ID Fan capacity
ID Fan capacity
(OP)
How could the capacity of the ID fan be determined for a bagasse fired boiler ?
I noticed on my boiler that at a certain flow rate, it is really very difficult to manitain a negative pressure in the furnace and the ID is almost at 100 %.
Bob
I noticed on my boiler that at a certain flow rate, it is really very difficult to manitain a negative pressure in the furnace and the ID is almost at 100 %.
Bob





RE: ID Fan capacity
RE: ID Fan capacity
rmw
RE: ID Fan capacity
Bob
RE: ID Fan capacity
I think the answer to your problem is in my post of Aug 6 in thread794-130923
You are pulling so much fan load from leakage, which doesn't have to be very large leaks that you aren't pulling enough air through the furnace to completely combust the fuel, resulting in the CO you mentioned and preventing your ID fan from maintaining a negative draft in your furnace.
Re-read that anectodatal story I told about the bark boiler. It is analogous to your situation. You don't have a door open, but you have enough leaks to simulate some portion of an open door furnishing cold air to your fan. Fix the leaks and notice the difference.
rmw
RE: ID Fan capacity
Hence my question.
Does your fan then have dampers for start up, and if so, then yogibear1's comment above is something to check.
rmw
RE: ID Fan capacity
RE: ID Fan capacity
There are so many variables in burning bagasse, however. Just a change in fuel moisture which is a function of how much imbibition water they put on the grinding mills for sugar recovery purposes can change the picture completely, even with a non leaky boiler.
rmw
RE: ID Fan capacity
Bob
RE: ID Fan capacity
With respect to the fuel, did you initially burn bagasse straight from the mill floor, or did you burn a combination of mill produced fuel and fuel from storage which had dried out somewhat?
Or, are you burning bagasse from outside storage that has been rained on to a large degree.
Also, is the boiler producing steam at the same pressure/superheat ratings as it originally did, or has that changed?
If you don't have a lot of leaks, (which I still suspect based on information from the CO thread) then something else has to hae changed.
The leaks don't necessarily have to be from external sources. Air bypassing the grates due to grate warpage, or due to inadequate fuel distribution can also overload ID fans.
Keep checking, and keep posting the results.
rmw
RE: ID Fan capacity
I did a test while the boiler was about 75 % loaded and the combustion was very good with only 250-300 ppm of CO and O2 around 3.5-4 % in the stack. This , I believe, eliminate the possibilty of big leaks in the boiler.
Do you think a high CO around 6000-7000 ppm is normal for a bagasse fired boiler at near full load.
Bob
RE: ID Fan capacity
I don't remember from other threads whether you said or not, but do you have combustion air preheat? If not, that would help a lot. The bagasse being half moisture needs a lot of heat released just to boil the water off of the fuel so it can begin to heat up enough to ignite.
Preheat helps this.
rmw
RE: ID Fan capacity
Many thanks again for your prompt reply. Yes , we do use preheat air. However,one astonishing thing I noticed is that the FD was almost totally closed at 75 % load.Two observations could be made out of this , either the FD damper is not totally mechanically closed at 0 % or the boiler might be sucking air somewhere to sustain combustion !!
Could the air supplied through the secondary air , although it is not mean for, sufficient to maintain combustion.
Bob
RE: ID Fan capacity
Besides we have actually installed a damper(will used in exceptional case if the vsd fails ) in the circuit and the pressure drop across the damper is 5 cm H2O.
In the previous years, there was no damper installed.
Bob
RE: ID Fan capacity
An item that concerns me is how you combustion controls as well as flame safety/burner managment systems actually measure unit air flow. On some (old) units it is via pressure drop across some back pass section of the boiler. On newer units there are air foils in the FD fan ductwork that measure air flow. If your air flow is not coming thru the proper burner registers then you could have less than satisfactory combustion which would show up in your flue gas analysis. And if the unit is not breathing right it could be subject to excursions into the wrong area of the air-fuel-ignition diagram.
RE: ID Fan capacity
Maypot, confirm what type of fuel feed system you have. Knowing if you have air swept spreaders or screw fed distributors will aid us in our analisis of your inquiry.
Also describe briefly what type of grates you have, flat, inclined, travelling, dump, water cooled, etc.?
rmw