Trace heating

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.

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.

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.

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.[7]

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.

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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.

A constant wattage cable is composed of multiple constant electric power zones and 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. 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 applied.[2]

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, on the other hand damaged sections of cable (usually 3 ft span) will stay cold and possibly lead to freeze ups on said section. Also, this cable can be cut-to-length in-field due to its parallel circuitry, however, due to the circuit only running to the last zone on the cable, when installing on site you normally have to install slightly beyond the end of the pipe work. When installing constant wattage, or any heat tracing cable, it is important to not overlap or touch the cable to itself as it will be subject to overheating and burnout. Constant wattage cable is always installed with a thermostat to control the power output of the cable, making it a very reliable heating source.

The disadvantage of this cable is that most constant wattage cables do not have soldered connections to the bus wires but press on type contact and are therefore more prone to have cold circuits due to loose connections caused by cable manipulation and installation.

Self-regulating heat tracing tapes are cables whose resistance varies with temperature - low resistance for temperatures below the cable set point and high resistance for temperatures above the cable set point. When the cable temperature reaches the set point, the resistance reaches a high point, resulting in no more heat being supplied.

These cables use two parallel bus wires which carry electricity but do not create significant 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 so the cable is inherently power saving and only delivering heat and power where and when required by the system. The cables are manufactured and then irradiated and by varying both the carbon content and the dosage then different tape with different output characteristics can be produced. The benefits of this cable are the ability to cut to length in the field. It is more rugged, and much more reliable than a constant wattage cable; it cannot over-heat itself so it can be crossed over, but it is bad practice to install tape in this way. Self-regulating and constant wattage heating cables have specific maximum exposure temperature, which means that if they are subject to high temperatures then the tape can be damaged beyond repair. Also self-limiting tapes are subject to higher inrush currents on cold starting up similar to an induction motor, so a higher rated contactor is required.