unit is a RAH047C. I can provide more info if needed. Any thoughts?

 

[fredb]

Hello,

Vibration can be induced on a fan wheel. One way is to have air flowing predominantly on one side of ductwork. This can occur for example when a turn is placed too close to the fan. The off center airflow basically acts as a concentrated weight on the fan wheel causing it to vibrate because the wheel is not now in balance. The vibration then destroys the bearing. A vibration check should be done on the bearings to determine if the displacement levels are within tolerance.

 

[TBP]

Is everything properly aligned? Nobody has substituted anything like a motor with a different RPM, sheaves with a different diameter, etc.?

 

[krb]

It looks like no changes have been made to the drive, however at this time I do not have information on what bearing was installed as a replacement after the first failure. But these things would not be related directly to airflow changes as a result of installing the ERV on the outside aire mak-up. I have asked for the old bearing to see if we can examine it for some clues.

 

[lilliput1]

Check bearing manufacturer's required fit tolerances for mating parts - shaft diameter and bearing housing. I know of instance where fan manufacturer used a different bearing and forced it to fit, shortening the bearing life.

Verify that the bearings have been lubricated with the proper grease.

Verify that the tension on the drive belts is proper, not excessive.

 

[sdfrench]

You have described EXACTLY a problem that has just ocurred at a building I am working in. Everything from the same mfg to the addition of an ERV, the second massive bearing failure since February and the vendors belief about airflow causing the problem. Any chance we are talking about the same building? I am located in Knoxville. BTW, I have been thinking about the airflow angle and the more I read the less likely it seems to me to be the problem.

 

[krb]

sdfrench,

Sounds like we may be talking about the same building. I'm in Oak Ridge. I am supposed to get the failed bearing today so I can look at it. Repairs are being made today, and I would like to have some vibration readings taken as well.

Sounds like from the posts that most people see other causes for the failure than the addition of the ERV.

 

[Yeldud]

I would be very surprised if the ERV is connected to bearing failure, unless the ERV flow is being directly injected into the fan cabinet section of the AHU, causing flow imbalance through the wheel. If that is the case, then when the ERV is operating there should be a measurable increase in fan vibration versus when the ERV is not operating. Otherwise I would suspect excessive belt tension, fan wheel imbalance, poor bearing lubrication, or even inadequate bearing capacity in the initial design. What type of suppy fan is it (DWDI Centrif or other?), and how is the ERV air introduced into the unit?

 

[krb]

yeldud,

Flow is not directly injected into the fan cabinet, it is connected to the outside air make-up hood. I believe it goes through a preheat coil before it gets to the fan cabinet. The fan is a DWDI centrifugal, 40 hp motor, I have to find out about speed of fan, and it is a Dodge 212 bearing. Thanks for the input.

 

[lilliput1]

Also make sure the fan is not and will not operating in the surge region (beyond the envelope of allowed fan operating curves. Operating in the surge region will cause excessive vibration. You may need to reciculate air flow around the fan and/or require thimble connection between the makeup & AHU.