NiChrome Furnace/kiln build - question on wiring

tcmtech

Joined Nov 4, 2013
2,867
There are lots of guys still doing it to make castings at home, the now defunct Lindsay books had several books on how to build one at home. The internet has plans too.
Yea Lindsey got replaced by YouTube.

I've read a lot and watched a lot of how to stuff on how to build one and it's well within my skill set to make a bigger unit could do 100+# batches of iron. :cool:
 

shortbus

Joined Sep 30, 2009
10,045
The historical ones around here were of the "cold blast" type Built against one side of a small cliff or hillside and using the prevailing winds to provide the air draft. One of the more famous ones is the Rebecca furnace, that president William McKinley's father was a part owner and the operator of.
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
We have 240 volts available in residential, but it's center tapped to provide a pair of 120 volt supplies.
A "standard" electric furnace for a home might use size 6 AWG wire on a dual 60 amp circuit breaker to drive a 10KW heater.

Now to the kiln.
Based on: http://www.bryantrefractory.com/uploads/products/20_9.pdf
3000 watts can raise 2/3 of a cubic foot of interior space to 2600F.
That's assuming I did the math correctly, you can get 3KW out of the 3KW worth of nichrome without melting it, and 3 inch thick firebrick.
Interior dimensions are an 8 inch cube.


Those numbers resemble this: http://www.bigceramicstore.com/paragon-sc3-kilns.html
costing about $1000 as a commercially available kiln which heats about the same amount of space to 2000F with 1800 watts, so I think my math is pretty close.
#12 - your calculation of 2/3 ft^3 being equal to a 8" cube is much less than 2/3 of a ft^3. 8" ^3 = 512 cu inches where a cubic ft is 1728 or about 30% of a cubic ft. IDK how this will effect your calculations, if you were using 2/3 as the space needed for the 3Kw then the 8x8x8 would need much less than the 3Kw. If you need to have 2/3 ft^3 then you are looking at about 10.5 x 10.5 x 10.5 which is very close to 2/3 of a cubic ft.

A per #12's comment about not providing the needed details for "proper" calculations, I didn't expect for the members here to do the "heavy lifting" in determining what heat outputs per in^3 of space and other calculations I would perform on my own. What I was mainly interested in is running 2 separate nichrome "coils", each with their own plug (so I can utilize 2 separate mains circuits - I do the same for dual element hot plates).

I do like the idea of using a diode and splitting a single AC voltage cycle between the two coils. I will probably try this either way.

What I have considered doing is adding some kind of fan or air flow mechanism that can circulate air over the coils on the bottom.

I have also considered using the 1500w halogen bulb from this article: http://forum.allaboutcircuits.com/t...d-how-do-i-get-165.125791/page-2#post-1021844 I am afraid that the glass would melt unless it is made of a quarts tube. 2 of these powered on 2 different DC rectified 120 mains would word I think if the glass isn't an issue.


Here is an attempt I made of making a hot plate for heating small beakers (3" - 4" diameter). I took a coil of nichrome and wound it in some Kaolin clay then heated to cure (this is where it fell apart as I've never fired clay before.) I thought this would make the nichrome last longer as it is in contact with the clay material.

heatplate1.jpg heatplate2.jpg heatplate3.jpg
 

tcmtech

Joined Nov 4, 2013
2,867
How hot are you aiming for? So far I have yet to see that mentioned?

As for overall power consumption the tow critical factor in that are simply temperature and level of insulation used so if it's well insulated you don't need a huge amount of continuous power to keep it hot.

Option three for your heating elements would be to simply wire them in series and then cut one down shorter until you reach the maximum wire tempurature you are comfortable running them at.

BTW the quartz glass used in halogen tube type bulbs has a melting temp of just over 3100 F (1700 C) whereas your nichrome hair dryer wire melts at around 2550 F (1400 C).
 

