Abstract
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.
Highlights
Industrial furnaces have become vitally important equipment since they are involved in the production of many consumer products, such as food, beverages, containers, machining tools, infrastructure materials, amongst other applications [1]
We focus on the temperature and fluid dynamic fields inside the furnace, as well as the heat transfer behavior and efficiencies when the speed is changed
It can be seen that the global thermal behavior of both chambers is well captured by the numerical calculations: the highest temperatures are obtained at the metal coils, the heat transfer is forced by the fan from the higher temperature zone to the lower temperature zone, and the thermal homogeneity occurs in the process chamber
Summary
Industrial furnaces have become vitally important equipment since they are involved in the production of many consumer products, such as food, beverages, containers, machining tools, infrastructure materials, amongst other applications [1]. Non-uniform temperature distribution inside the heating chamber may lead to low-quality products, thereby increasing production costs. For example, the quality of cooking can be controlled by keeping the uniformity of the temperature in the oven [9] In this case, it was shown that the heating coil placement on a side position guaranteed the homogeneity of the temperature inside the furnace. The achieved improvement was only about 0.1 K, which can be considered to be a very low value These studies recommended the use of CFD to create geometric prototypes that guarantee the uniformity of temperature in convection ovens. In another study [12], an electric nitriding furnace was simulated using a commercial CFD package and velocity uniformity was evaluated using a spatial criterion based on the mean and actual values of the velocity magnitude. Taking into account the fact that internal velocities and temperatures fields inside the furnace are difficult to directly measure, the numerical approach allows for the identification of the main phenomena governing the thermal and fluid behavior and could contribute to the furnace design for process improvements
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