Continuous furnaces

Whether it’s annealing and galvanizing lines or a roller hearth furnace, electric heating can make the process cleaner and more efficient, while greatly reducing maintenance and repairs. Krister Wickman, Global Product Manager, Kanthal, explains how.

Postproduction heat treatment of steel is vital for the quality of the final product, but the process itself consumes a lot of energy and generates substantial amounts of CO2.

“These furnaces are typically heated with W or U tubes, where each tube is fired with a gas burner, sometimes even two,” explains Krister Wickman. “Some furnaces can be up to 20 meters high and hundreds of meters long and require hundreds of tubes and burners. That generates a lot of flue gases and exhaust, which is released into the atmosphere.”

Gas-burner challenges

These tubes are typically designed to lengthen the time the gas takes to pass through, and thereby maximize the amount of heat that is released into the furnace chamber. Gas is often preheated prior to ignition as well in order to further improve efficiency, yet even with these measures thermal efficiency is still poor at around 50 to 60 percent.

Furthermore, the design of the W and U tubes can cause them to slump and bend, which dramatically shortens their service life. In the preheating section of the furnace, this means they often have to be replaced about every two years.

“In addition, because these tubes are usually made from NiCr (nickel-chromium), oxide spoilation will form on the inside, which needs to be removed at regular intervals,” Wickman explains. “So the maintenance work is continuous, with constant repairs and adjustments being made all the time.”

A beneficial solution

By replacing the gas burners with electric heaters, steel producers can increase thermal efficiency to over 90 percent. This would also eliminate all exhaust and flue gases, and effectively reduce CO2 emissions to zero.

In addition, a lot of the service life and maintenance issues can be solved by using Kanthal^® APM/APMT instead of NiCr.

“What we suggest is replacing a W tube with gas burner with a suitable number of straight Kanthal^® APM or APMT tubes with Tubothal^® heating elements inside,” Wickman says. “Because you have support at both ends of the tube, and because the material is stronger, you will not experience any slumping in the tube. And because Tubothal^® is made from Kanthal^® APM wire, there will be no spoilation of oxide inside.”

Customized solutions

In fact, a heating system comprising Kanthal^® APM/APMT tubes and Tubothal^® heating elements will provide a reliable and completely maintenance-free operation.

“By working together with Kanthal, we can devise a customized solution that will last a very long time. There will be no need for maintenance work, no tube bending – it will just be allowed to run.”

Krister Wickman, Global Product Manager, Kanthal

Five reasons for using electric heating in continuous heat treatment furnaces

By installing Tubothal^® heating elements combined with APM^® tubes in their continuous heat treatment furnaces, steel producers can achieve a number of benefits. Here are the top five:

Zero emissions

By electrifying the heating, all the exhaust and flue gases associated with gas burners are eliminated. This means CO2 emissions are reduced substantially, and if the electricity used is from a renewable source, emissions will be virtually zero.

Over 90 percent thermal efficiency

Despite measures such as gas preheating, the overall thermal efficiency of gas burners is poor at just 50 to 60 percent. However, with electric heaters, since virtually all of the heat is released into the furnace chamber, thermal efficiency is well over 90 percent, reducing energy consumption and costs.

Longer service life

Over time, the middle sections of the W and U tubes typically used in continuous heat treatment furnaces will tend to drop and sag, especially in the preheating section. This means they generally have to be replaced every two years. However, this issue can be eliminated by using straight Kanthal^® APM or APMT tubes instead of one W tube. The extra supports, combined with the superior strength of the Kanthal^® APM material, will allow the tubes to be used for years to come.

Uniform temperatures

Tubothal^® offers a far higher level of precision in temperature control compared with gas burners. It also distributes a uniform heat across the entire length of the tube, which ensures higher quality and yields in the end product.

Less maintenance

Unlike NiCr (nickel-chromium) tubes, Kanthal^® APM offers very good oxidation in addition to high corrosion resistance. If combined with a Tubothal^® electric heating element, because of its Kanthal^® APM wire, there will be no oxide spoilation inside. This means they will not require the same high levels of maintenance needed for NiCr.