• Electrical India
  • Sep 5, 2017

Self Regulating Heat Tracing Cable

Self-regulating trace heater cables on roofs or gutters are used to melt ice during winter months. When used in gutters the cable is not meant to keep the gutters free of ice or snow, but only to provide to free path for the melted water to get off the roof and down the downspout or drain piping. Similarly, hot water service piping can be traced, so that a circulating system is not needed to provide hot water at outlets. The combination of trace heating and the correct thermal balance where the heat output from the trace heating matches the heat loss from the pipe. Self –limiting or regulating tapes have been developed and are very useful in this application. In this paper authors like to discuss about development, uses and product classification approach to measure the temperature class of thermopads make self-regulating type trace heater model STF 15 J.

- Navin Kumar, A. K. Singh, Arvind Kumar,
Prasad Bhukya, Rakesh Kumar Mishra


 Every pipe or vessel is subjected to heat loss when its temperature is greater than ambient temperature. Thermal insulation reduces the rate of heat loss but does not eliminate it. Self-regulating heat tracing tape is used for pipe freeze protection application. Its resistance varies with temperature. When the cable temperature reaches the set point, the resistance goes high and then no more heat is supplied. This means heat is supplied only where needed along the length of the cable.

  A trace heating system shall be designated so that the sheath temperature of the trace heater is limited to a temperature class or ignition temperature, minus 5oK for temperature less than or equal to 200°C or minus 10oK for temperatures greater than 200°C. Maximum sheath temperatures of trace heaters shall be determined for proper application of the heater.

Development

  Electrical trace heating began in the 1930s. Mineral insulated cables ran at high current densities to produce heat and control equipment was adapted from other applications. Mineral insulated resistance heating cable was introduced in the 1950s, and parallel type heating cables that could be cut to length in the field became available. Self-limiting thermoplastic cables were marketed in 1971.

  Control systems for trace heating systems developed from capillary filled bulb thermostats and contractors in the 1970s to network computerized control in the 1990s, in large system that require centralized control and monitoring.

Figure 1: Self-regulating trace heater cable STF15J for freeze protection in pipes.

Uses

  The most common pipe trace heating applications include:

• Freeze protection
• Temperature maintenance
• Snow melting on driveways
Other uses of trace heating cables include
• Ramp and stair snow / ice protection
• Gulley and roof snow / ice protection
• Under floor heating
• Door / frame interface ice protection
• Window de-misting
• Anti-condensation
• Pond freeze protection
• Soil warming
• Anti-cavitation purpose
• Reducing condensation on windows.

Figure 2: Set up for measuring the sheath temperature of trace heater Cable, STF15J

Self-regulating

  Self-regulating heat tracing tapes are cable whose resistance varies with temperature – low resistance for temperatures below the cable set point and high resistance for temperatures above the cable set point. These cables used two parallel bus wires which carry electricity but do not create significant heat. They are encased in a semi-conductive polymer. This polymer is loaded with carbon as the polymer element heats, it allows less current to flow. The cables are manufactured and then irradiated and by varying both the carbon content and the dosage then different tape with different output characteristics can be produced. There is then an inner jacket which separates the bus wires from the grounding braid. In commercial and industrial cables, an additional outer jacket of rubber or teflon is applied. The benefits of this cable are the ability to cut to length in the field. It is more rugged but not necessarily more reliable than series or zone heaters, it cannot over-heat itself so in theory it can be crossed, but it is bad practice to install tape in this way. Self-regulating heating cables have a specific maximum exposure temperature based on the type of polymer which is used to make the heating core which means that if they are subject to high temperatures then the tape can be damaged beyond repair. Also self-limiting tapes are subject to high inrush currents on starting up similar to ‘induction’ motor so a higher rated contactor is required.

Figure 3: The variation between tracer heater sheath (Ts-To) and oven ambient temperature (To)

Thermo trace Self Regulating Trace Heater Cable

  STF15J is a self-limiting heating tape for applications for freeze protection applications in pipe lines. It comprises of the inner insulation of TPE material and outer jacket of fluoropolymer. As the cable self regulates its heat output, it limit the maximum sheath temperature. Thus, making it burnout proof. Table 1 shows the technical specifications of STF15J.

