Electric trace heating, also called "heat trace" and "heat tape", is a system used to heat pipes. Trace heating consists of a long cable-like electrical heating element run in physical contact along the length of a pipe. The pipe is then usually covered with thermal insulation to retain heat. Recent technological developments look to slowly phase out traditional trace heating, replacing it with more efficient technologies. These including heatable polymers that have the added benefits of providing uniform heat, being much more energy efficient (a key consideration due to rising energy prices and climate change) and having greater flexibility.
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Uses
The most common trace heating applications include:
- Maintaining pipe temperatures (includes pipe-freeze prevention);
- Underfloor heating;
- Snow and ice prevention / melting;
- Anti-condensation.
Maintaining pipe temperatures
Process pipes and vessels cool when their temperature is greater than ambient temperature. Insulation reduces the rate of heat loss but does not cancel 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 thermostat or proprietary controller can be used to control the trace heating to maintain a set temperature. Maintaining a pipe's temperature above 0°C will prevent it from freezing. Most commonly, a thermostat is used to control the trace heating to maintain around 3 to 5°C. The thermostat's set-point temperature is a few degrees higher than 0°C to provide a little margin for errors and wind chill. The temperature of domestic hot water service piping is often maintained using trace heating. A flow and return system can be abandoned in favour of a single leg or 'flow only' system. Trace heating is installed on virtually all of the pipework to maintain around 55°C pipe temperature. The combination of application-specific self-regulating trace heating and prescribed insulation maintains a thermal balance where the heat output from the trace heating matches the heat loss from the pipe. Underfloor Heating
to be added soon Snow and Ice Prevention / Melting
to be added soon Anti-Condensation
to be added soon 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. Industrial applications for trace heating includes use in places such as chemical factories and oil refineries. For example, wax is a material which starts to soldify 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. This can also be done using very thin steam pipes instead of an electric heating element but is prone to leaks etc... There also exist laboratory applications for trace heating. 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.
Different Technologies
Constant Wattage "Series"
Most people will be familiar with this style heater from a toaster or electric oven. A cable is produced that has an undersized bus wire. 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.
Constant Wattage "Zone"
Much like the series heater, the zone heaters produce heat by under-sizing the heating element. The difference is that zone heaters have two insulated bus wires, the heating element is attached to alternating bus wires at "nodes" which occur at predetermined spacing - typically every two to six feet. This allows the heating cable to be terminated in the field and if a section of the heating element is broken, the loss of heat is contained to only that two to six foot section. It is still subject to overheating and burnout if overlapped.
Self Regulating
The construction of SR cable is different from the other two. It has two parallel bus wires which carry electricity but do not create heat. They are encased in a semi-conductive polymer which allows electricity to flow between the bus wires and changes it's heat output based on the temperature around the cable. 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 and reliable than series or zone heaters, it cannot over-heat itself. Self regulating heating cables have a specific maximum eposure temperature based on the type of polymer which is used to make the heating core.
Electric Trace Heating Phases
Three Phase: This electric heating can be three phase or single phase. The three phase elements are tied to the pipe so that none of the element cross each other otherwise there is a 'hot spot' formed. The three element phases then connect to a 'star' point at the end of the run. If one element (phase) develops a fault then it is possible to disconnect that element from the 'star' point and supply and run on just two phases. The faulty section can also be bypassed on the outside of the pipe as a 'quick fix' (Note: Current will increase on the two remaining phases) Single Phase: The single phase trace heating is known by the trade name 'self regulating tracers' This cable has two bare bus-bars inside the cable insulation which run from end to end (ie: Live and Neutral) which are kept separated and must not, in this case, be joined in a 'star' point. The bare bus-bars are separated from each other by a carbon type material which has a resistance and conducts which produces the heat. When this type of heating is first switched on the resistance is very low and looking at the ammeter you will notice a very high current (Similar to an 'induction' motor start) As the cable heats up the resistance rises and the current drops dramatically. A light can be installed between live & neutral at the very end to indicate that voltage is on the circuit.
Control System
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.
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. Earth Leakage Protection: It is recommended that a 30 mA Earth Leakage Unit/RCD is installed in the feeder contactor circuit for personnel protection although if the system is old this has a tendency to nuisance trip.
