Metal Heating Elements

Metal heating elements are key components in industrial electric heating furnaces. Their structure, installation method, surface load, and operating temperature directly affect furnace heating efficiency, temperature uniformity, service life, and operating safety.

The selection and arrangement of metal heating elements should not be based only on power. It must also consider furnace type, furnace temperature, furnace atmosphere, material properties, element support method, oxidation resistance, creep resistance, and maintenance cost.

Types and Installation of Metal Heating Elements

The combination and installation of metal heating elements are determined by their type, shape, and the structure of the furnace. In wide and flat furnaces, if the heating element has sufficient mechanical strength, it can be fixed on the furnace roof. When the furnace temperature is lower than about 870°C, the heating element may also be placed below a heat-resistant alloy furnace bottom.

In most industrial furnaces, metal heating elements are arranged on the side walls. This layout is convenient for heat radiation, installation, maintenance, and replacement.

Cast metal heating elements are usually manufactured in sections. Each section acts like an independent frame. During casting, small defects must be avoided because these defects are prone to local overheating and oxidation. Some tiny defects are difficult to detect from the appearance, especially when they are distributed near pores or non-metallic inclusions.

Compared with rolled strip heating elements, cast heating elements cannot be made very thin. However, this also gives them better resistance to mechanical disturbance and chemical corrosion. Because cast elements have good rigidity, they allow high-speed forced circulation of furnace gas. Each section is generally suspended with hooks for easier installation and maintenance.

Supporting Methods for Heating Elements

The support of a heating element may be made of ceramic material or a metal material similar to the heating element itself. For furnaces with a shallow chamber, heating elements are often fixed on the furnace roof. In some designs, resistance strips or wires are installed in one or several rows on the same vertical plane.

To improve radiation heat transfer toward the furnace bottom, heating elements should not be suspended too low. For safety, a certain distance should be kept between the heating element and the furnace bottom.

A multi-row arrangement has several advantages. The thermal stress is lower, and even if scale deposits cause partial short circuiting near the furnace bottom, only the lower row of heating elements needs to be replaced. Since each row has a smaller height, bending caused by uneven heat transfer at the lower part of the element is also reduced.

Mo-Si heating elements are also commonly installed by suspension. Each U-shaped heating element is usually hung in a groove. Due to furnace lining limitations, long heating elements have a lower maximum allowable operating temperature.

Al-Cr-Co-Fe heating elements soften when they slightly exceed the specified temperature, so they are often placed on refractory bricks or heat-resistant alloy supports. In this installation method, the element must be prevented from sliding out of the furnace, and overheating during rapid heating must also be avoided.

Capacity of Metal Heating Elements

The capacity of a heating element is equal to its reasonable maximum surface load, usually expressed in watts per square meter or watts per square inch. The word “reasonable” is important because the capacity of a heating element is not a fixed value. It varies with many factors.

The main factors affecting heating element capacity include:

1. Service life of the heating element
2. Initial investment cost
3. Loss caused by furnace shutdown or production interruption
4. Material of the heating element
5. Supporting method of the heating element
6. Furnace temperature and furnace atmosphere
7. Furnace type, such as batch furnace or continuous furnace

Therefore, it is difficult to determine heating element capacity only by using a simple formula or fixed table.

The final heating temperature of the workpiece is a decisive factor. For example, if the workpiece only needs to be heated to 650°C, it is unreasonable to design the heating element temperature at 1200°C. This would increase energy consumption and shorten element life.

Surface Load and Furnace Temperature

To prevent accidents caused by improper use, heating element manufacturers usually provide technical charts and instructions. These references are useful for the design and operation of electric heating furnaces.

For common Ni80Cr20 heating elements, the chart usually shows the relationship between furnace temperature, surface load, and heating element temperature. The horizontal axis represents furnace temperature, while the vertical axis represents the surface load of resistance wire or strip under free radiation conditions. The curves indicate how much higher the heating element temperature is than the furnace temperature.

In general, the higher the furnace temperature, the lower the allowable surface load of the heating element. When the furnace temperature is above about 1040°C, some common heating elements are not recommended. When the furnace temperature is around 700°C, the heating element temperature can be designed higher because radiation is weaker at lower furnace temperatures.

The chart also reflects the combined effects of creep and oxidation. Where stress is highest, creep is also most serious. Thin or weakened areas of resistance wire or strip are more likely to fail. These thin sections operate at higher temperatures, oxidize faster, and eventually cause rapid failure of the heating element.

Heating Element Layout and Service Life

For nickel-chromium heating elements used in non-oxidizing atmospheres, similar technical charts may also be used. In many cases, the reasonable surface load can be about 50% higher than that used in oxidizing atmospheres.

If the goal is to extend the service life of the heating element, it is usually better to use about 80% of the recommended value from the technical chart.

Inside the furnace, if the spacing between heating elements is too small, nearby elements will influence each other and cause local overheating. The denser the arrangement, the greater the additional surface load on the heating elements. As a result, service life may be reduced.

The electrical power input to the heating element should equal the heat required by the furnace, including the heat absorbed by the workpiece and the heat lost during operation. When calculating one or more heating elements, many factors should be considered, such as cross-sectional area, surface area, length, spacing, voltage, and the furnace wall area covered by the heating elements.

For strip heating elements, calculation can be simplified by keeping the spacing uniform. The spacing is often set as a multiple of the strip width. When this ratio is constant, the watts per square meter of strip surface can be converted into the kilowatts per square meter of furnace wall covered by the heating element.

Based on the heat required per unit time and the allowable surface load, the required furnace wall area covered by heating elements can be calculated. In many cases, this area is about three-quarters of the total furnace wall area.

Conclusion

The design of metal heating elements must consider much more than power rating. Furnace temperature, furnace atmosphere, element material, support method, installation layout, surface load, oxidation resistance, creep resistance, and service life all affect performance.

A reasonable heating element design can improve heating uniformity, reduce local overheating, slow oxidation and creep, extend service life, and improve the stability of industrial electric heating furnaces.