Sciencemadness Discussion Board

Homebuilt 1300°C tube furnace

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garage chemist - 25-12-2007 at 05:02

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]

Nick F - 25-12-2007 at 07:39

Very nice! Nice neat construction too.

SecretSquirrel - 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?

Alan - 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...

garage chemist - 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]

Nick F - 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...

12AX7 - 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).

Tim

Rosco Bodine - 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]

Magpie - 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]

Fleaker - 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.

garage chemist - 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

len1 - 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

garage chemist - 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]

chloric1 - 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

garage chemist - 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.

len1 - 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

The_Davster - 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?

Rosco Bodine - 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 .

garage chemist - 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]

Xenoid - 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.

Magpie - 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

len1 - 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]

Xenoid - 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.

The_Davster - 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]

Magpie - 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&a...

Drunkguy - 25-12-2007 at 18:07

Ich habe jetzt deine furnace gesehen.

Jawohl! Bravo! Prima!

Now I know who i'll will be employing to make me my Raney Nickel catalyst.

Also, you say u are interested in oleum (SO3). That's also a chemical that could have uses to me although I dont know how far I want to go into the workshop when most things are commerically available already.

[Edited on 26-12-2007 by Drunkguy]

chloric1 - 25-12-2007 at 18:27

Quote:
Originally posted by Drunkguy
I dont know how far I want to go into the workshop when most things are commerically available already.
[Edited on 26-12-2007 by Drunkguy]


Well, for starters, it could be cheaper to make what you need with shipping charges and all. SecondLY, making something can give you hands on understanding.

len1 - 25-12-2007 at 18:30

Quote:

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.


@Xenoid
No problem. The controller circuit I have is more complicated than triac with thermocouple feedback - thats on/off control. I have one with microprocessor PID control which eliminates overshoot. It does mean you have to build a microcontroller circuit, and program it with the program downloadable off the Silicon Chip web site. Please tell me if you actually want a PID controller and I will email you the circuit.

@magpie

Yes they can be bought in the US for US$50 or so, thats no a problem. But these are cheapo types, you cant program them youraelf, and they dont come with computer control and interface such as the one Im referring to above.

@Davster

You are right in that the thermocouple needs only be as short as the junction of the two metals, and any two different metals will produce a voltage difference, the key is that they and their alloy shouldnt melt at the measurement temperature.

The need for having long leads comes from the fact that if you join the first thermocouple to a different wire set AT THE SAME TEMPERATURE as the measurement point the voltages will cancel and you will get no measurement. To get a result the only requirement is that the junction with the longer (cheaper) wire must be made at a point where the temperature is substantialy different from the measured temperature.

So the ideal approach for the amateur is to locate the Pt couple as close as possible to the exit of the furnace where you believe the temperature is representative of the furnace temperature. The wire needs to be taken as far as the outer end of the furnace, where the temperature is below the max temperature of Cu/Const. There you can attach screw-on terminals of copper wire, and place a normal thermocouple. The whole apparatus can now be calibrated at a few points, and there you have it. The only problem is where does one get the primary high temperature t-couple from.

Magpie - 25-12-2007 at 19:17

from len1 to Magpie:
Quote:

Yes they can be bought in the US for US$50 or so, thats no a problem. But these are cheapo types, you cant program them youraelf,


They are programmable in the sense that you can set the P, I, & D parameters, type of thermocouple, T in Celsius or Fahrenheit, etc. They also have an auto-tune feature. Did you mean something different than this?

[Edited on by Magpie]

len1 - 25-12-2007 at 23:23

Well I guess I meant you can program it to follow a temperature regime. Different temperatures for different durations, and the tuning I think is better too, and you have control over that because you have access to the program. Len

Fleaker - 26-12-2007 at 14:25

Quote:
Originally posted by len1

You are right in that the thermocouple needs only be as short as the junction of the two metals, and any two different metals will produce a voltage difference, the key is that they and their alloy shouldn't melt at the measurement temperature.

The need for having long leads comes from the fact that if you join the first thermocouple to a different wire set AT THE SAME TEMPERATURE as the measurement point the voltages will cancel and you will get no measurement. To get a result the only requirement is that the junction with the longer (cheaper) wire must be made at a point where the temperature is substantialy different from the measured temperature.

So the ideal approach for the amateur is to locate the Pt couple as close as possible to the exit of the furnace where you believe the temperature is representative of the furnace temperature. The wire needs to be taken as far as the outer end of the furnace, where the temperature is below the max temperature of Cu/Const. There you can attach screw-on terminals of copper wire, and place a normal thermocouple. The whole apparatus can now be calibrated at a few points, and there you have it. The only problem is where does one get the primary high temperature t-couple from.


That is very good to know. Platinum and rhodium are both trading at all time high prices, and even a few inches of wire are very expensive. I have maybe a 11g piece of 50cm long Pt wire, but no PtRh alloy wire, and that will be troublesome to procure. My main concern with connecting a type S to a type K is that the business end might conduct enough heat to melt the connections. How would one connect the type S from the center of the tube ( BTW, that is the best place for thermocouple, it is most representative of horiz. chamber temp.) to the cheap type K? That is a problem I think, otherwise it would be great: type K is very cheap, type S, damn expensive. Len, you seem to know what you're doing with electronics, how would you solve the conduction problem?

I am very interested in building a tube furnace for home that is about 75 cm/30 inch in length with a 6 cm inner diameter. Originally I planned on using a fused alumina tube and heating it with molybdenum disilicide elements but it would cost me about $1200 to do that. It would give me 1750*C though, suitable for many things!

Yet for really cheap, about $50, I can buy some A1 kanthal that uses 240V in series, 14.5 amp, and 3500 watts. I think that would be sufficient, the furnace won't heat as fast as garage chemist's but it will still be plenty powerful. A1 Kanthal is good til about 1450C/2550F. This would also mean that I could use a cheap mullite tube or silimantite tube. Probably under 200 dollars I would think. I think I would want a PID and some sort of stand so I can make the furnace vertical as well.

I have several hundred square feet of kaowool and plenty of Al2O3 and MgO, so that's no problem.


My biggest question is, how one would make a clamshell design? It would be difficult because you would not be able to just wrap the wire around the tube, plus you would need to have the tube cut in half.

Despite the evident construction problems, they are still very convenient--quick cool downs, allows easily introducing reaction system, best part I think is that it could make for easier ''zoning'' of the tube furnace.
That's another question for you electronics wizards: how would I make one end of the tube 500*C, while the other end would be 1000*C? I know I would need some sort of insulating barrier in between, but how do I control current supply to the elements, do I run several heating element circuits?


Edit: found one interesting supplier, and they're relatively local: http://www.clevelandelectriclabs.com/welcomeCEL.html

More:
http://www.sentrotech.com/accessories.php
http://www.zircarceramics.com/

[Edited on 26-12-2007 by Fleaker]

len1 - 26-12-2007 at 16:14

Here is an explanation of what I had in mind


____________V2(T0)_____________________
V(T)
____________V3(T0)_____________________


The high temperature thermocouple is at the temperature of interestT, the contact to the cheaper wire (which must be the same at both contacts is at a location where the temperature is T0.

