This website requires certain cookies to work and uses other cookies to help you have the best experience. By visiting this website, certain cookies have already been set, which you may delete and block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our updated privacy and cookie policy to learn more.
This Website Uses Cookies
By closing this message or continuing to use our site, you agree to our cookie policy. Learn More
This website requires certain cookies to work and uses other cookies to help you have the best experience. By visiting this website, certain cookies have already been set, which you may delete and block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our updated privacy and cookie policy to learn more.
Industrial Heating logo
search
cart
facebook twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
Industrial Heating logo
  • Home
  • Magazine
    • Current Issue
    • Digital Edition
    • Archives
  • News
  • Featured
    • IH Daily
    • IH MagEzine
    • Web Exclusives
    • IH Economic Indicators
    • The History of Industrial Heating
    • Heat Treatment Processes
    • Top 10 Heat-Treated Holiday Gifts
  • Topics
    • Additive Manufacturing / 3D Printing
    • Ceramics & Refractories / Insulation
    • Combustion & Burners
    • Heat Treating
    • Heat & Corrosion Resistant Materials / Composites
    • Induction Heat Treating
    • Industrial Gases & Atmospheres
    • Materials Characterization & Testing
    • Melting / Forming / Joining
    • Process Control & Instrumentation
    • Sintering / Powder Metallurgy
    • Vacuum / Surface Treatments
  • Columns
    • Editorial
    • The Heat Treat Doctor
    • Federal Triangle
    • MTI Profile
    • Academic Pulse
    • Heat Treat 5.0
    • International – Brazil
    • Next-Gen Leaders
  • Directories
    • Equipment Buyers Guide
    • Commercial Heat Treat Capabilities Directory
    • Aftermarket Parts & Services Directory
    • Materials Characterization & Testing Equipment Directory
    • Take a Tour
  • More
    • Classifieds
    • White Papers
    • Industrial Heating Bookstore
    • Organizations
    • Market Research
    • Custom Content & Marketing Services
    • FORGE Magazine
  • Multimedia
    • Podcasts
    • Videos
    • Webinars
    • Image Gallery
    • Mobile App
    • eBooks
  • Events
    • Meetings & Trade Shows
    • FNA
    • Heat Treat Show
  • Blog
    • Dan Herring - Heat Treatment
    • David Pye - Metallurgy
    • Dan Kay - Brazing
    • Debbie Aliya - Failure Analysis
    • Thomas Joseph - Intellectual Property
  • Contact
  • Advertise
  • Subscribe
    • Print & Digital Edition Subscriptions
    • eNewsletter
    • Online Registration
    • Customer Service
Home » Electric Heating Elements Part One: Silicon Carbide
Commentary/Columns

Electric Heating Elements Part One: Silicon Carbide

September 5, 2008
Daniel H. Herring
Reprints
One Comment
Fig. 1. Silicon carbide heating elements in operation[2] (Photograph Courtesy of Keith Company)


Electric heating elements are a popular choice of many heat treaters. They come in a variety of shapes, sizes and materials. One of the most common types are silicon carbide heating elements, known by several tradenames including Globar® and StarBar®. They are used extensively throughout the heat-treating industry when high temperatures, maximum power and heavy-duty cycles are required. Let’s learn more.

A silicon carbide (SiC) heating element (Fig. 1) is typically an extruded tubular rod or cylinder made from high-purity grains of silicon carbide that are fused together by either a reaction-bonding process or a recrystallization process at temperatures in excess of 3900°F (2150°C). The result is a chemically stable material with a low thermal-expansion coefficient and little tendency to deform.

Recrystallization forms fine grains of silicon carbide that act as “bridges” or connection points between larger grains, thus forming conductive pathways. The number of bridges formed dictates the material’s resistance – the greater the number, the lower the resistance. The secret to the creation of a good heating element is controlling this formation process within the material so as to develop a consistent electrical resistance.

The factors that influence the life of a silicon carbide heating element include the type of furnace atmosphere, watt density, operating temperature, type of service (continuous or intermittent) and maintenance. Furnace type, design and loading play an important role as well. Silicon carbide heating elements are extremely versatile, operating, for example, in air up to 3000°F (1650°C).

Transformers used for silicon carbide heating elements have multiple secondary taps in anticipation of a change in resistance of the elements over time. Silicon carbide heating elements, being 20–30% porous, oxidize or otherwise react with the furnace atmosphere and increase in resistance during their operational life. Oxidation causes a reduction in the cross-sectional area of the bridges, resulting in greater resistance to electrical flow. The oxygen in the air reacts with the silicon carbide grain, reducing it to silica (SiO2) as shown by the equation below:

SiC + 2O2 ® SiO2 + CO2

It is estimated that new silicon carbide bars will increase in resistance 10–15% on startup, which should be taken into consideration when considering replacing the bars. In most cases, silicon carbide heating elements fail mechanically long before they fail due to aging.

Tips for Extended Service Life

To maximize element life, be sure to do the following:

1. Handle the elements with care – Silicon carbide heating elements have low tensile strength and, therefore, are sensitive to mechanical damage from rough handling, dropping (even in the packaging) or forced bending that can occur during storage, unpacking or installation.

2. Match resistance – The purpose of matching resistance of elements is to improve their life and to improve temperature uniformity in the furnace. Silicon carbide heating elements are typically factory tested with the test amperage marked on the shipping box and/or the element. Elements can be connected in parallel (preferred since they tend to come into balance in use), series or series-parallel. Elements connected in parallel should be matched in resistance within ±20%, while elements connected in series should be matched within ±5%.

3. Choose the proper size element for the equipment – If there are any doubts about the size to use, check the design parameters with the original furnace equipment manufacturer.

