Heating effects on Nichrome 1

[KAP67]

I have a requirement for a Nichrome heat seal blade which is used to heat seal & cut a polythene bag. The problem I hope somebody could assist me with is the maximum heating of the blade under fault conditions, what temp will it reach ? or will it 'pop' like a fuse ?

Blade details
Material - Nichrome 80/20
Size - 65mm x 30mm x 0.5mm
Electrical resistivity @ RT - 108Microhm/cm
Temp coefficient of Resistivity - 100ppm/°C (25° to 105°C)
Thermal conductivity @ 100°C - 15.0W/múK
Supply 110vac

 

[dpc]

I'm not clear on what fault conditions you are referring to.

 

[KAP67]

Sorry if I didn't explain fully, the heat seal blade has a thermocouple connected to it, this is fed into a temperature trip unit, I also have a timer on the system to ensure the blade is only energised for a short period of time & am looking at current limiting. The fault condition is if these forms of protection fail and the blade continues to heat, what would be the max. temp this blade would reach, would it 'pop' or is there an equilibrium point at which the blade can heat no more ? and if so what are the formulas for calculating this based upon the original info supplied.

Hope this helps further.

 

[cbarn24050]

Hi, its not possible to say because the temp will rise until the heat energy lost equals the energy supplied. Nicrome melts at a much higher temp than copper but it can break if there are sharp bends in the wire.

 

[dydt]

hi..
the way i understand it is that you use this blade to seal/cut a polythene bag......questions..

do you have a temperature control for this blade?
is the thermocouple mounted beside the blade or on another casing?

i suggest to use a temp. control with alarm setting ...say set at 350 deg.F with +/- 10 deg...if temperature exceeds 360 deg. F then an alarm signal will activate cutting of the relay that is supplying power to the blade..

i hope i got you right
dydt

 

[ve7brz]

I would suggest a current regulator supplying the wire and a current sensor to alert you to a failure. If you find your wire is breaking too easily or frequently reduce the current.

 

[nbucska]

*The data seems to be incorrect:
Blade size 2.55" x 1.18" x .019" i.e. about 1.2" wide and 2.6 " long ???

If you connect 108 microOhm to 110V, it will take 112 Megawatts !!! <nbucska@pcperipherals.com>

 

[IRstuff]

It's 108 micro-ohm-cm, so the resistance of the heater is actually:

108 uohm * L/A = 0.0048ohm, better, but not by much, only 23,000 amps required ;-)

It looks like your cross-sectional area is waaaaaay too large. You may need to use a smaller cross-section and mount it on a passive heat spreader.

TTFN

 

[nbucska]

Sorry but I am impatient with nonsense... <nbucska@pcperipherals.com>

 

[KAP67]

Thanks everybody for your comments, this is based upon an old design & I have found some drawings which detail a 110/2.6V transformer the secondary is 350A, therefore the blade is rated at 910W. I am now using a Eurotherm system.