All junctions generate voltage, and as most multimeter type thermometers assume a linear relationship, Ill assume that too (but its inessential to the result, just makes thing seasier to see).

V(T) = a T
V2(T0) = b T
V3(T0) = c T = (a - b) T0

the last equation arises from conservation of energy. Then the voltage measured on the multimeter is

a T + b T0 - (a - b)T0 = a (T - T0)

(note that b wil cancel out even if its a function of T)

i.e. the tempearture at the measurement point is the sum of the temperatures measured by the two thermocouples, one with short leads located at the measurement point, the other at the point of contact with the copper leads. The later could be located on the outer iron casing of the tube, with holes cut for the high T thermocouple leads, which would need to be about 6 cm long.

There is also the ambient temperature. Most thermometers take this into account by adding it onto the temperature measured by the thermocouple (else at ambient we get zero). So the relation is actualy

V(T) = a T - a Tambient

To avoid adding this twice from the two multimeters the formula becomes

Tactual = T + T0 - Tambient

Tambient you can get by noting the thermometer reading prior to measurements.

I didnt quite understand why you want a temperature gradient across the tube, or a clam type arrangement.

PS Do you have suppliers for the molyb elements and alumina tube? Len

WizardX - 26-12-2007 at 17:18

Temperature Controller Circuits. http://www.schematicsforfree.com/archive/dir/Misc/Temperatur...

WizardX - 26-12-2007 at 18:25

Quote:
Originally posted by garage chemist
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.

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.


I would coil the 14.7m of 1mm (or thicker) Kanthal resistance wire around at tube with a outer diameter of 30-31mm to make a coil that is the 32mm internal diameter of the ceramic tube.

Similar to an induction coil.



The 14.7m of 1mm (or thicker) Kanthal resistance wire coil is then fed inside the length of the ceramic tube with spacers between the windings.

Xenoid - 26-12-2007 at 20:17

Whilst not being in the same league as garage chemist's tube furnace a smaller, simpler version could be built with components purchased from our friends the pottery suppliers.

In their "Kiln Furniture" sections they have tubular ceramic props. These are used for building up layers of shelves for pots in the kiln. They are about 45 mm diameter and come in a range of lengths from about 3cm upto 30cm (12"). They are very cheap, with a 30cm length costing about NZ$10. They also supply Kanthal wire and Kaowool blankets.

I have thought about using one of these 8cm "props" (in a vertical orientation) as a single crucible mini electric furnace.

Fleaker - 26-12-2007 at 20:41

Quote:
Originally posted by len1
Here is an explanation of what I had in mind


____________V2(T0)_____________________
V(T)
____________V3(T0)_____________________


The high temperature thermocouple is at the temperature of interestT, the contact to the cheaper wire (which must be the same at both contacts is at a location where the temperature is T0.

All junctions generate voltage, and as most multimeter type thermometers assume a linear relationship, Ill assume that too (but its inessential to the result, just makes thing seasier to see).

V(T) = a T
V2(T0) = b T
V3(T0) = c T = (a - b) T0

the last equation arises from conservation of energy. Then the voltage measured on the multimeter is

a T + b T0 - (a - b)T0 = a (T - T0)

(note that b wil cancel out even if its a function of T)

i.e. the tempearture at the measurement point is the sum of the temperatures measured by the two thermocouples, one with short leads located at the measurement point, the other at the point of contact with the copper leads. The later could be located on the outer iron casing of the tube, with holes cut for the high T thermocouple leads, which would need to be about 6 cm long.

There is also the ambient temperature. Most thermometers take this into account by adding it onto the temperature measured by the thermocouple (else at ambient we get zero). So the relation is actualy

V(T) = a T - a Tambient

To avoid adding this twice from the two multimeters the formula becomes

Tactual = T + T0 - Tambient

Tambient you can get by noting the thermometer reading prior to measurements.

I didnt quite understand why you want a temperature gradient across the tube, or a clam type arrangement.

PS Do you have suppliers for the molyb elements and alumina tube? Len



Thanks very much for clearing it up for me. That calibration looks simple enough. I still maintain that there will be a problem with finding a minimum amount of wire required to not melt your connection. Anyone want to volunteer to do the math on how much heat Pt/Rh wire conducts over a given distance at a given temperature with a given amount of insulation preventing radiative and convective losses?


I suggested taking the temperature in the middle of the tube with the thermocouple for the reason that it will be where the highest temperature is obtained, and will show you your max temp. for the furnace. Refer to my thread on sulfur trioxide from V2O5 catalyst tubes--I noted there how temp at one end of a tube furnace can vary by several hundred degrees.

As for why a clam shell is desirable, well, I guess you've never used a tube furnace before. I've used both a simple type and a clam, and I much prefer the clam for the ease with which I can install my reaction tube, thermocouples, and most importantly, how quickly I can cool the apparatus down.

IF you looked at those links you might have noticed that both zircarceramics and sentroTech sell MoSi2 and the requisite alumina or zircon tubing. It's not cheap, but a well built tube furnace is very nice to have.

http://www.sentrotech.com/custom.php (Moly disilicide)

12AX7 - 26-12-2007 at 23:17

Pt isn't very conductive, AFAIK. The wire will basically be at the same temperature as the tube, give or take a hundred degrees or so. The copper end might cool the joint by maybe up to a hundred degrees, depending on how well insulated it is. We're talking wire here, it's nothing amazing as conductivity goes.

The thermocouple wire I have (K I think) is wrapped in woven glass fiber. Nice.

Tim

Rosco Bodine - 27-12-2007 at 00:09

Watlow has some good reference charts and other information .

http://www.watlow.com/literature/catalogs/files/sensors.pdf

http://www.watlow.com/literature/catalogs/

len1 - 27-12-2007 at 03:15

No the contact V2 and V3 must be at the temperature of the outer (iron) tube. They must be electrically insulated from the tube, but well thermally attached to it ( high T thermal grease is best). A low temperature thermocouple must be attached between them. This ensures all three are at a stable intermediate T, a few hunder degrees. Theres hardly any point calculating their thermal flux as their mass is tiny and the entire temperature gradient will the fall across the wires. The high T couple must be well afixed to the alumina tube with a suitable bracket, and insulated from the heating wire. There is of course a temperature gradient along the length of tube, but that doesnt matter if you afix the thermocouple in the middle, if your reaction volume is small. Len

[Edited on 27-12-2007 by len1]

clamshell vs fixed tube

Magpie - 27-12-2007 at 15:05

Fleaker said:
Quote:

As for why a clam shell is desirable, well, I guess you've never used a tube furnace before. I've used both a simple type and a clam, and I much prefer the clam for the ease with which I can install my reaction tube, thermocouples, and most importantly, how quickly I can cool the apparatus down.


I have never used a tube furnace but understand your reasoning. My original plan was also for a clamshell design. I was going to make the shell out of a pipe with welded end-faces, or even use a 16 gallon steel drum. This would be cut in half lengthwise, and hinged. The center of the furnace would be two clam faces made from castable refractory. I have found refractories with nominal temperature limits of 2600F(1427C) and 3000F(1648C), available on-line for around $40/50 lb sack. It would take some craftsmanship to form a cavity with metal supports for the coiled resistance wire. These two clam shells, in turn, would have to be supported by steel rods that would extend out to the shell and be anchored there. The annulus would then be filled with ceramic blanket or ceramic wool. Removeable saddles would allow the use of test tubes of various diameters and lengths.

