Trace Heating for Pipe-work & Vessels
Electric trace heating is also known as electric heat tracing or surface heating, & is a system used to maintain or raise the temperature of pipes and vessels.
Trace heating takes the form of an electrical heating element run in physical contact along the length of a pipe.
The pipe must then be covered with thermal insulation to retain heat losses from the pipe. Heat generated by the element then maintains the temperature of the pipe.
A series heating cable is made of a run of high-resistance wire, insulated and often enclosed in a protective jacket. It is powered at a specific voltage and the resistance of the wire creates heat.
The downside of these types of heaters is that if they are crossed over themselves they can overheat and burn out, they are provided in specific lengths and cannot be shortened in the field, also, a break anywhere along the line will result in a failure of the entire cable.
The positive element is that they are typically inexpensive (if plastic style heaters) or, as is true with Mineral Insulated heating cables, they can be exposed to very high temperatures.
Mineral Insulated heating cables are good for maintaining high temperatures on process lines or maintaining lower temperatures on lines which can get extremely hot such as high temperature steam lines.
Typically series elements are used on long pipe line process heating, for example long oil pipe lines and quay side of load pipes on oil refineries.
A constant wattage zone cable is made by wrapping a fine heating element around two insulated parallel bus wires, then on alternating sides of the conductors a notch is made in the insulation.
The heating element is then normally soldered to the exposed conductor wire which creates a small heating circuit; this is then repeated along the length of the cable.
The benefits of this system over series elements is that should one small element fail then the rest of the system will continue to operate, one draw-back of this system is that is length is limited to the notch distance, so when installing on site you normally have to install slightly beyond the end of the pipe work .
It is still subject to overheating and burnout if overlapped, but this is generally bad practice to overlap when installing.
SR Cable. Bus wires (1), SR polymer (2)
Self-regulating cable uses two parallel bus wires which carry electricity but do not create 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 the 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 then 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 this means that if they a 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.
Power supply and control:
Trace heat cables may be connected to single-phase or (in groups) to three-phase power supplies.
Power is controlled either by a contactor or a solid-state controller. For self-regulating cable, the supply must furnish a large warm-up current if the system is switched on from a cold starting condition.
The contactor or controller may include a thermostat if accurate temperature maintenance is required, or may just shut off a freeze-protection system in mild weather.
Electrical heat tracing systems may be required to have Earth Leakage (Ground Fault or RCD) devices for personnel and equipment protection.
The system design must minimize leakage current to prevent nuisance tripping; this may limit the length of any individual heating circuit.
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