Designing high temperature electric heat tracing systems in hazardous areas can be a difficult challenge, especially when process temperatures approach the area classification limit. This restricts the allowable temperature differential between what is heated and the surface temperature of the heaters.
Mineral Insulated (MI) heat tracing technology has been used for over 100 years. MI cables have proven to be robust and reliable when used to operate as a series electrical heating circuit.
Unlike self-regulating or power-limiting cables, MI heating cables are supplied in fixed lengths, so determining and ordering the correct cable length is critical.
In many cases, designs utilizing Mineral Insulated (MI) trace heaters and conventional temperature controllers use an abundance of heater to the point where it becomes nearly impossible to install.
Recent advancements in trace heater designs, engineering design software, and controller technologies have notably improved the options available to heat tracing designers for these more difficult applications. The resulting benefits include fewer trace heater passes per unit length of pipe, lower installation and maintenance costs, and a better match of power output needs to the system heat loss. In many cases, the reduction in trace heater length can be as much as 50% over traditional design approaches. Lastly, the chances of creating designs that are difficult or nearly impossible to install are greatly reduced.
Trace heaters and their related application designs must meet numerous national and international standards, with many of these requirements having been recently harmonized under IEC/IEEE 60079-30. When used in hazardous area locations (i.e., areas defined as potentially explosive), electrical trace heating systems must comply with an additional operational constraint. This constraint requires that the maximum surface or sheath temperature of the trace heater does not exceed the local maximum temperature classification rating (often referred to as a T-Rating) that is governed by the Auto Ignition Temperature (AIT) of surrounding flammable liquids, gases, or vapors in the area.
Traditional methods have been bound by the attributes of the trace heaters. In many cases, this leads to designs that are difficult to install, maintain, and often increases the cost of the overall solution.
Recent developments provide alternative methods using recently improved heater constructions and controller algorithms. Used with engineering design software that can accurately predict heater surface temperatures, these options provide the design engineer with improved flexibility in creating solutions, with lower investment cost and lower operational costs.
New MI heater designs
Several manufacturers offer a wide range of resistances, heater diameters and sheath materials to help mitigate sheath temperature issues. This broad offering enables the designer to select a trace heater output that more closely matches the heat losses, minimizing high sheath temperature excursions. Larger diameter trace heaters inherently improve the sheath temperature versus watt density profile. A recent MI trace heater development that further improves the sheath temperature profile uses the principle of larger diameter and better emissivity.
New control systems with smart adaptive power control
The on/off temperature control algorithm used by thermostats and standard controllers operates on the premise that by monitoring pipe temperature, the heaters can be modulated on and off to supply just enough average energy to match the heat loss. These controllers have no way to adjust for the maximum sheath temperature excursions caused by duty cycling. They monitor temperature, turn on or off based on the temperature set point, and may have long cycle times of several minutes or more depending on the application.
If, in addition to temperature control, the power delivered to the heater is monitored and limited, then the heater sheath temperature can be minimized to closely match the heat loss. The use of solid-state relays and high-performance controllers allows modulation of the control output to be managed many times per second, ensuring that the desired effective (average) power delivered to the heater is precisely controlled.
New generation controllers have the necessary measurement and computational performance to implement nested control loops, with the outer loop delivering temperature control and the inner loop managing trace heater power.
Advanced design software
In addition to the standard trace heater and application considerations, engineering design software must also reflect these effects due to duty cycle modulation and controller algorithms to ensure accurate prediction of sheath temperatures.
Ultimately, Mineral Insulated (MI) heat tracing systems are the ideal choice for applications involving high temperature output requirements or harsh and hazardous environments.
Learn more about MI heat tracing systems here.