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.
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 and 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 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 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.
Constant wattage heat tracing cables do not vary its output depending on temperature and there are times where this is an advantage. Principally, if a higher maintenance temperature is required, constant wattage cable is sometimes preferred since a lower w/m variant may be used (as opposed to self regulating cable, where there would be the need to allow for the reduction in output due to elevated temperatures).
Since there is little inrush current with a constant wattage cable, it is also possible in general to use it in longer circuit lengths, particularly for higher output variants as opposed to similar self regulating versions.
One of the key benefits of parallel heating cables (self regulating or constant wattage) is the ability to cut the cable to length from a drum on site, without the need to consider the resistance of the wire itself.
HEAT TRACING EXPERIENCE FROM THE EXPERTS
Thorne & Derrick have almost 35 years trace heating experience.
Frost protection has always been core to our business growth with over 1 million meters of cable installed in the UK alone.
Schools, hospitals, sports stadia, prisons, hotels and other commercial buildings have T&D’s heating cables preventing their pipes from freezing. Some of our more prestigious projects include Liverpool Echo Arena, Wembley Stadium and The Shard and we are very proud to have secured such contracts.
ROOF & GUTTER HEATING
As well as frost protection of pipes, heat tracing cables can also be used to prevent frost, snow and ice build up in gutters and on ramps and access ways.
Roof and gutter heating cables can be installed in gutters and on rooftops to provide gentle warming which melts any snow/ice as it falls.
This maintains a flow path for the water and prevents gutters overflowing and water seeping back into the building. Melt water which cannot freely drain away will form pools which in turn can penetrate seams and joints in gutters and flat roofs causing serious damage to the internal building fabric and contents.
A leaking gutter or roof can cause extensive structural damage within buildings. Heat tracing cables create a flow path for the water allowing it to drain away correctly.
In some cases, the load bearing weight of any snow accumulation is the primary concern as the additional weight can negatively affect the structural integrity of the building.
Heat tracing cables can also be buried in concrete to heat up the surface temperature and prevent snow and ice accumulation. This is particularly beneficial to car park ramps which are exposed to the elements.
T&D were recently involved with the design and supply of a ramp heating system to Sainsbury’s in Bicester. At the time, it was the largest ramp heating installation in the UK with over 3 kilometres of heating cables installed. Both self regulating and contact wattage heat tracing cables can be used for this application.
Typically, the construction of the ramp governs which type of cable would be selected and offer the most benefits. For example, self regulating heating cables are not suitable for direct burial into asphalt as they can not be exposed high elevated temperatures whereas a major advantage of constant wattage cables is the ability to be exposed such temperatures.
Conversely a constant wattage cable gives out a constant power output and offers no energy efficiency compared to a self regulating cable.
Eltherm are a global leader in the manufacture of self regulating and constant wattage heating cables producing the full spectrum of variants with options for highly corrosive resistant fluoropolymer outer jackets. Research and development is a permanently on-going process, fed with input from their large projects and applications team, ensuring that products are designed and built to the highest standard and for the applications of the modern world.
TRACE HEATING SYSTEMS & HEAT TRACE CABLES
The heat trace cable ranges stocked and supplied by Thorne & Derrick International includes electric heating cables for process temperature maintenance, pipe and vessel freeze protection and roof and gutter de-icing applications where snow and ice removal is required – cables and systems are available for commercial, industrial safe area (non-hazardous area) and hazardous area heating applications with ATEX certification.
Electric heat tracing is a cable based system used to maintain, raise temperature and protect process pipework and vessels against freezing temperatures and associated frost damage – heat tracing cables mitigate and counteract cold weather effects as part of a Winterization strategy for industrial and process applications against low (sub-zero) ambient temperatures on both onshore and offshore locations.
Heat tracing systems utilising self-regulating, constant wattage and mineral insulated (MI) heating cables are available to provide the optimum electrical heating system for your application – we provide a trace heating system design service.
Heat Trace Cables: Ramp Heating | Roof & Gutter Heating | Water Pipework Frost Protection | Hopper Heating | Tank Heating | Hazardous Area Heating | Fire Escapes
By Thorne & Derrick International