|
|
|
NEI's Technical Info
"Do's & Don'ts" in Designing Heating Elements General Operating and Maintenance Instructions for Heating Elements Ohm's Law Chromel®: The original resistance alloy that first made electrical heating practical. Heating Element Design Fundamentals Selecting a Suitable Resistance Alloy Technical Fundamentals of Electrical Heating Current Carrying Capacity Chromel® "A" Wire
Selecting a Suitable Resistance Alloy
A prime requirement for any heating element is the ability to perform dependably in service over a long period of time. This quality of service life in NIKChrome® alloys, or any other metallic heating element is largely dependent upon (1) temperature, (2) element cross-section, (3) thermal cycling, and (4) the environment in which it is operated. Thus selection of a specific resistance material for a given application should be made on the basis of the designer's knowledge of these factors and their probable effects on service life of available resistance alloys.
Service Life of NIKChrome® Alloys-With regard to the NIKChrome® alloys, their service life characteristics are monitored on a regular basis using the standard ASTM B-76 accelerated life test. This test consists of heating a sample of .0253" diameter NIKChrome® wire to a prescribed temperature and cooling it by alternately applying and discontinuing power to the wire. Heating and cooling cycles consist of two minutes with power on and two minutes with power off. While this test is in progress, "hot" resistance measurements are taken on the wire at regular intervals and its durability, as a heating element is determined in terms of three characteristics:
- Total life-or the elapsed time until complete failure occurs.
- Useful life-or elapsed time until the wire's resistance has increased by a factor of 10%. And (3) total resistance increase up to the time of complete failure.
Effects of Temperature and Cross-Section. The accompanying graphs illustrate the effects of temperature on the total life characteristics of NIKChrome® resistance alloys under these test conditions-as well as the effect that cross-section has on the service life of heating elements. The life-time factor of 1.0 at 2150 F was assigned to provide a basis for comparison. In actual practice, however, it is not uncommon for .0253' diameter NIKChrome® samples to operate for several hundred hours at that temperature before failure m the ASTM B-76 life test. While these data are helpful in developing practical heating element designs, the ASTM life test characteristics of alloys should not be used to project the probable life expectancy of a heating element in service for any given application.
Effects of Environment-Consideration of the environmental conditions to be encountered in an application is also very important in selecting a heating element alloy for maximum service life. This is especially true of elements to be used in industrial furnaces where contaminants are quite apt to be present or where controlled atmospheres are to be used.
In this regard, resistance alloys operating in furnaces and ovens using carbon bearing and/or other semi-reducing atmospheres. While the metallurgical factors involved in such applications have been previously discussed in some detail, the following table summarizes basic characteristics of our Chromed alloys and gives general recommendations regarding their range of application. In the event you would like additional information or data on what effects a given environment may have on the life of any resistance alloy, our technical service personnel will be glad to answer any specific questions you submit.