Heat treat oven

timgunn said:
The staples seem to stay put.

Of the 7 or 8 HT ovens I've built so far, the oldest one I've seen lately is about 7 years old and is used regularly by a maker of fairly high-end Carbon steel and PW kitchen knives. It had an element failure early on (16 ga Kanthal A1. I now use 14 ga A1), fixed by shortening the element slightly: the failure was very near the end, but seems to have stood up ok otherwise. He's not sprung for an Evenheat or Paragon, so it is presumably getting the job done.

On the wire diameter, the 14 ga seems about right. I buy wound elements because my supplier is set up to make them to order and they only cost the same as buying the wire to make them myself. YMMV.

I bought the original 16 ga elements wound to .380" OD to fit in a 10mm routed groove. I got the 14 ga wound to fit the same groove width. I don't think anything much thicker would wind so tightly and the groove width and depth would need to increase, cutting down the wall thickness behind the grooves. It's another thing to consider.

In theory, my new 14 ga elements will fit as direct replacements for the originals. The stretch ratio will be reduced (though still well over 2:1) and I don't want to be the one that fits them for the reasons mentioned above. My original chambers were 18"L x 7"W x 6"H and I added another brick to the length (22.5") for the ones with 14 ga elements to increase the groove length and get the stretch ratio up. The 18" ovens had performed well enough that I was confident the extra length would not be a problem. These also worked fine. For the latest 2 ovens, I went another brick for 27" long. The elements are all rated for 3 kW, 2 elements, each rated for 1500W at 115V and connected in series, as UK domestic mains sockets are rated 13A at 230V.

The 27" ovens reach 1300 degC/2372 degF with ease (they max out the type N thermocouples I use) and I'm pretty sure I could add another brick to go to 31.5" and still exceed 2150 degF. The connections to the elements were a PITA to do because they come out of the sides and need to be boxed in. The back of each oven is removable to allow them to be bolted together for swords up to 56" long (the extra 2" being a result of using 1" ceramic fiber blanket for the door- and back-gaskets. This compresses down to about 1/2" in use). In case this is not enough, the doors can also easily be extended by adding in one or two, maybe even 3, 2" thick Ceramic Fiber board spacers each, and one, maybe 2, more 2" spacers could go in the middle. 6' seems entirely possible.View attachment 63148View attachment 63149View attachment 63150
Click to expand...
Just WOW!!! What a nice oven!!
 
emtore said:
Thanks for that.
I've been gathering formulas and methods from a couple of places, and if you have time to go through it I will calculate a heating element.
You can go over it if you feel like it.

All measurments are in cm and all results are in cm, cm ² and watts unless stated otherwise.

First I decide upon the size of the oven chamber.
length=44cm
height=12.5 cm
width=13cm

Calculate chamber area = 2569 cm ²

A sidenote:
This method uses two parameters called surface loading and power density.
Both are measured in Watts per cm ².

Power density applies to the chamber,-surface loading applies to the wire in the heating element(s).
So, what people usually refer to as surface loading at this stage of the calculation, I will be refering to as power density.


We'll need to know how powerful to make our oven, but first let's decide upon a suitable power density value.
The preferred value will be somehere between 0.8 and 1.0. Lower than this, and the oven will take a long time to reach desired temp.
To high, and the temp. will overshoot, and the PDI will struggle to keep temp. stable.
So, let's continue calculating the power of the element with a desired power density of 0.8.
P(watts) = surface area x power density = 2569 cm ² X 0.8 = 2055 watts.

Let's round this up to 2500 watts.
P=2500W

Calculate how much restistance the element(s) has to have.
R=V²/ P
R=230²/ 2500 = 29.5 ohms.

1.6 mm wire = 0.695 ohms per meter.

How long does the wire have to be to get a resistance of 29.5 ohms?
Length of wire = required wire resistance / resistance per meter = 29.5 / 0.695 = 42,5 meters.

Surface Loading:
To calculate surface loading we need to first calculate surface area of the wire.
What is the surface area of our wire?
Our wire has a diameter of 1.6 mm which equals 0.16 cm.
Length of wire = 42.5 meters which equals 4250 cm.
Then we calculate the circumference of the wire.
Circumference = π x diameter = 3.14 x 0.16 = 0.544 centimeters.
Surface area of wire = 4250 x 0.544 = 2312 cm ².
Surface loading = Power / surface area = 2500 / 2939 = 0.85 watts per cm ².

How long will our 42.5 meter wire become when coiled up tightly?
d = wire diam. (mm)
D = inside coil diam. (mm)
L = length of wire (meters)
x = length of close wound coil (mm)
Inside coil diam. must be at least 5-7 times wire diam. Let's go for 8 times = 12.8 mm. rounded up to 13 mm.
x = L x d x 1000 / π x (D+d)
x = 42.5 x 13 x 1000 / 3.14 x (13 + 1.6) = 1478 mm = 1.478 meters, rounded up to 1.48 meters.

The distance between the single windings should be 2-3 times the wire diameter.
For a wire with a diam. of 1.6 mm. this means the distance between windings should be between 3.2 and 4.8 mm.
Stretch the coil at least 2 times the length of the coil when it is tightly wound and see how the distance between windings end up.
Stretch more if distance between windings is below 3.2 mm.
How much you stretch the coil will always be a compromize between distance between windings and length.
If the oven needs a longer element, then stretch some more.
If the element becomes too long and the distance betwen windings is still below 3.2 mm. then cut off some of the wire, provided you don't cut off too much.
Click to expand...
Looks perfect as far as I can tell.
 
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