Heating Elements Dictate Life-Cycle Performance and Operating Costs of Electric Furnaces and Ovens
March 3, 2009
In this conversation, Custom Electric's Bob Edwards and Vic Strauss answer several important questions about electric heating elements. Custom Electric designs and manufactures original-equipment and replacement heating elements for electric thermal-processing equipment. Their products are used worldwide in heat treating, die casting, aluminum and steel primary-metals processing, forging, curing and related applications.
What do heating elements add to furnace operating costs?Bob Edwards, president of Custom Electric Manufacturing Co.: Energy consumption, atmosphere generation, racks and baskets are lead items on the list of electric-furnace operating costs. Another item on that list should be electric heating elements.
Because heating elements impact all aspects of thermal processing, it is difficult to precisely estimate what portion of total furnace operating costs they represent, but we have been working on a profile. With new large furnaces, we estimate 10% of operating costs can be attributed to heating elements. With new small furnaces, the number increases to 40%. With older large furnaces, the number is 60%, and with older small furnaces the number can be as high as 85%. Though not definitive, these numbers indicate a pattern and support our belief that more attention to heating-element selection and maintenance can significantly reduce electric-furnace operating costs.
What changes have there been in element usage?BE: One of the biggest changes I have seen in the heating-element business over the past 30 years is that many of the old rules-of-thumb no longer apply. We no longer take for granted that heating elements will self-destruct every three to six months. Furnaces do not have to be shut down for weeks or months at a time to replace elements. Higher furnace operating temperatures do not necessarily mean shorter element life. Today, heating-element suppliers, like Custom Electric, have products to address all of these issues.
The other change taking place relates to how furnace users view heating elements. Not too long ago, heating elements were simply perishable tools, an expense of doing business. Today, more companies see heating elements as an opportunity to not only reduce furnace operating costs but also as a way to improve furnace performance, shorten process cycle times, reduce maintenance costs, reduce downtime, improve the plant environment and eliminate potential quality problems. Captive heat treaters have taken the lead in looking at long term or life-cycle costs, but commercial heat treaters are fast catching up. These companies recognize the least-expensive heating element is not necessarily the most cost-effective one.
What changes have there been in element design?Vic Strauss, a Custom Electric vice president with 21 years of element design experience: The fundamentals of designing electric heating elements have not changed all that much. Wattage, wire diameter and coil length are still dictated by Ohm’s Law and standard circuit formulas. From a design standpoint, I would say the increased use of bayonet heating elements – particularly bayonet elements used in combination with radiant tubes – has had the greatest impact on minimizing downtime related to element replacement.
What changes have there been in element alloys?VS: From a materials standpoint, furnace temperature, furnace atmosphere and material costs dictate where we specify graphite, silicon-carbide or molybdenum-disilicide elements, pure molybdenum or tungsten elements, or resistance alloys containing varying amounts of iron, chromium, nickel and aluminum. We have a lot of materials to work with. That said, if we focus on industrial furnaces, 70/30 nickel/chromium, 80/20 nickel/chromium and iron/chromium/aluminum alloys available under a number of trade names account for the majority of our shipments. The other products we strongly recommend today are Kanthal APM powder-metal alloys (Table 1).
In applications like carburizing, APM elements and tubes can have a 4:1 service-life advantage over nickel/chromium. Because they contain no nickel, pricing is also less volatile. As heating-element suppliers, we are surprised more companies do not take advantage of these products. They are not new. Iron/chromium/aluminum alloys have been around for 70 years. What is new is we now know a lot more about how these alloys perform in different furnaces, different atmospheres and different applications because there are more electric furnaces in service today.
What is life-cycle economics?BE: In the past, plant managers and maintenance supervisors simply ordered replacements when elements failed and concentrated on getting the furnace back into production. Today, they track element life by furnace, process and workload. They track downtime and lost production related to element replacement. They track in-house and outsourced maintenance costs. Then, they project these costs forward five to 10 years. You only need look at a few examples to get a sense for how heating elements impact all aspects of furnace operations.
Example 1: Corrugated 330 stainless steel heating elements in eight austempering furnaces were replaced with APM iron/chromium/aluminum bayonet elements installed in APM radiant tubes. Element life increased from an average of 30 weeks to 208 weeks. More importantly, furnace cycle times were reduced by 50%. Over four years, the end user reported additional capacity related to shorter cycle times and less maintenance downtime equivalent to nine additional furnaces, which would have cost more than $4 million.