This would make a very flexible furnace. But it is all so complicated and would take so much work that I might never do it! That's why I like garage chemist's design so much.

The_Davster - 27-12-2007 at 15:12

At work we have a clamshell type tube furnace. I can take pictures of how they orient the heating elements, if noone is using it next time I am there.(if anyone is interested that is)

[Edited on 27-12-2007 by The_Davster]

Magpie - 27-12-2007 at 15:28

Quote:

At work we have a clamshell type tube furnace. I can take pictures of how they orient the heating elements, if noone is using it next time I am there.(if anyone is interested that is


Sure, take some pictures if it is not too much trouble. Try to get some good ones of the construction details.

LSD25 - 18-1-2008 at 00:27

Slightly off topic - but the nearest I could find at a glance - how would one go about cutting the vycor tubing in tungsten-halide lamp bulbs in order to make use of the vycor tube? Of some interest may also be the molybdenum seal, the platinum and/or tungsten wires, etc.

Will an ordinary glass-cutter do the trick? If not, would the scratch and chase technique work?

If either of these techniques work, any glass blowers out there with knowledge or suggestions how to join two lengths of vycor tube?

Also somebody mentioned above the possibility of using the tubular shelving supports offered by kiln providers - which are rated to around 1300C (and quite probably, indeed almost certainly, consisting largely of alumina). I have several differenr lengths of this on order at present and wish to know if anyone has tried using this?

Fleaker - 18-1-2008 at 09:21

Scratch and chase really sucks for quartz, I rarely ever get clean breaks when using the old file, rub, and snap method. Often I end up having to clean the edges (since it's hard to flame polish quartz) using a metal support gauze.

I would think a regular glass cutter would work. If not, I'm pretty sure a carbide pipe cutter would do the trick too if you were ginger with it.

The_Davster - 18-1-2008 at 17:44

I have rather extensive quartz cutting experiance... The key to clean breaks is not just to notch it in one spot, but the scratch/notch must go completly around the quartz tube, and must not have any 'mistake' scratches that are not where you want the break to be. The scratch also must be deeper than in regular glass; when you can see powder coming off when you deeply file, you are almost there.

EDIT: As for joining the quartz tubes, be prepared to fail on many before you get it right. I probably went through several metres of tubing before I was 'sufficient' at joining quartz. (although I was working with diameters varying by a cm or two, so it was more difficult). You will absolutley need a very high temp torch...oxyhydrogen is what I used. The procedure is to circularly heat the quartz tube, untill its glowing followed by drawing out the glass as if you were about to make pipettes, but instead only pull it untill you estimate that the thinner OD of the quartz is less than the ID of the undrawn out quartz. Notch and snap off excess quartz. You only want 5mm or so of the thinner quartz to be within the larger diameter quartz. Insert the drawn out part into another normal diameter quartz tube, and while only applying light pressure(only enough to hold it in place) axially forcing the tubes together, circularly heat the joint untill the quartz is glowing all around. Then and only then apply greater force causing the semifluidic quartz to smush together sealing the joint. If all the holes are sealed, you are done, there will be a lump on the inside, but that is unavoidable, and would probably require a real glassblower to fix. If there are any difficult holes, a graphite shaft can be used to manipulate semifluidic quartz, and push it where you want it. Have water nearby to dunk the graphite rod in when it starts glowing orange. Joints like this have survived 1000C to ice water shocks without incident. It is also why quartz rocks.

[Edited on 18-1-2008 by The_Davster]

homemade 2000w dimmer

Magpie - 8-3-2008 at 10:18

Wanting to have a 1300C tube furnace I began by installing a 230VAC outlet in the wall next to my lab bench. In consultation with garage chemist I decided that a dimmer using a thyristor would be best for a power controller. I wanted to use the Kemo 028 thyristor module as did GC but these are only available on the German eBay. So I decided to build my own dimmer using the circuit of figure AN1003.15 in Littlefuse document:

http://www.littelfuse.com/data/en/Product_Catalogs/PowerThyr...

The only exception I made to the circuit was to use quadrac Q4015LTH instead of the diac/triac shown. This allowed use of the higher current (8.7amp) needed to provide 2000w. A larger triac could have been used as well.

The first picture shows the assembly with the quadrac/heat sink mounted outside the enclosure for test purposes. The enclosure is a 5"x7"x3"H plastic box from Radio Shack.

The 2nd picture (next post) shows the circuit in full operation at 7.5amp using the power resistor bank I salvaged from my old houshold electric furnace. Sorry I didn't show this in dim mode - you'll have to trust me that it dims. BTW it dims smoothly and with no flickering, even at 0.33 amp load.

Some lessons learned:

1. Start off with a large enough enclosure before doing any hardwiring/soldering.

2. Try to get all your components in one order from as few suppliers as possible. For US builders I highly recommend Mouser and Newark. To a much lesser extent try Allied Electronics and Digi-Key. These suppliers, together, provide a great selection but if you are not careful shipping charges will kill you. For example, Allied charged $10 in shipping for that dinky little heat sink.

3. Most importantly, make sure that on the quadrac or triac that the Main Terminal 2 (MT2) has the same polarity as the gate (trigger).

[Edited on 8-3-2008 by Magpie]

[Edited on 8-3-2008 by Magpie]

dimmer circuit.jpg - 76kB

Magpie - 8-3-2008 at 10:20

The 7.5 amp test:

dimmer test.jpg - 67kB

garage chemist - 8-3-2008 at 13:52

Does the heat sink on the quadrac even get warm at full power now that the circuit is correct?



[Edited on 8-3-2008 by garage chemist]

Magpie - 8-3-2008 at 16:14

GC: Yes, but I am not sure how hot yet. I ran it at the 1800w load (7.5a) for ~15 minutes then felt it with my hand. I would guess about 60C. It wasn't a realistic test though as there was a fair amount of heat coming off the nearby resistor bank, and the quadrac/sink wasn't in the enclosure with the lid on. I have hardwired everything in its permanent configuration now. I need to run a good test at full load with a thermocouple in place to monitor the sink temperature.

The data sheet for the quadrac says to expect a heat dissipation of ~1w/amp. Overheating of the quadrac (max allow case temp @ 10a=95C) is a concern but the heat sink thermal resistance is rated at 2.6C/w. So at 8.7a the temperature shouldn't exceed 50C even if drawing 10 amps. But I don't know how realistic this is, especially when in the enclosure.

How hot does your thyristor module get at full load?

[Edited on 8-3-2008 by Magpie]

12AX7 - 9-3-2008 at 09:48

Your enclosure looks plastic. Don't be putting that heatsink in there!

A bigger heatsink could come in handy. Got any junk laying around with heatsinks? Monitors, power supplies, stereos, all sorts of things.

If your box is actually diecast aluminum, that might be conductive enough that you can bolt it to the case and go. You may still want a heatsink mounted flat to the outside, though (something your current heatsink wouldn't exactly be able to accommodate..).

Tim

Magpie - 9-3-2008 at 10:07

Tim, I hear what you are saying. I'm not comfortable with the situation either. It does bother me, however, that the heat sink is designed for the TO-220 component style and has the 2.6C/w rating that indicates that it will do the job. Yes, the circuit board and the case are plastic.

