HEATING ELEMENT MAINTENANCE RECORD
Furnace ID: _______________________
Number of Elements in Furnace ________
Number of Spares ___________________
Starbar heating element order information:
Type: ------------ Outside Diameter: ___________________ Heated Length: _____-______________ Overall Length: ___________________ Nominal Resistance when new:_________Ω Date Element Location Amps written On element Measured Voltage E Measured Amperage I Computed Resistance E/I Computed Watts E x I Notes Sketch of heating element layout
FACTORS THAT AFFECT HEATING ELEMENTS SERVICE LIFE
Silicon Carbide heating elements increase in resistance during their service life. The rate at which aging occurs is affected by many factors such as element watt loading, operating temperature, atmosphere, mode of operation--continuous or intermittent and power control methods. Element life depends on many factors. The elements can last several years if operated in a favorable atmosphere at a favorable watt loading.
Watt Loading and Operating Temperature
A heating element with a higher watt loading will operate hotter, as shown on the adjacent chart. This higher temperature is directly related to the rate at which the element increases in resistance. For optimum element life, the lowest possible watt loading should be considered, in the range of 20 to 50 watts per square inch (3-8 watts per square centimeter).
In clean, dry air atmosphere sic elements can be operated at furnace control temperatures up to 1600ºC. The maximum element temperature is 1625ºC.The three-piece W or U type sic elements are limited to 1425ºC. In inert atmospheres of argon or helium elements may be operated to 1700ºC.)For atmospheres containing water vapor, alkali metal vapors, flux vapors or oxygen enrichment, we recommend QS1 infusion glaze coated elements or elements of special type Please contact us for recommendations. In reducing atmospheres the maximum operating temperature is 1370ºC. There is a protective coating of silicon dioxide on the silicon carbide. Hydrogen reduces this coating and causes the sic elements to deteriorate. Very dry or very wet hydrogen is detrimental to long service life. Nitrogen atmosphere applications are limited to 1370ºC and 30 watts per square inch (5 watts per square centimeter) maximum surface watt loading. Too high of a surface temperature will result in a silicon nitride reaction. A thermally insulative layer forms around the heating elements resulting in very high surface temperatures which damage the sic elements.
Mode of Operation
sic heating elements can be used in both continuous and intermittent operations. For maximum service life above 1300ºC. continuous operation is recommended. The reason for this is that a new element is essentially 100% pure silicon carbide with very little silicon oxide. As it is used, the silicon carbide oxidizes and forms more silicon oxide and silicon dioxide. This is what causes the element to increase in resistance over its life. The chart entitled THERMAL EXPANSION OF DIFFERENT SILICA MATERIALS show that these various forms of silica - quartz, cristobalite and tridymite have considerable size changes as they go through these phases. When the silicon dioxide in the element is cooled or cycled, it goes through several phase transformations. The high-low quartz transformation at 570ºC. involves a substantial volumetric change, which can lead to the fracture of bodies containing large amounts of quartz. As the sic elements go through these temperature ranges, the expansion and contracting of the silica can fracture the silicon carbide lattice structure which increases the electrical resistance and can cause element failure. 571ºC temperature is theoretical and the exact element temperature can vary somewhat. For these reasons, we recommend a minimum idling temperature of 704ºC, to remain well above this inversion temperature. There are those customers who feel it is more economical to conserve on energy and suffer a shorter element life. There are other customers who prefer the longer element life at the expense of the energy used during idling. Since each application is different, the choice remains with the customer.