Figure 4: The variation between tracer (STF 15J) output (W/m) and oven ambient

Construction

1) 1.2 sq. mm coated copper bus wires
2) Semi conductive heating core extruded over bus wires
3) TPE jacket providing electrical insulation, mechanical strength and moisture resistance
4) Aluminum mylar with drainwire / coated copper braid to give a continuous ground path
5) Outer jacket of UV Resistant Flouropolymer (J)/TPE(JT) to enable use in corrosive area.

Determination of maximum sheath temperature

  At least one of the following two methods shall be used for verifying a trace heater’s sheath temperature or temperature class.
a) Product classification approach in which the maximum sheath temperature is determined in an artificial environment simulating adverse conditions.
b) Systems method in which the manufacturer demonstrates the ability to design and predict sheath temperatures of trace heaters by conducting tests on representative installations representing adverse design and operating conditions when installed according to the manufacturers installation instructions.

  We are adopting product classification approach for verifying trace heater sheath temperature or temperature class in which the maximum sheath temperature is determined in an artificial environment simulating adverse conditions. A detailed test procedure is described in the following section.

Determination of Sheath temperature as per Product Classification Approach

  A sample of trace heater at least 1500 mm in length is placed loosely coiled in a forced air circulation oven. For a trace heater pad or panel, a representative sample is placed horizontally in the oven. The sample shall be within the upper half of the trace heater’s thermal output tolerance. Representative thermocouples are used to monitor sample sheath temperatures and are placed approximately 500 mm from each end. One additional thermocouple is used to monitor oven ambient temperature. The trace heater shall be powered at 110% of rated voltage. Table 2 shows the measured data of Thermopads make tracer type STF15J.

  The oven ambient temperature is incrementally raised from room ambient in approximately 15 oK increments. Sufficient time is permitted at each temperature for the oven ambient and heater sheath temperatures to stabilize and attain thermal equilibrium. Oven ambient and heater sheath temperatures are recorded at each successive level until the difference (∆T) between the two approaches 5oK or less. Figure 3 shows the variation between, difference between tracer heater sheath and oven ambient temperature. A straight line is also drawn tangent to the curve at 5oK temperature difference point and extended to the X axis (oven temperature). The temperature read at this intercept is taken as the maximum sheath temperature which can be verified from the test results and curves obtained. Figure 4 shows the variation between tracer (STF15J) output (W/m) and maintain oven temperature °C.

  It is cleared from Table 2 and Figure 4 that tracer STF15J output (W/m) decreases as oven temperature increases.

Instruments Used to Measure Data

1) Digital Ampere Meter 0-10A
2) Digital Voltmeter 0-500V
3) Digital Voltmeter 0-300V
4) Digital Watt Meter 0-200W
5) Digital Watt Meter 15-1000W
6) Digital Temperature Controller 0-400 Deg.C
7) Digital Temperature Controller 0-300 Deg.C
8) Data Logger (8 Channels) 0-1370 Deg. C
9) Ageing Oven 0-300Deg.C with 3 Nos. K-type thermocouple Sensors
10) Ageing Oven 0-400Deg.C with 3 Nos. K-type thermocouple Sensors.

  This is an effective means of slowly heating an object to measure thermodynamic properties such as thermal.

Conclusions

1) Thermo trace self-regulating is used for freeze protection of pipeline.
2) Tracer maintains the temperature above freezing by balancing heat loss within heat supplied.
3) Tracers are cable whose resistance varies with temperature – low resistance for temperature below the cable set point and high resistance for temperature above the cable set point heat.
4) As the cable self regulates its heat output, it limits the maximum sheath temperature, thus making it burnout proof.
5) The maximum sheath temperature of thermo pads make tracer type STF15J was measured 72°C which comes under temperature class T6 (i.e. less than 85°C).
6) The output of tracer STF15J (W/m) decreases as oven temperature increases.


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