Graphite Heater

Please provide information about the context in which these heaters are to be used.

these are graphite bars with a 16 mm diameter & 230 mm length, I want to use them as heaters

What fluid is in contact with them and at what temperature and pressure - air? Water? Petrol?

medium is air & temperature : 700 C & pressure : atm

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One thing you may want to keep in mind is graphite's threshold of oxidation is around 600 °C. Most graphite heating applications are used in vacuum or inert atmosphere furnaces.

Quite. At the OP's 700degC, the graphite will turn to CO₂ over time at this temperature. So the project sounds like a non-starter so far in terms of the longevity of the heating materials.

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It would be so much better if the readership didn't have to tease the information slowly out of OPs.

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Yea, at that temperature the rods aren't going to last long unless you pull a vacuum or backfill with N2 (preferably both and hold it in the low millitorr range). Most I've seen use 208V single phase and monstrous current. Measure the resistance of the rods and calculate your max current. Ohms law for DC circuits won't be exact, but it will put you in the ballpark.

thank you all for this precious informations, i'll try .......

thank you again