RW80 Heating element ribbon - Page 1
Forum section: electronics / projects

author: HeatSealSteve
author_rank: Newbie
timestamp: May 23, 2014, 12:34:48 pm
content: Can anybody help me? I'm trying to find the W/cm" limits of an element. I've found various examples of suggested W/cm" but all are for spiralled design. I wish to use RW80 NiCr 80/20 element ribbon 3mm x 0.15mm coated with 50 micron Kapton tape - element length 1000mm folded in 2 to make 500mm and inserted into a stainless steel tube 520mm long 5mm Diameter x 0.2mm wall thickness, then pressed flat. The element is to be encapsulated in an aluminium bar approximately 20mm x 12mm x 550mm and heated using PID controller to 300 degrees Centigrade.

author: richard.cs
author_rank: Super Contributor
timestamp: May 23, 2014, 01:24:40 pm
content: This is very much a physics problem. You should calculate it as follows:Ambient temperature = 300 C I assume this is your aluminium bar.Max element temperature =The Kapton tape will limit this, lets say 400 C but you might want to pick lower for reliability.Thermal resistance element to ambient per cm - the kapton may dominate but there may be significant contribution from the contact between stainless and aluminium. Pulling a number out of thin air let's say this was 20 degrees C per Watt per cm. **Example only, do not use this number**Max power is (Tmax-Tambient)/Thermal_resistance = (400-300)/20 = 5 Watts per cm.That's an example, you will need to confirm your Tmax and measure your thermal resistance or calculate it based on the bulk properties of kapton and stainless and some estimate for the contact between the layers. i.e. for each cm of element you have roughly 0.3*1*2 square centimetres of 50 um kapton. Bulk kapton has a thermal conductivity of 0.12 W/m.K so you can calculate a thermal resistance based on this and sum it with the thermal resitance of your stainless, etc.Is there some reason you can't use a commercial mineral-filled heating element? The 400 C limit for kapton will make your allowable dissipation very low.

author: HeatSealSteve
author_rank: Newbie
timestamp: May 23, 2014, 02:33:55 pm
content: Thanks for your reply, we use these elements on unusual shapes with space constraints. Being in the stainless tube enables us to bend to the required shape. Perhaps I need to find an alternative to Kapton.

author: richard.cs
author_rank: Super Contributor
timestamp: May 23, 2014, 03:45:55 pm
content: The conventional mineral insulated elements are made from coiled resistance wire inside a copper tube. The insulating material is normally tightly-packed magnesium oxide powder. Those elements can normally be bent into complex shapes, the minimum bend radius being limited by the copper splitting. Thishttp://cross-electrical.co.uk/trace-heating-systems-productsis the kind of thing I'm thinking of. Those are stainless steel sheathed and the smallest is 3.2mm diameter with a 19.2mm bend radius. Copper ones do exist and will bend more tightly, especially if you anneal the copper between several stages of bending. Certainly the non-resistive (as in copper core) mineral insulated cable is very robust - you can hammer it flat and it still works, the sheath and the conductors both distort but are kept insulated and continuous.