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Author: Subject: Homebuilt 1300°C tube furnace
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[*] posted on 25-12-2007 at 05:02
Homebuilt 1300°C tube furnace


I completed my new powerful tube furnace. Look here for pictures:
http://www.versuchschemie.de/ptopic,168531.html#168531
You can see that I have successfully taken it to 1300°C (2370°F- thats above the maximum continuous use temp for quartz glass!) already before I put it into a casing.

As you can see, the construction is ridiculously simple. It can be thrown together and put into use on a single day.
No cements or castable refractories are used.

The core of the furnace is a ceramic tube, 500mm long, 40mm OD, 32mm ID. The ceramic is called "pythagoras" and is good up to 1400°C (some sources say 1500°C), it is also absolutely gastight (even high-vacuum tight).
14,7m of 1mm Kanthal A1 wire (1,8 ohm per meter, giving about 2000W of power at 230V) is wound onto the tube and covered with a paste of equal parts MgO (dead-burned magnesia) and Al2O3.

After drying, it is surrounded by two layers of 1400°C aluminum silicate-zirconia 25mm ceramic fiber blankets held in place by wire.
It is put into a 120mm ID sheet metal pipe (the fiber blankets have to compressed somewhat for it to fit into the pipe- a larger pipe would have been better, but I couldnt find one).

The power is regulated by a thyristor circuit (commercial dimmer module), a 60W incandescent light bulb is put in parallel to the furnace to serve as a crude indicator of power.

Temperature is measured by a selfmade Type K (Nichrome-nickel) thermocouple connected to a multimeter with a temperature measurement option.
The lifetime of such a thermocouple is limited above 900°C (but can be taken to 1350°C for short periods of time)- a Type S (Pt-PtRh) thermocouple would be a much better choice, but is very expensive.

I have a 20cm long quartz test tube with ground glass joint that fits into the furnace. This can be used to take substances to temperatures far higher than what can be reached by a bunsen burner.

Such a tube furnace is able to provide extreme heat for every application in the lab where a gas burner fails miserably.
The prime example for such an application is: SYNTHESIS OF PHOSPHORUS.

I am mostly interested in the production of sulfur trioxide from ferric sulfate in this tube furnace, as well as carbon disulfide from sulfur vapor and charcoal and ketene from acetone.

[Edited on 25-12-2007 by garage chemist]




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[*] posted on 25-12-2007 at 07:39


Very nice! Nice neat construction too.
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[*] posted on 25-12-2007 at 08:11


Nice. I like it very much.

What kind of "solvent" is used with MgO and Al2O3 to form a paste? Is it water or something else?
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[*] posted on 25-12-2007 at 08:25


Looks very nice! I have been wanting to build one myself.

Oh, i translated it to English for those who cant read German (i cant) http://translate.google.com/translate?u=http%3A%2F%2Fwww.ver...
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[*] posted on 25-12-2007 at 09:18


Yes, the MgO+Al2O3 is made into a paste with water.
Brauer recommends this mix for coating the wire, I found that it doesnt adhere at all after drying, but used it anyway.

I posted the important information from the german text in the first post. If questions remain, feel free to ask.

EDIT: I forgot to mention, the 4cm of insulation that I used is pretty thin. The outer sheet metal tube gets hot quickly (over 100°C). You need to place the furnace on a heat-resistant support, like a few ceramic tiles or bricks, otherwise a wooden table may char.
If you can get a suitable larger sheet metal tube, using a third layer of ceramic fiber blanket would be a very good idea.

The furnace still reaches 1000°C in just 10 minutes at full power with my current design, and takes less than 50% power to stay stable at 1100°C.

[Edited on 25-12-2007 by garage chemist]




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[*] posted on 25-12-2007 at 10:14


"I found that it doesnt adhere at all after drying"

What about mixing a little sodium silicate or something in with the water? It wouldn't make it strong, but would probably make it a bit stronger...
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[*] posted on 25-12-2007 at 10:49


If you use calcined (not dead burnt) magnesia, it cements together a little on hydration (forming Mg(OH)2).

