Once process equipment is designed and placed into service, the importance of each individual component is frequently forgotten. Unfortunately, heaters often find themselves in this situation until there is a failure. Since heaters usually play an integral role within the application, any failure can rapidly expand to a major issue or shutdown. The good news…many of these disasters can be avoided by increasing the mean time between failures!
Let’s discuss how heaters fail and what can be done to maximize their service life.
During normal heater usage, nickel-chrome (NiCr) alloys form a protective chromium oxide (Cr2O3) layer when heated to 800-900°C in the presence of air. The oxide layer is relatively thick and greenish in color and has a tendency to flake off during thermal cycling. This flaking exposes the base metal to further oxidation, eventually leading to element failure when the chrome is depleted.
At temperatures above 1000°C, Iron-chrome-aluminum (FeCrAl) alloys form an oxide comprised mainly of alumina (Al2O3). Like chromium oxide, an iron-chrome-aluminum oxide is very stable. However, it has one very important difference. It is very thin and adheres very well to the base metal, making it less likely to flake off and lead to product contamination. As the element is thermally cycled, small cracks may develop in the oxide, which eventually will lead to aluminum depletion in the base metal.
Below the temperatures mentioned above, the oxide layers are quite complex but less conducive to preventing heater failure. In general, all resistance heating alloys have a practically unlimited life below 600°C provided they are not exposed to any chemically, mechanically, or electrically damaging conditions.
Although resistive alloy heaters are inherently self-destructive, there are many practices that will aid in preventing a premature heater failure.
Contamination is a frequent cause of heater failure. Do not expose elements to any liquids, conductive or nonconductive solids, or contaminating gases. Always ensure your heater supply air is clean and contains no moisture or lubrication. Also keep in mind when not in use, many processes allow product and/or byproduct to flow upstream and contaminate the heater from the exhaust end.
Excessive temperature cycling is very detrimental to the life of a heater. The most detrimental is the cycle rate that allows full expansion and contraction of the heater resistance alloy. A heater’s wattage should be matched as closely as possible to the application’s actual load requirements to limit thermal cycling. The problem with cycling is the oxide will either crack or spall off exposing the base material to further oxidation and eventual failure. The most effective way to minimize heater element temperature cycling is to use a process thermal controller. A solid-state relay and PID temperature controller combination provides the best performance for both the thermal system as well as for the heater itself. Solid-state switching devices cycle power to the heater very rapidly. This fast-power cycling dramatically reduces heater element wire temperature excursions and substantially extends heater life.
Just as the expansion and contraction of thermal cycling will damage the oxide layer, excessive movement and vibration can be equally destructive. During high levels of movement and vibration, the element may contact conductive or nonconductive structures and supports within the heater assembly. Contact with conductive objects may lead to instant failure by ground short. Contact to nonconductive objects may physically damage the oxide layer and may even damage base metal structure under extreme conditions. It is best to isolate the heater from as much movement and vibration as possible to prevent this type of damage.
It is essential to ensure a heater’s rated voltage matches the supply voltage. Heater wattage increases with the square of the increase in voltage applied. This will negatively impact heater life by significantly raising element operating temperatures.
Perhaps the most important practice is to properly design the process around the heater. Ensure controls are in place to prevent heater failures caused by system failures. Use flow sensing devices to prevent heater operation during periods of insufficient airflow. Devices to monitor process air or heating element temperatures should also be incorporated to ensure temperatures cannot exceed established threshold limits. The use of these types of devices can nearly eliminate the possibility of immediate heater failure during an outside-of-normal system operating condition.
These are only a few examples of conditions that can significantly reduce the life of a heater system. Take time to evaluate your system to ensure you avoid premature heater failure.