Typically, these box furnaces are gas fired and operate in the 1250°C to 1300°C range. Most applications call for the use of high-performance nozzle-mix burners that deliver maximum efficiency through close control of the air/fuel ratio, furnace atmosphere, and furnace pressure. Reliability and stability are also important, especially when considering excess fuel and excess air situations, allied to the ability to withstand radiant heat.
Slot-style box furnaces – and a typical slot entrance might be around 10cm high – feature an air curtain that separates the furnace’s internal temperature from the surroundings. A turbofan would be incorporated for the air curtain and burner, while an exhaust fan is used to guide the combustion gases to a safe zone via a specially constructed exhaust hood.
These furnaces operate with automatic proportional combustion control. In a two-zone furnace, which would suit many steel heating facilities, there would be both furnace control and also high-limit control that shuts down the burner if the temperature exceeds the preset limit, in order to protect the equipment. That said, these furnaces can be designed as single-zone or multi-zone, as required by any specific application.
As far as reaction to burner failure is concerned, detection is via a UV flame detector and flame failure relay. In the event of a failure, the relay sends a signal to cut off the gas supply to the burner, while a visual and audible alarm located on the combustion system control panel is activated so that the operator is alerted and can reset the system.
Effective insulation is an absolute must for these processes, and most designs incorporate a 30cm thick castable refractory on the floor, and the same thickness of NUTEC’s proprietary ceramic fiber MacroModules on the walls. MacroModules for this application have a density of 190kg/m3.
NUTEC Bickley elects to use MaxWool® HTZ grade of fiber, which is composed of long, flexible, interwoven fibers manufactured through a spun process, creating durable, strong, and lightweight MacroModules. With the selected thickness, it is possible to achieve cold-face temperatures low enough to prevent damage to the external steel parts of the furnace and to eliminate any danger points for personnel operating the equipment. A port is usually installed in the furnace wall to facilitate observation of load and flame.
