Outlet gas temperature drifting above the normal operating window for a VRM raw mill is a symptom that the mill is no longer in thermal balance. Either drying demand has fallen (drier feed, lower output, separator drift) or the hot-gas inlet has been pushed beyond what the current operating point needs. Sustained high outlet temperature stresses bag filter media, dries out grinding-bed plasticity, and accelerates wear on internals that depend on the gas being at a controlled temperature when it leaves the mill.
Why this matters in the raw mill
Outlet temperature is the cleanest signal that the energy going into drying matches the energy needed to remove moisture from the current feed. When it rises, the mill is rejecting heat that it does not need — or worse, that the feed could not absorb because of low load or grinding-bed problems.
Bag filters downstream see thermal stress that shortens media life; the bed loses the moisture it needs for stable plasticity, and vibration trends start to climb in step. The trend is also a leading indicator of false air ingress and conditioning tower performance loss, both of which become more expensive to fix once the kiln campaign is well underway.