Maximizing Heater Performance
Guard against heater contamination
Contamination is the most frequent cause of heater failure. Any organic or conductive material that can be drawn into the heater by expansion and contraction during cycling can cause an arcing failure in the heater windings. If allowed to collect at the lead end of the heater, contaminants can cause an electrical short between the power pins. Do not allow lubricants, oils, low-temperature tapes or processing materials to remain in contact with the lead end of the heater. Specify a moisture seal if necessary.
Protect leads from damage by high temperatures
Standard fiberglass-insulated lead wire may be used to approximately 500ºF (260ºC) ambient. If the lead will be exposed to higher temperatures, high-temperature lead wire or ceramic bead insulation should be used. An unheated section of the heater, protruding from the heated part, could enable the leads to run cooler.
Protect leads from damage by excessive movement
When heaters are mounted in moving machinery, anchor the leads to prevent them from being damaged. Specify a lead protection option.
Heater selection and sizing are important
Match the heater wattage as closely as possible to the actual load requirements to limit ON/OFF cycling. For fitted-part applications, specify the heater size exactly so it can be tightly fitted. Even small air gaps cause hot spotting.
Ground the equipment
It's common sense and a safe practice to electrically ground all equipment on which the heater is used. Grounding protects plant and personnel in case of any electrical failure in the heating system.
Regulate voltage
A 5% variation in voltage results in a 10% variation in heater output.
Prevent excessive cycling
Excessive cycling can be very detrimental to the life of a heater. The worst cycle rate is one which allows full expansion and full contraction of the heater at a high frequency (approximately 30 to 60 seconds ON and OFF).
This problem can be eliminated by using SCR controls with either zero-cross or phase-angle firing, operating on a variable time base of less than one second. A solid state relay using SCRs operates on a one-second time base. This reduces the temperature difference between the overshoot and droop points and increases the life of the heater. The life-shortening effects of cycling can be nearly eliminated by using solid-state controls with voltage or time-proportioning outputs. Even with simple ON/OFF controls much less frequent cycling can be achieved by closely matching the total wattage to the actual requirements.
For immersion applications of heaters
Make sure the sheath material and watt density ratings are compatible with the liquid being heated. Immersion heaters used in tanks should be mounted horizontally near the tank bottom to maximize convective circulation. Vertical mounting is possible where limitations prohibit horizontal orientation. Both methods must be located high enough to be above any sludge buildup in the bottom of the tank. Vertical mounting is not recommended. The entire heated length of the heater should be immersed at all times. This makes vertical mounting difficult to properly effect. Do not locate the heater in a restricted space where free boiling or a steam trap could occur. Scale build-up on the sheath and sludge on the bottom of the tank must be minimized. If not controlled they will inhibit heat transfer to the liquid and possibly cause overheating and failure. Extreme caution should be taken not to get silicone lubricant on the heated section of the heater. The silicone will prevent the "wetting" of the sheath by the liquid, act as an insulator, and possibly cause the heater to fail.
Prevent excessive temperatures in process applications
A reliable temperature control system is imperative to the performance and life of your heaters. The temperature sensing device should be located in a built-in thermowell for maximum accuracy and responsiveness. The protection of a high-temperature limit control can be very important, especially when process temperatures are critical.
Heater Life vs. Power Controller Type and Switching Cycles
Problem:
Increased heater life can result from many factors: a minor change in material, configuration, application, method or component. Extending life improves productivity by increasing throughput and reducing downtime. At Watlow we wanted to know what affect a power switching method and mode had on heater life. Our test conditions used identical cartridge heaters, thermocouples and temperature controls; with the only variable being the power controllers and their minimum time base cycling rates. Power controllers included electromechanical relays, mercury displacement relays, solid state relays (SSRs) and SCRs (Silicon Controlled Rectifiers). Additionally, the SCRs were operated in both burst fire and phase-angel fire modes.
Results:
By using the fastest possible cycling rates on all power controllers to minimize temperature over- and undershoot, both burst fired and phase-angle fired SCRs improved heater life approximately:
1.75 times more than the SSR.
15.5 times more than the mercury displacement relay.
17.9 times more than the electromechanical relay.