Stack exhaust design revisited: the 1 in 6 taper myth?

[applejak_2000]

....I believe I have an answer to all of this. Every stack design since steam locomotives were built are covered in this book all the way up to modern designs....http://www.camdenmin.co.uk/products/the ... late-april

Errrrr....A great deal of this thread already mentioned pertinent topics in the book; as well opinions of others that differ somewhat. Is the book valuable? no doubt, I even have a copy. I don't think that the book is "an answer to all of this", though.

[Fred_V]

I don't think there is an answer to it. After 21 pages of replies and thousands of points of view you still have to start somewhere and adjust from there.

[makinsmoke]

I may be off my nut but several manufacturers' model locomotive
stacks appear at least on the outside to have no taper, yet they steam
just fine. Am I not seeing something?

[JJG Koopmans]

Yeah, third best solutions can also work!
Kind regards
Jos Koopmans

[jma1009]

This thread is one of the most important on here and is bookmarked by me.

Jos's book is now reprinted in the UK, and is giving much food for thought for those interested in this important subject. Do please buy a copy!

To go back to the original topic, and this relates to Bryce's going off at a tangent, the problem with the 'Greenly' 1:3 and 1:6 rules is that they do not set a height of 'choke' above blast nozzle for optimum results. Jos in his book and posts on here, and Sam Ell (ex GWR testing centre) in the UK in the 1950s showed that this was important. Also the slight divergent internal taper above the choke and a few other details.

Now, our American cousins seem to run miniature locos in a different way to the UK and in Europe. They want a strong loud bark, and dont 'notch up' their reversers like we Europeans always do to obtain efficient use of the steam and expansive working.

In the UK miniature locomotive performance and driving has been developed into a fine art since at least the 1920s. We dont worry about having a loud bark. We want as little back pressure as possible and optimum smokebox draughting to make best use of this so our locos perform more efficiently.

With this goes an inquiring mind about all things related including smokebox draughting. We also probably have better coal!

You can make up some 1:3 and 1:6 taper templates and get Trainman's and Bryce's draughting methods absolutely correct but you can easily end up with the wrong 'choke' diameter and distance of same from the blast nozzle (this is what Bryce mis-understood), and if you have a parallel ID above the 'choke' you are missing the advantages of Jos's 1:12 divergent taper. This is pretty much 'bread and butter' stuff to those of in the UK who have studied the Sam Ell Rugby Test Reports in the 1950s which were on UK fullsize locos and might be considered of far more importance than Young's research in the 1930s.

One of the particular problems with many UK locos both in fullsize and miniature is restricted 'L' length ie 'choke' to top of chimney due to the restricted UK loading gauge. Jos has re-affirmed this must be a minimum of 2 x 'choke' or 'D' using his and Ell's nomenclature. I dont think this applies with American prototypes, but is a real problem in many big express type large diameter smokebox locos in the UK, and Jos has advocated the multiple nozzle blast pipe to deal with this particular problem.

Jos's formulae in miniature is very simple, and has been stated by him on here (though you need to go back to 2009 to find it!). I disagree with Jos over the 'Greenly' ratio of blastpipe nozzle ID to cylinder diameter ratio of 1:7. Miniature loco design has moved on a bit since the days of Greenly, and I have had built award winning locos (performance wise) with proportions of 1:5 and even 1:4.4 for blast nozzle diameter to cylinder diameter, and is in line with UK miniature loco designer Don Young who used a percentage of grate area to determine blast nozzle diameter. Sam Ell used a rather odd method of evaporative heating surface, but did not use the cylinder diameter.

I hope Jos will not mind me disagreeing with him on the above - we are old correspondents and Jos has been incredibly helpful in re-designing the draughting on my GWR King.

Cheers,
Julian

[makinsmoke]

The Sant Fe 2900 locos had the bottom of their petticoat pipe nearly level with the top of the blast nozzle, which was of course sitting virtually on the bottom of the smoke box.

As discussed, as boilers and locos became larger, the stack essentially retracted into the smoke box in order to keep the locomotive height under maximums.

Designed and built at the apex of US superpower, many modern steam locos probably share this attribute. I am still awed by the minimum clearance between the exhaust and petticoat.

Somehow, I guess, Baldwin fought through all the design issues and came up with this development in the twilight of steam here in the US.

Take care,
Brian

[little giant]

Was the Santa Fe 2900 oiled fired? The set up almost seems like a propane design.

[Dick_Morris]

One of the specifications for the circa 1943 USATC Consolidations was that they could be converted between coal and oil and the prints show what needed to be changed. I have a list of everything that is different between the two, but only part of the drawings. It make for a good comparison as that same people designed both the coal and oil burning versions simultaneously for the same locomotive. A drawing I have shows a major difference in the front end - the bottom of the petticoat on the oil burner is 8" lower than for the coal burner.

[applejak_2000]

I decided to go back and read this thread a second time. In so doing I discovered this statement on page 3:
"Henry Greenly, who in many areas of miniature steam locomotives was THE leading light and in others was seriously retarded in his thinking...." (my emphasis added)
If I'm reading that correctly and it means what I thought.

Wow! didn't catch that the first time. Not that I'm criticizing, it was just unexpected. I had to read the line twice!


And as a side note: when I'm driving with my wife and use an expression like the above, for example when the car in front of me refuses to make a right turn on red (who is in the right turn only lane with the turn signal ON, btw), the result is I sometimes get konked on the shoulder/head. Happened just day before yesterday. HaHaHa!

[elm53]

Brian,the stack bore does not need a taper.the 1 on 6 simply creates a reference point between the blast nozel and a point in the stack that the expanded exhaust completely fills the bore of the stack so a vaccum is formed.

[JJG Koopmans]

Oh dear! Do I really have to go on explaining that this is nonsense? Around the jet an induced flow is formed so the expanded exhaust never never never fills the bore. It is the transfer of momentum within the stack that takes care of the suction by the stack.
Kind regards
Jos Koopmans

[makinsmoke]

Yes the 2900's along with most Santa Fe locomotives were oil fired.
Many were built in the early years as coal burners but were converted
to oil at a later date. There were a few holdouts particularly in the east
where coal was more readily available but the Santa Fe realized the
cost savings using oil soon after Spindletop blew out in Texas and the growth
in the industry along with Santa Fe's expansion south and west.

I've seen the Baldwin front end folios showing the nozzles and petticoats
in different arrangements with regard to oil and coal fuel.

Jos, just to clarify in a simple statement for my non-engineering background.
A tapered petticoat pipe and stack are not critical but having sufficient flow
whether from the exhaust and/or blower along with the design criteria of nozzle
size and location, stack and petticoat location will provide ample draft in our
model locomotives.

Thanks,
Brian