Winding Your Own Elements
If you cannot find a ready-made Kanthal element to meet your needs, you can easily wind your own. This operation is 50% calculation and 50% execution.
Calculation:Kanthal A1 is a high temperature heating wire. It contains iron, chromium and aluminum and can handle temperatures up to 1400°C (2550°F). Kanthal has a known amount of resistance per foot, generally labelled as Ohms/ft or Ohms/m (in metric). Tables are available online, but the place where you buy your Kanthal wire will be able to tell you exactly what the resistance per foot is. I bought some Kanthal A1 from Pottery Supply House in Oakville Ontario, Canada. They sell all gauges by the pound. I ordered 18 AWG, of which a pound was about 255 feet of wire. The resistance per foot is 0.5369 Ohms. In the USA I have found similar Kanthal A1 18 AWG on eBay.
Diameter: The next thing to look at is the diameter. I need my new element to be about 1/4" in diameter.
Length: Lastly I need the length of the element to be about 22" long. I am not super concerned about the exact length as it's going to get stretched to 75". The thing here is to go for shorter and stretching to length.
This is the calculation part...
Ohms required = 19
How many ohms per foot = 0.5369
Feet of wire required = 19/0.5369
Now that seems like a LOT OF WIRE to fit into the walls of a small oven! But the wire needs to be coiled into an element.
Coil calculation is roughly this:
pi = 3.14
Preferred outside diameter of coil = 0.25"
Each coil will use 3.14 * 0.25 = 0.785" of wire. (This is an estimation.)
But how long is the whole element going to be?
For this we need to know the thickness of the wire. The wire I have says 18 AWG is actually sold as 1.02 mm or about 0.0402".
Number of coils per inch is 1 / 0.0402 = 24.9
Number of coils per foot is 12 x 24.9 = 298
Number of feet of wire per foot of element 298 x 0.785 = 234 / 12 = 19.5
(Thanks for the correction here David!)
For 18 AWG wrapped into 1/4" coils, it takes almost 20 feet of wire to make a single foot of element!
So the length of the whole coiled up element is 35.38 / 19.5 = 1.81 ft (or 21.75 inches)
As I mentioned before, I plan on stretching this to become 75" so it will fit into the existing grooves in the fire bricks.
A quick check on the ohmmeter reveals we are right on track.
When installing the element, I know that I need it to go from anout 22" and stretch to around 75". This means that each coil of wire will be spaced apart from the next by about two diameters of the wire. I did this stretching in small sections and stapled as I went.
The LeadsFold back the wire on itself to pass through the fire brick. When you fold back the lead you create 1/2 of the resistance in that area and the lead acts more like a wire than a heating element.
If you want to design your own oven, we can take this process as part of the whole.
General Heat Treating Oven Design Guide
Step 1: Determine how big to make the INSIDE of the oven.
Your biggest knife will give you an idea, but smaller will take less power to heat. If you know the dimensions of the Insulated Fire Bricks (IFBs) this will allow you to make good size choices based on using full bricks.
Step 2: Determine the cubic feet of the inside of the oven.
Take the inside dimensions and convert them to decimal feet, so 6 inches becomes 0.5 foot and 9 inches is 0.75 foot.
A 6" x 6" x 24" oven is 0.5 cubic foot. This can be expressed as: 0.5 x 0.5 x 2 = 0.5
A 9" x 6" x 18" oven is 0.75 x 0.5 x 1.5 = 0.5625 cubic feet.
Remember that this volume will have to be heated and more volume takes more heat, so make the volume as small as practical. As a knifemaker, you'll likely never need a chamber that is 8" high, but you may need one that is 18" deep.
Step 3: Determine how many watts of heat you are going to put inside.
Steps 1 & 2 will point this out. Aim for 5000+ Watts per cubic foot. 3000 Watts or more for a 1/2 cubic foot oven is recommended. More power will heat up faster.
I did a survey a few dozen 'knife maker' units from both Paragon and Evenheat that puts the Watts per cubic foot in a broad range from 6,000 to 10,000. Of course the 10,000 W/cu.ft. unit is going to get there much faster. Note some of Paragon's Xpress models are over 10,000 W/cu.ft.
Step 4: Determine Voltage and Current requirements.
Larger ovens will need 240V supply. If you need more than 2400 Watts, you'll want to go to a 240 Volt supply. The most we normally get from 120 V circuits is 20A which is 2400 Watts. If your oven is over 0.5 cubic opt for 240 V, either 15A or 20A. Power is voltage times current. A quick way to work this out is Watts wanted, say 3500 W divided by 240 V = 14.6 Amps.
Step 5: Determine how the elements will be wired.
Elements in parallel will allow you to use smaller gauge heating wire.
Some basic arrangements are as follows:
3000 Watts @ 240 V
18 AWG Kanthal A1
|38||Ohms per run|
|0.54||Ohms per foot|
|6.32||Amperes per run|
|70.8||feet per run|
2200 Watts @ 120 V
16 AWG Kanthal A1
|13||Ohms per run|
|0.34||Ohms per foot|
|9.2||Amperes per run|
|38.5||feet per run|
Step 6: Determine how the elements are going to be arranged.
For example, connections to the elements are made at the rear of the oven or at the side. Now estimate how long the elements are going to be.
From here you should be able to build the element from bulk Kanthal.
Other Design Notes:
In some designs it makes sense to have several elements in parallel. One helpful formula is the parallel resistor formula.
Rtotal = 1/(1/R1)+(1/R2)+(1/R3)...
For my 120 Volt oven, I made three elements. I measured each one with the ohmmeter and recorded the values.
R1 = 19.6
R2 = 19.4
R3 = 19.5
Plugging in the numbers I got:
1/(1/19.6)+(1/19.4)+(1/19.5) = 6.499 Ohms
120 Volts / 6.499 Ohms = 18.46 Amperes
120 Volts * 18.46 Amperes = 2215 Watts
This works well for me on a dedicated 120V 20A circuit.
There is a thermal limit at which the interior temperature cannot increase any further with a given amount of power. This is due to losses. Essentially what this means is you need more and more power to make the same increase in temperature. Your target temperature is going to be around 1100°C or slightly less. Kanthal A1 is rated for 1400°C and is ideal for a knife maker's needs.