If the sink gets too hot then would an externally mounted fan, blowing through the holes drilled in the enclosure possibly suffice?

If I do have to mount the quadrac on a bigger heat sink outside of the box it too will have to be "enclosed" for safety as the terminals are at 230VAC.

12AX7 - 9-3-2008 at 14:48

It would, but then you have the hassle of a fan, which must run at all times, is noisy (compared to no noise whatsoever), attracts dust, may need another power supply, and probably other things.

Even a small fan moves a lot more air than convection does; one of those dinky microprocessor fans would suffice. Poke plenty of holes (or an opening covered with a fan guard), and use a loose filter type deal if you'd like to keep it somewhat cleaner at the expense of some airflow. And, of course, what goes in must come out... you need two sets of holes!

If you mount it outside, can't you mount it with insulating hardware, so the thyristor faces the chassis and no voltage is exposed? Some sheetmetal could block the ends of the slot the thyristor mounts in, so even a very determined pinky finger couldn't reach the leads.

Tim

Magpie - 9-3-2008 at 16:41

I ran temperature tests this afternoon with the sink mounted in the enclosure. Conditions were about 20C ambient and the power draw was 1800w (7.5a). With the lid off a steady-state temperature of 46C was reached after 25 minutes. I then put the lid on. After another 9 minutes the temperature had risen to 60C and was still rising. At this time I terminated the test.

I'm going to have to exercise one of the options we have been discussing.

Magpie - 10-3-2008 at 14:06

The quadrac/heat sink was remounted about 1" (2.5cm) outside the enclosure. When operating at 8.7a (2000w) the sink temperature quickly stabilized at 36C. Ambient was 27C and rising:D. I will have to make sure I don't get my "pinky" in the wrong place, however. It will help that nobody else will be operating this puppy but me.

12AX7 - 10-3-2008 at 15:40

Just put some duct tape over it. :D

Magpie - 19-6-2008 at 18:19

In consultation with garage chemist I have completed the tube furnace construction that I put aside last fall after completing the power control thyristor circuit. This furnace could almost be given serial no. 2 of the garage chemist design shown upthread.

Differences are as follows:

1. I used 8" (200mm) stove pipe vs GC's 4.8" (120mm) stovepipe.
2. I used an alumina tube vs GC's mullite tube.
3. I used ITC-100-HT to coat the Kanthal wire vs GC's Al2O3/MgO mix.
4. I used 1" (25mm) Kaowool insulation vs GC's 25mm insulation that he suspects contains some ZrO.

My design is for 2kW, same as GC. I used 18 ga Kanthal A-1 wire (1.02mm), GC used 1mm Kanthal A-1.

Edit: My tube is 40mmx34mmx500mm, GC's is 40mmx32mmx500mm.

(GC please make corrections where required.)

Below is a picture of the furnace in operation. It reached 1252C and rising before I shut it down. Maximum amps I put to the furnace was 9. At the end, temperature was rising at about 1C/min with Kaowool plugs in both ends. I am quite pleased with the performance and saw no reason to push my luck by taking the furnace to higher temperatures.

It should be noted that even though I had an extra 40mm of insulation the stovepipe was too hot to touch at the higher temperatures.

I will post 2 more pictures and comment about my use of ITC-100-HT.

[Edited on 19-6-2008 by Magpie]

[Edited on 20-6-2008 by Magpie]

furnace at 1252C.jpg - 59kB

Magpie - 19-6-2008 at 18:33

Here's a picture of the tube geing coated with ITC-100-HT. I had never worked with this before. The directions on the bottle said "Add water and mix thoroughly." That's all. So I added just the minimum amount of water to make it workable and brushed it on with a 1" paintbrush. It is sticky and was easy to apply, not dripping off at all. I just kept applying it until I had a thickness of about 5mm. I let it dry for about 3 days at room temperature.

putting on ITC-100-HT.jpg - 84kB

Magpie - 19-6-2008 at 18:38

This last picture shows the ITC-100-HT after drying. As you can see it cracked all over. But it stayed on and never spalled off. Dryed pieces of waste coating showed that the product has good strength.

cracked ITC-100-HT.jpg - 58kB

garage chemist - 19-6-2008 at 21:05

Beautiful! Great work!;)

I am very pleased that your project became such a success. So my design has proved to be workable even with somewhat different components.

Using the ITC-100 paste was a very good choice, I see. My Al2O3/MgO mix became very powdery and flaked off in some spots. If I wanted to build another tube furnace, I would definately not use this again (I'd probably use calcium aluminate cement, even though this would be good to 1250°C only).
ITC-100 does not seem to be easily available here.

A question: what kind of heatup rates did you get with your furnace on full power? I get over 100°C/min at low temperatures, and at 1000°C still 60°C/min.
You said you got 1°C/min at over 1200°C- I think mine still heats up much faster at those temperatures!
Does your alumina tube + ITC-100 have such a large heat capacity, or are there other reasons, like heat loss?

My mullite tube has quite a low thermal conductivity. I remember alumina ceramic having a much higher conducitvity. Does this lead to increased heat loss at the ends?
The free ends of the ceramic tube on my furnace stay below 600°C (no visible glow) even at 1250°C inside the furnace.
Is that different with your alumina tube?

Magpie - 19-6-2008 at 21:31

Thank you gc and yes this shows that your design is robust. But I'm sure that the selection of materials is still critical because of the high temperatures.

Yes, the ITC-100 was nice to work with. Time will tell how it proves out in the long run. Perhaps someone with more experience can tell us how to prevent it from cracking. How thick was your Al2O3/MgO layer?

The relatively slow heat up rate is something that I noticed also as I remember you saying how fast your furnace heats up. I have to think that you are right about the tube/coating heat conductivities being higher. My Kaowool conductivity may also be higher. I will see if I can find some conductivity data for these materials.
Quote:

The free ends of the ceramic tube on my furnace stay below 600°C (no visible glow) even at 1250°C inside the furnace.


This was true for me too.

[Edited on 19-6-2008 by Magpie]

Fleaker - 20-6-2008 at 09:23

I would've used much more kaowool than you did Magpie. I think 2" as a minimum.

I have a forced-air propane furnace that I've lined for 1760C/3200F and I used 3" of 2400F kaowool and 1" of the 2700F material. That is topped off with 3/4" inch of 3200F and an ITC wash. It is a large furnace, perfectly accommodating a #16 SiC crucible. Insulated as it is, if I hold that furnace at about 1600C for an hour or so, the outside of the furnace becomes hot enough to melt the rubber off of my shoes! Around 750C for aluminum casting, the shell is not warm at all.

Magpie - 20-6-2008 at 10:51

Quote:

I would've used much more kaowool than you did Magpie. I think 2" as a minimum.


Fleaker, I don't understand this comment. The alumina tube w/coating is 1"radius and the stovepipe has a 4" radius. My Kaowool thickness is therefore 3".

Magpie - 20-6-2008 at 11:23

GC I found the data for the 99.5% alumina. First of all I have an ID of 34mm vs your 32mm. Therefore your wall thickness is 4mm whereas mine is 3mm. This means your tube has nearly 33% more mass (assuming equal densities) than mine.