#12

Joined Nov 30, 2010
18,224
If you need to have 2/3 ft^3 then you are looking at about 10.5 x 10.5 x 10.5 which is very close to 2/3 of a cubic ft.
I don't need any cubic feet. I merely typed 2/3 instead of 1/3. You found a typo.
A per #12's comment about not providing the needed details for "proper" calculations, I didn't expect for the members here to do the "heavy lifting" in determining what heat outputs per in^3 of space and other calculations I would perform on my own.
What can I say? I thought I was giving you a ball park figure based on just about no specifications at all. I'm glad to see you have it all under control.
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
I don't need any cubic feet. I merely typed 2/3 instead of 1/3. You found a typo.

What can I say? I thought I was giving you a ball park figure based on just about no specifications at all. I'm glad to see you have it all under control.
I wasn't trying to be argumentative if that is how it came off. I always appreciate you input and help and I appologize for neglecting details in some postings as I often am trying to figure out basics and theory - making sure it is sound - then doing the "heavy lifting" on my own so I can learn and understand what is going on. Thanks again for your help here and previously!
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
How hot are you aiming for? So far I have yet to see that mentioned?

As for overall power consumption the tow critical factor in that are simply temperature and level of insulation used so if it's well insulated you don't need a huge amount of continuous power to keep it hot.

Option three for your heating elements would be to simply wire them in series and then cut one down shorter until you reach the maximum wire temperature you are comfortable running them at.

BTW the quartz glass used in halogen tube type bulbs has a melting temp of just over 3100 F (1700 C) whereas your nichrome hair dryer wire melts at around 2550 F (1400 C).
As far as temp, that is kind of up in the air now as I'm researching a number of different things which I may be able to use this device for from pottery, ceramic, porcelain to chemistry experiments and metallurgy projects, so I need to see what the requirements are for the range of these needs. The chemistry aspect probably has the largest range of temperature requirements.. It will be nice to be able to decompose some compounds without using an oil, coal, coke forge/furnace which often contaminates some of the compounds while processing - thus why I'm looking at electric (or may go to high amperage arc furnace).
 

shortbus

Joined Sep 30, 2009
10,045
As far as temp, that is kind of up in the air now as I'm researching a number of different things which I may be able to use this device for from pottery, ceramic, porcelain to chemistry experiments and metallurgy projects,
So what your really wanting is more of a kiln than a furnace? There are lots of plans on the net, with the experimenting already done, working plans. https://www.google.com/search?q=diy+kiln&ie=utf-8&oe=utf-8
 

Roderick Young

Joined Feb 22, 2015
408
I have some experience not with designing kilns, but with running them for ceramics at 2 different schools. What I noticed is that the two electric kilns never have heating elements on the bottom. The heating elements spiral up channels on the inside of the kilns. I'm guessing that this is because you will put your pieces on the floor of the kiln, and want a smooth place where there is no chance of the piece touching an element. Uneven heating can shatter your piece during a bisque firing. And if you're doing a glaze firing, you don't want any glaze to drip on the element. Also, a kiln could have multiple shelves in it on stilts (fireproof tables on columns), and those stilts need to be on a smooth surface, too.

For temperature control, you could use an actual thermocouple element intended for a commercial kiln, or you could do it the old way and use a physical cone. In the latter case, you would have a hole in the side of the kiln with a loosely fitting ceramic plug. From time to time, you would take the plug out and look at the cone to see if it melted yet.
 

gerty

Joined Aug 30, 2007
1,305
DSC00249.JPG DSC00250.JPG DSC00254.JPG "Here is an attempt I made of making a hot plate for heating small beakers (3" - 4" diameter). I took a coil of nichrome and wound it in some Kaolin clay then heated to cure (this is where it fell apart as I've never fired clay before.) I thought this would make the nichrome last longer as it is in contact with the clay material."


We had to replace a heating element in a heat treating oven for our machine shop class. The element is wound through channels in what appears to be a ceramic plate. Ordering a heat element gets you the whole block. We checked the element and according to our gauge it's #9 AWG.

edit: I don't know how the pics ended up above the quote ?
 
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