Resistance Heating Elements
Electric type resistance elements consist of a high temperature resistance alloy either Nickel-Chrome alloy or iron chrome aluminum alloy, usually formed in sinuous, loops or coils. The elements may be supported from the furnace sidewalls on refractory hooks or alloy hooks, suspended from the roof with alloy hanger; hooks, or may be laid on the floor in comb type refractory insulators.
Heating Elements are designed to deliver rated kilowatts at rated voltage only when hot. If actual voltage differs from rated voltage, the power delivered will vary as the square of the voltage. Remember a 1-% increase in voltage is about a 2% increase in wattage, and vice-versa, a 1% reduction in voltage is about a 2% reduction in wattage. The resistance of the heating elements will be lower at room temperature than when hot. The resistance of the elements will increase with age, due to the reduction in cross section by oxidation, and also, due to elongation of the loops. This will result in decreased power to the furnace and ultimate failure. Such failure represents the normal life of the elements.
Certain impurities in the atmosphere will attack the alloy in the elements. These impurities may be in the incoming gas, or may be given off by the work entering the furnace. Cutting oils/fluids are major sources of impurities, typically carbon and sulfur.
Sulfur even in small quantities; will cause rapid deterioration of heating elements. Carburizing atmospheres tend to increase the carbon content of the heating element causing it to become brittle and to develop a lower melting point. Lead, tin, or zinc, and halides will attack the element. These materials should not be put in a furnace.
Heating Element Inspection, Maintenance and Repair
This guide discusses general issues relating to the use, care and maintenance factors relative to obtaining the longevity of heating elements. The complexity of issues relating to resistance type heaters indicates the need for a universal guide as a starting point. This guide is just that, a guide only and actual specifications of heating units should be made only after consultation.
Electrical Lead Considerations
Consider the type of electric heater, placement and wattage requirements, the types of electrical leads used and the methods by which they exit and terminate inside the heated area. Some general considerations in selecting various types of lead are:
- Flexibility Required
- Temperature of Lead Area
- Contaminants in the Lead Area
- Accessibility to Controls
- Relative Cost
- Abrasion Resistance Required
Heating Element Leads And Power Connections
Check that line voltage matches the heater’s rated voltage. Electric wiring to the heater must be installed in accordance with the electric code. The polarity of the leads must always be observed. Adjacent leads should always be connected to the same polarity. Failure to observe polarity may cause premature heater failure.
Heating Element Lead Styles
Heating element leads are available in a wide range of styles, but they all can be generally grouped into a few categories such as:
- Single Conductor
- Twisted Pair
- Pad or Bar
Single conductor leads are standard for ceramic and vacuum formed fiber heating elements. With this form, the heating element conductor also serves as the lead. Caution must be exercised when this type of heating element is used as the lead becomes hot when the element is running near its maximum rating. The heat generated can create problems with terminations, interactions with lower grades of insulation, and overheating of the lead wire itself.
Twisted pair leads are where the element conductor is folded back upon itself and then twisted together in a specific manner. In this method, the effective cross-sectional area of the lead is in effect doubled. This allows the lead to run at substantially reduced temperatures. This feature greatly reduces the potential of element failures directly tracable to lead or termination problems. Twisted pair leads generally carry a premium over the single conductor type. This type of lead configuration is recommended wherever possible.
Rod leads involve fastening a lead of much heavier cross-sectional area to the actual heating element. This allows the lead to run at much lower temperatures than the actual heating element. Typically, the rod leads are welded to the heating element conductor. Although the rod lead is heavier than the element, it must be carefully handled since the welding process results in a fairly brittle area at the weld site. This brittle section is susceptible to cracking or mechanical breakage if mishandled. The rod type of connector can be used with either wire or strip heating elements. The material used for the rod type connector can be made of a lower temperature rating but similar chemistry alloy as that used in the actual alloy heating element.
