Tertiary air duct temperature falling noticeably below the design band means hot air from the cooler is losing heat between the take-off and the calciner. False air ingress, duct buildup restricting flow, or damper-position drift can each contribute. The signal looks small on the kiln board, but the calciner sees it directly: combustion temperature drops, AFR substitution becomes harder to hold, and the kiln picks up the calcination work that the calciner can no longer do at full duty.
Why this matters in the preheater
The tertiary air duct is the calciner's combustion-air supply, and its temperature directly sets calciner combustion stability. A sustained drop below the design band is enough to push the calciner into a window where AFR with high moisture or low reactivity stops burning cleanly, CO at the kiln inlet rises, and the volatile cycle intensifies as combustion shifts back upstream.
Secondary air quality also degrades, because the same loss point that is bleeding the tertiary duct often signals a wider seal-and-flange degradation around the kiln hood. Heat consumption rises across the kiln-calciner system in a way that is hard to pin to a single cause, and the operating window narrows on every other downstream symptom — free lime, ring formation, NOx — that depends on calciner stability.