Temperature Control Over Time

Raising the outer surface of a workpiece to a specified temperature and keeping that temperature basically constant is relatively simple. However, determining how long it takes for the coldest part of the material to reach the required temperature is much more difficult. To do this, it is necessary to identify the point that heats up the slowest and then place the temperature-measuring instrument at that point.

In most cases, the slowest-heating point can only be estimated. For some workpiece shapes, placing a pyrometer at this point is relatively easy.

For workpieces with ordinary shapes, however, the instrument may be damaged during furnace loading or unloading. When large workpieces are heated, the slowest-heating point is often inside the material and cannot be reached directly.

A more practical approach is to measure and control the furnace temperature rather than the material temperature. Furnace temperature is usually represented by the temperature of the inner wall of the protective tube of the measuring device. This closed tube extends slightly from the furnace roof or side wall.

The tube receives radiation from the furnace roof and walls, as well as from heating elements, radiant tubes, and furnace gas. At the same time, the tube shell also radiates heat toward the charge. While the material is being heated, the furnace temperature is therefore an uncertain “intermediate” temperature. Near the end of the heating process, the outer surface of the material reaches a temperature close to that of the furnace roof, furnace walls, and furnace gas. This process is shown in Figure 105.

The position of the pyrometer is important. Ideally, it should be installed near the point where the material first reaches the final required temperature. It must not be placed in the flow of burning fuel, near heating elements, or close to furnace doors that are frequently opened.

When the temperature curve becomes horizontal, it indicates that the outer surface of the material has reached the required temperature. However, as mentioned earlier, this does not indicate the temperature at the slowest-heating point.

If no pyrometer is installed at that point, the following method is often used: conduct a trial run with a typical workpiece. A pyrometer placed at the slowest-heating point will show the time lag, that is, the time difference between the point when the furnace temperature curve levels off and the point when the slowest-heating location finishes heating. This data can then be used for the same or similar types of materials.

During the trial run, it may be necessary to drill a hole in the material so that the pyrometer can be inserted. For workpieces with unusual shapes, the calculation methods given in the first volume can be used as a reference when applying trial-run data.

For continuous furnaces, where materials pass through the furnace continuously, furnace temperature is controlled so that the outer surface of the charge reaches the specified temperature at the hot end of the furnace. The temperature uniformity inside the heated material depends on how long it remains in the furnace.

The same principle also applies to furnaces that are always fully loaded, where one workpiece is removed and another is immediately loaded. The number of control points and the difficulty of temperature control when using different fuels will be discussed at the end of this chapter.