Firebox Update

We are in detailed discussions regarding the format of the firebox, see the below extract of recent emails:

With a welded firebox, increased water space over the full height and modern rules on ligaments we can just about get the original LNWR tube layout in, maybe just losing a couple of small tubes either side.  Thinking about possible improvements, an extra row of flues might be considered; however the three rows of flues reduced the number of tubes from 309 on the Precursor to 159 on the George, suggesting that 32 flues would leave room for only 109 tubes  There would be a reduction of heating area, and possibly a need to recess the tube plate for a larger superheater header.  Such a drastic revision might prove too much.  It would certainly increase costs significantly.

Modifying the flues to reduce the rear end swaged-down length to 6” (as Stanier standard) might allow slight lengthening of the elements, increasing the length inside the flues by 7 or 8%.  Further benefit might come from increasing the bore of the elements to 1¼” from 1⅛”, giving 10% increase in heating surface and 23% increase in cross sectional area. Steam would pass more slowly, increasing heat transfer.  One snag might be the reduced free area in the flues, potentially reducing the gas flow.  Perhaps the 5⅛” flues of most LMS engines would ease this a little.   None of this should cost much.

Variations in small tube layout would be possible.  Options:-

1. 140 2” tubes, 11SWG, 2.455 in² gas flow area, (S/A)^-1 331
2. 159 1⅞” tubes, 11SWG, 2.120 in² gas flow area, (S/A)^-1 356
3. 187 1¾” tubes, 11SWG, 1.810 in² gas flow area, (S/A)^-1 385

Length between tube plates is 12’-2¼”.

The 2A boiler had 5⅛” flues, 1⅜” elements with 13’-0” between tubeplates, which suggests we are in the right area.

Note that the numbers of tubes is estimated, and it may be necessary to fit fewer when the layout is done.