Trace heating
From Wikipedia, the free encyclopedia
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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[edit] Uses
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.
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..
[edit] 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.
[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 ensure 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.
