Wednesday, December 8, 2010

Heat Trace Cable - Heating Cable

Heating Cable

As winter is upon us, it is time to think of freeze protection and keeping pipes warm. Heating Cable is one way to do this, we sell electric heat trace. All heat tracing requires INSULATION. It will not work without it.

There are two main types of heating cable; self regulating and constant wattage.

Self-regulating cable
Self Regulating Heating Cable has two parallel conductors or bus wires extruded in a polymer core. The core's resistance changes porportionally with temperature.

The key to understanding Self-regulating cable is the output adjusts proportionally to the cable sheath temperature
not the temperature of the pipe heated.

There is
no direct feed back for the pipe temperature. Most self-regulating cables are designed to give their rated wattage at 50 degrees F. The wattage output willexceed the rated wattage when the cable temperature is below 50 degrees and the lower the cable temperature the larger this wattage i.e. amperage is. INRUSH!! A high start up current is the result. Tight temperature control is also more difficult with self-regulating cable, because the wattage output is always changing seeking 50 degrees F on the cable not the pipe or fluid in the pipe.

Self-regulating cable works quite well for freeze protection applications. Freeze protection and low temperature applications that do not require very tight control. The inrush can be considered on the design of wiring and breakers, and there are some other cost savings to consider. A thermostat is recommended for most applications. However, when there are several very short runs or several pipe size changes one ambient thermostat can save on a job.

The semi-conductive core contains a graphite network, which allows electricity to flow from one bus wire to the other. When the core is dense and colder, there are many paths for electricity to take through the graphite network, producing more heat.

Since the core material expands as it heats, the graphite network is elongated, disrupting some of the paths. More and more paths are disrupted as heating continues until the system reaches self-controlled thermal stability. When the core material cools it contracts, reconnecting some of the electrical paths in the graphite network, and more equivalent heat is produced.

This temperature response occurs independently at each point along the heater. If an externally produced high temperature occurs next to a low temperature in the cable, each section of heating cable will adjust its own heat output in relation to its own local requirements.

Advantages:
  • Cut to fit in the field.
  • Possible lower operating cost.
  • Possible lower installation cost.
  • Wide variety of wattages
  • Most tolerant to overlapping
  • Low Cost

  • Disadvantages:
  • High start-up currents
  • Difficult to precisely control temperature
  • Not well suited for procsss control
  • Low design temperatures

  • Constant Wattage Cable is made up with two parallel conductors or wires. These wires have a heater wire wrapped around and connected in approximately 2-foot intervals in parallel. This design provides constant wattage output or a uniform output all along its length. The heater wires are designed such that if any section of cable should fail the rest of the heater would continue to operate without change.

  • Constant wattage cable should NOT be overlaped. The effect of double the wattage where the cable crosses will cause high temp failures.

    The constant wattage output makes maintaining higher and more accurate process temperatures easier. This cable is well suited for freeze protection and many low temp process control applications.

    This cable can be cut in the field. When terminated the last 2-foot heater section becomes a cold sections "unheated". This provides an unheated section of cable to penetrate the insulation, enter the junction box and make up power connections. Constant Wattage Cable needs a thermostat on each pipe and each pipe size in order to maintain accurate temperature control.


    Advantages:
  • Cut to fit in the field.
  • Easily controlled
  • Built in cold length at every power connection
  • Wide variety of wattage outputs
  • Low cost
  • 480 Volt Cable available
  • Design Temperatures up to 500° F
  • Disadvantages:
  • Requires pipe sensing thermostats
  • CANNOT be overlaped

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