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Joule Heating Project

by SinguIarity
Tags: heating, joule, project
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SinguIarity
#1
Oct23-13, 02:31 AM
P: 1
Hi all, some background info, thanks for reading and helping!

I'm working on a project, the portable device is to vaporize glycerol(300 C). In an attempt to maximize vapor production, I've decided to skip the experimentation and calculate the optimal specs for each part. Also pardon my vocabulary, computer science major with about 5 hours of research done in resistance heating and general electrical engineering.

These are the limits on the circuit board:
PWM (Pulse Width Modulation) output, up to 5 amps
Variable Volt: 3~6.0V
(2) 1200mAh 18350 batteries
1.2ohm minimum coil
20mm maximum length of coil

My goal is to create a heating coil that will vaporize as much glycerol as possible on this board. I did some research and people say the lower the ohms the more heat is released so is it safe to assume we should stick to a 1.2ohm coil? We also want it to have as much surface area as possible, to heat up more liquid. The preferred heating wire is 28awg Kanthal A1. We also need the coil to reach the desired heat relatively quickly(0-2 seconds). So would it be better to use a lower gauge wire so we can put in more wraps and get a longer coil? Or should I stick with the common build of 10 wraps around a needle for around 1.5ohm. What would be the best coil design and wire material and wire type(round vs ribbon/flat).

Cheers,
Eric
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Rena Cray
#2
Oct28-13, 12:00 AM
P: 36
Your maximum power seems to be (6V)(5A)=30 Watts. 30 Watts electrical power in a resistor is also your heating power.

Your goal seems to require you both obtain this power and minimize loss.

So use a resistive heating element that gets you 30 Watts. Set your adjustable power supply to full voltage. If you’re really clever you might use some of the waste heat from the power supply to preheat the glycerol.

Minimize heat loss using low thermal conductivity containment materials. Dribbling down a coil using the vaporized glycerin itself as your first insulation layer may be best. Glycerin is very viscous---at least it is at room temperature. At room temperature its volumetric tension (called surface tension) should separate it into droplets protruding perhaps 1 mm from the surface on fine wire. If you use a coiled heating element keep the windings about 3 mm apart. Experiment.

This isn’t electrical engineering. Its geometry and materials.

I don’t know where 1.2 Ohms comes from… You need 1/1.2 = .83 Ohms. So use about 8,8 Ohms unless your power supply gets upset with it.

Search “American Wire Gauge” to get 8.8 Ohms. I see about 25 cm of 40 gauge copper wire which looks problematic. It's far too flimsy. Use Nichrome instead.

Good luck.
meBigGuy
#3
Oct28-13, 02:20 AM
P: 1,074
[edited due to an error regarding resistance change with temperature]

1.2 ohms is 6V/5A, so 1.2ohms is the correct answer. Not sure where Rena got 0.83 and 8.8.

Kanthal 28 AWG Gauge A1 Wire is 5.27 Ohms/ft Resistance (Kanthal A-1 is a ferritic iron-chromium-aluminium alloy (FeCrAl alloy) for use at temperatures up to 1400C (2550F).)

The first issue is whether the wire can handle 5 amps without fusing. (I didn't find the fusing current)

Also, you are discharging the 1200mah batteries at 5 amps. You need to know what you are doing with respect to load balancing the batteries both for charging and discharging. (do you have a circuit that takes care of that?)

The mechanical design question is how to expose the most wire to the chemical. I assume that would be to make the coil a flat disc and drop the chemical directly on the wires. I don't know about residue and cleaning though. You can use a single wire or multiple longer wires in parallel to get the desired resistance

This has good info starting at page 80 http://www.kanthal.com/Global/Downlo...NG-2012-01.pdf

Rena Cray
#4
Oct30-13, 08:41 PM
P: 36
Joule Heating Project

Quote Quote by meBigGuy View Post
1.2 ohms is 6V/5A, so 1.2ohms is the correct answer. Not sure where Rena got 0.83 and 8.8.
Yes, you are quit right. I don't know where-from either.


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