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Temperature control

Release time:

Aug 26,2025

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Time dependent Temperature Control

 

Raising the outer surface of the material to a given temperature and maintaining that temperature essentially constant is a relatively simple task. Determining the time required for the coldest part of the material to reach a desired temperature is more difficult, as this requires finding the point where the temperature rises slowest and then placing a temperature measuring instrument there. Determining the slowest heating point is largely a matter of guesswork. Placing a pyrometer at this point is easy for some workpiece shapes. For workpieces of typical shapes, the instrument is likely to be damaged during loading and unloading. 

 

When heating large parts, the slowest heating point is located internally and inaccessible. It is more convenient to measure and control the furnace temperature rather than the charge temperature. The furnace temperature is measured by the temperature of the inner wall of the measuring device's protective tube. This tube is closed and protrudes a short distance from the furnace roof or side wall. The tube is exposed to radiation from the furnace roof and walls, as well as from the resistor, the radiant tube, and the furnace gases. The tube's outer shell, in turn, radiates heat to the charge. While the charge is being heated, the furnace temperature is at some indeterminate "intermediate" temperature. Near the end of the heating process, the charge surface reaches a temperature close to that of the furnace roof, walls, and gases. This process is shown in Figure 105. The location of the pyrometer is important. It is best to place it on the charge so that it reaches the highest temperature first.

 

Temperature control

 

The pyrometer should be located near the point where the final temperature is reached. It should not be installed in the burning fuel stream, near heating elements, or near a furnace door that is frequently opened. When the temperature curve becomes horizontal, the outer surface of the charge has reached the desired temperature. However, as previously noted, this does not indicate the temperature at the slowest point. If a pyrometer is not installed at that point, the following method is often used: a test run is performed with standard charge. 

 

The pyrometer at the "slowest point" will indicate the time lag (the time difference between the flattening of the furnace temperature curve and the end of the temperature rise at the "slowest point"). This data can be applied to the same or similar charge types. The test run may require drilling a hole in the charge to allow the pyrometer to be inserted. For unusual charges, the calculation method described in the previous volume can be helpful when using test run data. In furnaces where the charge is continuously moved through the furnace, the furnace temperature is controlled so that the outer surface of the charge reaches the desired temperature at the hot end of the furnace. The temperature uniformity within the heated charge depends on the time it remains in the furnace. The same applies to a furnace that is always fully loaded (a piece of material is taken out and immediately replaced). The number of control points and the ease of control when using various fuels are also important.

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Application of penetrating heating

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Aluminum melting rotary furnace

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Application of penetrating heating

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Aluminum melting rotary furnace

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