The lower part of the coil will be constructed in three layers and the part in the stack will be a single layer in contact with the brass tube. The three layer section is wound with a 3.5mm (on average!) gap between the 22 spirals and with the vertical separation maintained with sections of 1/8th inch copper tube. I have a spread sheet that I can input the heating area required, tube size, gap size etc., and it calculates the overall dimensions of the boiler - so I can play around with the numbers to get the boiler shape that I require.
On my steam jet engine project (see the other thread) I used a 5 layered coil and I did achieve steam temperatures in excess of 720degC - yes the silver solder did melt!
I think with the horizontally coiled flash boilers, that Iíve been using, couldn't have had much natural circulation, since any density variation causing circulation must have been cancelled out along the length. In a normal boiler, as you know, the circulation occurs between the lower headers and the drum and requires lots of relatively large diameter tubes. With the monotube flash boiler with a small diameter tube the flow relies on continuous feed pumping to maintain flow through the narrow tube. Anyway, its all part of experimental fun, which, if it is successful, could be applied to larger vessels Ė all part of modelling I suppose.
I have been wary of using any ďheaderĒ in the system, since it might constitute being a pressure vessel, which is what Iím trying to avoid with all the testing etc.
Yes, Iíve got the Selway Fisher Edwardian 30 foot launch plans and their detailed booklet on strip planking techniques. Iím aiming to use the 5 inch prop thatís currently installed in Vital Byte. The boat is planned to be of 75 inch length and the boiler is scaled to fit. Iíve never tried strip-planking before, so its back to the bottom of the learning curve!