Pad or bar lead is similar to rod lead except that either a flat bar is used or, if the element in question uses a strip rather than wire, the strip is often folded back on itself to increase the cross-sectional area. It is typically provided with a hole near the end for terminating via bolted connections. If the pad has been welded to the heating element conductor, the same concerns about brittleness at the weld site apply. This lead style is used with fiber based heating packages and if the lead is not long enough to extend through the back up insulation, installation requires all bolted power connections in an area exposed to rather high ambient temperatures.
Lead Bending Radius
The lead wire extending from the heater elements usually can be bent to conform to specific requirements. Caution is required so the integrity of the internal connection are maintained to prolong the heating element life. To avoid placing excessive stress at this junction soft nose pliers are used to hold the lead wire secure where the wire exits from the heating element and then bend.
The minimum bending radius of the wire should be 4 to 8 times the diameter of the wire. This works for both nickel-chrome alloys and iron-chrome-aluminum alloys. However, in very cold ambient conditions, iron-chrome-aluminum alloy heating elements could still crack or break when any bending is done.
Heating Element Brittleness
Many high temperature metallic alloys used for heating elements suffer from poor ductility and brittleness, especially after they have been at their operating temperature for any length of time. This is especially true for iron-chrome-aluminum alloy based materials, which are often used in higher temperature applications. Traditional iron-chrome-aluminum alloy materials will become very brittle once they have reached a temperature of 950 °C, and this brittleness occurs almost instantaneously. The newer powder metal based iron-chrome-aluminum alloys also become brittle once they have been heated but this is a more gradual process and is strictly dependant on time and temperature. Once these alloys are cooled to room temperature, attempting to move them will lead to breakage. Heating these brittle elements to a color temperature should allow them to be moved or repositioned without mechanical damage.
The iron-chrome-aluminum alloy materials also exhibit a low temperature brittleness phase. This is typically a problem when the material is below 20 °C and becomes more of a problem as the temperature is decreased. Typically, attempting to bend, twist, or flex these alloy materials below 4.5 °C will cause cracking and breakage. If the units have been stored in an unheated area, allow them to warm up to as least 22 °C.
When these alloys are welded, the immediate area in the weld site will become brittle from the heat of the welding. These areas should always be given special treatment when handling, since excessive force or flexing applied to these joints will cause cracking and breakage. Because of this potential risk, it is often desirable to supply very large size element systems with the rod or pad terminals unattached. Once elements have been permanently secured, the terminals are positioned and TIG welded to the elements.
Heating Element Terminations
Proper terminations are critical to a successful heating element application and if not done correctly will adversely effect element life. One of the major goals is to insure that the largest amount of element lead wire is in close hard physical contact with the actual termination as is practical. In cases where insufficient contact exists, either through a lack of material or loose physical contact, a condition known as a High Resistance Joint can develop. This phenomena will cause localized heating in the element termination area causing further degradation of the connection leading to failure at the joint. Generally this will require the replacement of what is otherwise a perfectly good heating element. An added point of consideration is that the termination process requires metals of differing alloys to be joined together. While this joining process may produce chemical reactions at the junction, which can lead to early failure, it can be minimized if kept under 540 °C.
When terminating small gauge wire leads on ceramic plate or vacuum formed fiber heater panels, its recommended to use a mechanical compression procedure. This can be a bolt on a binding post with washers and jam nuts, split bolt with washers and nut, or a specialized terminal strip. In all cases, the lead wire should be thoroughly cleaned at the area of contact to insure a good electrical connection. The lead wire must be wrapped completely around the binding post and compressed between the washers and jam nuts or the terminal strip hardware. Insertion through the split bolt and compression between the washers generally suffices. The preferred terminal material is brass or stainless steel. Excessive or repeated bending causes work hardening of the material leading to cracks and breakage.
