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Dazza

Making an electric furnace.

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Hi Ante, Alun :),

The op amps that you suggested Ante, are not easy for me to get, that is not available from Jaycar or Dick Smith >:( but luck is on my side this time, I have two recycled (TL061) that should be ok :D.

Yes the calculating thing, haven't got the hang of that yet :o, and there is still that problem of having nothing on top, to pull out ;D ;D.

My thermocouple hasn't arrived from Jaycar yet :( , so I'm not sure what the voltage output will be, I know it's in the very low millivolt range. I do have a thermocouple that I think is from a convection oven or something like that, I connected it to my DMM set to the 200m range and applied heat, it rose to 00.6mV and went no further, although I don't really know if this thermocouple is working properly ???.


Alun, Yes it is true that thermal shock will damage this kind of element Nichrome 80 :), when they are first fired the aluminium within the wire burns off, creating a protective layer around the wire preventing oxygen from entering and rapidly deteriorating the wire, I have read about this on the net, and unfortunately I have already discovered it to be true, after I had fired my elements, I found that there was a small portion of the element protruding out from the furnace wall, I made a small kink in the wire to fix the problem and sure enough after a while it burnt out, not only where I made the kink, it burnt out in several places where I touched it.

Alun, I really wish I could go with the way you are suggesting, I really do it would make things so much simpler 8), but from what I have read it strongly suggests, to avoid switching the power on and off to these elements and it is also a good idea to leave the top on the furnace to allow the temperature to decrease slowly, to avoid thermal shock and the same would apply for heating the furnace up.

Ok I think I need to plan my angle of attack for this project, this is what I am thinking, start with getting the thermocoupler to register the temperature on my 3.5 LCD panel meter. Then I can use the output of the op amp to the LCD, to drive an LED via another op amp, I think I need to try to get it this far, then work on what needs to be done after that, does this sound like the way to go.

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Guest Alun

The best solution for this is to soft start the furnace by using phase or burst control (I would recommend the latter) then when it's up to temperature use the switching method to control the temperature. For example you increase the power gradually, when the temperature reaches 700C you turn it off then when it drops below 690C you turn it back on again. This 10 degrees of hysteresis will reduce the system's susceptibility to noise and I a 10 degree temperature change is unlikely to damage the element.

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Hi Ante, Alun,

Ok, I have the thermocouple and some more pic too ;D. On the 200mV range, my DMM reads 18.0mV.

It doesn't seem to want to go higher than that, the aluminium is Molten but not hot enough for casting, and I don't think I can give any more power to the elements :(, hmm maybe I need another element, or the refractory isn't keeping the heat in good enough, well this doesn't matter for now, I think I should concentrate on getting this temperature control sorted out first :).
%7Boption%7D

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I have a little more to add, the temperature did eventually rise to the point where it would have been suitable for casting 18.7mV.

I also measured the temperature of the molten aluminium 25.2mV, unfortunately I didn't think to measure the resistance of the thermocouple ::), that probably would have been handy.

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Guest Alun

Hi Dazza,

The reason the furnace is taking so long is the thermal insulation is not good enough and i't also black - very good colour for radiating heat. Try wrapping it in aluminium foil this shoul reduce the heat loss by radiation. Just as long as the external furnace temperature isn't hot enough to melt aluminium it should be fine. Even if you don't see how this can help please humour me and try it anyway, really do believe it will help.

I'm still in the process of thinking of a way to increase the power to the furnace slowly.

How slowly do you trun the power on?

Do you start a low power and turn it up a bit every 10 miniutes and leave it on full power after an hour?

Do you turn if off immediately or do you power down slowly?

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Guest Alun

Just thinking you could use an op-amp PWM circuit like the dome lamp dimmer and either connect it to a lamp and use an LDR like Ante says or reduce the frequency of the PWM circuit to about 5Hz and connect it to a solid state relay circuit as I would strongly reccomend.

Either way you're going to need a ramp generator that generates a ramp as long as a few hours - I don't think you can do this easily with simple RC circuit as you would need a very big low leakage capacitor. Maybe a DAC with a binary counter would do the job, even so I don't think this would be too complicated.

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Alun, Dazza;

I would prefer to wrap the furnace with the kind of bandage you use for turbo exhaust manifolds (the kind you can touch at full working temperature) and on top of that you just might put some foil (shiny side inwards). This will save some energy and time for you. 8)
BTW, I can see you have got the LF PWM to stick now. ;D But what

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Guest Alun

Hi Ante,
I would agree with everything you said but I would put the foil matt side down. If you dont't believe me the you could do my Thermos flask experiment. :) Take 3 plastic bottles wrap on in foil with the shiny side up, the other with the shiny side down and leave the last one unwrapped as a control. Fill them up with hot water and leave them to cool.

