firebox crown height above centerline

[Simon CollierParticipant @simoncollier74340]

I am preparing to make a Springbok boiler using 10 gauge copper but in order to fit the tubes specified in the design by Martin Evans, 18 1/2″ tubes and 4 7/8″ flues, the crown height has grown 1/4″ in height and has become flat-topped rather than curved. That would leave 2″ space between the crown and the barrel (6″). The boiler is a parallel, round top, narrow firebox type. Because of the shape of the tube nest, in order to reduce the crown height significantly, I would have to leave out 6 tubes, leaving only 12. What do people think about crown height? I have read the thread on tube number with interest, as obviously, the two are interrelated.

[Simon CollierParticipant @simoncollier74340]

[WearyParticipant @weary]

I’m assuming that you do not have a copy of the pertinent M Evans book so…..

Per M Evans, Model Locomotive Boilers:

‘For model locomotive boilers from 1/2 in. to 3 in. scale, a good average height for the firebox crown is 7/64 in. above the longitudinal center line of the barrel for each inch of diameter, this figure being for boilers of the parallel barrel round-top firebox type.’

 

‘……If these figures are adopted, the following approximate heights for the crown above the barrel centre-line are arrived at:

 ……..6 in diameter parallel round-top        21/32 in.’

 

Note that M Evans couches his advice with the phrases ‘a good average height’ and ‘the following approximate heights’.  So these are only guidelines;  If I have understood correctly the increase in height in your boiler over the recommendations above is only 11/32 in (9mm).  I see that the crown of Springbok is drawn as being 11/16 in. above boiler center-line so your increase over the drawn dimensions is minimal.

 

As regards the crown being flat…..M Evans writes:

‘it should be remembered that a plate curved so as to present its convex side to the pressure of the steam is always stronger than a flat plate, in addition a curved plate has a slightly higher heating surface.  The crown of the inner firebox should therefore be made curved whenever possible.’

 

I have had a very quick look through some of M Evans’ boiler designs and all that I have spotted do indeed have a curve to the firebox crown, although this is sometimes very slight – for example in ‘Highlander’.  There is room to add another central crown stay to the top of the Springbok firebox if you are concerned – but note the remark below about thickness of firebox wrapper.  Also, or as an alternative, the design crown stays could be extended to and silver soldered to the outer boiler wrapper.

 

(I guess that there is a slight advantage in that  your total firebox area is very slightly increased, see ‘How Many Tubes’ thread). 

 

M Evans recommends that :

The inner firebox wrapper should not however be made unnecessarily thick, or the steam raising capacity of the boiler would suffer to some extent.  If the inner wrapper is of copper or stanless steel it can be made two-thirds to three-quarters the thickness of the barrel material’ . 

 

In the light of all this the reduced water evaporation before the crown is at risk of being uncovered is probably more pertinent.  A driver of one of these locomotives might be able to give you some idea of the evaporative rate and boiler performance ‘in real life’ (rather than the theoretical rates) and thus advise you if this minor change to the design is going to have any noticeable or significant operating effect. 

 

[Simon CollierParticipant @simoncollier74340]

Thanks for the reply. I do, in fact, have the book, but I was wondering if builders in the modern era, i.e., now, have strong opinions about these things based on experience. Martin Evans’ statements are often very general and I doubt that he was infallible anyway, despite his vast contribution to the hobby. Mine will be 3/16 heigher I think. I will be using rod crown stays, not girder stays and I wish to avoid a thinner wrapper because of the greater number of stays required (AMBSC code- I’m in Australia). I suppose the essential question is: what is more important, more tubes or low crown height? Another possible disadvantage of a low crown is that it leaves a greater area above, unsupported on the backhead and firebox tubeplate, possibly requiring more longutidinal stays.

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