Cold blowing is an expression for a fault in the cupola furnace, describing a condition in which solid charge material (see Charge) is present in front of the air blast nozzles caused by lowering of the bed coke column to below the nozzles.
The reason for lowering of the bed coke column may be excessive supply of air blast (see Blast rate) or insufficient bed coke contents in the charge make-up.
Cold blowing is noticeable beforehand due to significant increase in the smelting output and to increased weight loss on iron-accompanying elements and iron. In this process, the iron itself is “overblown”, which means that it has a high oxygen content which may result in porous castings. The slag color appears darker and the slag quantity is increaseing.
Experienced foundrymen are able to identify the beginning of a cold-blowing phase by the color of the flame coming out of the open slag hole. The only effective remedial measure for cold blowing is to operate the furnace at low blast rates and to add a buffer coke charge immediately. As soon as the coke bed has reached it normal height due to the buffer coke charge, the furnace can be operated in regular conditions.