Ok, so back to the topic:
It will be some time before I can start making the boiler but I think it is the right time to discuss its design now. It will also take a while to make final changes and for the materials to arrive, so hopefully I can start without too much waiting when the running era is finished.
I used John Baguley’s loco design spreadsheet to determine the effects of my alterations. (http://www.modeng.johnbaguley.info/Loco%20design/design1.htm) First the original Kennion’s boiler design was entered into the spreadsheet and I found out that it should be improved. I won’t tell you every change I made but the major ones.
Below you can see an extract from my new drawing with some key dimensions. I didn't include stays in the drawing, but they are included in the design of course.
The grate area was increased to 72x115mm as the original one was way too small for the cylinders. As the fire tubes are rather short and the smokebox is very small, the boiler won’t have a superheater. This should make the smokebox piping much less crowded and the boiler tubes easier accessible. I don’t intend to open a can of worms with the non-existing superheater but I believe that it will make things unnecessary complicated with questionable effects on performance.
The firetubes were enlarged from 8,1mm to 9mm id, which results with 180mm length in Kt=55,4 (the Keiller ratio)
Some more data:
- Ratio grate area to tube area: 16,4%
- Engine and boiler performance values: Ee 0,185; Eb 33,3; Eo 6,2
Especially Eb is far from the ideal values but this is the best I could get it without resulting in a boiler layout that is impossible to mount on the existing chassis.
Now the first question to the more experienced people: will a boiler with these factors be sufficient?
The factors are suggesting, that the tube area is too big, resulting in fire lifting or clinkering, but I don’t like to reduce tube diameter. (The value for Kt is good and smaller tubes block easier). At least it should steam freely (?) Could a less fierce smokebox draughting compensate for too large a tube area or would that lead to further issues?
With 2 tubes less (19 instead of 21) Eb will be 36,7. Just by looking at the numbers this seems to be the better configuration. My gut feeling says to better fit 2 tubes more though. If 1 or 2 get blocked I should still have enough steam. If turns out that there is still too much tube area, I could still block one or two tubes with a cap.
The boiler barrel is supposed to be 4,5” (114mm) od which is not obtainable here. So I switched to 108x2,5 tube with a separate firebox wrapper. This will have an effective diameter of 114mm with 3mm thick plate. I will loose a bit of water and steam space due to the smaller barrel but I don’t fancy to buy the piece of tubing in the UK and mess with customs etc.
Now to the firebox:
The inner firebox sides have 3 radii: The transition from the crown to the side, the side curved concentric to the barrel, and the transition to the vertical part if that makes sense. From my understanding this makes it very difficult to bend the wrapper accurately. It will be easier with two radii and 2nd radius part flat (see picture). Difference will be about 1,1 mm on my boiler. The firebox sides are stayed anyway so my modification should not be a problem (?)
Kupferkessel V2 gerade Seiten 2 by
Technikfreak L, auf Flickr
After some calculations I came to the conclusion that 4mm copper rivets at 16mm spacing will be sufficient as stays for the sides, throatplate and backplate.
For the crown stays I also thought about rod stays. I’ve never made a locomotive boiler so far but from my general engineering understanding, rod stays should be easier to solder. At least on the outside where they can be left longer for heat conduction.
From what I have read here before, soldering girder stays to the outer wrapper is problematic to do. Most published boiler designs have girder stays though, so they can’t be that troublesome on the other hand. Am I overlooking something?
