If you are designing a control system for a resistance-heated process, you can't ignore the properties of the heater. This two-part series will look at the heater features that affect how you deliver power. They are: resistance change with material temperature and service life; connection arrangement for multi-element heaters; speed of response to power changes; and effect of thermal cycling on life.
I'll look at four of the most commonly used materials: nickel-chrome alloys, silicon carbide, molybdenum disilicide and tungsten. Figure 1 shows how resistance varies for nickel-chrome and silicon carbide, the two heater materials I'll cover in this issue. Note that figure 1 shows element -- not process -- temperature.
Nickel-Chromium AlloysNickel-chromium alloys are the most common -- and for control, the most docile -- class of resistance heater materials. Various formulations, which can include iron, aluminum and silicon, show usable element temperatures up to some 1,400oC. Change in resistance from room temperature over the working range is only some 4 to 6%. It also changes very little during service life, which makes it easy to detect and warn of partial heater failure, simply by monitoring resistance. Control usually is achieved with low-cost electromagnetic or solid-state contactors working in the time-proportioning mode.
Speed of Response. For some heaters -- for example, cartridge, band and metal-sheathed magnesium oxide insulated types with thermal time constants more than one or two minutes -- cycle times around 10 or 20 sec are acceptable. Because of their fast response, others -- low-mass spiral wound radiant heaters in quartz tubes or refractory panels and open-coil heaters in airstreams -- need fast cycling or phase-angle control. Slow cycling could lead to process temperature variations in sympathy with the cycle time. With air heaters, a flow-failure power-cutoff switch is advisable to avoid wire overheating and burnout.
Silicon CarbideThis class of materials has a permissible element temperature close to 1,600oC. The control system has to cope with resistance changes of some 4:1 over the useful life of the element and up to about 3:1 over the working temperature range.
Resistance Change with Life. A new element may need, say 60 V, so a string of four in series would suit a 230 VAC supply. Power modulation by a magnetic or solid-state contactor is common choice. Thermal fatigue of slow cycling magnetic contactor control can severely limit the working life of the elements. Though many such systems are still in service, fast solid-state contactors are being installed to replace them.
Over the service life, element resistance can increase, gradually, up to fourfold. To maintain the power, you may have to reconnect the elements into two parallel paths or find a higher voltage supply. You could use a multi-tap transformer and adjust it periodically. While still widely used, these arrangements are tedious to monitor and adjust. With a series string, the higher resistance elements dissipate proportionally more power and show accelerated aging relative to their partners.
Resistance Change with Temperature. From the silicon carbide curve in figure 1, you can see that you have to cope with resistance change, but the variation is by the minute as you run up to working temperature.
Conclusions So Far. An arrangement of all elements in parallel makes the best self-compensating setup because the higher resistance elements take proportionally more power and age more slowly than their partners. If you must connect elements in series, use the least possible number and select them so that their resistances match to 5% or less. Though you will never need both at the same time, your power supply has to provide enough voltage for maximum resistance and enough current for minimum resistance.
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