Stack temperature climbing above the WHRS design band is heat the boiler should have absorbed but did not. The most common cause is fouling — accumulated dust on tube surfaces blocking heat transfer — but tube damage, gas bypass through internal leaks, or simply a kiln that is running with lower exhaust temperatures than the boiler was sized for can each push the stack number up. Whatever the cause, the heat going past the boiler is heat the WHRS economic model assumed it would keep.
Why this matters in the whrs
WHRS stack temperature is one of the cleanest signals of how well the boiler is doing its job. A drift upward means heat-recovery efficiency is falling, and steam generation is following. The annual generation target gets harder to hit, and the payback period on the WHRS investment quietly extends.
The second concern is environmental: stack temperature affects bag filter inlet conditions, conditioning tower performance, and any downstream pollution control. Sustained drift can shift the operating point of equipment further downstream into regimes the original design did not account for. Treating stack temperature as a process indicator that touches both the kiln and the dust system — not just a WHRS metric — usually catches the trend earliest.