The resistance of sic elements increases gradually during their useful life. Therefore, some means of maintaining the power input to the furnace at a level sufficiently high to maintain the desired temperature, is required. Historically this involved the use of mechanical contactors and multiple tap transformers with up to 16 taps to be able to adjust the voltage upwards in steps, to maintain the desired power output as the elements aged. In more recent times, the use of SCR’s (silicon controlled rectifiers, also known as thyristors) have become more popular as they are less bulky, and enable very precise control of the voltage and power input to the heating elements. To provide a large voltage reserve to enable maximum life of the element elements when designing higher temperature and more industrial furnaces, the use of a combination of step down transformers with only 3 or 4 output tappings in combination with SCRs, is the most common system today. Sic elements can be used directly on the line (fixed voltages) at temperatures up to 1300ºC. To compensate for the reduced output as the sic elements gradually age or increase in resistance, the furnace or kiln is typically initially overpowered by 25% to 50%. This type of arrangement has proven very satisfactory in many applications
SiC heating element life is limited due to several reasons, but the major reason for failure is rarely due to faulty material or workmanship. The most common reasons for element failure, which we have seen have been due to:
Terminal holes should be aligned and straight in the refractory wall. Any binding of the Starbar can cause breakage during heat-up or operation due to expansion or movement of the furnace walls. If the elements are tight in the terminal holes or something bumps or jars the elements, they will break. Please make sure the elements can be moved north and south and east and west about 4-6mm (1/8"-3/8") with a slight force such as can be applied with a thumb and a finger pushing on a pencil. This test should be done both at room temperature and at maximum furnace operating temperature. Contact straps should be long enough so that no stresses are transferred to the elements.
Arcing occurs when the element is powered. If an element is not free to radiate freely in all directions, the section too close to a wall will overheat, arc and fail.
Handling of element packages by the carriers while in transit. Handling of elements by the customer after being removed from the package.
Silicon carbide heating elements should be operated in a clean environment. Glass deposits, for example, can cause mechanical breaks. The differences in coefficients of thermal expansion causes mechanical stresses in the element structure and breakage to occur. If possible, keep excessive moisture, methane and hydrocarbon vapors out of the high-temperature zone of the furnace, where the elements are located. If moisture is unavoidable, and for more aggressive furnace atmospheres, we recommend LMA infusion glazed coated elements. Please contact us if you have any questions.
Center the element in the chamber so no portion of the heating section is in the refractory wall. This can be done by measuring the cold end length sticking out of the furnace shell and make sure they are both equal
CHECKLIST OF INFORMATION REQUIRED TO
TROUBLESHOOT SILICON CARBIDE HEATING ELEMENT PROBLEMS
Customer Name:_____________ QS Contact:_______________
Contact Name:_____________ Contact email:_______________
Address:______________ Contact Phone:_______________
Furnace builder:_________________ Furnace type:___________________
Production capacity, lb/h:__________________
Production Process:________________ Furnace Power Rating,Kw:______________
Furnace Temperature, ℃:________________ Firing Cycle:__________________
Number of Zones_________________ Time to temperature:______________
Refractory/insulation thickness:_____________ Refractory Insulation Composition:___
Diameters of holes in refractory:_____________ Ceramic Terminal Tube used, size:____
Width--- Height --- Length---
Furnace atmosphere:___________________ Is atmosphere
Hat is the atmosphere dew point?_____________ Contained in muffle:_______________
What volatiles are given off from the load during heating?____________________________
Is there any steam?____________________________
Electrical orientation (Wye/Delta/Single-Phase):______ SCR Rating:_______________
Transformer Voltage taps:_________________ SCR Manufacturer:_________
Transformer Rating:__________________ Method of Control SCR:_______
Transformer Manufacturer:__________________ SSR or Contactor):___________
If SCR, what firing method is employed?(phase-angle, zero-switching and type i.e single cycle.,
Slow cycle and time base , if known):_______________________________________________
Volt(AC) Phase Hz
Main Line Power__________________ ____________ _______
Type of element & part number:__________________ Number of elements:_______________
Element loading:_________________ Power per element, KW:_______________
Element temperature:__________________ Hot zone Length:_________________
Element dimension:__________________ Cold End Length:________________
Element Spacing(Centerline to Centerline)_________Element Spacing from chamber walls:____
Connections and installation
What orientation are the elements installed in?
(Horizontal ,Vertical ,at an Angle, etc.)___________________________________
How are the elements supported?___________________________________
Are terminal tubes/lead-in sleeves being used? ____________________________________
Are the terminal holes in line with each other from one
Side of the furnace to the other ?(where application)____________________________________
Are the terminal holes free of debris?(any signs of
Condensates in the holes? ___________________________________
Are the elements being packed around with fiber at the
Ends where they pass through the efractories?)____________________________________
Are the elements still able to move freely in both linear
And radial directions(important for thermal
Is there sufficient stack in the length of the aluminum
Straps so as to not transfer stress to the elements?____________________________________
Are there any signs of residues on the element cold
Do you know/what do you think the residues are? __________________________________
Are the elements being installed in well matched groups?_________________________________
(if unclear, we can explain and help)__________________________________
Are new and old elements being installed in the same Connection group?__________________________________
Have the transformer trapping and/or SCR outputs been
Adjusted when replacing spent elements with new ones?_________________________________
Are the aluminum braids showing signs or oxidation
Arcing or heating up?__________________________________
Are the connection clamps loose on the ends of the elements?_____________________________
How are the elements connected in each control group?
