EREMA heating elements are subject to gradual oxidation, formation of Silica and increase in electric resistance while in use. These are some of the characteristics associated with ageing of the material.
The oxidation process is shown by the following equation:
SiC + 2O2 &rarr SiO2 + CO2
Silicon carbide (SiC) reacts with oxygen (O2) in the atmosphere and the surface of the heating elements gradually oxidize, while the amount of Silica (SiO2) increases. This raises electrical resistance. Oxidation occurs when the temperature reaches 800°C and is accelerated as the temperature increases. Rapid oxidation will occur in the early stage of use, however the rate of oxidization will diminish and stabilize. General variation in electric resistance is shown in Figure 3. For Types D, E and F, the service life of EREMA expires when its resistance increases to about 3 times the initial resistance. (The life of SG and SGR lasts until resistance reaches 1.7 times the original value).
With approximate threefold increase, variation in resistance becomes too great and heat
distribution worsens, causing inefficient temperature distribution inside the furnace. The time
required for electric resistance to increase threefold the initial value i.e. the life of heating
elements, varies according to the following application conditions:
(1) Operation Temperature (2) Surface loading (Watts density) (3) Atmospheres and Materials
to be Processed (4) Furnace Operation (5) Electrical Connection (6) Installation Method of
Heating Elements
Further explanations are given directly below.
Fig 3) Increase in Resistance (Type D, E and F)
The higher the temperature of EREMA elements - the shorter the life due to oxidization. Oxidation accelerates when the furnace chamber temperature exceeds 1400°C (for Types D, E&F) or 1600°C (for Types SG&SGR).Therefore it is recommended to keep the surface temperature of EREMA as low as possible and minimize the difference in temperature between the furnace chamber and EREMA elements.This point will be discussed in greater detail the section on surface loading.
Surface loading of EREMA elements is expressed in terms of electric power dissipated per square centimeter (cm2) of the heating surface. This surface loading is also called watt density and is expressed as Watts / cm2. The higher the surface loading of the EREMA the higher the surface temperature. The temperature rises in accordance with the increase in the surface loading (watts density) and has a detrimental effect on the life of the element. Under uniform furnace chamber temperature, the surface temperature of heating elements rises in relation to the increase in surface loading as shown in Figure 4.
Temperature, Surface loading
(Watts density), and Surface
Temperature of Heating Element
Surface load (watts density) limit:
The application limit line is indicated in Fig. 4. It is recommended to use 1/2 to 1/3 of the max. surface load.
EREMA voltage and current markings:
The voltage and current marked on EREMA elements conform to the JIS (JAPAN INDUSTRIAL STANDARD). This JIS requires these ratings to be calculated in atmospheric air at 1000 Celsius with a theoretical surface loading of 15 w/cm2. Under no circumstances should the marked voltage and current be applied to the Erema element as overloading and rapid oxidization will occur. The voltage and current allows for accurate calculation of the element resistance at 1000 Celsius.
Caution:
For all Erema elements, and in particular elements with a 30mm + diameter, it is important to avoid thermal shock.Thermal shock is caused by the rapid increase in the element temperature and presents as fracturing and subsequent premature failure of the element.It is recommended that the voltage be applied gradually - 1/3 of the rated voltage gradually increasing.Thyristor control with "soft start" and current limit is most commonly used to achieve optimum control and element life.
Furnace Operation and Continuous Power Supply:
EREMA element life is maximized with continuous use - intermittent use reduces life. During operation silicon carbide heating elements oxidize at the silicon carbide grain boundaries where a silica film is formed. With long, continued use this film gradually thickens resulting in an increase in the resistance of the heating element. The silica film behaves abnormally by expanding or contracting around the crystal transformation temperature of 270°C. If heating elements are cycled repeatedly through this temperature, the accumulated film will be destroyed and removed encouraging further deposition of silica film.