The alumina has a sp. gr. of 3.8.

Its thermal conductivity is 50 BTU-in/(ft2-hr-F) at 800C, which, if I have done the conversion correctly, is 7.2W/(m-K).

I'm still looking for the conductivity for the 8 lb/ft2 Kaowool.

Magpie - 20-6-2008 at 13:09

Here's a nice data sheet that compares mullite to alumina:

http://www.ortechceramics.com/material_compare.htm

It shows the mullite thermal conductivity at 3.5 W/(m-K) vs a value of 30 W/(m-K) for alumina. But this is at 20C. At 800C the k for the alumina drops to 7.2 W/(m-K). I don't know what happens to the k for mullite at 800C. But it is likely still no more than half of that for the alumina.

2nd Edit: This website gives k = 3.3W/(m-K) for mullite at 800C:

http://www.techceramics.com/pdf/mulliteMV20.pdf

[Edited on 20-6-2008 by Magpie]

[Edited on 20-6-2008 by Magpie]

12AX7 - 20-6-2008 at 15:51

Quote:
Originally posted by Magpie
I'm still looking for the conductivity for the 8 lb/ft2 Kaowool.


Of course, you would have to integrate over the radius, because the thermal gradient cross-section has cylindrical geometry and is therefore expands with radius, while at the same time the temperature falls and the kaowool becomes a much better insulator. If you want real numbers, that is. As I recall, in the limit it comes out logarithmic, so you don't gain much by using a lot of insulation around something narrow.

120V * 8A is just shy of 1kW. Evidently, this furnace has an efficiency of roughly 1.3C/W.

Tim

Magpie - 20-6-2008 at 16:07

Tim I'm not trying to calculate a thermal resistance. I'm just trying to compare conductivities of my materials to those used by garage chemist in an effort to explain my relatively slow heatup rates.

2nd Edit: The maximum voltage I can put to my furnace using my thyristor control circuit is 230VAC. At 230 volts and 9 amps the power would be 9*230 = 2070W.

[Edited on 20-6-2008 by Magpie]

[Edited on 21-6-2008 by Magpie]

Magpie - 20-6-2008 at 21:13

GC I have measured the electrical resistance of my furnace at 18 ohms at room temperature. It should be 26.5 ohms. So instead of delivering 2070W at 9 amps I was actually delivering something like 1458W. This is no doubt caused by internal shorts (coils touching). I suspect this is the main culprit for my slow heatup rate.

It was difficult to keep the wire coils tight and evenly spaced. I tried to correct uneven spacing but the coils wouldn't stay in place. I should have given this more attention, throwing on more nooses or even rewinding.

Could you measure your furnace resistance for comparison?

S.C. Wack - 21-6-2008 at 00:30

Not sure if this is of interest, but the resistance of my 2070W Lindberg is 6 ohms. BTW it did not come with the controller that connects to the 4 contacts at the base...if anyone happens to have an extra one...Not that I really need it, because the Watlow 988 controller and Din-a-mite SSR are outstanding little items. Works great on my little electric bunsen even. Glas-Col's too, using their built-in thermocouple, after switching the controller from K to J. Highly recommended.
Quote:
Originally posted by Magpie
I have measured the electrical resistance of my furnace at 18 ohms at room temperature. It should be 26.5 ohms. So instead of delivering 2070W at 9 amps I was actually delivering something like 1458W.


Now, how did you figure your wattage? Please humor someone a little slower than many here with electricity; why isn't the observed potential and current combined with Ohm's thingy relevant to wattage calculation in this circumstance?

[Edited on 21-6-2008 by S.C. Wack]

Magpie - 21-6-2008 at 07:55

Quote:

Please humor someone a little slower than many here with electricity; why isn't the observed potential and current combined with Ohm's thingy relevant to wattage calculation in this circumstance?


Yes, Ohm's law is relevant here. E = IR and P=IE, where E is voltage, I is current, R is resistance, and P is power.

By substitution then P = (I^2)R. So for a power of 2000W at 9 amps, the resistance should be:

R = P/(I^2) = 2000/81 = 26.7 ohms

With R at 18 ohms and I at 9 amps, P = (I^2)R = (81)18 = 1458W

For your Lindberg you say you measured a resistance of 6 ohms. Assuming your voltage is 120 volts, then the power calculation would be

I = E/R and P = EI = E (E/R) = (E^2)/R = (120^2)/6 = 2400W. I don't know why the discrepancy with your rating of 2070W, but the calculated result is in the ballpark.

12AX7 - 21-6-2008 at 09:18

Power dissipated by a resistive load is defined as V*I. If you measured 240V and 9A, you were dissipating 2160W. Evidently, the resistance at that moment was 240/9 = 26.7 ohms. Anything else contradicts either the voltage or current you in fact measured; 9A through 18 ohms develops V = I*R = 162V, far less than the 240V you know you have; likewise, P = V^2/R = 240^2 / 18 = 3200W is a current of 13.3A, which is not the 9A you measured.

The cold resistance will, of course, be much lower than the hot resistance. A factor of two difference shouldn't be at all surprising; over the same range, the resistivity of pure iron increases something like tenfold.

If you measure the current draw at the instant of turn on, I think you will find nearly 13.3A. It's a much safer assumption that line voltage is constant rather than current draw!

Tim

Magpie - 21-6-2008 at 09:33

I'm using a thyristor circuit to supply power to the furnace so don't really know the effective voltage applied to the furnace. I did not have it set at full voltage.

I used a clamp-on ammeter to measure the 9 amps, which I'm assuming is a fairly reliable value.

The resistance of Kanthal A-1 increases 4% from 100C to 1200C as can be seen here:

http://www.kanthal.com/

[Edited on 21-6-2008 by Magpie]

garage chemist - 21-6-2008 at 10:04

The clamp-on amperemeter only gives a sensible reading if you don't use a thyristor circuit.
The thyristor circuit "chops up" the sine wave of the current and makes an accurate measurement impossible for the amperemeter, which needs a sine wave to function properly (that's what I think, although I don't have a clamp amperemeter. What I've said is certainly true for a normal electronic amperemeter.).
You would have to use an analogous, electromechanic amperemeter in order to obtain the true value of the current.

With your ca. 18 ohms (as you've correctly pointed out, the resistance of Kanthal wire changes only little with temperature), the power of your furnace will indeed be over 3000W, which is currently reduced by the thyristor circuit.

Try running the furnace without the thyristor circuit (not over 1000°C!)- I am fairly sure that you will get a temperature rise of over 100°C/min then.

You didn't use preoxidised Kanthal wire, did you? Preoxidised wire avoids short-circuited windings due to the insulating properties of the Al2O3 oxide layer on the wire.
The wire you used looks bright, so it was probably not preoxidised- this seems to be the reason for the too low resistance.
I did not use preoxidised wire either, but I did not have any touching windings.
The resistance of my furnace was the calculated 26,5 ohms.
Now, after repeated use, it has increased to 27,5 ohms (measured yesterday) due to oxidation of the wire, so the power has dropped to ca. 1926W (not that this would be a big problem, but it demonstrates that the power of an electric furnace that uses resistance wire will slowly decrease with continued use, and heating spirals are always a wear part).