Example 2: When rebuilding a gas-fired batch furnace, a Minnesota plant replaced silicon composite outer tubes with Kanthal APM-alloy radiant tubes. The initial cost of APM tubes was higher – between the price of silicon-carbide tubes and nickel/chromium tubes. The APM tubes had a four-year service life, however, compared to one year for composite tubes. This eliminated three furnace shutdowns, hundreds of hours in lost production and thousands of dollars in maintenance charges.
Example 3: In a captive heat-treating plant, iron/chromium/aluminum elements provided seven times longer service life than the elements replaced in eight carburizing furnaces. Over four years, downtime attributed to element replacement was reduced to 1% from 28%, and production capacity increased by the equivalent of 13 furnaces.
Example 4: Not long ago, we were involved in rebuilding a gas-fired integral-quench furnace. In addition to a new combustion system, the company replaced nickel/chromium tubes with Kanthal APM tubes. Tube life more than tripled. Reduced downtime allowed the new combustion and control system to perform at a level that otherwise would never have been possible.
How do furnace builders, users and element suppliers interact?VS: Furnace builders specify power and zoning parameters along with their suggested choice of elements for common heat treatments performed by their equipment. End users focus more on specific applications and costs. Element suppliers make recommendations based upon input from the furnace builder and the end user based on their years of experience. Furnace setup and mission frequently change, so a straight one-for-one replacement element often is not in the best interest of the customer.
It is estimated that 10% of furnaces under 10 years old continue to operate with original-equipment-style elements. In furnaces from 10-25 years old, Custom Electric would probably recommend elements of a different design or alloy 20% of the time. With furnaces in service longer than 25 years, we would probably recommend upgrading heating elements 100% of the time.
What can be done to reduce heating-element expenditures?BE: There are many things a furnace owner can do to manage heating-element costs. Most involve little time or expense. Not stocking spare elements or having a formal element replacement program in place is probably the number-one cause of prolonged, unscheduled downtime. Larger element suppliers, like Custom Electric, stock different alloys so that in emergency situations, they can ship replacement elements in 24 to 48 hours. Because most elements are manually formed and assembled, however, lead times can easily stretch to a week or more, depending on circumstances.
In many situations, particularly with older furnaces, updating the layout and positioning of elements to create heat zones is another way to improve furnace performance and extend element life. Something as simple as installing extra thermocouples in critical areas of a furnace to obtain actual working temperatures can also yield significant gains.
VS: When I encounter instances of premature element failure, 90% of the time it has nothing to do with the design of the element. All too often, we encounter elements operating at temperatures two and even three times higher than what they were designed for. Continuously operating elements at or above their maximum designed output can seriously reduce element life.
I also look for contamination from upstream operations like machining, quenching and part washing when investigating the cause of premature element failure. Simple changes in cutting oils or die lubricants can disrupt furnace performance because elements don’t react well to sulfur, phosphates and carbon. Other common problems we encounter include:
- Contamination, even oil on the gloves of maintenance personnel, can damage the oxide barrier on metallic elements
- Not running a scheduled burnout or oxidation cycle to eliminate contaminants
- Improper element installation
- Non-operating furnace fans
- Debris buildup on furnace hearths
Are element suppliers keeping up with electric-furnace usage?BE: Heating-element manufacturers have the technology to allow electric furnaces to operate at efficiency levels beyond what are commonly achieved. With more electric furnaces in service, we know more about how different element and tube materials perform in different situations. We are working closer than ever with furnace builders and furnace end users to optimize heating-element performance. Today, most of our heating-element recommendations are based on an application, not a furnace design. Therefore, the answer to your question is yes. Heating-element manufacturers like Custom Electric can support the growth in electric furnace heating. IH
For more information: Contact Bob Edwards, president, or Vic Strauss, vice president, at Custom Electric Manufacturing Co., 48941 West Rd., Wixom, MI 48393; tel: 248-305-7700; fax: 248-305-7705; e-mail: email@example.com; web: www.custom-electric.com.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: heating element, Ohm’s Law, silicon carbide, molybdenum disilicide, powder metal, austempering, radiant tube, integral quench