Direct Fired Furnaces

Direct Fired Furnaces like the Muffle or Tube Furnaces are built for the continuous in-line austenitizing or annealing of multiple wire strands. High velocity burners are normally mounted along both sides of the furnaces in a staggered pattern in order to produce temperature uniformity. The combustion systems are on most modern furnaces arranged in multiple zones of control and can be either oil, gas or dual fuel fired. Layers of refractory line the furnace to maintain efficient operation and establish low skin temperatures. Todays furnace shells are of welded steel construction divided into modular units with bolted flanges.

Direct Fired Furnace

sketch courtesy of qed

With the burners firing above the wire field, an effective convection flow pattern is established for improved heat-transfer. The wires are supported above the hearth on special wear resistant castable piers. Careful control of each zone’s combustion ratio minimizes scale formation while optimizing fuel efficiency. A charge end adjustable height door assures precise furnace pressure. Many furnaces are designed with a rather long pre-heat section using counter flow exhaust gasses to increase efficiency. A special discharge chamber at the furnace exit provides the wires a protective atmosphere during transition to the subsequent process.