Magpie - 21-6-2008 at 10:27

GC thanks for measuring your resistance.

No, I did not pre-oxidize my wire. Never had heard or thought of such a technique.

I know the voltage sine wave is being chopped up but I still suspect that I'm measuring reasonable values for effective current at the higher amperage levels.

I just now plugged in the furnace, turning up the power until I measured 9 amps. Then I measured the voltage at the furnace terminals. It was 140 volts. This would yield a power consumption of 140*9 = 1260W. Again, I know this is just a rough value.

The reason I have been limiting my current to 9 amps is concern over burning out my Kanthal wire.

Edit: Using the two crude measurements of 9 amps and 140 volts R = E/I = 140/9 = 15.6 ohms is calculated. This is not too far off from my measured 18 ohms, which should be solid.


[Edited on 21-6-2008 by Magpie]

Fleaker - 21-6-2008 at 18:05

@Magpie,

Sorry, for some reason I read
"4. I used 1" (25mm) Kaowool insulation vs GC's 25mm insulation that he suspects contains some ZrO."
and actually thought you'd be fool enough to use an inch of insulation. Reading it over again, I wonder what exactly I was thinking when I read that!

Still though, it seems odd that your furnace is becoming too hot to touch on the outside and that it takes so long to heat up?

Magpie - 21-6-2008 at 18:44

Fleaker I was pretty sure you had just misread something, but I wanted to clear it up. ;)

Now that I have realized my lower power input, it is not surprising to me that my furnace heats up much slower.

My shell being too hot to touch did surprise me, but when you think about it the power has to be dissipated somewhere. Garage chemist said his shell might char wood at 1300C. I don't know if mine was that hot but it was getting there. It seems that the commercial tube furnaces all have a secondary shell of perforated metal, probably for the same reason.

An interesting experiment would be to compare the power required to keep our respective furnaces at a selected temperature, say 1000C or 1200C. Getting believeable, or at least comparable, E or I measurements would be tricky, however, since we are both chopping the sine wave with thyristors.

not_important - 21-6-2008 at 19:54

Pre-oxidising is a good idea, a friend of mine that salvaged and restored old pottery kilns ,to sell to potters without much money, did that. They also applied a thin layer of a zirconium silicate based wash over the resistance wire before the final buttoning up, the layer is too thin to affect the radiative properties but enough to reduce the occurance of shorts from coil creep.

You might try building a box of backing board (usually calcium silicate based) around the tube furnace, with spacers/supports for the tube cut from some of the board. Most of this type of product is good up to 1100 C, and is low density giving good insulating properties. This makes the outside of the kiln square, which is easy to build a metal shell for.

Alternatively, or in addition, to backing board you could put on an outer layer of castable insulation, something like Kastolite light weight.

Magpie - 24-6-2008 at 13:28

Earlier I reported that the insulation used in my tube furnace was 8 pcf (lb/ft3) Kaowool. Upon further questioning of my vendor I find out that it is actually 8 pcf Superwool 607. This has a classification temperature of 1100C and a heat conductivity at 982C of 0.28W/(m-K).

jarynth - 6-10-2008 at 19:52

Hey Stefan, congratulations on your success! I also started building a tube furnace following your design. I'm really staying on the cheap side this time, which is always a good challenge for me. Used professional furnaces of comparable power can be bought for the amount you spent so far, if not less, although the fun and the learning experience are of great value too.

My tube is 30cm (12") long sillimanite; my heating coil is probably much like yours (so rogue, it took me two days to wind it all up, leaving about 2mm between successive turns); instead of the alumina coating I used some gypsum-based protective mixture... it might get brittle when heated, but the kaowool will be tightly bound around it by then. I plan on avoiding the oven pipe (way overpriced, for my liking); instead I'll place decreasingly heat-resistant layers of mineral fibers going from the core out; I still have to decide about an external case. An orientable stand would clearly come in handy. About that, you could mirror a Newton telescope mount to get a firm but versatile stand.

So far, my major concern is the temperature control. Kiln thermocouples are on sale at pottery supply stores here for well over $100. You said you built your own because the one that came with your PID had silicon insulation that wouldn't take your working temperature. My question is, is there a particular reason why you couldn't just remove this silicon (and, if necessary, replace it with ceramic fiber tape or similar) and go without? Or is this silicon worked inextricably into the construction of the sensor?

One more thing. It seems customary to place the sensor inside the chamber. This way you have no control on the temperature of the wire, which could easily pass the limit in case of quick heating, assuming a hundred or more degrees temperature gradient across the tube wall.

chief - 7-10-2008 at 08:35

It doesn't need to be Kanthal-wire. At welding-supply they sell Cr/Ni-wire (for stainless-steel-welding), by the kg. I paid for 1 kg 30 EUR, several years ago, in a hardware store. Probably can be got much cheaper ...
This Ni/Cr-wire (0.6 mm2) has a resistance of 3 Ohm when cold, and this doubles up to 1000 [Celsius]. It withstands at least those 1000 Celsius for quite a while, and I numerous times repaired my furnace with self-made-windings of this wire (which fails whenever some carbonate-melt gets out and fuses to a glass with the ceramics of the furnace).
Usually after many hours of 900-1000 [Celsius] the wire seems still like fresh, when it was within clay, but when on air it has a lifetime of maybe several 100 hours only.
It helps to glow it (electrically) before trying to make any windings of it (using a slow drilling engine), because it's much softer then.

I usually calculate the necessary wire-amount by the Stefan-Boltzmann-law of emission (assuming only radiation-emission cools the wire), constrainted to the desired electrical data, so the wire is only 100 [Celsius] hotter than the desired furnace-temperature, thereby the furnace-temperature may be optimally high.

Besides only 500 [Celsius] are necessary for Fe-vitriole-decomposition, according to other threads here.

Automatic temperature control

jarynth - 7-10-2008 at 17:56

Quote:
Originally posted by chief
This Ni/Cr-wire (0.6 mm2) has a resistance of 3 Ohm when cold, and this doubles up to 1000 [Celsius].


This means that at constant voltage, the power is halved. Correct me if I'm wrong, but this effect, common of most metallic conductors to some extent, causes the temperature and current to eventually reach an equilibrium dependent only on the voltage and the response rate of temperature to emitted power (which in turn depends on the physical construction, materials, content of the chamber etc). It would then be possible to calibrate the temperature by adjusting the constant voltage and get rid of an electronic controller, as long as the changes in the thermic properties of the reactants are negligible during the reaction. It would suffice to interpolate a few data about the equilibria and print out a table of T to V :cool:

[Edited on 7-10-2008 by jarynth]

IrC - 7-10-2008 at 20:20

I learned two things running and rebuilding my kiln many times over the years. one is the importance of keeping the kiln airtight until it has cooled to a minimum temperature. The other is to coat my heating coils with a thin coat of ITC-100. In other words the most important factor was keeping oxygen away from the hot metal coil as much as is humanly possible. I also noticed an amazing increase in heating VS power efficiency and a decent increase in acheivable temperatures inside my kiln by coating every possible area inside the kiln with a nice thick coating of ITC-100 HT Ceramic Coating.

