First some general information on this technology:
This fluidized sand bed (FSB) can be used inside or outside of a furnace at temperatures ranging from -200°C to + 880°C / + 1100°C depending on the version. The basic working principle of is as follows: solid particles with a very high melting point and good chemical stability are loosened up and suspended by a flow of gas from the bottom of the container to the top. By the upwards motion and pressure of the gas, the ceramic particles move almost freely as they are fluidized. This mixture of gas and solids has properties very similar to a fluid, hence the name. The transfer of heat is very high, and the temperature uniformity is excellent due to the axial and radial mixing. A precise temperature control helps to get the same results each time and is the basis for optimization. High heating rates between 600 °C and austenization temperature help to achieve a fine austenite grain and therefore result in a tougher knife at the same hardness.
The big advantage over using liquid salts or metals is that all those substances have a relatively narrow usable temperature range and they are often toxic and dangerous to use. Therefore, the fluidized sand has many advantages for knife making and you can achieve excellent results with it.
I offer two different versions (one goes up to approx. 880°C and one to 1100°C recommended maximum operating temperature) – Why?
On the one side it is a matter of price as the materials get way more expensive above 900°C, on the other side it is also a matter of design. I optimised each version with the available materials, so it is basically two different approaches:
- with 880°C version Fluidised sand bed (FSB) the entire vessel is relatively cheap and it could be replaced as a whole unit if need be one day, therefore I could make a more complex design with a gas preheating coil around the external diameter of the crucible. This means the air / gas already is at process temperature when it enters the sand, resulting in a faster response time, higher heating and a compact size. This is perfect if you plan to only use it for carbon steels.
- with the 1100°C version FSB I had to go a different path, the crucible is rather expensive (and thick, and durable) and the expected weak point was the gas inlet tube, so I had to design it in a way that is easier to replace, therefore with this version it does not go around the external diameter and is welded into the crucible - but instead it just goes straight down from the top on the inside. To compensate for the not-so-much preheated gas, I added 5 cm (2 inch) of height at the bottom which are not meant for the workpiece but rather to equalize the temperature and make sure there the temperature is as uniform as possible. So overall this retort has more mass, is a bit more expensive, is 50 mm taller, is a bit slower with heating up but can be used at all temperatures from very cold up to 1100°C. (you can see a comparison of the two vessels in one of the pictures)
If you want to go for the 1100°C version FSB you will have to take it up to >1000°C from time to time, since this material tends to form a brittle phase in continuous use at 700-900°C. When you heat it above 1000°C this brittle phase is dissolved again.
I hope that explains a little better why I offer two versions - let me know if you have any questions!
These prices include everything you see in the picture of the kit (basically all you will need additionally is a furnace around it and a gas supply / air compressor)
- Retort (Vessel)
- User manual - installation instructions
- Gas connectors, hose, regulation valve
- Manometer for use with pressurized air
- ceramic media
If the shipping cost/weight is a problem in your case we can also reduce the total weight shipped by removing the ceramic media from the kit as this is a big part of the weight - you can instead source it locally and I will help you find the right grade. Please just send me an email.
Fotocredit: Sabine Miorini
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