Heating elements do degrade over time due to several factors including embrittlement, contamination, corrosion, damage to the oxide layer, formation of hot spots, and oxidation. These factors can lead to reduced efficiency, increased risk of failure, and ultimately, the need for replacement.
Embrittlement Due to Grain Growth: Heating elements containing iron tend to form large, brittle grains when subjected to high temperatures. This makes the elements very brittle and prone to rupture, especially when cold. This condition can be exacerbated by improper handling or sudden jerks, leading to premature failure of the element.
Contamination and Corrosion: Elements can be contaminated by gases in the furnace atmosphere, fumes from fluxes used in brazing, or oil fumes from heat-treated components. These contaminants can cause dry corrosion on the elements, further degrading their integrity and performance. The most critical factors affecting element life in such conditions are the temperature of the hottest point and the ratio of intermittent to continuous working.
Damage to the Oxide Layer: The oxide layer on heating elements is crucial as it prevents further oxidation of the inner metal. However, this layer can be damaged during shutdowns, exposing fresh metal to oxidation. To prolong the life of the heating element, it is recommended to avoid frequent shutdowns once the machine is turned on.
Formation of Hot Spots: Hot spots are areas in the heating element that are at a higher temperature than the rest. These can be caused by local oxidation reducing the element's cross-section, increasing resistance and heat locally, leading to breakdown. Other factors include improper support, high element temperature, and wrong fuse material selection, which can cause sagging and warping, leading to uneven spacing and potential shorting of adjacent sections.
Oxidation and Intermittency of Operation: Continuous and tenacious oxide scale forms on the surface of the heating element at high temperatures. However, frequent heating and cooling cycles can cause this scale to crack and flake off, exposing fresh metal to oxidation and leading to increased local oxidation and the formation of hot spots.
Maintenance and Replacement: Regular maintenance is crucial to prevent premature failure of heating elements. This includes monthly inspections for cracking and sagging, annual replacement of thermocouples, and periodic replacement of insulation and seals. Heating elements should only be replaced when they fail, and individual elements can be replaced rather than the entire set, which is more cost-effective.
In summary, heating elements degrade over time due to physical and chemical changes induced by their operating environment and usage patterns. Proper maintenance and operation can significantly extend their lifespan and ensure efficient performance.
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