Sciencemadness Discussion Board - Nichrome furnace.

Nichrome furnace.

TheMrbunGee - 28-7-2016 at 06:15

So I have been busy few days now trying to build a furnace from old stove nichrome ribbon.

i had a few tries connecting NiCr directly to mains and I got few moments of glory, but I always end up overheating NiCr and ruining the furnace.

I tried dimmer switch, but it won’t go for long, because it is rated for only 400w.

Is there any suitable solution for not overheating the element? I mean without pricy, hard to come by power supplies?

Loptr - 28-7-2016 at 07:23

Talking about using easy to come by equipment there is a project on YT called the "Scariac". Its a take on a variac using common components, salt, and a jug of water.... a neat idea, really.

They also have higher wattage dimmer switches.

[Edited on 28-7-2016 by Loptr]

TheMrbunGee - 28-7-2016 at 07:54

Quote: Originally posted by Loptr

Talking about using easy to come by equipment there is a project on YT called the "Scariac". Its a take on a variac using common components, salt, and a jug of water.... a neat idea, really.

They also have higher wattage dimmer switches.

[Edited on 28-7-2016 by Loptr]

OMG, I had seen that video, I forgot about it! I will try to get that work!

Thanks!

That wont help, I was able to heat the furnace for about 2 minutes and a liter of water started to boil.

[Edited on 28-7-2016 by TheMrbunGee]

careysub - 28-7-2016 at 11:24

The problem with furnace power supplies using the salt water rheostat for power control is that 80% or so of you power goes into boiling the water. 20% efficiency has me looking for other options.

Fulmen - 28-7-2016 at 13:19

Basically you're trying to fix a poorly constructed heating coil. Three are resources out there for designing a good coil. It's not just about ohms law, you also have to consider the power density. Basically you need more wire, more mass to dissipate the power.

As for fixing your current predicament without redesigning the coil, you need a method for reducing the current. A variac (or fixed step-down transformer) is probably the best and simplest solution, an inductive ballast would also work. Resistive ballasts works in a pinch, but that heat will be wasted and most likely cause practical problems.
Another option would be power regulation by fast switching. I suspect you can use solid state relays for this, if so it just needs a simple timer circuit to drive it.

subsecret - 28-7-2016 at 19:48

Kanthal A1 kiln heating coil would be great for this; they're usually made in lengths suitable for 240 volts (here in the US), so whether you have 220/120/240 you'll be able to get something to work. Fulmen's point of switching the power input on and off is a good one. It doesn't even have to be too fast...

Bimetalic car blinker thing (off an older car) in series with a varistor to change current flow, thus changing how fast the bimetal heats up. I think you could connect this little setup in series with the "control" side of a relay. It would be good to use a beefy bimetalic setup and strong resistor so that the relay affects it a little less. I have little experience with electronics, but that's an idea; definitely do some more research before trying this.

markx - 28-7-2016 at 23:22

Quote: Originally posted by TheMrbunGee

I tried dimmer switch, but it won’t go for long, because it is rated for only 400w.

Is there any suitable solution for not overheating the element? I mean without pricy, hard to come by power supplies?

Variacs and salt water resistors are not a very civilized option for controlling AC power these days. It is also worth to mention that the safety acpect of these devices is ranking quite low. As an alternative there are dirt cheap and efficient SCR regulators available that are excellent for the job.
For example 2000W:

http://www.ebay.com/itm/High-power-2000W-220V-SCR-Voltage-Re...

4000W:

http://www.ebay.com/itm/High-Power-4000W-220V-SCR-Voltage-Re...

10000W:

http://www.ebay.com/itm/10000W-220V-SCR-Super-Power-Regulato...

I suspect there are even larger ones available for higher power, but I think the 10kW one shall mostly suffice for an amateur furnace. You can also scan on Aliexpress....perhaps find better price and broader selection.

If you need feedback and temperature control then consider the following set of equipment:

http://www.ebay.com/itm/Digital-PID-Temperature-controller-D...

A rather straight forward and ready to use solution for an efficient AC furnace temp control.

[Edited on 29-7-2016 by markx]

Fulmen - 29-7-2016 at 01:19

Quote: Originally posted by markx

As an alternative there are dirt cheap and efficient SCR regulators available that are excellent for the job.

Excellent idea, I forgot about those.

TheMrbunGee - 29-7-2016 at 05:18

Quote: Originally posted by markx

Quote: Originally posted by TheMrbunGee

Ordered 4kW one. It will do for starters! My furnace is not big - 70*100*100mm Element has 16.something ohm rezistance, what gives 3kW power.

careysub - 29-7-2016 at 06:51

I was just searching on "kiln" on CraigsList iin my area and was amazed at how many ceramic kilns at a fraction of the price of a new one were listed.

This has me thinking about my own future plans...