4. Install carefully – Check that the terminal holes through the insulation are in alignment so that the elements slide in without striking the opposite side or are put under tension due to forcing them into position. Be sure to center the elements in the furnace chamber so that no portion of the heating section of the element is in the brickwork.

5. Pack the element with ceramic fiber to a depth of about 1 inch (25 mm) so as to avoid heat loss, but be sure that the terminal ends of horizontally mounted elements lie flat in the terminal holes and are supported by the furnace walls.

6. Use the lowest voltage that will maintain the desired furnace operating temperature. This will ensure the lowest possible surface temperature of the element and lengthen its service life.

7. Run the correct silicon carbide element watt density for the required furnace atmosphere (Table 1).

8. Perform in-service inspections – Check the amperage as an indication that the elements are operating correctly.

9. Maintain matched-resistance circuits at all times. Don’t mix old and new elements in the same circuit.

10. Be sure that the elements are loose in the terminal holes not only when the furnace is cold but also hot.

Fig. 2. Spiral-cut silicon carbide heating-element design provides increased resistance for applications up to 3000°F (1650°C)

How Do You Know It's Time to Change Heating Elements?

Furnace type, design and cycling make a difference. Let’s answer this question by considering a mesh-belt copper-brazing furnace with a muffle operating at 2050°F (1120°C) in a hydrogen/nitrogen (75%/25%) atmosphere. The furnace is run six days a week with a typical belt loading of 10–12 lbs/linear foot (15–18 kg/linear meter). Typical element life is expected to be in the range of 12–24+ months. Here’s how to determine when its time to change elements:

1. Beginning with a change of elements, measure the amperage and voltage to the individual elements on a quarterly basis. Calculate their resistance and watt input to determine whether they are balanced or not. If a particular element shows erratic readings or large changes in resistance from one set of readings to the next, it is time to replace it. Note: Silicon carbide heating elements should be changed in sets depending on how many elements are in series with one another (sometimes in pairs, sometimes more).

2. Also, there is a relatively simple procedure that should be done to take the guesswork out of knowing when to change elements. With all new elements, lower the furnace temperature (and monitor it during this procedure) and place the power controller (SCR) in manual mode calling for 100% power. Take a digital meter with extra long leads and, CAREFULLY touching the element on either end, measure the voltage drop across the element. Note: Do not take this reading by touching the straps as they run slightly cooler and your reading won’t be as accurate. Measure the current using a clamp-on ammeter around the straps and calculate resistance and watt input. Record this information for future reference. When the elements have doubled in resistance, it is time to change elements. Repeat these measurements every quarter.

3. When you see that the elements have degenerated to about 2/3 of their original resistance value, it is time to change to the next highest tap on the transformer.

4. If you are fully “tapped up” – on the highest tap setting on the transformer – and a particular zone begins to “struggle” (when it has difficulty maintaining your temperature set point), then you know it’s time to change all of the elements in that zone.

Summing Up

The choice of heating element depends on many factors. For example, silicon carbide elements are capable of higher operating temperatures and higher watt loadings than say metallic elements. They are self-supporting and can be used in furnaces either too wide or too long to be spanned by other element types and are relatively easy to change while hot. Silicon carbide heating elements are used extensively in brazing and sintering furnaces running continuously at or above 2050°F (1120°C) and for other processes where the temperature range lies between 2375°-2725°F (1300°-1500°C). IH

ih-subscribe

Recent Articles by Daniel Herring

Facts about the Elements: Actinium

What Constitutes a Good, Workable Vacuum Maintenance Program (part 2)

Facts about the Elements: Chlorine

Facts about the Elements: Neodymium

Vacuum Maintenance (part 1)

Dan-herring

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance). He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center. tel: 630-834-3017; e-mail: dherring@heat-treat-doctor.com; web: www.heat-treat-doctor.com

 

Related Articles

Electric Heating Elements, Part Two

Tramp Elements and Their Influence on Steel and Steel Heat Treatment

What Happens to Steel During Heat Treatment? Part One: Phase Transformations

Influence of Alloying Elements on Austenite

You must login or register in order to post a comment.

Report Abusive Comment

Subscribe For Free!
  • Print & Digital Edition Subscriptions
  • eNewsletters
  • Online Registration
  • Subscription Customer Service

More Videos

Popular Stories

Editorial 2019: Reed Miller

Noel Nuggets

1219IH-Fabrisonic-slide5

Metal Additive Manufacturing without Melting

Industrial Heating Web Exclusives

Steel Mill Powered by Wind

Industrial Heating Industry News

Novelis to Expand, Upgrade Georgia Facility

Industrial Heating Industry News

Steel Dynamics Expansion Includes Induction Furnace, Billet Welder

IH Ipsen 360x184customcontent

Events

January 1, 2030

Webinar Sponsorship Information

For webinar sponsorship information, visit www.bnpevents.com/webinars or email webinars@bnpmedia.com.
View All Submit An Event

Poll

Additive Manufacturing

Has additive manufacturing had any impact on your business?
View Results Poll Archive

Products

Vacuum Heat Treatment Volume I

Vacuum Heat Treatment Volume I

See More Products

The History of Industrial Heating 1000 BC - Present Day


Industrial Heating Employment Marketplace

Industrial Heating

1219IH-cover144x192

2019 December

Check out the December 2019 issue of Industrial Heating, featuring "Metal Additive Manufacturing without Melting", "Furnaces with Tungsten Heating Elements Make High Product Quality Possible", and much more.

View More Create Account
  • Resources
    • List Rental
    • eNewsletter
    • Manufacturing Group
    • News
    • Want More?
    • Featured
    • Product / Event
    • Industry Links
    • Connect
    • Privacy Policy
    • Survey And Sample

Copyright ©2019. All Rights Reserved BNP Media.

Design, CMS, Hosting & Web Development :: ePublishing