HIGH FLUE GAS TEMPERATURE 1

[gladkenn]

Can anybody help me on this. Our firetube boiler has a progressive increase in flue gas temperature due to the formation of unburned residue or soot inside the boiler. We have already check the flue gas analysis and its corresponds to the standard setting. CO2 = 12-15%, O2 = 3-5%. Any suggestions regarding this. We suspect the nozzle of the burner but no conclusive data.

 

[metengr]

What type of fuel are you burning - oil or natural gas?

 

[hacksaw]

you'll not get much soot from NG, but whatever you are burning is fouling the tubes if the flue temperature increases and the steaming rate is declining. of course you could be pulling in dusty combustion air depending on your location


good luck

 

[quark]

Blow the soot and descale the shell(or outer side of the tubes)

Proper combustion is not an indicator for good heat transfer across the tubes.

 

[GeneratorGrrl]

Are you certain that the watersides are clean? Increasing stack temperature is one of the indicators of scaling in the boiler. Since your stack gas composition is within expected limits, I would inspect the watersides. Quark is right, fuel combustion efficiency does not indicate heat transfer efficiency.

Hope this helps
-==- GeneratorGrrl


"Eat well, exercise regularly, die anyways."

 

[combustionman]

Four pass firetube boilers that utilize a refractory shelf for the chaning flue gas direction (like Cleaver Brooks boilers) can fail cuasing a short circuiting affect that will affect boiler output and increase stack temperature. If your boiler is a C-B I would check teh rear door shelf.

 

[gladkenn]

You share a couple of causes for this problem. But just think about this. I am using CB600 boiler. From the info I recieve CB600 boilers only fires with No.6 oil, ie, bunker C. Other Cleaver Brookes boiler like CB100, CB200, and CB400 has their own classification of what kind of fuel it fires. But for a fuel that is a combination of Diesel(70%) and Bunker(30%) fired in a CB600 boiler, will it be ok. As for our boiler, we experience gradual increase in stack temp and sooting. Will changing the nozzle diameter would further atomized the fuel and get rid of soot at first pass of the boiler? Please help me guys.

 

[quark]

When you use lighter oils with same fuel pump and nozzle, more fuel flows into the boiler. There are fair chances for unburned fuel accumulating in the combustion chamber and subseuent carryover onto the tubes. You should contact the manufacturer for correct pump discharge pressure settings and nozzle sizes. Further, gear pumps generally get lubricated by the pumping fluid and you should also seek recommendation from the manufacturer about using the same pump.

Check your combustion chamber internals for deposits.

 

[ApC2Kp]

gladkenn,
The mixture with 70% Diesel fuel has lower viscosity, and the nozzle(s) are probably not atomizing as effectively. You could check nozzle feed pressure - might be on low side. The burner vendor may have a scheme for re-sized smaller nozzle and higher feed pressure range to compensate for different ranges of fuel viscosities.
Combustion characteristics of Diesel versus Bunker C could be so different as to require separate nozzles or staged combustion zones for complete combustion of fuel.

 

[spector]

As a commissioned boiler inspector, I suggest the following:

-Consider employing a qualified specialist to evaluate your boiler combustion controls and burners. Yes, often these guys seem like they want to sell you something, but just as often they will pinpoint the problem.

-Your boiler will gradually soot up over time and the stack gas temps will increase as a result of it. If the rate of fouling is abnormal for your boiler, then and only then do you have a problem. Your type boiler requires routine cleaning of the firesides to remove soot. If the fouling is rather rapid, then yes, you are correct to suspect that the burner is at fault.

-Also consider that the other half of the fuel/air mix could be the problem. Investigate to see if you are starving your boiler for air. Even tho the burner registers, dampers and blower components are working properly, there might be times where the boiler room gets insuff air flow. A boiler can quickly evacuate the building or equipment room of air and end up in a negative pressure causing the boiler to soot up. If you are in the habit of closing off the windows/doors at night or during bad weather, then the combustion fuel/air mix will go fuel-rich on you if the building/room isn't properly vented.

Picture this: Your boiler is located in an equipment room that is normally sealed off from casual access....You go there to service the boiler, so you prop open the doors to allow full access for people and equipment. You check and adjust the burner combustion settings....Then you pack up and shut the boiler room door....And in doing so, completely change the fuel/air ratio!

-Examine how your boiler operates...how often does it cycle off? If a fully modulating boiler, how often does it idle at low fire or struggle at high fire? Frequent cycling or prolonged firing at either extreme indicate an improperly sized boiler or an improperly managed load.

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It's great that you are monitoring the stack gas temps and then investigating the cause of the increase....Go one step farther. Once you have corrected the present problem, start yourself a graph. Use that graph to plot all your boiler readings over time. Look for trends. Trends will indicate possible problems. Trends are hard to detect unless they are graphed. So record and plot out fuel consumption, make up water consumption, stack temps and anything else you have indications for. Those graphs will also prove helpful and indicate your progress as you make repairs and other changes.

 

[chicopee]

I can not believe that the flue gas temperature is increasing thru each pass of the firetube boiler.If soot is building up inside the firetubes, punch'em once in a while.

 

[spector]

I believe gladkenn stated that the flue gas temps were progressively increasing "over a period of time", not as they passed/progressed thru the boiler....

He is correct that such an increase indicates either fouled firesides, watersides, or both.

 

[chicopee]

spector-thanks for the clarification-yes I agree ,the tubes are getting dirty and need to be punhed out periodically.