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Hot spot risk for flexible conductive or heating yarns

What is a hot spot?

A hot spot is a specific section of a heating structure where overheating occurs.

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What are the risks?

In addition to a breakdown of the textile structure, the main risk is fire. This is very often initiated by the fire of the materials in contact with the section of the yarn with a very high temperature.

How is a hot spot created?

Let's go back to electricity basics. Let's consider the below resistive yarn.

The following yarn is considered: 10Ω/m

I = U/R I=12/10 = 1,2A

P=UI =12*1,2=14,4W

The dissipated heat per yarn length is 0,144 Watts/cm. We can then calculate the charge rate : (Watt/cm²):

0,144 / (0,138x π)= 0,33 Watt /cm²

Note: in order to facilitate your understanding, we have simplified this case study: the load rate actually depends on the exchange surface with the environment: in case of a multi-filament yarn, the equivalent exchange surface should be calculated. Its value should be between the total external surface and the external surface of the equivalent mono-filament yarn.

The damaged part of the yarn would correspond to a much thinner conductive path, for instance 1/10th of its surface, but with very short length (ex: 1mm).

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    Let's go back to the load ratio*(areal dissipated heat):

    In the section 1: P1 = 9,99*(1,19)² = 14,15 Watt soit (14,15/99,9)/0,44 = 0,32 Watt/cm².

    In the section 2 : P2 = 0,10*(1,19 )² = 0,14 Watt, soit (0,14/0,1)/0,44 = 3,19 Watt/cm².

    With high temperature withstanding alloys (stainless steel , Kanthal etc…) the load ratio should not be over 2 to 2.5. For copper or even worse aluminium, the melting points are even lower! Therefore, even with low voltage, a very high temperature hot spot can be created. It will quickly destroy the remaining strength of the metal, weakening even more this specific point, leading to an ever higher temperature or the complete break of the conductive element.

Possible solutions with our Thermotech yarns

The Thermotech yarn (for instance the 14Ohm version with 50 micro-filaments of stainless steel) has a very good fatigue resistance. Its outstanding flexibility optimizes the positioning and therefore the supply of energy where it is needed

Application example for industrial heating

By applying a 24V tension, the electric intensity is 1.7A. A constant heating around 250°C can be obtained reliably

In that case the dissipated power is: 24x1.7 = 40,8W, which means:

40,8/100/0.06xπ = 2.16 W/cm². Equivalent cross-section of 0.06mm²

The yarn, thanks to the quality of its alloy, withstands these temperatures and keeps an outstanding fatigue resistance within the composite part

  • In the case of heat collection or transfer (short term conductivity), we recommend to associate Spuntech fibers with our Thermotech yarn

  • For electricity transfer, we recommend the Conductib yarns

Examples of mistakes which could lead to serious hot spot and fire risks for an automotive seat

In specific cases, the mechanical strength of the conductive yarns has to be increased.

To do so, the association of a copper yarn with one or more ultra-thin stainless steel yarns (35 to 100 micrometres for instance) is considered.

What would be the consequences for actual application within heating automotive seats?

  • The mechanical strength of the yarn will obviously be greatly improved.
  • Initially, the product will work properly. But going forward, the many bending strains applied to the structure when the user is sitting down or standing up will lead to a weakening point on the copper yarn.
  • The fatigue consequences will be worsened by the higher stiffness and robustness of the stainless steel yarn which will damage again the copper yarn.
  • As a result, all the electric power will go through a very thin but strong stainless steel yarn, creating a brutal temperature rise which will weaken even further the remaining strength of the copper part. The stainless steel thin wire will become incandescent and set the synthetics parts in its vicinity on fire, fire will be unavoidable!

Be careful of too simplistic concepts !

For instance, a multi-filament copper yarn can be not resistant enough to fatigue issues.

An association with a stainless steel mon-filament seems to be a good idea, but it would actually be a very dangerous solution!

Why?

If a few copper filaments are cut because of oxidation, fatigue or contact with the stainless steel mono-filament, the most resistive yarn will create a punctual heating at the same weakness point of the yarn.

If the cutting is on a very short distance (micro crack), the filament can get incandescent and melts the other copper yarns (with only 12V applied). Fire is unavoidable!

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