Pumping air fan problem

[pekjaer]

Hi collegues,
Working with industrial furnaces. Gas burner system where we have a centrifugal fan producing combustion air. Its an 45kW electric motor direct coupled to the impeller. Back folded blades for a "flat pressure curve". Pressure 0,15 bar and 5000Nm3/h, atmospheric air. The fan is installed on rubber feets bolted to the concrete floor. A flexible compensator is installed right after the fan connecting to the steel pipework diam. 410mm. The flow varies with the throughput of the furnace. 1m downstream the fan a 90dgr. bending is installed then 1,5 m straight and then another 90dgr. bend. Thereafter 10m straight pipe before deviding into 2 lines each side of the furnace. The fan speed is 2850rpm fixed.
Problem: We have a very strong pumping constant of approximately 1,5 Hz independent of flow. Pressure variation approx. +/- 20mb. The fan manufacturer decline that the problem is the fan construction, but says we have to remove the 2 bendings right after the fan "to get more straight flow before the bendings"??, but he cannot explain why. Trying to find the natural resonnance for the pipe system, I get much higher frequency than 1,5 Hz.
Does anyone of you have similar experiences and found a way reduce this kind of pumping? If so I would be gratefull for some tips where to start looking.
Thanks in advence.
Poul Erik, Denmark

 

[MikeHalloran]

I've seen strong pulsations at ~1.5Hz independent of flow in a boat's wet exhaust, where the lift tube in the muffler was too big and the gas flow couldn't quite blast the liquid water out of it to establish annular mist flow. It shook the whole boat ... and it was a big boat.

But I wouldn't expect to find water or any other liquid in a furnace air supply.

Can you move the two elbows downstream as a unit, so the fan discharge goes through 10 meters of tube, then 2 bends, then 1 meter to the burner? If it doesn't work, then you have a basis for demanding a site visit by the fan manufacturer. More so if you use >0.7m radius bends.



Mike Halloran
Pembroke Pines, FL, USA

 

[pekjaer]

Hi Mike,
Thank's for your reply.
You are right, no water here.
We can (but cost and time!) move the elbows, but the reason for the question to the forum is, that I am not sure it will help - there might be another overseen problem. Therefore I hoped, that somebody in the forum had an experience, which could confirm whether it might help to move the elbows or not, and maybe point at some other possible solutions.

I would be gratefull for any help.

Thank's in advance

Poul Erik

 

[MikeHalloran]

No, I don't seriously believe that moving the elbows will make a difference. But since the fan manufacturer recommended doing so, I'd do it.

And when/if it doesn't work, I'd send his boss a bill for labor and materials. That should get his attention.





Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

I suppose the next logical question is, "Where does the fan get the air?" I.e., is it trying to evacuate a closed room? Assuming the room has e.g. shutters, are they open?


Mike Halloran
Pembroke Pines, FL, USA

 

[pekjaer]

Hello Mike,
Very good point, but unfortunately this can be ruled out.
Its a very large production hall, and about half of the side walls can be opened by large "damper-windows". The pumping problem does not change at all whether the dampers are open or not.

Poul Erik

 

[btrueblood]

Another issue - what is the volume of the burner/combustion chamber? Could that be what is pulsing at 1.5 Hz?

 

[Strong]

I don't think pulsation is associated with the fan. Flow induced vibrations can be difficult to track down. I agree that boiler volume may possibly act as a Hemholtz resonantor. I would concentrate on the flow divider and at the air entry registers to the boiler. You state that flow variation does not affect pulsation, but I would expect that there is a threshold flow when the pulsation starts. The flow divider may act as a fluidic oscillator. The turbulent air register flow might exicte the throat of a Hemholtz resonantor. One technique that I have used is to disturb the flow near the suspect location and determine if the pulsation frequency changes or if it terminates. These kind of problems can be fun and a challenge, so don't give up.

Walt

 

[pekjaer]

Hi
Thanks for the responce. As the answers evaluate, it becomes obvious, that more and more specific data is needed (well, so big surprise).
From the fan we have this 12m 410mm pipe to a tee. Dividing into 2 times 350mm lines each side of the furnace length 10m. Min flow (pilot burners ON) is 100 m2/h (2% of total) furnace chamber 80m3. Its hard to evaluate the Hemholz resonance, but if the entire pipe system can be considered as the resonnance tube with a length of 30m, the frequency would be 0,4 Hz(tube 410mm). Looking at only "one side" with 20m, the frequency would be 0,4 Hz (tube 350mm). Considering that only the side tubes at 10m should be considered as the resonnnace tube, the frequency would be 0,6Hz(tube 350mm).
I agree, that we are looking at frequences near the problem, but to reach 1,5Hz quite a change must be introduced. As example: To reach 1,5 Hz, the tube length at 410mm should be only 2m. As the resonnance doesn't change with flow (from 100m3/h to 5000 m3/h)surely indicates a frequency related to mechanical data, I just didn't find a plausible structure creating this frequency. Also the Hemholz resonnance doesn't seem plausible ?

Poul Erik

 

[Strong]

Poul,
Have you reviewed the gas system for pressure variation? I am not sure what you mean by "surely indicates a frequency related to mechanical data"? Other than a fluctuating pressure gage, what is the significance or impact of this dynamic pressure on the boiler?

Walt

 

[pekjaer]

Hi Walt,
Yes we have reviewed the gas system, and it has variations as well, but that has to do with the governor and the throughput. Which means no variations related to 1,5 Hz.
What I mean is: It seems as the frequency is not depending on flow or other process variables, but only on some kind of fixed (mechanical data) conditions like tube sizes, length, volumes etc.
The problem is ustable combustion as the air pressure varies, and we guess life times of some components exposed to the constant pressure variations.
Even nobody has answerred: Yes I have seen this before, and the reason/cure was ......, I still havn't figured out where to look and where to change something. We can in principle change everything, but we might end up with the same problem again without knowing why. We could change on part at a time, and see if it change anything, but both solutions are very expensive and time consuming - I really had the hope that some of the experts here had seen something like this before, and was able to help me to decide where it was most likely to get to problem.

I still hope....

Kind regards
Poul Erik

 

[MikeHalloran]

No, I wouldn't start changing parts just yet.

I'd start drilling small holes and measuring pressure/time, temperature, acoustic noise levels, etc., and overlay the data on a scaled map of the system. The intent is to find the forcing function ... and perhaps to stumble over an 'ah-Ha!' while measuring and mapping.






Mike Halloran
Pembroke Pines, FL, USA