hi nigel,
with the greatest respect i think you are about to make a complete cock up of a boiler. ive built a few in 5"g and done IMLEC. i think you are way off the mark. try it as an experimental design by all means, but be prepared to throw it in the scrap bin! i would recommend a copper boiler with 3 x 1" dia superheater flues as the top row, and 12 x 1/2" OD flue tubes underneath. this will give ample free gas flow (though i think your grate area is excessive… beware the 'Greenly' lazy grates problem…. in which case what size are the cylinders?) i would have thought a total of 15 square inches grate to be sufficient for a 5" dia boiler. the keiller formula (as amended by jim ewins ie slightly larger bore tubes) hasnt been bettered in 5"g as a formula. i would depart from it at your peril!
dont forget that the last 1/3rd of the tube length contributes nothing to heat transfer as discovered by jim ewins in a standard type boiler. 22" long tubes for a 5" dia barrel seems rather long to me…makes me wonder what type of loco it could possibly look like! i would be inclined to recess the front tubeplate quite a bit. if you have a wide firebox it is possible you could go up to 9/16" dia flue tubes but you wouldnt get so many in or have the even spacing as described above (from someone who has built a few boilers with 5" dia barrels). in your proposed design you are basically having a huge firebox with a very long barrel with enormous sized flues contributing nothing to heat transfer… the main heat transfer will take place only in the firebox… so what is the point of the long barrel? it does nothing at all for steam production. btw, if you consider heat transfer in the flues, the slower the gasses flow some say the greater the transfer…but if you take this to the extreme and reduce the flow to zero there is no heat transfer!
good luck!
(sorry for the edits… my spelling or keyboard playing up!)
julian
Edited By julian atkins on 21/05/2012 23:03:05
Edited By julian atkins on 21/05/2012 23:18:34
Edited By julian atkins on 21/05/2012 23:22:32