Ceramic Industry

An Elemental Analysis

February 1, 2002
Understanding how your heating elements operate can help you improve productivity and avoid unnecessary maintenance and downtime.

SiC elements, such as the Kanthal Globar® elements shown above, are designed to deliver economical high temperature performance.
No heating element lasts forever. No matter how well built your furnace or how sophisticated your controls, the heating elements will eventually need to be replaced. A little knowledge of how heating elements operate and how they should be maintained can go a long way toward helping you improve productivity and avoid unnecessary maintenance and downtime.

Metallic elements, such as those supplied by National Element Inc., are typically the least expensive and are ideal for lower temperature (less than 2300¿F) operating conditions.

Understanding Element Types

While a number of different heating elements exist, three basic types are used in the ceramic industry: metallic (wire), silicon carbide (SiC) and molybdenum disilicide (MoSi2). Many suppliers also offer proprietary variations of these three types that offer better performance, higher temperature capabilities or other benefits compared to their standard counterparts.

Metallic, SiC and MoSi2 elements vary in temperature range, longevity and cost. Metallic elements are the least expensive in terms of upfront cost, but they also have the lowest temperature rating and, in some cases, the shortest lifespan. Additionally, the resistance of metallic elements increases with age due to the reduction in cross section by oxidation, as well as elongation of the loops. This will result in decreased power to the furnace and the ultimate failure of the element.

However, according to Steve Best, general manager of National Element Inc. (Brighton, Mich.), “there are many furnaces that use metallic elements—mainly because they cost less, they operate on less expensive line voltage, and they’re ideal for processes that require temperatures lower than 2300?F (1250?C).” Additionally, Best notes, not all metallic elements feature a short lifespan. “Many furnaces, like those in the ceramic field, are very clean, and metallic elements can last many years in clean operating environments,” he explains.

SiC elements are used most frequently in processing ceramics, and they fall within the middle range in both price and operating temperature. They can be used between 1200 and 3012?F (650 and 1650?C). Under optimum operating conditions, they can last up to five years.

According to Gary Schwartz, sales manager at I Squared R Element Co. (Akron, N.Y.), “the lifespan of an SiC element primarily depends on the furnace temperature of the element. The temperature of the element depends on the furnace temperature and the amount of power per unit area on the hot zone. At a low watt loading, the element operates cooler and therefore lasts longer.”

MoSi2 elements are the top of the line in terms of temperature, longevity and cost. They can reach temperatures up to 3452?F (1900?C), can be used in a wide variety of atmospheres and demanding applications, and can last for several years depending on your usage. MoSi2 does not age or change in resistance, so elements can be replaced one at a time as necessary. “MoSi2 elements can really give you a cost-per-part advantage because they operate at optimal temperatures with a high surface load. This improves the furnace efficiency,” says Kevin Foston, senior sales and marketing coordinator at Micropyretics Heaters International Inc. (MHI) (Cincinnati, Ohio).

Your existing furnace typically determines which type of replacement elements you can use. “If you already own a furnace, your furnace manufacturer has decided the type, size and shape of element that fits your furnace,” Foston says. However, companies that are looking to upgrade their furnace don’t necessarily need to limit their options. Many furnace and element suppliers offer engineered solutions that are custom-designed to fit specific applications.

“You need to determine what you’re hoping to achieve, then approach your supplier with that goal, as well as with all the necessary information about your process. There’s always more than one way to do a job,” says Chet Popilowski, engineering manager for Kanthal (Bethel, Conn.). “What it generally comes down to is performance vs. price—what it’s going to cost upfront, and what it’s going to cost (or save) over the long term.”

MoSi2 elements, such as the Molycoil™ from MHI Inc., can operate at temperatures up to 3452¿F (1900¿C).

Maintaining Element Quality

Companies that aren’t quite ready or willing to invest in all new elements or a major overhaul of their furnace can take other steps to ensure that their heating elements remain efficient and last as long as possible. For instance, a common practice is to replace worn-out elements with the same size elements from a furnace that is offline due to slower production schedules. Another common practice is simply to replace old elements with new elements of the same type, sort of like switching out a light bulb. However, with SiC elements, the resistance of the elements increases over time. The elements should be replaced in matched sets to avoid firing problems.

“If you have two elements in series, and both elements started out at 1 ohm of resistance, both would see the same voltage drop and would be doing the same work,” explains Schwartz. “After maybe a year or a year and a half in service, the elements might have doubled in resistance, putting each element at 2 ohms. If one of those elements fails, and you put a new element, which has 1 ohm of resistance, in series with the older element, the older element is going to be doing more of the work because it has higher resistance and because power is equal to I2R. As a result, the older element will eventually fail sooner than it would have otherwise.”

Schwartz suggests that companies instead replace both the elements in series—“but don’t throw away the good element with 2 ohms of resistance,” he says. “Save it, and when the next element fails and you need an element with 2 ohms, you’ve got one on the shelf.”

According to Popilowski, MoSi2 and metallic elements are not susceptible to failure as a result of this type of mixing and matching.

All elements, however, can benefit from a periodic check of the terminals where the elements protrude through the outside furnace wall. “Most operators neglect the space around the terminals,” says Popilowski. “These spaces are supposed to be packed to keep the heat from escaping, but over time, as the elements expand and contract, some of that packing may become loose or fall out. Operators can help themselves in many cases by periodically checking those feed-through holes and making sure that no excess heat is escaping. This will keep the terminals cool, which is important, and may prevent some maintenance issues down the line.”

The Starbar® Type SER, an SiC element manufactured by I Squared R Element Co. Inc., offers maximum performance up to 3000¿F (1650¿C).

Maximizing Synergies

Every situation is different, and one solution typically does not work in every application. But whether you’re looking to replace your heating elements, upgrade your furnace system or are simply trying to develop a better maintenance program, it’s important to remember that you’re not alone. Your furnace and element suppliers have the knowledge and expertise to solve a range of problems, and are usually more than willing to help you maximize your thermal process.

For More Information

I Squared R Element Co., Inc., 12600 Clarence Ctr. Rd., Akron, NY 14001; (716) 542-5511; fax (716) 542-2100; e-mail isquaredr@worldnet.att.net; www.isquaredrelement.com.
Kanthal, 119 Wooster St., Bethel, CT 06801-0281; (203) 744-1440; fax (203) 744-2703; www.kanthal.com.
Mycropyretics Heaters International Inc. (MHI Inc.), 613 Redna Terrace, Cincinnati, OH 45215; (513) 772-0404; fax (513) 672-3333; e-mail sales@mhi-inc.com; www.mhi-inc.com.
National Element Inc., 7939 Lochlin Dr., Brighton, MI 48116; (248) 486-1810; fax (248) 486-1649; e-mail stevebest@nationalelement.com; www.nationalelement.com.

Additional heating element suppliers, as well as furnace suppliers, can be found in CI’s 2001-2002 Data Book & Buyers’ Guide in print or online at www.ceramicindustry.com.