Heat Tracing Cables – Self Regulating v Constant Wattage Heating Cables

Heating Cables
What Is Heat Tracing?

Trace heating is the application of a controlled amount of electric surface heating to pipework, tanks, valves or process equipment to either maintain its temperature (by replacing heat lost through insulation, also referred to as frost protection) or to affect an increase in its temperature – this is done by using heating cables commonly referred to as heat trace or heat tracing cables.

The primary function of heat trace cable systems is the prevention of freezing within water pipes and subsequently bursting.

With temperatures dropping during the Winter months, freezing pipes are always a major concern for homeowners, businesses and industry alike. By maintaining the ambient temperature inside the pipe, frost cannot build up and pipes will not freeze.

This means pipes will not burst due to ice expansion – in this article we discuss the pros and cons of 2 types of heating cables; self-regulating and constant wattage.

 💡 Did You Know….. A gallon of water, when frozen, will expand to a volume 9% greater than the original gallon.

In addition to frost protection, heat tracing cables provide heat maintenance and heat recovery in the process and hazardous area industries.

How Do Heat Tracing Cables Work?

This can be achieved simply by connecting a voltage across a length of wire, which will then dissipate a fixed level of power, based on ohms law. In application, such a simplified solution presents certain complications in application.

For one thing, it results in the need to bring both ends of the wire together for connection to the electrical supply, which is not always practical when installing heating cables.

Additionally, it requires the need for a high variety of different resistances to be available, in order to facilitate the design of different outputs at different lengths of heating cable. There are many occasions where this approach is in fact still the best solution.

However, there is also an alternative in the form of parallel heat tracing cables.

Parallel Heat Tracing Cables
Constant Wattage & Self Regulating

Parallel heating cables are typically available in two distinct variants; constant wattage and self regulating (also known as self limiting).

Parallel Heat Tracing Cables

Parallel heat tracing cables use two ‘normal’ copper conductor wires which run in parallel along the length of the wire and form the basis of live and neutral. The heat load is then created by two different methods. In the case of constant wattage cables, a fixed resistance filament is then spiralled along the length of the cable and soldered alternately to the live and neutral wire in fixed distances creating what are referred to as heating zones.

Essentially, every zone is a fixed resistance circuit supplied by a fixed voltage, providing a constant wattage along its length. Since each zone of heating is essentially in parallel with the zone before it, the supply voltage will remain constant along the length of the heating cable, aside from a small voltage drop brought about by the summation of the tiny resistances of the live and neutral wires as the cable gets longer and longer.

The first and most widely needed benefit of heat tracing systems is the prevention of freezing within pipes and with temperatures dropping daily, freezing pipes are now a major concern for homeowners, businesses and industry alike. By maintaining the ambient temperature inside the pipe, frost cannot build up and pipes will not freeze. This means pipes will not burst due to ice expansion.

Self Regulating Heat Tracing Cables

Self regulating or self limiting heat tracing cables in effect also provide a controlled wattage per metre of cable but with a distinct difference, both in terms of construction and performance.

The live and neutral wires are co-extruded into a polymer based material containing particles of carbon, providing a resistance path and hence circuit along the length of the heating cable.

However, this resistance and therefore the output of the heating cable varies depending upon the temperature, due to microscopic expansion and contraction of the polymer.

This type of cable then has the feature of reducing its power output as temperature increases and conversely at lower temperatures, the power is increased.

Self regulating heat tracing cables have an improved level of inherent efficiency as well as increased safety, if its application is correctly considered. Starting with the former, at higher temperatures the heating cable backs off its output, saving power even if not connected via a controller or thermostat.

This is not to say it will hold a fixed target temperature without external control, but the reduction of output as the work-piece temperature increases is a desirable feature from an energy conservation perspective.

This also gives rise to another highly desirable characteristic of self regulating cables, which is the ability to assign a T class (temperature rating) for ATEX purposes and safe installation in hazardous area locations. With the decrease in power output as the cable temperature increases, it is not possible for the cable to affect an increase to a temperature beyond a certain level, regardless of the level of thermal insulation used.

Read more here.