Unbalanced fan

[macmet]

We have a customer that is having problems with their ID fan coming unbalanced. The system has been running for only about a year and we are about to balance and replace the bearings for the third time. The pulleys/belts and everything appear to be in order and properly alligned.

The system runs about 8000acfm @ ~375F. We are burning wood.

Any thoughts? Our fan manufacturer has not been helpful.

 

[MikeHalloran]

Are you sure the flue temperature is always above 250F?

I'm just wondering if the fan is condensing creosote.



Mike Halloran
Pembroke Pines, FL, USA

 

[macmet]

Mike,

I would be very surprised if the flue gas temperature ever gets below 350F. To be honest, we were thinking the temperature may be too high, increasing the volume and overloading our fan.

Having said that though, we heard that the fan was cleaned and it was running much better now. We have seen problems with flyash buildup problems before but nothing significant or as frequent as this job site. Not sure why we seem to be having such a problem with build up here though.

 

[MintJulep]

Increased volume, or decreased density would reduce the load, not increase it.

 

[macmet]

Maybe it's because it's late in the day and I'm about to head home, but...If the flue gas temp is higher the volume increases and the fan has to run harder to maintain the same draft -- since the FD fans are running at the same speed.

Maybe I'm overlooking something...

 

[MintJulep]

Fan power is a mass-flow function.

 

[macmet]

If it is a mass flow function why are fans sized using CFM?

 

[macmet]

I should point out that I am not a fan expert. To me it does not seem to make a lot of sense that if ACFM increases that I would need less power. If I think about the extreme and there is no heat exchanger at all it would mean I would need a larger fan to move the significantly larger volume of air.

I am thinking of an expression Work = flow * delta P [cannot remember if there was a constant in it].

If I'm wrong, tell me, I am just not seeing it.

 

[MintJulep]

Volumetric flow rates (such as cfm) are related to mass flow rates through the density of the fluid.

Work is force x distance and has an equivalence to energy,

 

[macmet]

I am sorry, that "work" is supposed to be power.

Power = volumetric flow * pressure difference

Also, my pressure required to maintain draft will go up as my volume increases.

 

[macmet]

Alright, I have been thinking about this all morning. I see what you're saying. My power equation assumes constant density which would not be the case in my scenario because my mass flow is constant while my density is decreasing.

Like I said, I am not a fan expert.

 

[MintJulep]

There you go. Good teachers simply give the student time to figure things out for themselves

I actually wrote more in my last post, then accidently deleted it, then it was time for lunch...

These may help.

http://www.tcf.com/TCFBlower/pdfs/engr_data/ED1600.pdf

http://www.titan-air.com/techtips/6_2003.pdf

http://www.electronics-cooling.com/Resources/EC_Articles/MAY96/may96_01.htm