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Show 2 reveal important trends and design conditions for the furnace, and to identify means of optimizing furnace performance and productivity. Recently a new indirectly-fired heat treating furnace, called low inertia furnace (LIF), has been proposed [2]. In an operation of a batch-type furnace such as the LIF, the basic design problem is not only to optimize the thermal performance of the furnace, but also to insure the uniformity of heat transfer to the load at high energy efficiency. The pilot LIF developed appears to meet these challenges. The LIF is an indirect heating furnace in which either the gas-fired radiant tubes or electric resistance heaters have been replaced with new high-efficiency and low emission flat radiant heaters (FRH). The FRHs developed are compact and can readily be installed in the sidewalls, roof, and if desired, in the furnace hearth. The heaters provide for complete combustion of natural gas with low excess air, high level of flue gas heat recirculation, high temperature uniformity of the load surface, complete isolation of the combustion products from the furnace atmosphere and low N O x levels in the combustion products [2]. A schematic diagram of a LIF equipped with flat radiant heaters with the stock placed at the bottom of the furnace is depicted in Fig. 1(a). The flat radiant heater panels are placed on the sidewalls and the roof of the furnace. The various modes of heat transfer in the furnace are illustrated in Fig. 1(b). Fuel (natural gas) and preheated air introduced burn and the flue gases are exhausted. The high temperature combustion products transfer a part of their energy to the panel wall by convection and radiation, and a part of the sensible energy leaving the flat radiant heater exhaust is recovered in a recuperator. The high temperature heater wall then radiates thermal energy to the load, the refractory walls and neighboring heaters in the furnace. Part of the incident energy on these surfaces is absorbed and the rest is reflected back into the furnace enclosure. The absorbed energy raises the temperature of the surface which then radiates to the other surfaces in the enclosure. Buoyancy induced or forced (i.e., circulating fan) motion of the gases in the furnace help to transfer and distribute heat by convection to the various surfaces of the enclosure and load. The net radiative and convective heat transfer rates at a surface are then balanced by conductive through the refractory walls and lost to the ambient air and surroundings by convection and radiation, respectively. The net heat input to the load is distributed within by conduction in the stock or by conduction/convection/radiation if the load is a basket filled with small metal parts. It is appropriate to note that there is no published literature on mathematical modeling of LIF. Combustion and heat transfer in the flat radiant heater have been studied experimentally [2] but not theoretically. For the sake of completeness, reference is made to Ramamurthy et al. [3] for the discussion of heat transfer in indirecdy-fired radiant tube furnaces. This paper presents a mathematical model for simulating the dynamic thermal performance of a LIF coupled to the load placed in the furnace. The intent is to aid the furnace designer or user in making rational predictions of spatial and temporal temperature distributions in the load, deciding on the fuel-firing strategy in F R H s for optimum performance of a given furnace and through the simulation assist in commercialization of the LIF concept. The model has been simplified through several reasonable assumptions without considering the complexity of the physicochemical processes occurring in the FRHs. In the analysis the heaters have been represented by flat heat sources having uniform average temperatures or heat fluxes on their outside surfaces. Efforts are underway to include turbulent mixing, combustion and heat transfer processes in the heaters in an advanced and complete furnace system model. The local dynamic heat transfer rates at the load surface, temperature in the load and thermal efficiencies are predicted, and parametric calculations are performed to identify the most important factors influencing the furnace performance. |
