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The first heating cable that was produced was of a series resistive type. Today there are several types of series resistive heating cables available. These are manufactured in qualities from PVC to mineral insulated high-temperature cable with rustless sheath. The biggest advantage with these is the possibility to obtain long element lengths, from only one point of connection.
In contrary to parallel resistive and self-limiting heating cables which maximum length are limited by the voltage drop in the conductors, this is used as a heat emitting part in a series resistive cable. The heating conductor is manufactured of an alloy that gives required resistance per meter. By combining a required length with the available cable resistance’s and supply voltages can advantages be obtained such as varying outputs, lengths from a couple of meter to lengths on 800-1000m from one point of supply.
It is a disadvantage that the cable normally must be finished on the factory, which means you have to know the pipe lengths in advance to be able to pre order required lengths. (At long high-temperature lengths where the heating conductor contains copper (CC-cables) shall consideration be taken to the temperature coefficient of the heating conductor which influence the output of the length negatively).
For installations within the EX-surface a number of supplementary safety devices is required, and exemption from the touched authorities.
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Self-limiting cables can be bought in running metre for make-up on the site. The cable has a varying output depending on the ambient temperature, which guard against overheating even if the cable crosses itself. This also allows installation in Ex-surfaces (all Värmekabeltekniks self-limiting cable types are Ex-rated).
The self-limiting cable have a unique capacity in proportion to the sheath temperature of the cable, reduce the emitted output. These cables are often mentioned as self-regulated cables but this is a wrong denomination, as a required temperature not can be guaranteed without temperature control.
On the other hand, the cables make it possible to give an even temperature on a pipe even if the ambient temperature varies along pipings.
Värmekabeltekniks self-limiting cables are approved within Ex-surfaces when the cables have a stated T-rate, i.e. a maximum temperature that the cable reach. The T-rate varies for different outputs/m.
The self-limiting cables are built up round a semi-conductive bed embedded in the outer edges. The bed between the conductors has a capacity to lead current. In conducting capacity is in proportion to the temperature. At rising temperature the conducting capacity decreases, overheating is prevented. At sinking temperature, the current increases and the increasing heat requirement is satisfied.
The emitted outputs of the self-limiting cables are depending on the ambient temperature and the method of installation, a good dissipation of heat increase the emitted output. (Aluminium tape that covers all the length of the cable and is attached against the pipe, cool down the side of the cable, which borders against the insulation and thereby, increase the emitted output with 30-50%).
All self-limiting cables have a starting current which is higher than the operating current. This must be taken into consideration when designing circuit-/connection lengths, (see data sheet).
|PARALLEL RESISTIVE||MINERAL INSULATED|
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Parallel resistive cables can be bought as running metre for making-up on the site. This admits a good flexibility both at new production and repairs. The cable has a constant output per metre irrespective of length and temperature and can be cut on regular distances, most often between 0.5 to 1.2 metres dependent on module lengths from different suppliers.
The heating element in a parallel resistive cable consists of a resistance wire which is coiled round the insulated front conductors, at the so called contact points (these have been marked as waist on the outer side of the cable) have the resistance wire contact against one of the conductors alternating for each contact point.
The insulation material and the sheath consist normally of teflon-material.
There is more detailed information in the cable data sheets.
The heating cable is designed with earth braid that also works as an armouring and corrosion protected sheath of teflon where this is not possible by high temperatures.
Parallel resistive heating cables gives a solid output per meter independent of the ambient temperature. They have no starting current and can therefore be connected in relatively long lengths, (see cable data sheets).
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To satisfy the different usage needs, resistance wires consist of copper, Kumanal or Nickel / Chrome 80/20 while the outer sheath can be made of copper, Copper-Nickel 70/30 AISI 321 and Inconel 600.
Mineral insulated heating cable manufacturing process consists of a tube of the desired quality of the outer jacket are provided with pre-calculated leader in its center, then the tube is filled with magnesium oxide (insulation) which are packed into a homogenous insulation. The tube is heated and rolled in batches to the correct diameter. In the process, the insulation material is compressed and the leader rolled out propotionellt the outer jacket.
When mineral insulated cable are to be installed in corrosive environments, cable type with outer sheath resistant to the topic selected or cable protected by a sheath of polyethylene (HDPE), high density; in this case, is the maximum operating temperature 105 ° C.
Mineral insulated heating cable is manufactured according to IEC 800, their main characteristics are:
Mineral insulated heating cable is cost effective in construction and industrial applications, and in most cases, significantly cheaper and lasts longer than the self-regulating heating cables