ID Fan capacity 1

[maypot]

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

 

[yogibear1]

Have you made sure that the ID Fan controls really are at 100% stroke when the inlet vanes are likewise at 100% fully open (this would have to de done when the unit is down to verify. It is possible that the linkage could have gotten bent out of shape or somethign has changed to short stroke the dampers (do they all move in unison and reach full open)?

 

[rmw]

If your ID fan is steam turbine driven are you getting full governor stroke, full steam pressure, and can your speed be increased (within the RPM limitations of the fan wheel of course)?

rmw

 

[maypot]

Thks for the rplies. The fan is driven by an electric motor driven itself by a vsd. The vsd is most of the time at 100 %.

Bob

 

[rmw]

Maypot,

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

 

[rmw]

And, just as a point of reference, in my experience, most all of the ID fans on bagasse boilers in my little corner of the world are steam turbine driven with the exhaust going to the supply header for the evaporators, heaters, and pans.

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

 

[fredt]

I have followed through this thread and the other thread with interest because of my fan background. I agree with most of rmw's comments - especially those regarding tramp air. However, one big question is whether the boiler ever did perform satisfactorily - maybe it has never had enough draft to produce complete combustion and you simply need more fan capacity. Also, do you have enough FD. Generally with sugar mill boilers shortfall of ID capacity means difficulty maintaining a negative furnace pressure.

 

[rmw]

I thought that maypot had stated in one of these threads that once upon a time the boiler did make rating with relative ease.

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

 

[maypot]

Thks rmw for your always valuable replies. You are right , the boiler did perform well in the past. We have a close check for leaking air but could not find many worth causing problem.I think again you`re right about the issue of bagasse humidity.We are measuring humidity of 45-47 %, which is according to the mill norms, but nevertheless it`s becoming problematic to keep the boiler performing well.

Bob

 

[rmw]

Can you measure steam flow from this boiler accurately. Sounds to me like you might be demanding more from it than it was designed to produce.

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

 

[maypot]

Hi rmw,

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

 

[rmw]

Without knowing your furnace size and your heat release rate, I would suspect so. Your readings at 75% load indicate that at that point of operation, you still have enough furnace volume to get good heat release. However, at full load, you might be crowding it, especially if your boiler wasn't specified with conservative heat release.

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

 

[maypot]

Hi rmw,

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

 

[maypot]

Hi rmw ,

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

 

[yogibear1]

Maypot: an item in a recent post merits looking at, i.e. the FD Fan dampers position. As an electrical engineer you can appreciate how load increases motor amperage- so check that aspect to see if your FD Fan is actually pumping any air. The FD Fan inlet is a lot easier to check than the ID, so check the actual vane postion as well as the position feedback loop.
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.

 

[rmw]

And, adding to yogi's post, in the case of your FD fans, if you use radial vortex inlet vanes, they reach their maximum capacity at something in the neighborhood of 60-70% open, and if they are 100% open, they actually impede air flow. The 60-70% should be determined with your fan/damper supplier. The angle of the vanes actually gives an inlet swirl which aids the fan, while a damper 100% open will be detrimental to the fan.

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