The use of ring connectors is not recommended due to insufficient contact area between the lead wire and ring sleeve. It can cause lead wire deformation or damage caused during the crimping process. Ring connectors must be used of stainless steel and must be either TIG welded or silver soldered to the lead wire. The use of joint compounds is not recommended as it could adversely affect the integrity of the termination causing corrosion and early failure.
The lead wire can be bent to conform to specific requirements. Caution must be taken to insure integrity of the internal connections. To avoid excessive stress on this junction use soft nose pliers to hold the lead wire secure. The lead wire can then be bent as required.
A certain amount of slack must be provided in the element leads to allow for expansion and contraction during heat up and cool down cycles. If this is not done, the lead wire may be damaged or break due to mechanical stress. This is a compound problem because in addition to the wire expansion, the furnace shell and insulation and internal support structures are also moving during thermal cycling.
For heavier gauge wire elements, a rod lead is usually supplied. The rod is generally machined to allow factory specified connection. A common procedure is to provide a threaded rod with washers and jam nuts. When tightening these connections do not twist or flex the rod since this can cause cracking or complete failure of any welded joints. Other concepts used are slots or holes which allow welding of other leads of heavy cross section conductors directly to the element. Special mechanical compression connectors can also be used where required.
The terminations should be checked for tightness after the first operation and periodically thereafter to insure a high resistance joint does not develop through looseness. The length of time for followup examinations depends on factors such as cycle rates, ambient conditions, physical vibrations, etc. Incoming electrical power must be disconnected and locked out on systems to be examined as per electrical maintenance standards.
It is advisable to provide a protective covering over the heating element leads due to electrical or mechanical considerations. As a best practice, run the lead either inside a high temperature ceramic tube or place high temperature ceramic beads over the lead. Either of these methods can also have a flexible sleeve placed over the top for additional protection. Self sticking tapes should not be used since even high temperature grades typically use organic based adhesive which can break into the carbon based substances. These can cause corrosion, carbon infiltration and embrittlement.
The grades of insulation used should be carefully examined. Many of lower rated materials have a significant amount of free silica. When iron-chrome-aluminum based alloys are used for heating element conductor, generally for higher temperature applications up to 1300 °C, the protective alumina oxide coating formed on the outside of the conductor will react with the free silica starting at temperatures around 1000 °C. This reaction leads to eutectic melting occurring at the point of the reaction. Excessive insulation of the leads can also develop overheating conditions both of the lead and in the area of the terminations.
Fiber based heating systems are treated on the outside with some substance to make the fiber rigid and self supporting. However, undue pressure causes permanent deformation of the fiber surface and cracking which adversely effects the insulation qualities of the refractory fiber pad. Force fitting units will cause the fiber to crack or break off. The leads or terminal pads provided on the fiber pads should be supported to prevent twisting or flexing during the attachment of power leads. This prevents the fiber from damage in the lead exit area.
In larger elements of rod style and on certain lead assemblies, it may be possible to repair a break of mechanical nature or where the conductor is not extensively melted. To do this for nickel-chrome alloys the oxide must be cleaned off, the wires joined together, and then welded using approved methods. For iron-chrome-aluminum alloys, a similar operation is used except the material should be heated to red color temperature before it is moved. This will allow the bending of the conductor segments without causing additional breakage.
Handling, Storage amd Environmental Factors
One of the reasons modern metallic based heating elements can operate at such high temperatures upto 1400 °C for extended periods of time, is that they form a protective oxide layer on their outer surfaces. Surface contamination by a variety of substances interferes with this oxide formation process which occurs only at elevated temperatures. This leads to premature failure of the heating element. Since most heating elements are shipped in a green state with no oxide on the surface, it is important that the material be kept as clean as possible until the heating element is installed and has been heated to form the oxide layer.
