Electronics > Beginners
Newbie understanding joule heating
rthorntn:
OK, so I actually have some "Dernord" 12v 100w "17-11" cartridge heaters here, similar to these (the amazon ones say 17-03 not sure what my 17-11 means, anyone?):
https://www.amazon.com/Dernord-Cartridge-Electric-Stainless-Replacement/dp/B0741664G5
So with what I'm learning are they 1.44 ohms, I'm just asking OK, so if I connected one of these to a big bench supply at 24v supply it would draw 16.66 amps and put out 400W, say I used a bench power supply and gradually increased the amps from 1 amp, would it be safe to do? Essentially at 26v it would output pretty near to 480W but I just want to know if it would be OK at a shade under 20 amps?
Update: the "Dernord" is 100mm x 8mm do we have any idea is it would have a density of over 50W/cm², if it did would that impact anything, is there a way to check if a resistor can handle a specific amount of watts without a datasheet?
IanB:
If a cartridge heater is rated for 12 V 100 W then it is not safe to try and operate it at 24 V 400 W. If you try it will likely destroy itself in short order.
If you wanted 400 W from these, you would use 4 of them giving 4 x 100 W = 400 W.
Also bear in mind these are in no way suitable for heating air. They are not meant to run hot, and in any case they do not have a suitable surface area for contact with the air stream to transfer the heat. One could in principle fit some kind of finned heat sink to it, but it would still not be a very convenient arrangement.
The usual way to make an air heater is to obtain nichrome resistance wire (or similar) of the right thickness, form it into coils, and place the coils into an air duct. You can figure out the right thickness and length of the wire by considering the resistance per unit length.
rthorntn:
Thanks IanB!
I was thinking of using a finned heatsink encasing the cartridge heater and that placed in the air duct, more for safety (and ease of serviceability) than anything else.
I need to find more out about nichrome resistance wire, a calculator I found seems to suggest that at 48v I need 18.7m of 2.3mm wire to do 462w, clearly that's not happening.
larsdenmark:
A few points:
Resistance changes with temperature so you have to take the operating temperature of the wire into account. Since you don’t know the temperature you have to perform a few experiments.
If the fan fails the wire could get extremely hot and start a fire. Monitor the fan and make you don’t have any power on the heating wire when the fan doesn’t run.
rthorntn:
My maximum temperature I want to create is 250 degrees C in 12 minutes.
I'm not liking this wire idea, coiling up lengths of wire by hand, it seems to be more for AC mains.
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