Ideas anyone? Making electric coals

 Hi guys, girls 'n robots!

I intend to build an electric coal (for my hookah) and I need advice from all you electronic dudes and dudettes! If I use nichrome what length and gauge should I use to achieve the following

Specs:
1.  12 V power supply  OR 220 VAC (South Africa)
2. Minimum 400 Celsius / 700 Fahrenheit
3. Preferably 600 C / 1112 F

The element will be wrapped around a "doughnut" which I will fabricate from heat resistant cement, it will be +/- 2 inches diameter with an inner diameter of about 3/4 inch and 1/2 inch thick, therefore keep in mind that the nichrome must be able to heat all that ceramic.

If, during my testing, the nichrome expands sufficiently to short out then I'll cut grooves in the "doughnut" to keep them in their places. In the image orange represents nichrome wire ONTOP of the "doughnut" while yellow is BELOW

I am in no way adept at electrickery.

As a side note of lesser importance, any ideas on building a circuit to add a dimmer or potentiometer to the mix to regulate the temperature?

Thanks in advance!

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waqar13513 years ago

hi every1.

i think this idea of using resistance for heat generation is quite difficult you have to manage large length of nichrome wire or nichrome strips (used in old electric irons) in small area and its a big deal.

i'm thinking for induction heating, it wont work on dc supply but it seems little easier. i'll start work on this tomorrow.

pls let me know if someone has any experience in making of induction type of electric coal.

Orngrimm4 years ago
http://www.jacobs-online.biz/nichrome/NichromeCalc.html
Gives you for 2 meters of wire @ 599°C @ 220V a needed gauge of AWG 40 (0.076mm diameter).
Also this gives you a power-needed (and radiated) of 104 Watts.

If you go by 12V, it looks like this:
70cm of wire @ 12V @ 599°C gives you a needed gauge of AWG 19 (0.914mm diameter). The power is 116.94 Watts.

See how much wire you can fit on the doughnut and go from there and let you tell (from the link i posted) what gauge you need.
Sam Singer4 years ago
Hey - so i noticed you from SA ( i am to) and i came across your article and thought i'd let you know that i'm building an electric hookah as well ( only i'm making mine portable and car-12V rechargeable adapter). So thought it would be cool to share ideas if you keen ?

- Awe
Hey Thompson, you still working on that heating element?

If you are, I think the key to solving this problem is realizing that any given diameter NiCrome wire has approximately constant resistance per unit length

For example, suppose you have a 1m (100 cm) length of NiCrome wire, and you measure the resistance from one end to another using an ohmmeter, and find that this length has a resistance of 10 ohms. So the resistance per unit length is: 10 ohms/100 cm = 0.1 ohm/cm.  Thus if you want to fashion this wire into a 1 ohm resistor, you want a 10 cm length of it.

You can do this trick with found NiCrome wire that has been pulled from a toaster, hair-dryer, etc.

But now you're probably wondering questions like: How big/small of a resistor do I need?  What voltage should I be driving it with?

Stop me if you've already seen these equations:
Ohm's Law:   V=I*R  <=> I=V/R <=> R=V/I
Power dissipated: P=V*I = I2*R = V2/R

Also note the resistance of red-hot NiCrome wire is roughly 10% greater than than its resistance at room temperature.  So that will affect your calculations, a little bit.

The answer to the power question is that to make something glowing red hot you'll want power density in the neighborhood of 10 to 100 W/cm2  (watts per square centimeter) where the area part of this equation is the area of the surface through which heat is escaping.

Another way to think about this is to imagine a toaster. Now imagine what the heating element would look like if you stretched it out into a straight line. Suppose the whole toaster, the whole length of NiCrome wire, dissipates 1000 watts.  For your application you probably don't want that much power.  You only want a fraction of that, a small part of the whole toaster. A picture, of the way I sort of imagine this, is attached.

BTW, have you ever seen one of the old car cigarette lighters?
toaster-as-resistor.png
Baronrc (author)  Jack A Lopez7 years ago
Thanks Jack, I had shelved the project. You know how it goes, you get busy and sort of forget about the idea.....

Your advice, however, is truly appreciated. If i resume this project in the near future I will definitely come and reference your reply! It's all the same equations from physics class but you've put it together nicely and joined some of the dots that I hadn't thought of.

Thanks and peace!
lemonie8 years ago
The arrangement of wire doesn't look good for best heating. I'm thinking - dismantle a device with coiled heating wire, and fit it into your disk:

L
temp.bmp
Baronrc (author)  lemonie8 years ago
en.wikipedia.org/wiki/Nichromeniiiiiiice! thank you so much lemonie!

My only concern is how to keep them in their places as my tests so far show that they LOVE expanding and subsequently shorting out.

Question, do you reckon that the fire cement (used for building pizza ovens and the like) will be:
1) happy with these temperatures, and
2) easily machinable once cast into a disk shape?

for the record, I'm using a .0157 inch AWG 26 wire from a hairdryer, problem is I am not sure that it is nichrome because it doesn't adhere to nichrome's technical specifications as found on this wikipedia article and another very useful site.

I'm therefore considering buying some nichrome from a supplier near me who can tell me exactly what it's technical specs are so that I can be more confident in calculating the length needed for my needs.
lemonie Baronrc8 years ago
Yea, the hair-dryer was the kind of source I was thinking of. In coils it doesn't short so long as they're not compressed heavily. Fire cement might do it for you, it'll take the heat, but you want to cast it as is rather than machine I think.

L
Baronrc (author)  lemonie8 years ago
 ahhhhhh, coils say you? I'll experiment with that a bit.

Problem with casting is that (And I've tried this twice) when I open the mold after two days of drying the cement hasn't set so it just crumbles.

I'm thinking of using a mold with an open top to allow the cement to set better. Also considering setting the coils in the WET cement so that they are 100% enclosed. Hope their thermal expansion doesn't crack the disk.

I also figured out that the best way to straiten the nichrome (due to handling they have minor kinks) is to get it red hot then stretch it ever so slightly, works a charm.

K, thanks for the ideas, your insights are exceedingly useful.
lemonie Baronrc8 years ago
Make the mould out of card - moisture will seep through. Be careful heating wet fire-clay it has a tendancy to blister.
The enclosed was a thought, depends upon the thermal expansions, anyway make an Instructable if you've got enough material for one?

L