Luoyang Judian Metal Hot Processing Equipment Co., LTD is mainly engaged in the manufacture of complete sets of equipment in the metal thermal processing industry and the integration of the entire production line.
The title of this chapter is not entirely accurate, because what actually needs to be controlled is the temperature of the furnace charge, or workpiece, rather than the temperature of the furnace itself. However, this title is retained because it has been widely used.
From the metallurgical point of view, the purpose of temperature control is to make the surface of the charge reach a specified temperature and maintain that temperature until the required temperature uniformity is achieved inside the charge. In some cases, it also means making the charge temperature change according to a specified heating and cooling cycle.
In both cases, temperature uniformity throughout the furnace is required. The inside of the charge should reach the specified temperature without exceeding it. The reason is simple: for every process, such as rolling, forging, bending, extrusion, annealing, or heat treatment, and for every material, there is a minimum temperature required for the process to proceed smoothly. If the temperature greatly exceeds the required value, it not only wastes heat but can also cause harmful effects such as excessive scale formation, decarburization, and operating difficulties.
Temperature Control in Different Areas of the Furnace
If every exposed surface of the charge receives the same amount of heat per unit time, temperature uniformity has been achieved inside the furnace. This statement mainly applies to batch furnaces. In continuous furnaces, the temperature distribution across the width of the furnace must also be uniform.
It should also be noted that, in some processes, complete furnace temperature uniformity does not always produce the best result. One example is an expanding mill. When a long tube reaches the mill, all parts of the tube should be at the same temperature. Therefore, when the steel tube leaves the furnace, its rear end must be slightly hotter than its front end, because the tube rolls out from the side of the furnace.
True uniformity inside a furnace is difficult to achieve through automatic control alone. It is the result of good furnace design and proper operation.
Factors Affecting Furnace Temperature Uniformity
The following factors affect temperature uniformity inside the furnace:
Arrangement of electric heating elements or radiant tubes
Temperature uniformity of each radiant tube
Burner layout
Burner type, such as luminous flame or transparent flame
Speed and direction of combustion products that create circulation
Forced circulation
Height of the burners and furnace roof above the upper surface of the charge in high-temperature furnaces
Heating passages
Flue outlets
Muffle wall arrangement
Operational factors also matter, including charge placement, burner adjustment, and shutting off certain burners during the heating process.
In some heating processes that do not require a furnace, temperature uniformity is easier to obtain. For example, if electric current passes through a bar with a constant cross section, the temperature rise along the length of the bar is the same except at the ends. The same effect can also be obtained when heating is produced by induction from alternating current in a coil surrounding the bar.
Many design factors that promote furnace temperature uniformity have been discussed in topics such as muffle furnace capacity and gas movement. Other related factors are also discussed in the section on furnace design. The following points focus on additional influencing factors.
In a high-temperature furnace, if the burner is located higher above the furnace bottom, the heat flow entering the charge is usually more uniform than when the burner is placed close to the charge. However, in high-temperature furnaces, installing only one high-position burner is rarely sufficient.
Lower-Fired Furnaces
In a lower-fired furnace, the combustion chamber is located below the furnace bottom. It is usually a manifold with several outlets. The outlets for combustion products on the manifold are called ports. These ports can be made relatively large, and their correct size is usually determined by using shaped bricks or special refractory blocks.
To obtain uniform temperature over a wide heat-load range, the ports must be small compared with the cross section of the combustion chamber. If the manifold area is too small and the velocity of the combustion products becomes too high, the pressure at the far end may become higher than that at the burner end, and weak negative pressure may occur. If the ports are adjusted to offset this pressure difference, the balance may be lost again when fuel consumption is low.
In a large combustion chamber, recirculation can almost eliminate this pressure difference. As a result, the temperature can remain uniform both under high heat load and under partial-load operation.
In some large lower-fired furnaces, many ports are arranged on the furnace bottom. As mentioned earlier, the workpieces must be placed at a certain height above the furnace bottom.