I don't really know whether shiny side up is better than shiny side down I don't think it will make that much difference in this case. I just know that the more shiny a surface is the less will radiate the heat. The side of the foil touching the hot surface can't reflect the radiation back as it's in physical contact with the surface so the heat will be conducted straight to it. I don't really want to discuss this at great length just do the experiment and see. :D

About the ramp generator
Regardless of whether Dazza wishes to use your idea or mine a ramp generator is required. Because Dazza says that the nichrome wire will be damaged if the furnace is turned on too quickly it needs to be turned up very slowly. When the power is first applied the ramp generator could be connected to the PWM unit that's connected to a bulb coupled to the phase controller via and LDR or as I recommend a lower frequency PWM unit could be connected directly to a solid state relay, as there are no bulbs or spiky RF causing waveforms to worry about.

To make a ramp generator you use a constant source to charge a capacitor. A 1F capacitor will take 1 second to charge to 1V with a constant current of 1A, So for 10s to 1

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Alun,

Why to you think a picnic thermos flask is shiny inside? 8) The shiny side of a foil will reflect light and heat; this is why I want the shiny side inwards to keep the heat in the furnace. Light and heat are radiation just with different wavelengths. This is used in space to protect satellites and spacecrafts from heat and radiation (often using gold foil). And here on the ground or almost on the ground we use it in our shoes on insulating soles to reflect back the heat to the underside of our feet. And it

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Guest Alun

Alun,

Why to you think a picnic thermos flask is shiny inside?


I bet it's more shiny on the vacuum side (the part you don't see)


8) The shiny side of a foil will reflect light and heat; this is why I want the shiny side inwards to keep the heat in the furnace.


This is a common misconception as I said the foil's in contact with the furnace, thus it can't reflect any heat, so the heat is immediately conducted to it. Try this, hold an open biscuit tin up to your face and will feel warm because the shiny reflective surface on the tin is reflecting your body heat back at you, but if you touch the tin it will feel cold.

Light and heat are radiation just with different wavelengths. This is used in space to protect satellites and spacecrafts from heat and radiation (often using gold foil).

Yes, it works both ways.

And here on the ground or almost on the ground we use it in our shoes on insulating soles to reflect back the heat to the underside of our feet. And it

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Guest Alun

Try this, it's not a very good linear ramp generator it will just output a voltage wave form of 16 steps - more than adequate for this application.

post-0-14279142133812_thumb.gif

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Hi Ante, Alum,

Ante, sure I like the idea, but it's the putting it all together I'm not too sure about, by the time I'm finished that is if I can work out where to start or how to start, I should be able to pick up satellite TV with it, the way things are going ;D.

Alum, Sorry my electronic knowledge is very limited :-\, I don't understand the circuit you have provided or how to use it, I understand it is for creating a slow rise and fall response to the elements ???.

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Yes Ante :), amplify the voltage to the correct level, to display the temperature on to my 3.5 LCD panel Meter, then I can work on the inverting amplifier to drive the LED, then look at what else can be done ones that is working correctly :D.

I can borrow a thermometer from my neighbour that he uses for his homebrew, its maximum temperature read out isn't very high, but it should be enough to get a fairly accurate idea of what my thermocouple voltage output represents.

I will post the results tonight :D.

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Guest Alun

Hi Dazza,

By the way it's Alun not Alum :D

The circuit I've posted is a 4 bit binary counter with a poor man's analogue to ditital converter on the output.

the circuit counts from binary 0000 to 1111 thus connecting resistors on the A B C D outputs in various combinations these in turn form a potential divider with the 1K connected to ground.

The counter is counts a stream of pulses on the clock pin I've not shown it here but when the counter reaches 1111 it needs to stop or it will keep generating the ramps every 16 clock cycles.

I would connect this to the burst control unit. This will then turn the power up slowly. Then you use switch control to turn the furnace off when it gets too hot and back on again when it's too cold. Suppose you set the turn on temperature to 750

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Hi Ante, (Alun ;D)

Ante, here are the results of comparing the thermocouple voltage output to the thermometer. There is quite a difference between the temperature of the Molten aluminium and from where I measured the temperature of the furnace. I think I should be able to use the 200mV range of my 3.5 digital panel Meter, to represent the temperature. Ok 18.8mV = 560 so 188mV as it would be seen with out the decimal point on my digital panel Meter, times it by 3 A gained of 3 = 564mV, so what value resistors should I use for R2 and R1. Also I should be able to use the second amplifier of the TL 061 as an inverted amplifier to drive the LED? Amateur I M does it show ::) ;D.

18.8mV = 560C air temperature inside furnace.
25.2mV = 780C temperature of Molten aluminium.

200C = 07.3mV
190C = 07.0mV
180C = 06.6mV
170C = 06.1mV
160C = 05.6mV
150C = 05.1mV
140C = 04.7mV
130C = 04.2mV
120C = 03.7mV
110C = 03.3mV
100C = 02.8mV
90C = 02.4mV
80C = 01.9mV
70C = 01.6mV

Thanks Alun, I understand much better now how it works. You still want me to go with your idea, of the on-off switching, very persistent aren't you ;D ;D. I helped my neighbour lay a few slabs of concrete, I told him that he needs to soak the ground before pouring the concrete, as the ground is very dry and it will draw the water from the concrete very quickly and it will crack, he just wouldn't have it, and sure enough it cracked ::), man I was very persistent as I can be ;D, and he was starting to get very annoyed with me >:(, he has moved house now not because of me, we are still good mates, last week I went for a visit and he had laid a slab of concrete leading from the front of his house, yet again he still didn't soak the ground first and it was full of cracks, he believed that it would be sufficient to occasionally hose down the concrete, sometimes there is just no changing someone's mind ;D.

