Tube length and anthracite

I recently found (it's probably been there for a long time, but I only looked recently) on the WWWeb, at George Elwood's invaluable "Fallen Flags" rail image site, a diagram of the Delaware & Hudson's big E6a class 2-8-0:
http://www.rr-fallenflags.org/dh/dh-stm-s27a.gif

(Change "s27a" to "s28" for a diagram of number 1219, the E6a engine that got an all-welded Alco boiler; change to "s27" for a diagram of the prototype E6, with 15 foot tubes but a smaller firebox.)

What surprised me was the very short boiler tubes: 13 feet 4 inches to 13 feet 11 inches on different locomotives of the class. This is very short for a modern (built in 1918) American steam locomotive: the U.S.R.A. 0-6-0 and 0-8-0 switchers had 15 foot tubes, and large road engines (big 4-6-2 to 4-8-4) usually had at least 19 foot tubes. So,

QUESTION: is there some reason why an anthracite-burning locomotive ought to have short tubes, or is it just that once you had designed in the immense (104.f square foot grate) firebox, the boiler barrel you could fit on a 63 inch drivered Consolidation wasn't long enough for optimum tube length?

(For comparison, the Pennsylvania Railroad's H-8, H-9 and H-10 Consolidations were of roughly comparable overall size: 62 inch drivers in stead of 63 inch, and about 5/6 the total engine weight. They had 15 foot boiler tubes, but only 5 square foot grates, with-- crucially-- a grate almost two feet shorter than that on a D&H E-6a.)

HI
The answer to the question , the combustion chamber which is part of the firebox , is the reason for such a small tube length , now what is the combustion chamber , this you will have to look up , and learn , Pat

Thank you for the suggestion, Pat Fahey! Combustion chambers in later, larger, locomotives certainly kept the tube length down: large 4-8-4 had long enough boiler barrels to accommodate tubes much longer than the 19feet (or so) many of them had, but afire-to-eight foot combustion chamber was bette at extracting energy from the fuel than additional tube length would have been.
If you look at the diagram of the D&H E6, however, you will see that it had at most a very short combustion chamber... leading to another question: does the very large firebox volume used with anthracite fuel in effect substitute for a combustion chamber?

Sorry, maybe I should be more explicit about the thinking behind my last question. Anthracite burners had (compared to locomotives burning bituminous coal) very large grate areas. So (I am guessing) the amount of fuel burned in a given period of time on each square foot of grate would have been less than in a bituminous burning locomotive. So (I am guessing, with less confidence than last time) the velocity of the gas going up from the grate, around the arch, and forward to the tubes might have been lower in the anthracite burner. So (the thought occurred to me, though I can see enough problems in it that I am far from confident of it even on the assumption that the first two guesses are right) maybe... MAYBE... the additional combustion that is supposed to take place in the combustion chamber in a locomotive burning bituminous coal might occur within the firebox proper in an anthracite burner.

Is there a thermodynamicist in the house?

HI Allen

Maybe this mite answer your question, Pat
Wootten fireboxFrom Wikipedia, the free encyclopediaJump to: navigation, search
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4-6-0 camelback locomotive, complete with Wootten firebox.The Wootten firebox is a type of firebox used on steam locomotives. The firebox was very wide to allow combustion of anthracite coal waste, known as Culm. Its size necessitated unusual placement of the crew, examples being camelback locomotives. The Wootten firebox made for a free-steaming, powerful locomotive, and the cheap fuel burned almost smokelessly; the combination made for an excellent passenger locomotive, and many camelbacks operated in this service.

[edit] HistoryJohn E. Wootten was the Superintendent of Motive Power for the then Philadelphia and Reading Railroad (later simply the Reading Railroad) from 1866, and General Manager of the system from 1876. He saw the vast spoil tips (piles of anthracite waste) in the area as a possible plentiful, cheap source of fuel if he could develop a firebox that could burn it effectively. Through experiments, he determined that a large, wide firebox with a slow firing rate worked best, with a thin layer of the fuel and moderate draft.

