When selecting heating element leads, be sure that the line voltage matches the heater’s rated voltage.
Although electrical resistance-type heating elements have been around for many years, with new applications occurring daily, there still exist some misunderstood aspects of use. Also, quite frequently, the elements are misused due to assumptions or lack of readily available information.
This article discusses general issues relating to the use, care and maintenance to help ensure the longevity of these heaters. Although the complexity of issues relating to resistance-type heaters indicates the need for a universal guide, it can be just that - a guide only. Actual specifications of heating units should be made only after consultation with knowledgeable engineers.
Application and Maintenance Factors
With regard to the electrical lead considerations, it is important to consider the type of electric heater as well as placement and wattage requirements. It also is necessary to consider the types of electrical leads used and the methods by which they exit and terminate the heated area. Some general considerations in selecting various lead types are:
- Temperature of lead area.
- Contaminants in the lead area.
- Flexibility required.
- Abrasion resistance required.
- Relative cost.
- Accessibility to controls.
With heating element leads and power connections, be sure that the line voltage matches the heater’s rated voltage. Electric wiring to the heater must be installed in accordance with local regulations and the National Electric Code. Polarity must always be observed, and adjacent leads should always be connected to the same polarity. Failure to observe polarity may cause premature heater failure.
The single conductor concept is usually the standard form of supply for ceramic and vacuum-formed fiber heating elements.
Element leads are available in a range of styles but they generally can be grouped into categories. They include:
Single conductor. The single conductor concept is quite common and normally is the standard form of supply for ceramic and vacuum-formed-fiber heating elements.
Twisted pair conductor. Twisted pair indicates a lead in which the element conductor has been folded back upon itself and then twisted together in a specific manner. In this method, the effective cross-sectional area of the lead has been in effect doubled. This allows the lead to run at substantially reduced temperatures. This type of lead configuration is recommended where possible.
Rod conductor. This lead design involves fastening a lead of much heavier cross-sectional area (typically a minimum of twice) to the actual element. Again, this allows the lead to run at much lower temperatures than the actual element. Typically, the rod will be welded to the heating element conductor. Although the rod is heavier than the element, care must be exercised when handling because the welding process generally will result in a fairly brittle area in the immediate area of the weld site. The rod type of connector can be used with either wire or strip heating elements.
- Pad or bar conductor. The pad or bar lead is similar to the rod concept. In this design, either a flat bar is used or, if the element in question uses strip rather than wire, the strip is folded back on itself once or twice to increase the cross-sectional area. It typically is provided with a hole near the end for terminating via bolted connections. If the pad has been welded to the element conductor, the same concerns about weld site brittleness will apply.
Lead wire extending from the heater elements usually can be bent to conform to specific needs. Caution must be taken so that the integrity of the internal connection is maintained to prolong the life of the heating element. To avoid placing excessive stress on this junction, use soft nose pliers to hold the lead wire secure where the wire exits from the heating element and then bend. Be aware that some pliers can gouge the wire, creating a weak spot.
The minimum bending radius of the wire should be four to eight times the diameter of the wire. This works for both nickel-chrome alloys and iron-chrome-aluminum alloys. However, even at the proper bending radius, in very cold ambient conditions, iron-chrome-aluminum alloys still crack or break when any bending is done.
In some of the larger elements and on certain lead assemblies, it may be possible to repair a mechanical break where the conductor is not extensively melted.
Many of the high temperature metallic alloys used for heating elements suffer from poor ductility and brittleness, especially after they have been at operating temperature for any length of time. This is especially true for the iron-chrome-aluminum based materials that are often used in higher temperature applications. Traditional iron-chrome-aluminum materials will become very brittle once they have reached a temperature of 1,742°F (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 dependent on time and temperature. Once these alloys are cooled to room temperature, attempting to move them most likely will lead to breakage. Heating these brittle elements to a “color” temperature (above 500°F [260°C]) should allow them to be moved or repositioned without mechanical damage.
Although electrical resistance-type heating elements such as this flat plate example have been around for many years, there still exist some misunderstandings about how to use them.
Proper terminations are critical to a successful heating element application; if not done correctly, poor terminations can adversely affect element life. One of the major goals is to ensure that the largest amount of element lead wire is in as 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 phenomenon will cause localized heating in the termination area, creating 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.
When terminating small gauge wire leads such as those commonly found on ceramic plate or vacuum-formed fiber heater panels, the recommended practice is to use a mechanical compression procedure. This can be:
- A bolt (binding post) with washers and jam nuts.
- Split bolt with washers and nut.
- A specialized terminal strip.
In all cases, the lead wire should be cleaned thoroughly at the area of contact using steel wool or light sanding to ensure 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 will suffice. The preferred terminal material is brass although stainless steel is acceptable in many applications.
Often, it is desirable to provide a protective covering over the element leads. This may be required due to electrical or mechanical considerations. Great care should be taken when selecting a protective shield for the leads. The most common practice is to 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.
In addition, the grades of insulation used should be examined carefully. Many of the lower rated materials contain a significant amount of free silica. When iron-chrome-aluminum based alloys are used for a heating element conductor (generally higher temperature applications up to 2,372°F [1,300°C]), the protective alumina oxide coating formed on the outside of the conductor will react with the free silica starting at temperatures around 1,832°F (1,000°C). This reaction will lead to a eutectic melting phenomena occurring at the point of the reaction. Excessive insulation of the leads also could contribute to overheating conditions in both the lead and in the area of the terminations.
In some of the larger elements (rod style) and on certain lead assemblies, it may be possible to repair a break that is mechanical in 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 that material should be heated to “red” color temperature before it is moved. This will allow bending of the conductor segments without causing additional breakage.
Proper care and maintenance of heating elements such as this large air heating element will help ensure a longer operating life.
Handling, Storage and Environmental Factors
One of the reasons modern metallic-based heating elements can operate at such high temperatures - to 2,552°F (1,400°C) - for extended periods of time is they form a protective oxide on their outer surface. Surface contamination by a variety of substances will interfere with the oxide formation process, which occurs only at elevated temperatures. This will lead to premature failure of the element. Because most elements are shipped in a “green state” (no oxide on the surface), it is imperative that the material be kept as clean as possible until the element is installed and has been heated to form the oxide.
Another important area of consideration is storage of the elements. They must be protected from the weather and must be stored inside a cool, dry location. Ideally, this would also be a low humidity location but this is not always possible. Many of the alloys used for heating applications have a high percentage of iron in them and they are susceptible to rust when exposed to high moisture. The rust will interfere with oxide formation and lead 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 because in many cases, the alloy will be embedded and not visible for inspection.
Another area of contamination is body oil present on your hands. It is recommended that clean, cotton gloves be worn when handling the exposed 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 element material, the more significant this contamination becomes, especially for wire sizes below BSA 18 gauge and strip thickness of 0.04".
In general, all petroleum-based products and most “shop dirt” will adversely affect oxide formation. Therefore, never place elements directly on the shop floor without first putting down a protective barrier such as clean paper or cardboard. If there is a lot of oil vapor present in the atmosphere, do not expose the elements to the atmosphere any longer than absolutely necessary.
When heaters are removed from storage, they should be warmed to a minimum of 68°F (20°C) before attempting to install. Many of the high temperature alloys show increasing problems with ductility and brittleness at lower temperatures. If the leads or elements are below this temperature, attempting to bend or shape them could lead to cracking or breakage.
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.