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[*] posted on 25-12-2007 at 10:51


One of these would probably be useful

http://cgi.ebay.com/PROFESSIONAL-INTELLIGENT-PID-TEMPERATURE...

You'll need a better thermocouple than what comes with it ,
and you'll need a solid state relay too .

[Edited on 25-12-2007 by Rosco Bodine]
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[*] posted on 25-12-2007 at 12:40


Nice work gc! I love the simplicity of your design. ;) I am surprised that your furnace pipe OD is only 120mm (4.7 inches)! That means your insulation thickness is 40mm (1.6") maximum. Unbelievable!

This should open up the home chemist's ability make all kinds of useful reagents.

I have a couple questions. Your Pythagorus tube is 500mm long, with 450mm heated. Will you still be able to use your 20cm quartz tube w/ground glass fittings? Do you just insert the quartz tube and connect to borosilicate glass tubing on both ends, or what? Will you need to use supporting saddles for your quartz tube or will it center itself adequately?

[Edited on by Magpie]

[Edited on by Magpie]




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[*] posted on 25-12-2007 at 12:57


Very nice, and very simple. It could definitely be improved upon, but for the amount of time you spent building it, very well done. Garage chemist, you and I will have to have some synthesis competitions with these tube furnaces :) I'm tempted to leave phosphorus to you, but CS2 would be an interesting one.



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[*] posted on 25-12-2007 at 13:18


Magpie, the kaowool is indeed a very good insulator.
However, the 4cm are really less than optimal. The furnace gets really hot on the outside, well over 100°C. It still has no problems reaching 1300°C though.

The quartz test tube I am talking about is closed on one end, and has a ground glass joint on the other side. It will be inserted into the furnace at one end and connected to e.g. a condenser for product vapors. Yes, the top 5cm of the test tube will be in the unheated zone then, but you have to have a temperature gradient zone to protect the joint from the furnace heat.
I also have a 10cm quartz extension tube with two joints.
The test tube will not be centered in the ceramic tube, the heat distribution is even. The temperature at the bottom of the tube is nearly the same as at the top due to heat transfer via radiation.

I also have a 70cm long quartz pipe that is open at both ends, with two joints. This can be used for continuous flowing gas reactions like ketene synthesis or CS2.

Fleaker, I would prefer to leave the phosphorus synthesis to you if you can build a tube furnace too. I can simply buy red P, so I would rather sublimate this.
Also, I havent read up as much on the different raw materials from which P can be made as some other members here, like Polverone and BromicAcid, and probably you as well.


Currently, I have a bowl of agricultural FeSO4 sitting around for it to oxidise in air. Guess what that makes when I put it in the furnace. :D




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[*] posted on 25-12-2007 at 14:07


Hi garage chemist. This is indeed very good. The supplier of ceramic tubes you list appears to be a custom fabricator. Is the tube you used custom, or is it an of-the-shelf? Can one order on-line from them? What about the insulation manufacturer? The wire source appears to be order-on-line, can one get Kanthal that way? regards Len
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[*] posted on 25-12-2007 at 15:21


The ceramic manufacturer is one that normally only sells to companies, you cant order online from them. I called them up and said what I wanted (originally I wanted a sintered 99,7% Al2O3 tube) and they were able to find a tube from their stock that suited my needs.

The wire seller carries the wire diameters listed on his website. 1mm isnt one of them, but I got it on special request via email.

The insulation seller carries the 1400°C fiber blanket as a normal item in his online store, and you can order online.


Rosco, you're right, such a controller would be a very good addition to the furnace.
I currently control the temperature manually via the dimmer, when I've reached the desired temperature I reduce the power and see if the temp goes further up or down- if it goes up, I reduce the power further, if it goes down I increase it again a bit so that the furnace is in equilibrium and doesnt change its temperature.


[Edited on 26-12-2007 by garage chemist]




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[*] posted on 25-12-2007 at 15:21


I've seen Kanthal wire on ebay especially A1 grade and not terribly expensive neither.

