Electric Tracing

 

Electric tracing transfers heat differently than other heating systems (i.e., jacketed piping, bolt-on jacketing, and tube tracing).  In steam/liquid jacketing systems, the heating medium is held at a particular temperature.  The temperature difference between the heating medium and the process establishes a predictable heat transfer rate.  In electric tracing, a certain amount of electrical power is supplied to the tracing.  This power is converted to heat energy through resistance heating.  Since the power is fixed rather than the temperature, the heat transfer rates can vary depending upon changes in ambient and/or tracing conditions.
There are three broad categories of electric tracing:

  1. Series Constant Wattage (including MI Cable).  As the name implies, this category of electric tracing provides a constant heat output based on the voltage supplied to the tracing, the length of the tracing, and the resistance of the tracing.  MI (mineral insulated) cable is a special type of series constant wattage tracing that can be used at very high temperatures (up to 1200°F).  The maximum service temperature for other series constant wattage tracing is ~250°F.  MI cable also claims to have better corrosion resistance than other series constant wattage tracing.  RTD temperature indicators are required as over-temperature protection to ensure the tracing/piping does not overheat.  Series constant wattage tracing is pre-fabricated, and field adjustments are difficult and costly since they typically require a brazing specialist to perform.

  2. Power Modulating.  Power modulating tracing is encased in a semi-insulating polymer (tuned for a specific temperature range) whose electrical resistance changes with temperature.  For a constant electrical power input, this results in less heat output at higher temperatures, which reduces the possibility of overheating for some processes.  For other processes that have a smaller operating temperature range, the temperature change required to instigate a resistance change may be too high.  The temperature dependent nature of power modulating trace may require a temperature change of 50°F to adjust the power output 20%.

  3. Skin-Effect.  Skin-effect trace features a small magnetic metal tube that is welded to the pipe wall.  Inside the tube, a non-magnetic conductor is inserted and welded at the far end.  Applying an AC voltage to the inner tube creates an electromagnetic current on the inside wall of the outer tube.  The current in the heating tube generates heat through electrical resistance, causing the entire tube to heat up.  Because of the welded contact between the tube and the pipe, higher heat transfer rates are attained with skin-effect versus the other two categories of electric tracing.  Two major considerations with skin-effect trace are the need for welding the entire length of each trace element to the pipe and the possible need for custom-made transformers to accommodate large power demands. 

 

Thermal Capability
The thermal capability of electric tracing is similar to tube tracing.  Both technologies contact the pipe circumference in a single point, and the limited heat transfer surface area limits the heat transfer rates.  Heating pipe supports, flanges, and other large heat sinks which draw a significant amount of heat away from the process line is difficult with electric tracing.  Electric tracing is most commonly used in freeze protection applications where there a low process freezing point allows for broad temperature control.

 

Safety and Reliability
All types of electric trace eliminate the possibility of cross contamination.  However, electric tracing is considered a fire hazard in some locations because of its potential to spark.  It is prohibited in many plants where processing of combustible gasses or liquids occurs.  While its reliability depends on the specific tracing type and process application, electric tracing generally has high moisture sensitivity.  Particular consideration must be given to protecting the electrical connections from weather.  RTD’s are typically used to protect against overheating, but this is only broadly effective and does not prevent local burn-outs.


Cost
Electric tracing is most often considered in locations where no steam or liquid heating infrastructure exists.  Provided both steam/liquid and electrical infrastructure exists, electric tracing systems are generally at cost parity with bolt-on jacketing and tube tracing.