Trace heating

Electric heat tracing, heat tape or surface heating, uses heat tracing cables to maintain or raise the temperature of pipes and vessels. This system involves an electrical heating element contact with the pipe, which is typically covered with thermal insulation to retain heat and reduce losses. The heat generated maintains the pipe's temperature. Applications include freeze protection, maintaining flow temperature in hot water systems, and ensuring process temperatures for substances that solidify at ambient temperatures. Electric trace heating cables offer an alternative to steam trace heating when steam is unavailable or undesired.

Development Electric trace heating began in the 1930s but initially no dedicated equipment was available. Mineral insulated cables ran at high current densities to produce heat, and control equipment was adapted from other applications. Mineral-insulated resistance heating cable was introduced in the 1950s, and parallel-type heating cables that could be cut to length in the field became available. Self-limiting thermoplastic cables were marketed in 1971. Control systems for trace heating systems developed from capillary filled-bulb thermostats and contactors in the 1970s to networked computerized controls in the 1990s, in large systems that require centralized control and monitoring. One paper projected that between 2000 and 2010 trace heating would account for 100 megawatts of connected load, and that trace heating and insulation would account for up to CAD $700 million capital investment in the Alberta oil sands. International standards applied in the design and installation of electric trace heating systems include IEEE standards 515 and 622, British standard BS 6351, and IEC standard 60208.

Uses The most common pipe trace heating applications include:

Freeze protection Temperature maintenance Snow Melting On Driveways Other uses of trace heating cables include:

Ramp and stair snow / ice protection Gulley and roof snow / ice protection Underfloor heating Door / frame interface ice protection Window de-misting Anti-condensation Pond freeze protection Soil warming Preventing cavitation Reducing Condensation On Windows

Freeze protection

Every pipe or vessel is subject to heat loss when its temperature is greater than ambient temperature. Thermal insulation reduces the rate of heat loss but does not eliminate it. Trace heating maintains the temperature above freezing by balancing heat lost with heat supplied. Normally, a thermostat is used to energise when it measures temperature falling below a set temperature value - usually between 3 °C and 5 °C and often referred to as the 'setpoint'. The thermostat will de-energise the trace heating when it measures temperature rising past another set temperature value - usually 2 °C higher than the setpoint value.

Gutter and roof de-icing

Placement of heat trace cable on roofs or in gutters to melt ice during winter months. When used in gutters the cable is not meant to keep the gutters free of ice or snow, but only to provide a free path for the melted water to get off the roof and down the downspout or drain piping.

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.

Anti-cavitation purpose As heating a thick fluid decreases its viscosity, it reduces losses occurring in a pipe. Therefore, the net positive suction head (pressure difference) available can be raised, decreasing the likelihood of cavitation when pumping. However, care must be taken not to increase the vapour pressure of the fluid too much, as this would have a strong side effect on the available head, possibly outweighing any benefit.

Types

Constant electric power "series" 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 heat 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.

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