blow in

To blow in a blast furnace means to commission it after construction, repair, or extended shutdown by drying out the refractory lining, heating the internal structure, and then introducing a sustained air blast with fuel to reach operating temperature. This is distinct from normal operation: a furnace in blast runs continuously, but blowing in is a controlled, staged process that can take days or weeks depending on furnace size and condition.

The initial phase removes moisture absorbed by the refractory bricks and mortar during idle time. Heat is applied gradually, typically using external heating or low-intensity fuel burning, to prevent thermal shock that would crack the lining. Once the furnace is thoroughly dried and warmed, the main air blast is turned on. Coke and other fuels are charged into the top, and the ascending hot air ignites them, driving temperatures upward through the hearth and crucible.

The pace of blow-in matters. Too rapid heating causes refractory failure; too slow an approach wastes time and fuel. Operators monitor furnace temperatures at multiple points and adjust air volume and fuel feed accordingly. Large furnaces (those rated for 1,000 tons per day or more) may take three to four weeks to reach steady state. Smaller furnaces can come in faster.

Blow-in is hazardous because the furnace is not yet thermally stable and the system is incomplete: seals may leak, hot gases escape unpredictably, and uneven heating can trigger sudden failures. The refractory is weakest during this phase. If a furnace cools below a critical threshold before full operation begins, the blow-in cycle must restart.

Once a furnace is in blast, it typically stays in blast for years. Stopping it is avoided because a new blow-in is expensive and disruptive. This is why furnace uptime is a major economic driver in ironmaking and why preventive maintenance during operation is standard practice.