Understanding Tungsten Mesh Heating Elements
Tungsten mesh heating elements offer an ultra-clean heating source in controlled atmosphere applications.
Tungsten mesh has been in use for high-temperature furnace heating since the early 1960s. Tungsten mesh elements came into popularity because they were shown to be more durable than the earlier sheet-style elements. They also have greater watt density than rod-style heating elements.
Tungsten mesh heating elements have an upper temperature operating range of 3,000°C (3,273°K) in an inert atmosphere; Table 1 lists compatibility details for other atmospheres. Tungsten has a very high vapor pressure, which allows for this high-temperature operation.
The tungsten mesh heating element operates in the same manner as an incandescent light bulb. A 100-watt light bulb runs at about 2,200°C. The partial-pressure argon gas in the light bulb protects the element from oxygen and dissipates the heat. The distance from the element to the glass bulb protects the glass (the glass would melt before reaching 2,200°C). Incandescent lights produce more heat than light; this is why alternative lighting technologies are being increasingly used today.
The tungsten mesh heating element uses the heat component of this equation. If you look into a furnace at 2,200°C, the element will be as bright as a 100-watt bulb in light intensity.
Tungsten metal has the highest melting point and lowest vapor pressure of all metals, as well as a high resistance to many chemicals. Of course, tungsten is a refractory material, both in use and during manufacturing.
Most of the material fabrication is done with the metal heated for forming, which requires special handling in welding. The welding is done in a glovebox with a tungsten inert gas (TIG), which is also known as a gas tungsten arc welding (GTAW) system. Part of the difficulty when welding tungsten is that a tungsten electrode is being used, so the welder needs to keep the electrode from becoming part of the element. Other welding techniques are also used, such as electron beam and laser.
Mesh Heating Elements
In tungsten mesh heating elements, the element wires are wound around a mandrel to create concentric coils of wire. The coils are twisted into each other, which forms the mesh. The ends of the wires are sandwiched between solid bands made of tungsten. The interlacing of the wires in the panel design allows the individual wires to move during the heat cycles without binding and/or breaking against each other.
The unique quality of tungsten mesh allows the element to withstand multiple heat cycles without distortion, so these elements typically have a long life span. The large amount of surface area gives the element very good watt density, which is a factor of the total surface area of the wires used in the element. The higher the surface area, the greater the watt density.
The elements are custom-designed for each application. Resistance is a design factor that can be adjusted to maximize efficiency. The power leads and top and bottom bands are also key factors in the design. The tungsten mesh element can be circular or a flat panel, and they can be manufactured for single- or three-phase power operation. Most heating elements hang vertically, although some are operated horizontally. The position of the element becomes a design factor. The heating elements are generally supported by liquid-cooled power feedthroughs. The style and position of the feedthroughs determine the style of power leads (round rods or flat bars are most common).
The resistance and heat loss determines the actual wire size and configuration of the mesh. The element and power supply need to be matched for efficient operation. As a general rule, the elements run on low voltage and high amperage. Depending on the application, the voltage is kept under 50 volts (typically below 20 volts). Lower voltage reduces the risk of arcing and plasma effect, especially if the system is running in partial gas pressure. Plasma or ionization is undesirable and will cause damage to most furnace systems.
Another design factor involves how the element is going to be supported. Large elements may have multiple power pins that the element hangs from. A small element for three-phase operation would only have three power pins. A larger element may have six to 18 or more power pins. (The number of power pins for larger elements is always divisible by three.) Single-phase elements are usually smaller and have two or four power pins.
Understanding tungsten mesh heating elements can help ceramic manufacturers fire their products more effectively. These heating elements are the first choice for an ultra-clean heating source in controlled atmosphere applications.