Garage chemist,or anyone else, I have two questions:

1. The Al2O3/MgO water paste mix when you applied it and allow to dry, did you need to run the furnace to "set" this?

2. The design of your furnace is based on 230V as a power source. Wouldn't it be OK to use half the length of simular wire, different size tubing, make this suitable for use with 120 V source current?

I realize that these are somewhat obvious ideas but I know once a furnace gets a certain size 120 Volts is no longer practical. Although the cubic measurements we are dealing with here are quite small though.



With a 1300 C furncase I can make europium doped alkaline earth aluminates for glow pigments!!:D:D




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[*] posted on 25-12-2007 at 15:41


Yes, I baked the coating of MgO-Al2O3 mix after drying by running power through the wire without insulation. It became really powdery and fragile after that, I think it would be better to not bake it and just pack it into the fiber blanket as it is and then heat it up.
Dont use sodium silicate as binder, this attacks Kanthal wire above 1000°C and makes it oxidise very fast. You need to be really careful about what you put in contact with the wire. SiO2 attacks it as well, so you cant use a quartz tube instead of a ceramic one.

With 120V, your best bet would be to use a thicker wire that has half the resistance per meter of the 1mm one so that you get 2000W as well.
If you use the same wire as me and make it half as long to get the same power, you will have a special problem: the surface loading of the wire will be too high, e.g. the wire temp will exceed 1400°C and melt even if the inside of the tube is still far below 1300°C because the wire surface is too small to transfer all that power to the tube.

Look in Brauer in the section about tube furnaces. It explains the problem and gives maximum surface loadings for heating wire.
I already exceeded the maximum surface loading strongly with my current design- but only at full power, and the furnace is always run with reduced power through the dimmer.
Especially at high temperatures one must not heat at full power- as a general rule, dont ever give full power to the furnace when it already is above 1100°C. If you need 1300°C for your experiment, you need to increase the temperature slowly and gradually above 1100°C.




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[*] posted on 25-12-2007 at 15:52


I have quite a bit of experience with designing PID's. You only really need them if the oven you are trying to control has a max. temp way hotter than the temp you are running it at. If you want to operate the furnace at 1200C and its max temp is 1300C PID is overkill and on/off control via thermostat would work just as well. Len

@garage chemist. Im planning to ring them. Do they speak english? ARe they likely to have another tube in stock? Len
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[*] posted on 25-12-2007 at 15:56


Great work GC!
How easy is 1 mm kanthal to work with? Most resistance wire I have used is much less bendable than copper wire of equal diameter.

Your multimeter has a temperature function?!:o I know I am amazed by something so simple, but I have never seen one like that for sale. What is involved to set up such a multimeter to accuratly measure temperature, especially when using different types of thermocouples?

What is the current price on a type S thermocouple anyway?
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[*] posted on 25-12-2007 at 16:23


You really do need some sort of active feedback control when you have a reaction going inside the tube , because
many tube reactions have a sort of "ignition temperature"
where the tube furnace really just serves as kindling ,
and once the fire is started inside the catalyst chamber .....
the furnace becomes a catalytic converter and needs
little extra heating . You won't be able to keep up with
the process on a manually operated percentage controller , but need something that can track and adapt
to the demand for maintaining a setpoint .
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[*] posted on 25-12-2007 at 16:34


Len, what do you mean with PID? A thyristor circuit like I use it?

I dont know if they speak english. Perhaps you could search for a manufacturer closer to you. Do you live in the UK?


Davster, Kanthal A1 wire is somewhat springy and properly winding it onto the tube is quite a chore. It helps to secure it every 10 cm or even more often with wire circles.
I was not able to get a neat uniform winding onto the tube, it is kinda crooked and of inhomogenous winding density in some places.
A user on the german forum suggested in a different thread to wind the wire on a tube of smaller diameter first and then transfer that spiral onto the ceramic tube after it has unwound a bit due to the springiness.

