Put WFC triode picture here
To achieve Ravi's efficiency, a strict selection and preparation of stainless steel tubing is important. The following parameters need to be considered.
1) Choice of the grade of stainless steel
2) Tube gap and Thickness / SWG / AWG of tubes
3) Pre preparation and conditioning
Choice of the grade of stainless steel
Stan said he used T304 in line 52 of patent # 4936961.Ravi's choice was to use 316L seamless pipes. Use ONLY SEAMLESS PIPES and not seam welded. These tubes were annealed for 3 hours in inert atmosphere of Argon to remove all residual magnetism and cold work stresses before they were assembled. Even Nitrogen can be used as the inert atmosphere. The tubes are annealed to get rid of the crystal lattice imperfections induced due to cold work and any traces of residual magnetism. They have to be in bright finish only you don't want oxides of nickel chromium or iron on the surface (more details in preparation below).
You can use most of the 300 series Nickel-Chromium Steels but 316L would be the most preferable and next would be 304L. Never go for 310 as this has the highest resistivity among the 300 series. Avoid Inconel grade (High Nickel Alloys) pipes as well due to their high coefficient of resistance. Just between 316 and 316L there’s a lot of difference in resistivity of the material due to carbon presence. Seam welds have magnetized seam lines along the length of the tubes. You must have them annealed after machining/cutting/sanding before being assembled. If considering 316L stainless, the slight increase in Molybdenum, Nickel and Chromium would increase the Electric Specific Resistance of the material ever so slightly, but the thicknesses can offset this problem. One needs to find out if there is any Aluminium content in the grade. If the aluminum content is less than 0.5 Wt% or nil, you could use this if it is easily available in seamless form and your required size.
The reason why you need to check for Aluminum content is that it is used as a deoxidizer during the melting and alloying process. So there is a possibility of it remaining in trace amounts based on the amount of O2 available in the bath for it to turn to Alumina and float up in the slag. Aluminum is the main component in Fe-Cr-Al alloys which increases the electrical specific resistance. Incase these manufacturers use Cerium mischmetal or some other Rare Earths for deoxidizing, we don't even need to consider Al. The 316L seamless tubes Ravi used were sourced from a retailer of ‘Sandvik, Sweden’.
Stan claimed he was using T-304 SS. As there is no caustic electrolyte as such, is right to say that 304 grade s/s can be used. However, expect there to be a good deal of brown gunk generated during the conditioning process, with the additional part coming from the 304 material. There are some higher percentages of Ni and Cr in 316 and 2% of Molybdenum. Some of the manufacturers use Cerium Mischmetal (Rare earths) during the melting and pouring operations and this is done in the more expensive alloys like 316 to increase the hot life and the surface layer strength and in case of seamless tubes a little extra silicon is added for free flow in hot condition (Hot Extrusion of seamless tubes).
All these put together add up to different metallurgical properties of the material in our case. When in hot condition the Rare Earths, Silicon, Molybdenum tends to migrate towards the surfaces of the metal and this helps in the formation of a strong protective surface layer (The reason why I said annealing was important). SS 316L is the only SS that can be used in human Medical Implants other than titanium, that’s how stable 316L grade is.
In summery -The best grade of SS to use is 316L Next preference is 316,304L and 304. L stands for Low Carbon in the SS alloy. 316L composition: % Carbon : 0.03 Manganese: 2.0 Phosphorous : <0.45 Sulphur : 0.03 max Silicon : 1.0 Chromium : 16 to 18 Nickel : 12 to 14 Molybdenum : 2.0 to 3.0 Plain 316 SS nickel range is 10 to 14% and carbon being 0.08% 304 SS has lesser % of Nickel and Chromium and doesn't have Molybdenum at all.
Tube gap and Thickness / SWG / AWG of tubes
Ravi's tubes sizes used were:
Outer Pipe OD : 25.317 mm
Thickness : 14 SWG or 2.032 mm
Outer Pipe ID : 25.317 - (2.032 x2) = 21.253mm
Inner Pipe OD : 19.930 mm
Thickness : 14 SWG or 2.032 mm
Gap is 1.323mm ( 21.253 - 19.930 )
This was adjusted to both the sides as the inside pipe is centered is 1.323/2 = 0.6615 mm on either sides of the inner tube. So effectively the gap between the pipes is less than 0.670 mm. Ravi went for a lesser gap by increasing the thickness of the outer tube. Ravi had some difficulty in the alignment of pipes as they were shorting. He had to get them straightened on a pipe alignment machine. Ravi does not suggest that people without engineering skills go for this small a gap, but is convinced that the higher output of my setup could be due to this small gap. SWG stands for Standard Wire Guage
Suggestions are, you really don't want an outer tube smaller than one inch, so that pushes us to the 2 mm wall thickness. The inner diameter of a 25.4 mm tube 2 mm thick (14 SWG) is 21.4 mm, so for a 1 mm gap, the inner tube needs to be 19 mm. The outer diameter of a 3/4" tube is 19.05, which would give a gap of 1.175 mm. For a 1.5 mm gap, you would need an inner tube diameter of 17 mm.