EFFICACY AND RISK OF AERATION TO PREVENT SELF-HEATING POWDER BED FROMSPONTANEOUS IGNITION

The effect of aeration on the thermal stability of a self-heating powd er bed is mathematically discussed. Judging from the thermal character istics in the steady state, aeration greatly improves the thermal stab ility. It is also shown, however, that a steady state solution for the temperature distribution in the bed always accompanies an unstable eq uilibrium solution. It follows that once the steady state temperature distribution is forced through the unstable one by some external distu rbances, the powder bed starts to run away towards an ignition and los es stability even after the disturbances are removed. An aerated powde r bed is rather susceptible to this kind of ignition. To avoid possibl e risk of such an ignition, a restorable limit of the temperature dist ribution for a given aerated powder bed is proposed. Aeration should b e controlled so that the temperature at any position in the powder bed is kept well below the restorable limit distribution.