Heaters that incorporate one or other of the many nickel/chrome alloy wires and ribbons are widely popular, due to attributes such as low cost; a wide choice of size and shape; versatility of construction; robust performance; easily controlled due to minimal resistance change with temperature and service life; and tolerance to thermal cycling.
First, a note on part 1 of this column (April 2006). Near the end of the article, it read: 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.
The following sentence should have said: 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.
Now, continuing my discussion of these versatile tools, I'll pick up with heaters for plastic-processing machinery, and I turn my attention first to cartridge heaters.
Cartridge and tubular heaters are used in dies, platens and molds. As with band heaters, a snug fit and good thermal contact are essential. The hairpin type employs a D-shaped profile, doubled back to make a resilient round profile that provides internal pressure to the cavity.
Plastic Bag-Making Machines (Impulse Sealing). Here you may see a single nickel/chrome wire, suitably supported, taking the place of the familiar sealing bar. The wire or ribbon receives a precise and brief dose of energy that corresponds to the amount of heat required to make the seal.
Choice of AlloyIf you are making that choice, you will consider atmosphere tolerance, sag and the working temperature of the wire -- as opposed to that of the process.
Too Many Names. Each of the different alloys bears the manufacturer's or supplier's pet name. For example, at http://www.resistancewire.com/mainpage.php?page=crossref, there is a table of 16 suppliers and 22 alloys, all with different proprietary names.
When making your choice, look at the composition. This defines the generic description. At this stage, the alloys will become comparable between different suppliers; in effect, they become commodities. That means you can shop for the best buy. This is not welcome news for suppliers because they like to distinguish their products in some way and get better prices. (To the great advantage of users of thermocouple alloys, this kind of name confusion has been largely avoided by the worldwide adoption of letter codes -- Type J, K, T, etc.)
For all that confusion, your best source of specifications, application information and advice comes from the suppliers themselves who may have created the confusion. Excellent sources are the web sites of such companies as watlow.com, kanthal.com, resistancewire.com, pelicanwire.com and the many other respected suppliers.
One manufacturer, Sandvik, after a description of a particular alloy, states: “This data sheet is only valid for Sandvik material. Other material, covering the same international specifications, does not necessarily comply with the mechanical and corrosion properties presented in this datasheet.” This might imply some distinguishing properties that place its alloy in some way superior to the international specification. I recommend diligent comparison of specifications.
A Popular Alloy. One alloy -- 60 percent nickel, 16 percent chromium and 24 percent iron -- has been referred to as the world's standard of comparison for resistance wire. It is the accepted material for heating devices operating up to 1,832oF (1,000oC). This includes those heating units of medium temperatures that do not require the quality of Ni/Cr-80/20 and most power corded domestic heating appliances. These two alloys are compared in the table 1. The unit of resistivity shown here is meant to convey the microhm-cm unit.
Other Alloys. In addition to the NiCr alloys, there is a class of CrAlFe-based alloys that pushes the wire temperature limits to some 2,550oF (1,400oC). They have a place, particularly in glass and pottery kilns. The design is open coil, nested in grooves in the ceramic or ceramic fiber walls of the kiln.
Common (not generic) names for this class of alloys include Kanthal A-1 by Sandvik. Sandvik's figures 1 and 2 neatly summarize their place in relation to the NiCr alloys, shown in the lower temperature group in the pictures.
Warning on Units of ResistivitySandvik's unit on the graph (mm2 m-1) is confusing and is not the commonly used microhm-centimeter (microhm-cm) unit. To use this graph in microhm-cm, read the figures 0.900 to 1.600 as 90 to 160 microhm-cm. For comparison, copper has a resistivity of 1.6 microhm-cm at 68oF (20oC), so it is rated as a good conductor.
If you don't want to deal with the rest of the world, you can of course stick to that antique unit, the circular mil foot (the resistance of a foot of the material 0.001" dia).
Change in resistance from room temperature over the working range can be 4 or 5 percent to some 25 percent, depending on the alloy. Resistance changes very little during service life. This makes it easy to detect and warn of failure of a single heater or one of several parallel branch heaters.
For alloys that change resistance significantly above safe working temperatures, you have the possibility of detecting this change by your control instrumentation and heading off a burnout. Control is usually by low-cost electromagnetic or solid-state contactors working in the time-proportioning mode.
Speed of Response. Heaters such as cartridge, band and tubular metal magnesium oxide (MgO) insulated types have thermal time constants upward of 1 to 2 min. Here, cycle times around 10 or 20 sec would acceptable.
Low-mass spiral-wound radiant heaters in quartz tubes or refractory panels -- and open-coil heaters in airstreams -- all need fast cycling or phase-angle control because of their fast response. Slow cycling could give process temperature variations in sympathy with the cycle time.
RMS analog ammeters and voltmeters on your heaters can give valuable clues to heater condition and help the operator sense the heartbeat of the process. Unlike most high-temperature heater materials, those described here are in the docile class. Current goes up largely in proportion to voltage, and you can readily monitor heater health. PH
Author's Note: I thank Zesta Engineering, Watlow, Hyndman and Sandvik for valuable information used in this column.