The typical locomotive firebox of the day was long and narrow, fitting in between the locomotive's frames. The successful design of a trailing truck with the firebox mounted behind the driving wheels had not yet been developed. Wootten instead mounted his huge firebox above the locomotive's driving wheels. The problem now arose that with a cab floor at the then standard tender deck height, it would be impossible for the locomotive's engineer (driver) to see forwards around the firebox shoulders. Instead, a cab for the engineer was placed above and astride the boiler. The fireman, however, remained at the rear with minimal protection from the elements.

Pat Fahey--
Thank you for your reply. The fireboxes on the E6 (and on many other locomotives of railroads in the anthracite region-- Lehigh & New England, for example) followed Wootten's idea of a large wide firebox (though most more modern versions managed to combine this with a single cab in the usual position rather than the "camelback" cab used at the beginning of the 20th C). The bit about "slow firing rate" in the W~ia article you quote is consistent with one of my guesses.

So maybe the huge firebox of the E6 gave it the "moral equivalent" of a combustion chamber. I'm still curious as to whether its tubes were optimum length, or whether they were just short because of the constraints of overall locomotive size.

HI Allen

So far we have had a good discussion on the matter , the reason for the short tube length and combustion chanber was because of the Boiler design . Allen look at the Muhlfeld design and you should have your answer . Pat.

what you are discussing is the l/d ratio of the tubes. that is length over diameter. This is more significant parameter than the specific length of the tubes.
For a given tube material, flue gas temperature and boiler water temperature, - there will be an optimum gas velocity at which all of the heat in the flue gas will be transfered to the water by the time it reaches the exit of the tube. if the l/d ratio is too high the gas will be moving too slowly and cool too soon, and the remainder of the tube will produce no steam and be wasted. if the l/d ratio is too small the gas will be moving too fast and exit hotter than it might and heat will be wasted.

IF the design of the boiler constrains the length of the tubes, there are two solutions to reaching the optimum l/d ratio, either use more tubes to slow the gas velocity down, which may or may not be possible, or use smaller diameter tubes to increase the l/d ratio. going to a higher heat transfer material such as copper tubes would also work, but we dont need to go there.

Furthermore, since gas velocity changes with firing rate, there will be only one load at which l/d and hence boiler efficiency will be optimum. this can be chosen to coincide with 100% of boiler rating, or some other load level, the intended service would be studied to determine the load at which the maximum service hours will be spent to maximize lifetime efficiency.

That's a general discussion, but the nature of anthracite fuel burning is significant too. As others have mentioned the large firebox is doing a much larger proportion of the heat absorption from the wide, low heat release fire, so we can expect the peak gas temperature at the inlet of the tubes to be somewhat lower than it is burning higher volatile fuel. if that's the case then its clear that delta T is lower and maintaining optimum l/d for a normal diameter tube means shortening it.

at the opposite end of the spectrum from anthracite, are boilers burning fuel oil or gas, where there is almost no "furnace" and return chambers and baffles are set up to create multiple passes in order to achieve sufficiently high l/d. on many such boilers in stationary practice the gas passes through the length of the boiler four times before exiting.

Thank you, Eliphaz! I hadn't considered tube diameter. (Tube diameter didn't vary very much in 20th century U.S. steam locomotives, though, did it? My impression was that they were pretty much all something like 2.5 inches, with larger diameter "flues" containing the superheater elements... though, I guess, one of the considerations in choosing the superheater arrangement to use was what effect it would have on average gas velocity through the boiler.)

(The scans Pat Fahey posted are from "The Steam Locomotive in America" (W.W. Norton, 1952, reprinted some time or other by Bonanza) by Alfred W. Bruce, who had been the chief of steam locomotive engineering for Alco. The Muhlfield boiler is NOT the boiler on the E6, but an experimental design used on a few D&H high-pressure steam locomotives: diagrams of these locomotives are available at the same "Fallen Flags" site as the E6 diagram I linked to.)

static draft loss also varies by tube l/d. shorter & thinner = longer & fatter.