Yes, some new multimeters have a temperature measurement option and use K type thermocouples. You could always use any thermometer that uses a K type thermoelement.
There are also special ICs like the AD595 that can be connected to a thermoelement and give out a voltage of 10mV per °C that can be measured with a multimeter.
The thermoelement itself gives a voltage of 54886 µV at 1372°C, so it could theoretically be measured directly with a sensitive multimeter- but you would have to calibrate this very carefully and make a calibration curve, I dont recommend this. Just get an electronic thermometer and make the thermocouple yourself (and have enough spare thermocouple wire to make more thermocouples).
The temperature regulator from ebay that Rosco posted can also be directly connected to a K thermocouple and used both for measuring and regulating the temperature!

You need to get the two wires that make up such a thermocouple (those are special alloys, not just nickel and chromium, they contain more metals) and weld them together at the tip.
This can then be connected to the thermometer or regulator. Test for accuracy by comparison with a mercury thermometer.

A type S thermocouple (Pt and Pt/Rh 90/10)... well, lets say your thermometer is 50cm away from the point at which you want to measure the temperature, then you need 50cm of both Pt and Pt/Rh wire. You can imagine that this could easily cost more than the whole furnace, and such a length of Pt wire would serve better in a perchlorate cell.

Read up http://en.wikipedia.org/wiki/Thermocouple . Lots of good information there.

[Edited on 26-12-2007 by garage chemist]




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[*] posted on 25-12-2007 at 16:37


Quote:
Originally posted by The_Davster
Your multimeter has a temperature function?!:o I know I am amazed by something so simple, but I have never seen one like that for sale.


Hey.. What...! Where do you live! It's hard to find a multimeter WITHOUT a temperature function these days!

My old digital multimeter is over 20 years old, and came with a K-type thermocouple and temperature function. I calibrated it the other day using ice, boiling water and moten lead and it was within a couple of degrees for all three.
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[*] posted on 25-12-2007 at 17:15


From len1:


Quote:

The wire source appears to be order-on-line, can one get Kanthal that way?


For those that prefer the US market:

http://www.resistancewire.com/mainpage.php




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[*] posted on 25-12-2007 at 17:21


@garage chemist

I was replying to what someone further up the thread wrote about a PID controller. Thyristor (or triac) control without microsprocessor is on/off (and with opamps can also be made proportional control) where the load gets turned on/off when a preset temperature is reached. The trouble with that can be if the system has substantial thermal inertia, after the heater is turned off there will be overshoot, then undershoot etc. Its particularly bothersome with distillations, and when you are operating far from maximum temperature. I have a pdf article and circuit if you (or anyone else) is interested).

I live in Australia which is technological desert country. We dont have specialised resistance wire manufacturers or, ceramic laboratory fittings manufacturers here. We just dig ore and shear sheep. We dont even have Nickel metal, whose ore were one of the biggest exporters. However I plan to be in europe next month, and will go to this ceramics place and see if the will be able to sell me a tube. Aber ich nicht sprechte Deutch, so maybe it wont work.

[Edited on 26-12-2007 by len1]
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[*] posted on 25-12-2007 at 17:39


Quote:
Originally posted by len1
I have a pdf article and circuit if you (or anyone else) is interested).


Hi len1, I've been looking on the internet for a triac temperature controller circuit with thermocouple feedback for a while now. I'm sure there used to be heaps of them, but I can't find anything suitable now! I'd be interested in anything you have.
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[*] posted on 25-12-2007 at 17:43


Thanks! I was under several misconceptions about thermocouples. I have also used several tube furnaces in university labs, and I had no idea that a thermocouple was that long; I thought they were short with only a small metal-metal junction, I had no idea that the legwires had to be the same metal, producing high cost.

As for multimeters, I have 4 different ones, apparently they are all very old(or cheap), becasue not one has a temperature function.:P

[Edited on 25-12-2007 by The_Davster]
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[*] posted on 25-12-2007 at 17:43


PID (proportional-integral-derivative) controllers can be had for very low cost off e-bay.

I built a control circuit for a small relatively low temperature furnace using a PID controller and a solid state relay. It works quite well. You can see it in my last picture at:

http://www.sciencemadness.org/talk/viewthread.php?tid=2171&p...




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