I'm sticking with the way I am going with the temperature control, but you have convinced me that your way will work fine ;), once I have got everything set up for my foundry, I want to build a small furnace that is capable of casting a large heat sink or a good size project box, and offer it as a project in the project section 8), that is if mixos wants it there. So when the time comes I would greatly appreciate your help in setting up the temperature control, your way ;D ;D.

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Guest Alun

It's like when my boss at work told me to cover a table in a very thick coat of varnish. I told them it would better to use 3 thin coatings and one thick coat would take weeks to dry and look awful. But he insisted I used one thick layer and it would take too long to do 3 coatings. So I did as I was told and applied the thickest layer possible and it took more than a week to dry and looked terrible he was not amused.

Anyway Dazza, your results seem to be very linear and go up by approximately 0.35mV per 10

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Hi Ante, Alun

Thanks for your help, its greatly appreciated :). I shouldn't have too many problems getting this project to work now 8).

Ante, I was thinking there was two opamps in the one package, but of course there isn't ::), so I will have to use another opamp to drive the LED to influence the LDR.

I have been busy setting up the rest of the equipment I will need for the foundry, it's coming along nicely with the exception of obtaining a product called (Bentone) >:( which is an ingredient used in high-quality moulding sand. I have done a test casting using (green sand) which is a very basic moulding sand, and I'm not happy with the results :(, so I need to use a better quality moulding sand which is called K-Bond or O-Bond, which contains the Bentone.

I have come up with a few product ideas, that I can apply electronics and aluminium casting to, to produce a product I can market locally :D.

I now have to get my foundry set up, and produce a marketable product as quick as possible :o, to produce some extra income, as we have recently found out that there will soon be a new member to the family :D 8) 8).

Here are some smiley faces to show how excited and happy I am.

[move]8) :D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D :D[/move]
[move];D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D[/move]

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Hi Ante,

I will post some pic soon, of my test casting, the mould/pattern I used and what I have put together so far of the foundry set up :). But there will be no pic's of the new family member project ;D ;D ;D.

I have also been trying to finish off some of my unfinished projects, I have got a few of them laying around ;D.

Oh yes don't forget the hydrogen project, this is a project I will continue on with until I succeed 8), this is one of the reasons why I'm setting up the foundry, once the foundry project is making me some money, I will move on to making a metal turning lathe, then I will be in a better position to continue on with my hydrogen project.

Progress on my hydrogen project may slow down from time to time, but it will continue ;).

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Ante, here are the pic's as requested, and I'm very happy to take the opportunity to show off my work. As this was my first test casting I wasn't expecting very good results, the whole process went well with the exception of the casting sand being too coarse, resulting in a very rough surface finish which I was expecting, although it was much worse than I thought it would be, hence my quest for obtaining Bentone to be able to make a better quality casting sand.

Please do ask questions if there is anyone that is interested in this process, and I will be happy to answer any questions if I can.

Ok, a quick run through of the pic's, first up is the flask #1 consisting of the cope and drag, top and bottom half containing the sand and the mold cavity, unfortunately I didn't take any pic's when I created the mold cavity, but I will on my next casting if someone requests it. Next my furnace and my pooch #2. Then my work bench #3 for creating the mold not yet complete but still functional/usable. Next is the molding sand #4 contained within the half drum which makes up the top part of the molding bench. Then is the mold #5 that is made from MDF wasn't quite as difficult to make as I expected but still rather tricky, it has been coded with acrylic paint to prevent water damage from the molding sand. Next is a pic of the flask before casting #6. Ah then the pouring of the Molten aluminium into the mold #7. Then #8, after the poor. Then "lookout" #9 it seem to of worked, the surface finish was much coarser than expected as mentioned above but at least nothing else went wrong. Last of all #10 is the finished product, which is the Chinese symbol for (tranquillity) with the riser and filler still attached.

I should mentioned that pic number 6,7and 8 was taking using nightvision with my camera, pic number seven looks as though the Molten aluminium is white hot, "it is not" to describe its actual colour is more like a brighter orange.


%7Boption%7D

post-2744-14279142155777_thumb.jpg

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Wow! The casting of the dog looks almost like a real one!! ;D

(Just joking) ;D ;D

Dazza, looks like you have had quite a success with this project. Perhaps you could put it together in a "How To" form and Mixos could publish it on the site.
Good Work!

MP

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Hi MP,

Don't you think it looks more like a rabbit on a surfboard when it is turned upside down ;D, that's what I refer to it as (the surfing bunny) ;D ;D

It would give me a real kick to have my project published on this site :D, and I will put together a "How To" form, once I have successfully completed my foundry setup, and I am confident in all aspects of this process, so as I can be confident that I am not misinforming anyone, who wishes to take on this project ;).

Thanks for the thumbs up MP :).

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