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Electric Ceramic Heating

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R_Pyste

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Hi there, Im trying to design a heating module to quickly heat a small particle (~1-2mm) to around 1500C in air. Im going to use a MoSi2 coil or similar to do this. The coil i am considering has a diameter of 25mm with wire diameter 3mm, 5 turns and hot zone length 50mm. This equates to a total of around 20cm of hot wire.
I am a mechanical engineer and so am a bit lost with the electronics side of things. Could anyone give me a hand?
The way i figure it is, thermodynamics tells us that
Q=Aσε(T^4) and Q=hΔT
and Q=I²R
So does this mean we can deduce the required current if we know the properties of the element (surface area A, convective heat transfer coefficient h, emissivity ε, temp T and resistance R)
I would like to use Uk mains voltage and I understand that I will need a thyristor and transformer of some sort. Does anyone know of any beginner guides to designing a system like this?
I was planning on using a feedback system with a series, cooled resistor in order to approximate the resistance and hence temperature of the element.

If anyone has any advice or comments, they would be much appreciated!
 
I'd think you need to start out by finding the total resistance of the "hot zone". Once you know that and the power it must dissipate (from your heat transfer analysis), you can calculate the required current using P=I²R. Unless you somehow find the exact amount of current it takes to maintain 1500 C in your application, you're going to need some kind of control circuit also.
 
I can find out the resistance of the hot zone quite easily through graphs provided by manufacturers.
Im not sure if the power required should be the black body radiation plus convection or if it should be Q=mcΔT

With regards to the control circuit, what would you recommend? I was thinking of a logic circuit with either the resistance or a thermocouple as an indicator of temperature.
The resistance of the element goes up with temperature.. does this mean that more power/current will be required initially?
 
Im not sure if the power required should be the black body radiation plus convection or if it should be Q=mcΔT

Its hard to recommend how you should go about performing the heat transfer analysis. There are important details that have to be taken into account like, is the particle touching the heating element? Is the particle going to be in an enclosed space? What is holding the particle in place and what is it's thermal mass? Will there be air present? Etc.

In reality all 3 modes of heat transfer will be present. Exactly which one or ones you can neglect depend on the specifics of your system. You'll have to make some educated guesses and use some good old fashioned engineering judgment. In the end, I'd say your analysis should be considered a success if it yields a rough order of magnitude for you to use as a starting point. My guess is you'll have to experiment a little to get everything just right

With regards to the control circuit, what would you recommend? I was thinking of a logic circuit with either the resistance or a thermocouple as an indicator of temperature.

That was more or less what I was thinking too. If you have a sensor that shows temperature through varying resistance (i.e. a thermistor) then you can easily make a circuit to turn the heater on when the temp drops below a certain value and off again when it reaches an upper temp limit. This type of circuit is common and can be made using an op amp or comparator with some hysteresis. It has been discussed numerous times on this board before. Do a search for temperature control or something like that. As far as the sensor to use, I'm not sure. I have never done anything that high temperature before. I'd have to do a little research. Or better yet, you could!

The resistance of the element goes up with temperature.. does this mean that more power/current will be required initially?

This doesn't that more current will be required initially, it just means that the heat up rate will be non-linear. I wouldn't worry about it unless your specific application requires a uniform heat up rate. Besides, even if the resistance of the heating element was constant, the heat up rate will still be non-linear. It would be asymptotic as it approached a temperature where the system could reject heat at the same rate it was input by the heating element.
 
Build and test. Then build again. ;)

At 1500'C you will get a massive amount of convection cooling so a lot of the build is going to be controlling heat loss, ie insulation etc.

You will also need a sensor good for 1500'C, have fun there.

This is right up in kiln temperatures (near the top!) so you might want to google for "electric kiln" or "electric kiln controller" to give you an idea of the components.
 
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