Categories: Silicon Carbide
Published 30 Sep 2020

The choice of heating element has a significant impact on the overall energy efficiency of a furnace, but it is not the only factor. In fact, if other considerations are not taken into account, the savings achieved from an energy-efficient element can quickly disappear.

“There is a lot that manufacturers can do to make heating elements that are more efficient, but this will not always translate into energy savings in the furnace,” says Dean McCabe, Technical Support Manager, Kanthal. “We have done a lot of testing, both in the laboratory and out in the field, to make our elements more efficient, and in our experience the level of energy savings can vary greatly from application to application.”

Heat transfer and power ratios

A key consideration when it comes to heating elements is the ratio between the power dissipated in the hot zone of the element (the section that is inside the furnace) and the power dissipated in the cold end (the section that passes through the furnace walls). In some elements, the ratio can be as high as 60:1. However, in others it can be lower, resulting in a higher percentage of element power being dissipated in the cold end part of the element.

“To make a more energy-efficient heating element, the power ratio is what you would look to try and change,” McCabe says. “As a furnace operator, looking into energy-efficient elements is a good starting point and could generate savings. But it’s important to remember that it is not the only thing you should be looking at.”

Other factors affecting energy consumption

Other key considerations include the quality and standard of the insulation used in the furnace. If it has deteriorated or is in need of repairs, then the amount of heat escaping could be driving energy consumption up. The furnace’s surrounding environment is also important factor. Open doors or windows in the vicinity can affect convective heat transfer and have a major effect on furnace energy consumption.

“We saw this in the field,” McCabe says. “A customer in the automotive industry operating a very large heat treatment furnace saw large fluctuations in energy demand. Investigations revealed that this increase in demand coincided with their loading bay doors being opened and closed. This caused a huge change in airflow in the factory, increasing convective heat transfer away from their furnace. That’s how sensitive furnaces are to their surroundings.”

Another reason why an energy-efficient element might not necessarily deliver energy savings can be because in some cases the energy dissipated in the cold end of the element is contributing to the overall energy balance of the furnace. By reducing the cold end power, the hot zone has to deliver more, and therefore there is no net energy savings.
“The power dissipated by the cold end is not all wasted,” says Dean McCabe. “Some of that energy contributes to heating up the furnace and keeping the furnace at set-point temperature.”

No one-size-fits-all solution

In Kanthal’s experience, a number of furnaces have seen significant energy savings after installing its Globar® SR heating elements. Customers in some cases have reported savings of around 15 percent. This is achieved by designing lower-resistance cold ends, which improve the element’s overall power ratio. However, that does not necessarily mean that every furnace will see the same savings. The customers that typically benefit most from Globar® are large-scale operations with multiple furnaces, high power applications or furnaces using elements with long cold ends.

“It’s very difficult to say that a certain element will reduce energy by a set amount, because in our experience it will only be true for a specific set of operating conditions,” McCabe says. “We can produce energy-efficient elements but if all other factors, such as insulation and the environment, are not taken into consideration, it could still prove ineffectual. If you operate a furnace, certainly you should be considering energy-efficient elements, but you need to keep in mind that it is just one part of the jigsaw for your furnace.”