Figure 1. Heat transfer from the wire to air is direct, fast and gives excellent control response.


There is no limit to the ways in which you can deliver resistance heating to the material in your process. Chances are you will be looking for a heater that incorporates one or other of the many nickel/chrome alloy wires and ribbons. Reasons why:

  • Low cost.

  • Widest choice of size and shape.

  • Versatility of construction.

  • Robust.

  • Easily controlled due to minimal resistance change with temperature and service life.

  • Tolerance to thermal cycling.

  • Widely available, which means you'll have little downtime.



    Figure 2. Ovens and kilns can use the open-coil design, but they are supported in the grooves of the cast-ceramic wall or embedded in a ceramic-fiber liner.

    Shapes and Sizes

    The bare wire helix is as simple as they come (figure 1). Examples range from the hair-dryer to large open coil air-duct heaters. Heat transfer from the wire to air is direct, fast and gives excellent control response. Make sure that your temperature sensor also is fast responding and close to the air outlet. A fast flow-fail switch is essential to avoid heater burnout. To minimize sag and withstand overheating, an 80/20 nickel/chrome wire is a popular choice. Pressure drop across an open-coil heater is low because of the high percentage of open space.

    Some radiant heaters use open coils inserted into a quartz tube where both sag and exposure to bare wire are prevented. Avoid the term “quartz heater” for this design. The wire is the heater. This term also is misused to describe the large tungsten-in-quartz tubular radiant heater.

    Figure 3. With calrod heaters, the heater coil is enclosed in a metal tube that is hard packed with magnesium oxide (MgO) powder.
    Ovens and kilns can use the same open-coil design, but this time they are supported in the grooves of the cast-ceramic wall or embedded in a ceramic-fiber liner (figure 2). Heat transfer to the work is more radiant than convective.

    Tubular Heaters. Commonly called Calrod heaters, tubular heaters take their name from the GE registered trademark, which has become generic by common usage (figure 3). The heater coil is enclosed in a metal tube that is hard packed with magnesium oxide (MgO) powder. You see them on stove-hobs, inside ovens, on immersion heaters, beaten down into the grooves of platens, or cast in aluminum.

    It is important that the heat be allowed to get away and do its job; otherwise, the wire, in its cosy MgO insulation, will burn out fast. Radiant heaters and hobs usually come to a safe equilibrium temperature by radiating into the air. Immersion heaters readily burn out when they become exposed or when there is local buildup of calcium and other dissolved solids that impede heat flow into the water. There is usually a cold (low resistance) section, built in at each end of the heater to minimize heat degradation at the terminations.

    Tubular heaters are offered with a choice of sheath materials for different temperatures and environments. Examples are steel, copper, titanium, Incoloy 800, Incoloy 840, Inconel 600 and 304 and 316 stainless steel.



    Figure 4. Barrels, nozzles and dies use metal-sheathed, mica- or MgO-insulated, slightly flexible, band heaters.
    Heaters for Plastic-Processing Machinery. Injection molders, extruders and blow molders, for example, take in heat by conduction. Barrels, nozzles and dies use metal-sheathed, mica- or MgO-insulated, slightly flexible, band heaters (figure 4). A cast-in-aluminum design comes in two half shells, often incorporating water or oil pipes for liquid cooling, or fins for blown-air cooling. Precise machining to the barrel dimensions is essential for this type.

    Both types clamp snugly around the cylindrical surface that takes in the heat (typically a barrel) and conducts it through the thick wall to melt the polymer.

    To maximize heat flow, the mating surfaces must be clean and tight with no air gaps. Retightening when hot improves thermal contact. A loose fit will certainly lead to heater burnout and an underheated zone. The working temperature of the wire even under normal conditions could be some 900oF (500oC) greater than that of the polymer. You have a few grams of wire, eager to unload about 5 kW into some 50 kg of steel barrel so, to avoid rapid wire-burnout, you had better make sure that it can get there.

    Some heater bands -- and linear heaters -- are constructed like tank tracks made with segments of ceramic. Heater coils are threaded through the tunnels in the segments and radiation plays a part in heat transfer. Oil and polymer spills can be a threat to the live-wire coils. PH

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