ITC-100 HT Ceramic Coating

chief - 7-10-2008 at 22:50

Yes, the power decreases with tempoerature, and this does self-limiting. But as the power decreases, with thermal losses increasing (with temperature), the last 100-200 degrees [Celsius] are reached quite slowly; best ist: Heat up with full voltage, then change the voltage to hold the the temp.
This can be done by using a transformer with several windings, to add up the voltages as needed: {1,2,4,8,16,32,65,128}-Volt-windings allow every voltage between 1 and 256 V, to the accuracy of 1 V ...

IrC - 7-10-2008 at 23:44

When I said "heating VS power efficiency" I meant the ITC-100 acts like a heat mirror, in that thermal radiation losses through the bricks went down quite significantly. This allowed for lower "ON" cycle times while maintaining Cone 10 or hotter temperatures for many hours.

chief - 8-10-2008 at 02:58

Just to share 2 Ideas (not from me, but tricks of the trade) on reaching really high temoeratures:
==> up to 1700 [Celsius]: Mo-wire around Al2O3-ceramic-tube [also commercially available as furnaces]
==> up to 3000 [Celsius]: Wo-tube, current directly through the tube

The latter may be adapted,using cheaper stainless-steel-tubes (limited lifetime, but throw-away available from hardware-stores), which should hold up to 1300 [Celsius] too; then it would be only a matter of contacting them, wrapping some isolating-mat (even maybe rockwool) around them. Since the resistance rises with temperature the electrical power would be dissipated within the isolated zone, contacting therefor should be easy enough (if necessary through welding).
As power-supply: One thick additional secondary winding around the welding-transformer, eg. . Within such stainless-steel-tubes then, at lower temp., even pressure-experimentation could be done (catalysts in the tube, have fun making HNO3 etc.), since in the hardware-store there are also other stainless-steel-components available,usually to make waterpipe-connections etc. .

Maybe someone could succeed in Fischer-Tropsch-gasolining from coal-water (50 Bar, 200 [Celsius]) ?
[Edited on 8-10-2008 by chief]

[Edited on 8-10-2008 by chief]

chief - 9-10-2008 at 07:52

There was a thread on this somewhere, but here is a useful link upon reaching 1300 [Cels]within the microwave (700 W) within less than 5 minutes (!):
http://alp.dillingen.de/chemiekongress/doc/Workshopergebniss...

(usinc "activated carbon" as susceptor, maybe charcoal will work too ?)

Rosco Bodine - 21-10-2008 at 15:16

Surplus sales are great aren't they. I just scored a 340 meter roll of 2mm diameter Kanthal A-1 and a half dozen
Omega 150mm long quartz sheathed dual type K element
furnace thermocouples with Watlow solid state micro-controllers . Yeee ha! yipppee !
All of it is new unused but ancient stockroom surplus, all
of it for about eighty bucks :D One of the thermocouples alone lists for three times that :D Also got a few slightly used 95mm X 400mm mullite tubes. Let's see, now for what would these things be good ? Any ideas :P


[Edited on 21-10-2008 by Rosco Bodine]

chief - 25-10-2008 at 08:21

How did the old chemists reach the 1300 [Cels] ??
==> charcoal, with air-blowing
The to-be-decomposed material would before the heating be made to a dough, then dried as little spheres, so it wouldn't fly away as dust ...

Then these spheres would be mixed with the coal, the coal would be heated via air-stream, and the reaction would occur.

Now: With the "hot-air-gun"one may reach the extra 500 degrees above the usual coal-temperature, and it goes into white-glowing ! I do this every day now to start the fire in the oven ...
It's definately a high-temp heat-source to blow charcoal with the 10-$ hot-air-blowing-thing.

[Edited on 25-10-2008 by chief]

Extreamly simple mini 600*C tube furnace.

Sedit - 20-5-2009 at 20:05

If theres one thing I love more then chemistry thats tinkering with junk left over in my garage. Seeing some heating elements in an old hairdryer gave me the idea to see just how hot these elements could get a tube furnace.

My first attempt was to take some of the wire and wrap it around some glass tubing I had laying around. This was a big mistake. Within seconds the glass tubing almost
turned into a puddle. This gave me enough incentive to move along but since I did not have any quartz tube around I used 1" hard copper tubing. To prevent the wire from shorting out on the copper I preped the tubing with pieces of scrap pink insulation that had been diped into clay that was turned into a slip. This was allowed to dry overnight and the wire element was wraped aroung about 5 inches of the pipe and held in place with clay and insulation. More layers of insulation where built up and was toped off with a piece of metal flue pipe. The ends where coated with spackling compounds to hold the fiberglass insulation in place and to ensure a good low oxygen enviroment for the heating element.

Lastly as seen in the picture a dimmer switch was used to control the flow of electricity thru the circuit.

Here is a photo of the over all tube. It may have took me no more then two to two and one half hours of work to finish this little piece of crap but once she was fired up I was honestly impressed. In a matter of ten to fifteen minutes the temperature measured it reached a blistering 600 degrees Celsius and was held there for a matter of one half hours or so. This was done with the dimmer switch only half way up so I feel I could eak a few extra hundred degrees or more out of this thing if I allowed it to crank all the way up. Also since the tube is copper alot of loss is comming off the ends.
Over View


Side View



Pyrometer


Sadly the camera had trouble focusing on the numbers but what you are looking at here is the needle of my pyrometer resting directly on 600 degrees Celsius. It did raise a little above this and was slowly going up but for now I will call the limit as 600 even though I know it will surpass that number with ease.


Now....

What is one to do with such a creation?

I have considered Adapting the ends for a feed and exit tube to attach to and filling the tube with Cu powder in an attempt to synthesis formaldahyde and acetaldahyde and see how that goes. Could this same thing perhaps be used to oxidise BnOH to benzaldahyde? I already thought it was neat to stick A piece of Polystyrene in one end cap it off and watch liquid cracked styrene monomers to come out of the tube and condense on a cold glass.

Any other suggestions on how to play with this thing before I push it to its possible breaking point? Or melting point for that matter:D.

~Sedit

garage chemist - 21-5-2009 at 17:29

Nice work!
Electric heat is capable of much more, but this is a good start.
Acetaldehyde and formaldehyde production is a good idea for this temperature range.
A carrier for the catalyst instead of plain copper powder would be a good idea. You could, for example, soak pumice in copper nitrate solution, dry, and heat to decompose it to CuO.
This gets reduced once the alcohol is passed over it at elevated temperature.
A copper wire mesh could also be a good catalyst.
About benzyl alcohol dehydrogenation- try it if you can get benzyl alcohol.

And try to move on and make a furnace that can reach 1000°C. With this, you can decompose sodium pyrosulfate.

barbs09 - 22-5-2009 at 00:26

Hi, it seems that a limiting factor to making a truly useful tube furnace is the rarity/cost of a quartz glass tube. I notice that in most bar heaters (those cheap 1kw varieties) the resistance wire is wound inside some type of opaque glass tube which considering the temperature they get to must be quartz glass? While all the tubes I have seen are only in the order of 10mm diameter they could be useful for smaller reactions. Also someone recently told me that some of the bigger industrial sized light bulbs have sizable quartz glass tubes that may be serviceable.

If these tubes are too small for the actual “tube” they could be used as small reaction vessels when inserted into a metal tube furnace not unlike Sedit’s copper furnace.

