Nichrome Wire Temperature Calculation

[Hank Wallow]

Hey there,

I've scouted this forum for years, and it's nice to have finally joined. :)

I come to you with a problem. The objective is to find the correct voltage needed to heat air and another metal to a certain temperature, and then to maintain this certain temperature indefinitely within the metal and air.

The resistance of the Nichrome will be fixed by the specifications and geometry of the rest of the design. So you can imagine that the length and gauge is known. This isn't what we're designing for.

The problem's geometry: Imagine Nichrome running the length of a paper towel, once it reaches the upper edge it kinks and returns down the same side, snaking its way down the length of the paper towel, continuing in an up-and-down fashion. Now imagine that the paper towel is very porous (this is the other metal sheet).

There have been a few problems, and there seem to be two approaches.
1. I really like the idea of solving the problem precisely with a tried and true approach. The issue that is really pulling me apart is how the geometry of the problem figures into the calculations. And also how timing (if at all) figures into the calculation.

Of course you would go about finding the power, equate electrical power to heat gained. But if nichrome is heating both sides, does this further complicate the process? How would you account for both the amount of energy lost to radiation and the amount of energy lost to conduction? If the heating element is at a low enough temperature (ie nichrome isn't red), do you still need to account for the energy lost through radiation? Or would conduction be considered the big energy contributor? I really do not know how to go this route. But I see the precision of this sort of calculation as a great incentive to power through some dense arithmetic.

2.You could use the NiChrome temperature - amperage specs that are iterated for a straight wire. This wire will be snaked around in a very UNstraight way though, and further frustrates the method of problem solving. How would you compensate for the geometry in this approach?

Of course the resistance of the nichrome increases as the temperature spikes as well. The electrical resistance of the other metal is magnitudes higher than the resistance of the nichrome, so there is little risk of shorting out the circuit.

 

[the_emi_guy]

This is really a thermal problem, the electrical part is easy.
The amount of heat you need to pump to maintain a certain temperature depends on the rate of heat loss out of the system.

For example if you had this device in the open air with a strong fan blowing over it, you would need a higher voltage to create the necessary source of heat (via ohmic loss).
On the other hand, if the whole thing is mounted inside of a well insulating Dewar flask then you would need very little voltage.