Trace heating
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Electric trace heating, 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.
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[edit] Uses
The most common trace heating applications include:
- Frost protection
- Temperature Maintenance
[edit] Frost protection
Every pipe or vessel is subject to heat loss when its temperature is greater than ambient temperature. Thermal insulation reduces the heat loss but does not eliminate it. Trace heating is used to replace the heat that is lost. If the heat replaced matches the heat lost, temperature will be maintained. A temperature sensor can be used to regulate a set temperature.
For example, a 15 mm copper water pipe at 5 °C with 25 mm insulation in an ambient temperature of -15 °C will lose 4.39 watts per metre. To prevent the pipe from freezing, much thicker insulation could be applied but this is both bulky and expensive. A trace heating system can supply the extra heat to prevent freezing. A thermostat is used to sense the ambient air temperatures and control the trace heating to maintain around 3 to 5 °C in the pipe.
Typical residential applications for trace heating are the protection of water pipes against freezing and placement on roofs or gutters to melt ice during winter months. When trace heating is used in conjunction with common foam pipe insulation, the insulation will often melt, and precautions should be taken to avoid this.
[edit] Temperature maintenance
Hot water service piping can also 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 insulation for the operating ambient temperature maintains a thermal balance where the heat output from the trace heating matches the heat loss from the pipe. Self limiting or regulating heating tapes have been developed and are very successful in this application.
A similar principle can be applied to process piping carrying fluids which may congeal at low temperatures, for example, tars or molten sulfur. Hit-temperature trace heating elements can prevent blockage of pipes.
Industrial applications for trace heating range from chemical industry, oil refineries, nuclear power plants, food factories. For example, wax is a material which starts to solidify below 70 °C which is usually far above the temperature of the surrounding air. Therefore the pipeline must be provided with an external source of heat to prevent the pipe and the material inside it from cooling down. Trace heating can also be done with steam, but this requires a source of steam and may be inconvenient to install and operate.
In laboratories, researchers working in the field of materials science use trace heating to heat a sample isotropically. They may use trace heating in conjunction with a variac, so as to control the heat energy delivered. This is an effective means of slowly heating an object to measure thermodynamic properties such as thermal expansion.
[edit] Different Technologies
[edit] Constant Wattage "Series"
A series heating cable is made of a run of high-resistance wire, insulated and often enclosedin 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 upside 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.
[edit] Constant Wattage "Zone"
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, a 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.
[edit] Self Regulating
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.
[edit] 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.
[edit] Control System
[edit] Industrial
The Supply: The three phase systems are fed via contactors similar to a three phase motor 'direct on line' starter which is controlled by a thermostat somewhere in the line. This ensures that the temperature is kept constant and the line does not overheat or underheat.
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Boost: If a line becomes frozen because the heating was switched off then this may take some time to thaw out using trace heating. This thawing out is done on the three phase systems by using an 'auto transformer' to give a few more volts, and so amps, and make the trace heating elements a bit hotter. The boost system is usually on a timer and switches back to 'normal' after a period of time. Again this is not necessarily true, this is commonly known as heat raising, the problem with this is that you require approximately 4 times the heat loading per meter to heat raise pipe work or vessels, most designs allow for either maintain or heat raise, it is practically easier to set up a system capable of heat raising the pipe work and then control it using a thermostat, this ensures that the system ran be turned up should the need ever arise.
