Trace heating
From Wikipedia, the free encyclopedia
Electric Trace Heating, also known as Electric Heat Tracing and Electric Surface Heating, is a system used to maintain or raise the temperature of pipes and vessels. Trace heating takes the form of a electrical like 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.
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[edit] Uses
The most common trace heating applications include:
- Frost protection
- Temperature Maintenance
- Under floor heating
- Snow and ice prevention / melting
- Anti-condensation.
Maintaining pipe temperatures
Every pipe and vessel is subject to heat loss when their temperature is greater than ambient temperature. To counteract this heat loss insulation is used which reduces the rate of 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 thermostat or proprietary controller can be used to control the trace heating to maintain a set temperature.
Typically a to maintain the temperature of 5°C on a normal 15 mm copper cold water pipe with 25 mm insulation in an ambient temperature of -15°C will lose 4.39 watts per hour, to ensure that the pipe work will not freeze then nearly 5 – 6 times the thickness of insulation is required, this is both unrealistic and impractical on installations.
So to ensure that the 15 mm pipe does not freeze in an ambient temperature of -15°C you will need to replace the heat loss of 4.39 W/hr with a heating element that produces 4.39W/hr.
Maintaining a pipe's temperature above 0°C will prevent it from freezing. Most commonly, a thermostat is used to sense the ambient air temperatures and control the trace heating to maintain around 3 to 5°C. The thermostat's set-point temperature is normally between 3 -5°C to ensure that the heat is being provided before the pipe temperature drops below 3°C where water begins to crystallise.
Recently temperature of domestic hot water service piping is 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 all the hot water service pipe work up a minimum of 1 m from the tap or outlet point to maintain around 55°C pipe temperature, this ensures that hot water can drawn on demand with having to run a flow a return system where the water is being constantly heated by the boiler. 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.
Underfloor Heating
Has become very popular over the last couple of years and is commonly referred to as comfort heating. It is used in one of two was, to remove the chill from tiled floors so that the feel more comfortable to walk on bare footed or the heat the room as opposed to normal hot water heated radiators. In the latter form it has been proved that floor heating is a more efficient way of heating rooms at the heat is directed up into the room form below as opposed to having to convect in form the wall, unsure that the room get a more uniformal heating.
Two forms of under floor heating are available, one that is set into the screed of a floor prior to the tiles being laid over the top of the element, the other being a mat that is laid underneath wooden flooring or carpets.
The under floor heating can also be provided from the hot water system by installing a manifold and pipe work loop into the screed but this takes a little more installation time to complete.
Snow and Ice Prevention / Melting
This form of trace heating provides car park ramps, runways, walk ways, steps, roofs, guttering and even satellite dishes with a system that prevents the formation of Ice and snow.
By sinking the heating element below the surface of the tarmac of block paving it is possible to heat the floor to ensure that the ice and snow do not form on the surface, they are used in conjunction with Ice detectors to ensure that the heating elements only operate when there is moisture in the air and the air temperature is cold enough to form Ice or snow.
The system is also used on satellite earth station systems to ensure that ice and snow do not form on the receiving surface which causes the signal to be distorted, and the TV signal to literally look like a snow storm.
Anti-Condensation
Is where the heating tape is provided to maintain the air temperature above the Dew point either inside an enclosure of a vessel, this heat ensures that the surface and air temperature are kept above to dew point to ensure that moisture does not form on the surfaces.
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 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. 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.
[edit] Different Technologies
[edit] 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.
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"
Does exactly what it says on the tin, it is produced 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
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, this polymer is loaded with carbon and when an electrical current is applied will vibrate on a molecular level, generating heat, as these molecules heat up they will move apart and stop conducting electricity, hense they will self limit themselves, on cooling back down the molecule will then start to vibrate again. The Tapes 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 eposure 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] 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)
this is normally on large industrial applications or long pipe lines.
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.
This is an incorrect statement the trade name is not self regulating tracers, Single phase is your normal house hold supply as we know it. The bus wires are each connected, one to the live and one to the neutral, but the ends of the cable are not connected.
[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.
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 know 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.
Earth Leakage Protection: It is recommended that a 30 mA Earth Leakage (Ground Fault or RCD) Unit be installed in the feeder contactor circuit for personnel protection, although in an older system this may result in unnecessary shut-offs ("nuisance tripping"), with consequent loss of protection if not reset promptly.