(Please describe and/or send a sketch or connection schematic)__________________________
Element radiant protection Tube data(where applicable)
Tube material_________________ Tube dimensions(mm):________________
Type(Straight ,U,W,Inner)________________ Tube length(mm):________________
Tue orientation (Hor./Ver.)__________________ Tube loading W/cm2:______________
Number of tube:__________________ Net power per tube,Kw:______________
TIPS FOR MAXIMUM PERFORMANCE
1. Start-up -- If a transformer is used, apply minimum voltage during start up. This is usually 70%-80% of full load voltage. Any silicon carbide element may be weakened or fractured by heat shock if excessive voltage is applied in bringing the furnace up to temperature too rapidly.
2. Replacement -- Avoid mixing new elements with old ones. Reduce the voltage to the lowest value when replacements are made. Be sure to turn off and lock out power before changing elements. Never overload elements by applying an excessive voltage to get the furnace up to operating temperature in a hurry. If the broken element is in a series connection, it is usually necessary to replace all the elements connected in the series circuit. Elements in series must be matched closely. If an element has broken after the elements in the group have increased more than 10% in resistance, the entire group of elements should be replaced. If a used element of similar resistance (one which has increased in resistance approximately the same amount as the group) is available, then it could be used as a suitable replacement
.3. Watt Loading -- Don't exceed the recommended watt loadings or temperatures shown in the chart below
Our heating elements are calibrated in open air by applying a voltage through them that will cause them to dissipate approximately 15.5 watts/cm2 (100 watts/in2 )
The sic heating elements are calibrated in amperes rather than ohms, for it is a more convenient unit to use. Experience shows that a sic element when being calibrated in open air at a loading of 15.5 watts/sq. cm (100 watts/sq. inch) will come to a surface temperature of 1073ºC (1960ºF) in anywhere from three to eight minutes, depending upon the size of the Sic element. Then, current will drop slightly and another two or three minutes is required before the element becomes stable at a temperature of approximately 1073ºC (1960ºF).
If the element that are calibrated in free air are placed in an enclosure or in a semi-enclosure backed by a reflector, the ambient temperature will rise above that of the surrounding room with the result that the resistance of the element will be affected. It is possible to check the ampere rating of an element while it is at temperature in the furnace. A clamp-on ammeter is required and readings are taken by clamping around the aluminum braided terminal strap.
The value is a comparative figure which can be used to relate other elements in the furnace or other elements measured with the same method. To compute the resistance, measure the voltage drop across each element, then divide the voltage by the current. This will give you the resistance in ohms of the element. It is possible to calibrate used elements on a higher voltage than was originally used, the advantage being a higher voltage will make it easier to detect any unevenness in resistance or to detect any defects or nonuniformities that might have developed during the operation of the elements.
If used elements are calibrated on the same voltage as new elements, the elements will not have the same surface temperature because they have increased in resistance, but if the elements are matched in this manner they will still be very close to being matched at a higher voltage. If you elect to calibrate used elements at a higher voltage, the simplest way to select a voltage for recalibration is to multiply the initial nominal resistance of the element by the estimate of the increase in resistance.
If you had an element that had doubled in resistance multiply the nominal resistance by two. If the element has tripled in resistance, multiply the nominal resistance by three. The recalibration voltage can be determined by using the equation E=√WR. Where W equals the surface area of the element (square centimeters or square inches) times 15.5 (watt/cm2 ) or 100 (watts/in2 ) and R is the used element resistance. The crayoned markings on the element indicate the calibration amperage, month and year the Sic heating elements was manufactured.
Holding the heating elements so that the markings are right side up and reading from left to right, the first letter indicates the month. The letter A through L appear in this space (A is for January, B for February, etc.).
The second is a number 0 through 9. This is the year the sic heating elements was manufactured . The next mark on the sic heating elements would appear near the other end and will be the calibrated amperage.