Understanding BellPaire (sp?)

[johnpenn74]

Fellow Livesteamers,
While researching my latest armchair projects, I found my self reviewing a Great Northern 2-8-8-0 and ad a loss for the construction technique on the bell paire firebox. Specifically, any ideas why the bell pare top would extend infront of the throat sheet? I would have thought the Bell pair would only be around the firebox and not extend beyond the throat sheet. Clearly, it does here, the question is why? Something to do with combustion chamber maybe? Is this common on other Pennsy designs?

JP

[ThisGuyTwo]

I believe that the stayed section of the boiler extends past the throat sheet to try and increase the amount of heating area. I believe this is a picture of what you're looking at.

[Fender]

The other reason for using a combustion chamber on long boilers (besides increased heating area) is that the flue/tube lengths would otherwise be so long as to be impractical. The combustion chamber reduces the needed length of the flues and tubes. Since the boiler has a belpaire firebox, it makes sense that the combustion chamber would use the same configuration, rather than changing to a different arrangement of stays between the firebox and combustion chamber.

[Glenn Brooks]

Don't know much about Garretts, so Trying to understand the physical connection inside the boiler. My question is: what actually happens in the combustion chamber and is it physically separated from the fire box in some way - maybe it's a shelf or void that protrudes into the boiler in front of the fire box? I think that's what I interpreted in the above posts, but not sure... Is this an area that just traps hot gasses or does some additional combustion actually occur?

Thanks
Glenn

[James Powell]

Glenn, if there is an arch in the firebox, then it certainly does lengthen flame path. The idea is to allow for enough time for the chemical reactions to run to completion in a slightly over oxygenated environmenent- it being far less fuel efficient to have too little O2 and strangling the conversion from CO to CO2 than to run with excess air, which in and of its self is a loss. So, yes, there is a reason why to add the combustion chamber. Afterburning (burning in the tubes, or even worse, in the smokebox/open air) is something to be avoided if possible, because it is a substantial indicator of not enough air in the firebox.

A good intro thermodynamics book will probably cover some of this if you are interested. There is some of it covered in "The Red Devil", but it mostly assumes you have the thermodynamic background to understand what is happening. If you are going to get into experimental steam, a college course on thermo is probably a good investment of time. I'm very fortunate, I have more than just a college background in it, and find the math involved to be fairly interesting. (all the math to design a good loco). It's much easier now to look and criticize some designs which go far from the normal (in a statistical way), and understand why some designs were as comparatively sucessful as other much less ballanced designs.

(ps, question is no where near as dumb as you might think...)

James

[Boiler Builder]

Lima Double Belpaire Boiler article from Railway Mechanical Engineer on RYPN

[boaterri]

From what I have read, one of the other advantages to this type of boiler is that the outer shell of the boiler, the crown and side sheets of the fire box are parallel which simplifies the plan for the stays. Flexible stays are eliminated entirely.

Rick

[Marty_Knox]

From what I have read, one of the other advantages to this type of boiler is that the outer shell of the boiler, the crown and side sheets of the fire box are parallel which simplifies the plan for the stays. Flexible stays are eliminated entirely.

Rick

While it simplifies laying out the stays it does not eliminate the need for flexible staybolts. Flexies are used in the 'Breaking Zone' because the inner and outer firebox plates do not expand equally.

[Glenn Brooks]

I think I see now, that if we turn the photo above around to view from the side, the combustion chamber would be above and forward of the actual firebox. Sort of a step design with the chamber occupying the interior, open area forward, say a foot or two or three, inside the back end of the actual boiler tube, and at the forward end of the chamber, the flues are eventually rolled in. Makes sense now. Thanks!

[NP317]

I think I see now, that if we turn the photo above around to view from the side, the combustion chamber would be above and forward of the actual firebox. Sort of a step design with the chamber occupying the interior, open area forward, say a foot or two or three, inside the back end of the actual boiler tube, and at the forward end of the chamber, the flues are eventually rolled in. Makes sense now. Thanks!

Glenn:
On the largest locomotives, the combustion chambers are nearly as long as the fire boxes!
That hugely increases the surface area available for radiant heat transfer.
~RN