Sure, you will spend at least a few hundred on such a second-hand kiln (unless you get a really, really good deal - not impossible if you are patient and persistent I expect), but it should be a highly capable furnace (assuming it works at all). Look for a "cone 10" model for the maximum possible temperature, which is rated for 1300 C.

careysub - 29-7-2016 at 07:01

Quote: Originally posted by Fulmen

Quote: Originally posted by markx

As an alternative there are dirt cheap and efficient SCR regulators available that are excellent for the job.

Excellent idea, I forgot about those.

Is there any advantage in a variac over one of these thyrisistor regulators? Variacs can do some step up, I know, (but not all do) but lets say you have 220 V and don't care about that.

TheMrbunGee - 29-7-2016 at 07:35

Quote: Originally posted by careysub

Quote: Originally posted by Fulmen

Quote: Originally posted by markx

As an alternative there are dirt cheap and efficient SCR regulators available that are excellent for the job.

Excellent idea, I forgot about those.

Is there any advantage in a variac over one of these thyrisistor regulators? Variacs can do some step up, I know, (but not all do) but lets say you have 220 V and don't care about that.

I have 220V

Fulmen - 29-7-2016 at 08:59

Not really.furnace

WGTR - 29-7-2016 at 11:28

Quote: Originally posted by TheMrbunGee

Ordered 4kW one. It will do for starters! My furnace is not big - 70*100*100mm Element has 16.something ohm rezistance, what gives 3kW power.

I think you're using almost 10 times too much power for this amount of volume, assuming the use of insulative firebrick. My own kiln achieved about 1200°C after an hour at 400-500 watts. I used 15' of Kanthal A-1 24 AWG resistance wire at 120V. This length of wire costs about $2-3.

Fulmen - 29-7-2016 at 17:11

I agree, I'm feeding a larger furnace (90x180x250mm) with 900W, it reaches 850° in 3 hours. I actually wired it with both 1500W and a 2000W elements as I assumed I would need more power, but ended up running them in series. Figured a the gentle heating would wear less on the refractories, and the reduced load on the elements should also increase the life span.

byko3y - 29-7-2016 at 23:30

High power is good for fast start, but as elements of your furnace become hotter, you need to lower the power. All commercial furnaces have some kind of thermal regulator that turns off the power if some temperature is reached. This way heating element never overheats.
In my reagion there's sold some cheap 2kW power regulators and also temperature controllers, for 50-100$ some really advanced controller can be build.

TheMrbunGee - 29-7-2016 at 23:37

Quote: Originally posted by WGTR

Quote: Originally posted by TheMrbunGee

Ordered 4kW one. It will do for starters! My furnace is not big - 70*100*100mm Element has 16.something ohm rezistance, what gives 3kW power.

I think you're using almost 10 times too much power for this amount of volume, assuming the use of firebrick. My own kiln achieved about 1200°C after an hour at 400-500 watts. I used 15' of Kanthal A-1 24 AWG resistance wire at 120V. This length of wire costs about $2-3.

on early prototypes I got to 1100°C in 2 minutes, but I was not using firebrick but some kind of much more isolative stuff (not sure what is it.) because of lack of any power control after 5 minutes it reached about 1400°C and the nichrome melted..

Now I am making inner walls from termoconcrete (rated for at least 1400°C)

I would like to use less power, but it was not an option, because the nichrome went straight to mains.

Melgar - 15-8-2016 at 15:50

Use a photoresistor in series with a relay. When it starts to glow bright enough to trigger the photoresistor, it'll shut off. You can also have it charge a capacitor that's connected to the gate of a transistor (choose a capacitor such that it'll discharge internally fast enough so you don't need to wire in a resistor to do that) so that it doesn't turn on and off very quickly, but that probably wouldn't be necessary. You can vary the aperture on the photoresistor so that it takes more or less light to trigger it. You can either turn off that circuitry manually when it gets hot enough, or figure out some way to do it automatically.

This would work well because it detects the actual problem, (overheating) as opposed to using some arbitrary means of detecting it.

[Edited on 8/15/16 by Melgar]

MrHomeScientist - 16-8-2016 at 07:47

Quote: Originally posted by markx

If you need feedback and temperature control then consider the following set of equipment:

http://www.ebay.com/itm/Digital-PID-Temperature-controller-D...

A rather straight forward and ready to use solution for an efficient AC furnace temp control.

Would something like that be suitable for a heating mantle controller? I have a mantle but no way to plug it in!

markx - 30-8-2016 at 08:53

Quote: Originally posted by MrHomeScientist

Would something like that be suitable for a heating mantle controller? I have a mantle but no way to plug it in!

Yes, of course...it will do excellent as a heating mantle control. You can have the feedback sensor installed in the mantle or directly in the environment (flask, reactor chamber etc.) that the temperature needs to be controlled in. The controller can work in PID mode with feedback or just like a SCR regulator without any feedback.
I've found this set of equipment to be the most versatile tool ever when any kind of temp control is required. I have used it to execute temperature control for soldering irons, melting point apparatus, various reactors, sous vide water bath, cold smoke generator, heat mantles and basically any other temperature related device that operates on 220 mains voltage.