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Heat treatments, such as steel tempering, are temperature-controlled processes. It allows ferrous steel to stabilize its structure after the heat treatment and quenching stages. The tempering temperature also determines the hardness of the steel, preferably to its optimum working strength. In a tempering furnace, a heat-resistant fan is commonly employed to generate moderate gas circulation to obtain adequate temperature homogeneity and heat transfer. Nevertheless, there is a tradeoff because the overall thermal efficiency is expected to reduce because of the high rotating speed of the fan. Therefore, this study numerically investigates the thermal efficiency changes of an electric tempering furnace due to changes in the rotating speed of the fan and the effects on temperature homogeneity and the heat transfer rate to the load. Heat losses through the walls were calculated from the external temperature measurement of the furnace. Four different speeds were simulated: 720, 990, 1350, and 1800 rpm. Thermal homogeneity was improved at higher rotating speeds; this is because the recirculation zone caused by the fan improved the flow mixing and the heat transfer. However, it was found that the thermal efficiency of the tempering furnace decreased as the rotating speed values increased. Therefore, these characteristics should be modulated to obtain a profit when controlling the rotating speed. For example, although thermal efficiency decreases by 20% when the rotating speed is doubled, the heat transfer rate to load is increased by up to 50%, which can be beneficial in decreasing the process of tempering times.

Numerical Simulation of the Flow and Heat Transfer in an Electric Steel Tempering Furnace