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Bogie hearth furnaces are classified as low temperature furnace and high temperature furnace. Low temperature furnaces work within 100 – 450oC. Generally, as this temperature heat transformation can take place only by convention, air circulation is provided which circulates air from the chamber to the impeller, the air passes through heaters and reenters the chamber. This circulation continues. There is counter flow type air circulation where in air is circulated perpendicular to the hearth and enters the chamber at the bottom or either side. These are usual flow type furnaces wherein air passes across the hearth. Generally, to get uniformity the furnaces are designed to 20, 30 & 50 times the volume of circulation of the chamber per minute.
High temperature furnaces do not require air circulation as radiation starts emitting above 800oC which conveys heat throughout the chamber. In order to conserve energy, usually high quality ceramic fibers and boards are used in these furnaces. But if the furnace is continuous in nature wherein the question of switching off the furnace does not arise. Ceramic bricks of appropriate quality is used. This retains heat and incase of intermittent power supplies the parts are not subjected to huge variations.
These furnaces are used for stress relieving, normalizing heating of forging billets. They are also used for pack carburizing, annealing wherein temperature raising and casting rates are to be controlled precisely to meet the quality requirement of the material. Furnaces are designed with rate of heating and cooling by forced air circulation and venting of hot air etc.