Homebuilt 1300°C tube furnace

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. 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]

Attempting to make a furnace from Molydbenum Disilicide elements and insulating bricks. These elements can be driven up to 1800C giving a furnace temp. of around 1700. I will not be going even close to those temperatures. Coming from the kanthal handbook the diagram shows you how much power you need for a given furnace size. The diagram assumes a wool lined furnace. If you use bricks you must increase power by about 25% to get the same temperature. For a 10 cubic decimeter oven you need around 4 kW to get 1500C. (arrow on page).

If I were to use a double layer of bricks will I get much higher temperature?

My problem is I have only 2 kW worth of elements. I will have to make a smaller furnace that the 10 (if using single layer of bricks).
How much difference will a second layer of bricks make. I suppose this is not a simple question to answer. Any educated guesses welcome. I am unable to use heat transfer/modelling software.

TIA,
Yob




[Edited on 8-3-2018 by yobbo II]

I just built a tube furnace which is capable of reaching 1100 C.

The heart of the furnace is an alumina tube, 25/20/200 mm (OD, ID, length). This cost me about $15. Around it I wrapped around 100 turns of 22 gauge nichrome wire, which took around 8 meters. Cost of nichrome was $2. Then I painted on two thin layers of high heat mortar. The first was just enough to keep the wire in place, the second a little thicker and covered all the exposed wire completely. I bought a 10 ounce tube of the mortar but only used ~5% of it, for a cost of $0.25. While letting it cure, I cut a section of 4 inch duct slightly shorter than 200 mm. I also cut some caps for the duct out of aluminum flashing (adding holes for power and the tube itself in the middle). I already had these materials, but they would cost ~$5 to buy. I then wrapped the tube with a ceramic fiber blanket. It took about one square foot of 1/2 inch thick ceramic fiber to make a good fit. This was rated to 1300 C and cost $3.50. The final part was putting everything together with a couple of large hose clamps ($3).
Total cost: $28.75

The insulation works well, it only takes ~80 watts to maintain the temperature at 1100 C. The outside gets hot but it's not too bad, especially considering how hot the interior is.