Using PTC Heaters in Metal Finishing Processes
Metal finishing businesses, which often use immersion heaters in their chemical baths, emphasize continuous improvement as well as improving safety and avoiding downtime.
Heaters that use positive temperature coefficient (PTC) chips as the heat source provide metal finishers, and others, with heaters with a self-limiting temperature capability. When used properly, heaters heated with PTC chips eliminate the potential for overheating and equipment damage.
Figure 1. When voltage is applied to resistance wire, it heats up.
Electric Heaters Made with Resistance Wire
Traditionally, immersion heaters use resistance wire as the heat source (figure 1). Although they may not know it by its proper name, most people are familiar with resistance wire. It is used in many heating devices, including household items such as toasters, electric space heaters, electric water heaters, and electric ranges and ovens. Whether used in consumer or industrial heaters, resistance wire works the same way. When voltage is applied to resistance wire, it heats up.
Industrial heaters that are designed to heat water such as those used in metal finishing operations typically operate around 900°F (482°C) internally. The heat output is a function of the supply voltage and the heater resistance, represented by the equation
Watts = (Voltage2/Resistance)
Because the heater’s resistance measurement is constant, the heat output will be constant regardless of the surroundings, operating environment or application.
When an electric immersion heater using resistance wire is subjected to less than ideal operating conditions - for instance, being covered by buildup, or operation in air due to a liquid level drop - the heat generated by the wire is not able to dissipate quickly enough. This results in a rapid increase in surface and internal temperatures. These elevated temperatures shorten the heater service life and can damage surrounding materials such as plastic tanks or piping.
To protect surrounding materials from high-temperature damage, immersion heaters utilizing resistance wire are required to include a heater sheath overtemperature cutoff device. Also known as a protector, the cutoff device is installed in a thermowell alongside the heater tube. Protectors are designed to detect the surface temperature of the heater and trip if the temperature exceeds a predetermined value. Most often, protectors are wired into the temperature control circuit, but some may be wired directly in series with a low wattage heater operating below 240 V.
If a properly wired protector trips, the heater will shut off, thus preventing the heater from overheating and possibly starting a fire. One downside is that once the device trips, the heater remains shut down until the protector is replaced.
Electric Heaters Made with PTC Chips
Figure 2. The resistance value of the chips will increase as their temperature increases. This resistance change is not linear.
Electric heaters that use PTC chips do not use resistance wire as a source of heat. Instead, they use ceramic PTC chips as the heating source.
PTC is an acronym for positive temperature coefficient, which means that the resistance value of the chips will increase as their temperature increases. This resistance change is not linear (figure 2).
PTC technology has been available for many years as a low-wattage source of electric heat. Not unlike the resistance wire, PTC ceramic chips are used in many household heaters such as curling irons, heated car seats and some hair dryers. Whether used in household or industrial heaters, PTC ceramic chips are manufactured from barium-titanate along with a few key doping materials. The specific combination of construction materials provides the desired resistance/temperature characteristics (figure 3).
Figure 3. The specific combination of construction materials provides the resistance/temperature characteristics desired in the PTC chip heater.
As noted, when electrical voltage is applied to a PTC chip, heat is not generated at a constant rate. Rather, as the PTC chips heat up, they reach a designed temperature at which the heat output decreases drastically and prevents the chip from getting hotter. Thus, PTC chips have a designed temperature limit (figure 4).
Because the resistance change vs. temperature is not linear, the reduction in heat output is not linear. Therefore, if the hot zone of a PTC heater is exposed to air or covered in buildup, the heat output quickly drops by more than 80 percent of its normal rate while the internal temperature stays at its designed value.
In this way, PTC chips inherently limit the surface temperature of the electric immersion heater and thus do not require a protector. Instead, the overtemperature protection is built into the heater core itself.
Figure 4. As the PTC heater heats up, it reaches a designed temperature at which the heat output decreases drastically and prevents it from getting hotter. Thus, PTC chips have a designed temperature limit.
PTC technology offers advantages over traditional resistance heaters.
Increased Safety. The designed temperature limit of PTC immersion heaters results in a maximum surface temperature of approximately 518°F (270°C). This surface temperature is independent of the surrounding environment - even if the heater is operated in air. This temperature also is far below the ignition temperatures of materials used in immersion tank and liner construction. When used correctly, PTC heaters operating in air will not ignite tanks made from polyethylene (PE), high-density polyethylene (HDPE), fiberglass, polypropylene (PP), CPVC or PVC (figure 5).
Although it cannot ignite plastic tanks and tank liners, the surface of a PTC heater can get hot enough to melt them if in direct contact. Therefore, PTC heaters still require bumpers (minimum 0.5" recommended) to ensure the surface does not touch surrounding materials.
Longer Life. As discussed, a resistance heater will overheat if operated in air or covered by a layer of scale buildup or sludge. The elevated internal temperature actually shortens the service life of the heater, anywhere from a few months to several years.
Figure 5. When used correctly, PTC heaters operating in air will not ignite any of the materials used to construct immersion heating tanks.
The service life of a PTC heater is not adversely affected by operation in air, scale or sludge buildup because the heat output is reduced in these conditions. PTC heaters only generate enough heat to maintain the designed limit temperature. If heat output falls due to buildup or air exposure, the operator must restore the heater to normal operating conditions to return to its designed output. To do so, the user would clean the insulating material from the surface of the PTC heater or raise the liquid level. This will optimize the operating environment and increase the heat output accordingly.
In order to improve the methodologies of wet process heating, PTC industrial heaters have now been developed to heat solutions with additional ease and safety. The concept of PTC heating has been proven with everyday items and it now is suitable for electric immersion heating. PH