High Stack Temperature and a 6% Fuel Increase After Burner Tune-Up
A 600 hp firetube boiler fires natural gas at 150 psig and supplies a steady 70% load. After the annual burner tune-up, the operator reports a 6% increase in fuel use at the same steam flow. At the stack sampling port, 12 ft downstream of the boiler outlet, the portable analyzer reads 8.5% O2, 8 ppm CO, 30 ppm NOx, and a stack temperature of 640°F; ambient air is 70°F. Before the tune-up, the same load produced 4.5% O2 and 480°F at the same port. The burner windbox has a separate test port, and the breeching has several bolted inspection doors and an expansion joint. The operator proposes closing the air damper until the stack analyzer shows 4% O2, saying the boiler will then be back to its old efficiency. The boiler has no economizer or air preheater. The draft fan is at 60% speed and the stack damper is 45% open.
1. Calculate the approximate excess air at the stack port after the tune-up and at the pre-tune condition. What do the two numbers tell you, and what do they not prove?
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Use the dry-O2 approximation: excess air % = O2/(21 − O2) × 100.
After tune-up: 8.5/(21 − 8.5) × 100 = 68% excess air. Before tune-up: 4.5/(21 − 4.5) × 100 = 27% excess air.
If the sample is representative, the burner is now running with far more excess air. That is consistent with the 6% fuel increase because excess air raises mass flow and stack loss. But the numbers do not prove the burner is lean. The stack port is downstream of bolted doors and an expansion joint, so the sample can be diluted by air in-leakage. The correct next step is to compare burner-windbox and stack O2/CO at the same time, verify analyzer calibration, and inspect the breeching before changing the air damper.
2. The operator proposes closing the air damper until the stack reads 4% O2. Explain why that could be unsafe and what tests you would run before changing anything.
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Closing the damper until the stack reads 4% assumes stack O2 equals combustion O2. If the breeching is leaking, the stack reading is already falsely high; closing the damper would reduce true air to the burner and could drive the flame rich, producing CO, soot, or flame impingement. Even without leakage, driving to 4% at the stack should be done in small steps while watching burner O2, CO, NOx, flame signal, and stack temperature.
Before adjusting, I would:
- Inspect breeching doors, gaskets, the expansion joint, and the sample probe/tubing.
- Calibrate the portable analyzer and compare it with the fixed analyzer.
- Read O2 and CO simultaneously at the burner test port and the stack.
- Check draft at the furnace and stack and confirm damper/fan linkage position.
For natural gas, a typical target is about 3–5% O2 at the burner, not a blind 4% at the stack. If CO rises or the flame signal becomes unstable, stop and back out.
3. If simultaneous readings show 8.5% O2 at the stack and 4.0% O2 at the burner, what is the most likely cause and what would you do?
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A rise from 4.0% at the burner to 8.5% at the stack indicates air infiltration between the burner and the sampling point, not a true 68% excess-air condition. The stack CO is also diluted (8 ppm). Likely sources are an open inspection door, a failed door gasket, a leaking expansion joint, a cracked breeching seam, or a leak in the sample line or pump.
I would first check burner CO and O2. If burner CO is normal and O2 is 4%, repair the leaks and re-test before touching the air damper. A sample-line leak can produce the same false-high O2, so inspect the probe and tubing. If both ports truly read 8.5%, then the burner is running lean and the air damper or linkage should be adjusted slowly. Do not correct a measurement problem by changing combustion.