Another important area of consideration is the storage of the heating elements. They must be protected from the weather and must be stored inside a cool and dry location. Many of the alloys used for heating applications have a high percentage of iron in them and are susceptible to rusting when exposed to high moisture. The rust interferes with oxide formation and leads to premature failure. In cases where ceramic based or vacuum formed fiber elements are used, the ceramic and fiber can absorb moisture either directly from the air or from direct exposure such as condensation, leaky overhead pipes, or spills. This absorption characteristic can compound the rusting potential since the alloy will be embedded and not visible for inspection.
Another area of contamination is body oil present on hands. Clean cotton gloves should be worn when handling the exposed heating elements to protect them. If this is not possible, thoroughly wash hands with soap and water before handling the elements. It should be noted that the smaller the heating element material, the more significant this contamination becomes.
All petroleum based products and most shop floor dirt adversely effects oxide formation. Therefore, the heating elements must never be directly placed on the shop floor without a protective barrier such as clean paper or cardboard. If a lot of oil vapor is present in the atmosphere, do not expose the heating elements to the atmosphere any longer than absolutely necessary.
When heaters are removed from storage, they should be warmed to a minimum of 20 °C before installation. Many high temperature alloys show increasing problems with ductility and brittleness at lower temperatures below room temperature. If the leads or heating elements are below this temperature, attempting to bend or shape them can lead to cracking or breakage. The danger of this occurring increases dramatically as the temperature decreases. In practice, it is advisable to use a higher temperature of upto 38 °C as small variations in batch consistency could shift the critical temperature point up or down several degrees.
Ceramic based heater systems by their nature are susceptible to mechanical damage from mechanical shocks and stresses, thus do not drop them or force fit them.
For locations experiencing excessive vibration, it should be a prime consideration for shock mounting using industry standard shock mounting techniques. Excessive vibration can also affect wire connections. Make sure that connectors used can withstand the vibrations and still remain tight.
A 20% reduction in maximum loading should be allowed for if a contactor is used in lieu of an SCR control. This SCR control should either be a phase angle fired or variable time base fired zero cross over unit. A zero crossover unit is more advisable but actual application determine the practical choice.
Drying Out Procedure for Embedded Elements
Before initial heating up of a furnace, check for any embedding cement which has broken loose from the ceramic heaters and if the heater wire is visible. Apply embedding cement where needed following the instructions given for patching heaters.
Drying Out Procedure for Refractory Materials
Run the temperature up slowly to dry the moisture out of the refractory lining while open to air. Then increase the temperature thereafter until normal operating temperature is reached. If steaming appears at any time during run-up, do not increase temperature until the steaming stops.
The best procedure to get long life is to use a large cross sectional area heating element with moderate watt loading and never shut it off. The problem with cycling is that the oxide layer will crack or spall off exposing the base material to further oxidation and eventual failure.
Helpful Suggestions And Practices
While heating elements do not have a projected service life in most applications, the possibility of ultimate failure should be considered. Provisions should be made for ready replacement if the potential down time will be expensive or critical to production or operations. Replacement parts should be stocked as necessary so that a failed heating element can be replaced in a short period of time without completely stopping or disrupting the process.
Keep the equipment clean, particularly around the terminals, wiring enclosure, and the heater itself, through a regular maintenance program. In highly contaminating environments or hazardous atmospheric conditions, special attention should be directed to the terminal boxes and electrical enclosures. Heater terminal enclosures can be designed with special fittings to use positive inert gas pressure to prevent the entrance of contaminants or explosive gases.
Use field wiring suitable for the temperatures involved. Heater terminal boxes and enclosures get quite warm during operation and may require special wiring techniques. For field terminal connections inside the heater enclosure, alloy wire with high temperature insulation is recommended unless specifically copper or low temperature insulated wire is suggested. Rubber, wax impregnated or thermoplastic insulated wire should not be used on high temperature heater applications since these materials will deteriorate very quickly with heat. Some insulating materials may give off fumes which could cause injury or damage to the heating equipment. Use thermal insulation wherever possible to reduce heat losses. Insulation is relatively inexpensive and reduces heat losses and operating costs.
Article courtesy of Thermcraft Inc