Anyone here considered these options?

AB

watson.fawkes - 22-5-2009 at 06:45

Quote: Originally posted by barbs09  
it seems that a limiting factor to making a truly useful tube furnace is the rarity/cost of a quartz glass tube. I notice that in most bar heaters (those cheap 1kw varieties) the resistance wire is wound inside some type of opaque glass tube which considering the temperature they get to must be quartz glass?
Costly, somewhat, but rare, not at all. You can order, off the shelf, for less that $50, a four foot length (standard) of 25mm ID medium wall quartz tube for less that 50 USD. (Sample vendor.) That's enough for 2-4 reaction tubes, depending on what length you need. Quartz can be cut with a diamond saw, such as is used to cut tile. Fire polishing of cut ends is possible with an oxyacetylene flame.

The opaque tube might have been quartz, as opaque varieties are used in heaters. More commonly opaque heater tubes are mullite, which is a bit less expensive. Mullite is also used for reaction tubes, though.

dann2 - 22-5-2009 at 11:55


Hello,

I think the glass tube that Barbs09 is referring to would be more accurately described as translucent as opposed to opaque. The ceramic (probably Alumina) tubes with a 'thread' on the outside for the Nichrome wire to sit in are not what he is referring to (I think).
I have wondered what exactly the translucent glass tubes were made from. The wire is wound in a coil on the inside of these tubes.
They can be had from the dumpster.

Dann2

garage chemist - 22-5-2009 at 12:18

The translucent or opaque tubes from heaters are made from quartz glass. They are sometimes a lesser quality of quartz glass, an opalescent one which contains many small air bubbles and is cheaper to produce, but is equal in properties to clear quartz glass except for the transparency.

barbs09 - 23-5-2009 at 02:21

Hi thanks for your replies. The opaquely translucent ;) tubes I am familiar with also have fine striations parallel to the long axis of the tube which I guess could be extrusion marks.

Watson.fawkes thanks for the heads up on the availability and reasonable price of a good-sized quartz tube. I will look into this but I bet they are not so available/cheap in New Zealand:(

Also when winding Nichrome around a tube can anyone see any problems with twisting wires together if I don’t have on continuous length? Obviously I can’t solder them. I would imagine it would be much less than ideal due to localised resistance etc..

Regards, AB

pHzero - 23-5-2009 at 03:11

Quote: Originally posted by chloric1  

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?

P=IV and V=IR, so R=V²/P. That means that if you're using half the voltage and want the same power, you actually need 1/4 (ie 1/2²) of the resistance

dann2 - 23-5-2009 at 04:40

Hello,

Often wondered about sterilization lamps. Are the glass in them quartz, (in order to transmit the UV). They would be very expensive to buy just to scavange the tube but they must be replaced on a regular basis (not just when they are blown) so they may be available for zilch if you knew where to look.

There are also 'sun lamps' like here. You get four tubes (I presume they are quartz). You could just pinch one of the tubes for the good cause and give the remaining three legged sun-maker to the wife to alleviate any SAD syndrome she may be suffering from!

I presume regular sun tanning lamps (as you see in sun tanning shops) are not made from quarts but are just similarish to regular flouresent tubes.

Dann2

watson.fawkes - 23-5-2009 at 08:57

Quote: Originally posted by barbs09  
Also when winding Nichrome around a tube can anyone see any problems with twisting wires together if I don’t have on continuous length? Obviously I can’t solder them. I would imagine it would be much less than ideal due to localised resistance etc..
You can't solder, but you can weld them. Twist together, flux with a little paste made of borax and water. Put under regular power. Local resistance of a "bad" joint (before welding) provides the heat. Let cool down, wash off and neutralize the flux.

Sedit - 23-5-2009 at 09:36

Garage chemist I really like the idea of the pumice copper catalyst. I was going to just take the fine powder copper I got from precipitating CuCl solution with zinc and grind that together with sand to give surface area but your idea is way better.

After a little time to mess around with the thing it has proven to be a steady 600 degrees Celsius. This is caused more then likely due to heat loss thru the copper tuding itself and since I have a kiln next time I fire it up I will make some high alumina porcelain tubes for use with it instead of copper. The fact that I have seen it liquify the clay around the wires means that without the heat loss due to the copper extreamly high temperatures should be achieved. Im going to attempt to insulate the copper once I get the end plugs on for attachments but I have to find my high temp epoxy before I can do that.

Mind you all this was just tossed together pretty much on a whim so im impressed with its handling and wounder how long a life time it will prove to have. I only used partial of the elements that a hair dryer has but welding them like is mentioned above is a good idea because it will give me more working surface for any reactions to take place.

Question: Is this a high enough temperature to produce Keten? It produces a nice red glow which should be enough for acetone pyrolysis but am unsure if other factors come into play there.

User - 13-12-2009 at 09:14

Today I am 'playing' with electric heating.
First of my goal is to familiarize myself with this concept and see how far I could push some scratch that was lying around.

Materials available:
Old toaster
Flowerpot
old ceramic vase
Thermocouple
Sand

What did do:
Took apart the toaster and used 2 parts, the first was a glow wire and the second was the current limiter ( the part where you can set the amount of carbon you want on your toast )
4 spirals were found in parallel, I decided to use one for this test.
If course I tested if the single spiral would hold up when cut off from the rest, it did without burning.

Then I took the vase and spiralled the wire around it (about 1,5 meters of wire)
The vase is about 40 cm in length and 12 cm in width.
Only 15 cm was used for the spiral.
The vase was placed in a big flowerpot and surrounded with dry sand.

Test:
The juice was turned to about 20 % and I waited to see if the construction would hold up.
A temperature of 280 degrees was measured on the inner wall.
The power was further upped to about 40 percent and a temp of 360 degrees was measured.
(outside temp climbed to 100 deg C )

Finally the temp was slowly increased to 100% and a temp of 630 degrees was measured , it was still climbing but to coating of my thermocouple wire started to melt so fast that I had to stop.
(anyone any suggestions to fix this problem)
surprisingly the outside of the flowerpot only measured 140 degrees.

Anyway.
I found out that sand is a quite good insulation material and that even such a simple and primitive construction could reach such enormous temp within a short time spawn.
The temp that could be reached with all 4 of the wire's and better insulation is yet to be tested.
This all without paying a single dime (except for the electricity)
Ill report further tests if anyone would like that.



[Edited on 13-12-2009 by User]

densest - 13-12-2009 at 12:07

The "standard" insulation for high temperature thermocouple wires is braided glass/silica/asbestos substitute up to their limits, then little ceramic hollow tubes (long ones for rigid support, short ones for flexibility) threading a wire up through the holes up to the thermocouple wire limit. A two-foot-long (60 cm, more or less) ceramic type K probe without wiring (spot weld your current wire to it) costs $8.50 3859K56 from <a href="www.mcmaster.com">McMaster-Carr</a>. Fully fabricated ones with 304 stainless steel covering the probe cost $30 or so with cord and plug.

A truly cheap person with a Dremel tool and a small carbide bit might take broken pieces of pottery and drill pairs of holes in them to make improvised insulating beads.
I haven't seen unglazed ceramic beads for sale anywhere lately...


[Edited on 13-12-2009 by densest]
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