Galloway Tubes Vs Radiant Superheaters

[Isaac Lester]

Hi All.

LBSC's Bantam Cock boiler features a combustion chamber with galloway tubes followed by flue superheaters. My wonderings to the internet ether are as follows:
 - Is a combustion at this scale useful by any means?
 - If so, are galloway tubes with flue superheater the best option or would a radiant superheater produce better steaming?

It does perhaps feel like splitting hairs, but any simplification in build difficulty without sacrficing performance seems like a no-brainer.

A penny for your thoughts.
 - Isaac.

[brucevoelkerding]

Isaac - Model Engineer re-ran LBSC's Bantam Cock series in 2013 (I do not know in which Issue the series began). However, in Volume 212 No. 4472, 27 Dec - 9 Jan 2014, there is an Article by Paul Tompkins in which he describes the Design of a new Boiler for Bantam Cock. I recommend you read this Article first as Paul makes some good Suggestions (I'm updating a Boiler and I happen to have this Article open on my Desk).
Bruce

[tenor]

I am familiar with LBSC's Brittania design with similar combustion chamber arrangement and it is a remarkably good boiler. However, the combustion chamber is a tricky thing to make. There must be plenty of scrap boilers with a tiny pin hole in the combustion chamber joints that is completely inaccesible for a repair.

The throatplate flange, then the combustion chamber wall and then the tube plate flange take up a lot of area which could be better used to cram in a few more tubes. Add that to as many superheater flues as you can reasonably get in (4 or more hopefully). You might want to go up a size on the tubes as they will be longer.

I have an Excel spreadsheet that will work out the heat transfer and thermodynamics of the various options. You are welcome to a copy, just p.m me.

Martin

[suctionhose]

Ive only made one boiler with a combustion chamber - they are a bit tricky to establish the development for. They are also tricky to stay and, as tenor says, fix any leaks after the final assembly.

In principle though, my observations of running 5"g for several decades, lead me to believe that a wide firebox boiler benefits from a combustion chamber to provide sufficient firebox volume above the grate for good combustion. Without good combustion, especially with the less-than-ideal-fuels we are burning these days, sooting of tubes is more pronounced. That is certainly my experience.

For my New Zealand loco, I will be using a combustion chamber - without galloways (they take up the internal volume that we want and make tube access difficult from the rear) - with shorter tubes of a smaller dia than would be workable without a combustion chamber. Having owned engines that deviated beyond either side of the extremes of the L/D^2 ratio for tubes, to their detriment, I am convinced that adhering to this ratio is a fundamental element of success. (is it L/D^2 = 60 to 80? I have to check...)

To maximize tube area, the back tubeplate will have the flange mounted externally on the combustion chamber. Stays will be threaded into blind bosses silver soldered to the outside of the combustion chamber. Those stays are screwed in and later silver soldered to the outer wrapper upon final assembly. Superheaters will be radiant.

The additional heating surface provided by the galloways has to be weighed against the acquired combustion volume in my view. Typically, a well proportioned copper boiler will produce far more steam than the locomotive can use and therefore, in light of lost firebox volume, lost radiant supers, added work / risk of issues, I am quite willing to sacrifice the galloways.