Thanks to you both for continuing the conversation. There is a lot of good info for me to absorb here!
The problem, you see, is that they don't recognize what the combustion plume does, or what the 'combustion chamber' does, and only a few actually figured out why deep fireboxes are so much better, syphons are usually a scam, and what needs to be provided in circulation to optimize it. Straight GPCS is almost like a little threnody that says 'I don't understand Stefan-Boltzmann enough'.
People with too much time or training on IC engines tend to miss the whole thing while still in the assumptions. There, your flame travel to stoich is in inches, and while you may want to avoid detonation waves (e.g. in the wrong kind of pilot-injection promotion) your full combustion is in some ridiculous fraction of a second or rejected heat concerns go up, up, up. Nothing like this is true for a Stephenson boiler once you recognize the water in legs and over crown is the most important part of the Rankine cycle, not just expeditious 'shielding' that keeps the inner sheets from melting.
What actually happens in coal or proper oil combustion is that the flame plume remains luminous. Anything it touches that quenches the luminosity detracts from the radiant heat transfer, which goes as the fourth power of the sum of radiant frequencies. Anything, by contrast, that allows the luminous flame further travel in what is comparatively fast gas speed and thus short TOF while at high emission temperature will also be optimized with better heat transfer. (A possible caveat here is that chamber syphons may do as little good in causing early quench as regular thin staybolted ones do in the main plume, which may explain why ATSF installed them very expensively in some big engines in the late '40s and promptly ripped them out again!)
So increasing the 'fetch' for carbon-particle entrainment over the coal bed and lengthening the plume past surfaces circulated to absorb the additional heat uptake without flirting with runaway spot DNB and 'all that implies' becomes clearer... as does a ceramic rather than stainless arch. It is not difficult to see why back-firing burners for oil firing are better for big engines, too.
But substituting largely transparent gas release and combustion of transparent gas without luminous release dramatically reduces the radiant power spectrum -- the assumption, of course, being that the 'real' heat transfer is in the convection section ...everybody knows that. A gauge of what is lost can be gotten from Besler tubes, though, which channel gas into an annular space but also absorb peak energy in the few gas-excitation bands for CO, CO2 and H2O and then re-emit with more of a blackbody distribution essentially normal to tube/flue walls. (There is some refraction of IR in the moving gas stream but the practical import is likely small)
Equally obvious is recapture to the Rankine cycle of the colossal energy still in the combustion plume as it passes the front tubesheet and exposed parts of the superheater header. Even capturing enough in various air and water heaters to get to condensation of water is helpful; at that point the heat injected in steam for NOx reduction becomes net recuperated... and it alone is a meaningful amount.
With respect to the Big Boy and that notorious Trains article, what was it titled, "Big Boy or Big Mistake", there is a lot of criticism of a "high drivered" design applied to a freight engine lugging trains up mountain grades.
There is an element of "don't you think they (the people who designed that locomotive) knew that." William Withuhn in his posthumously published book appears to appreciate this in his discussion of the Big Boy and reasons for why he regarded it as a resounding success.
The original name for the Big Boy was the "Wasatch" type because the application was to climb the Wasatch grade in Utah with a certain size train without requiring a pusher. Having climbed the grade, the same locomotive was supposed to take that train and run with it across a much flatter rail line.
It may have been more thermodynamically advantageous to optimize the locomotive and train tonnage to running on the flat lands and add pushers to climb the steep portions of the route, and Withuhn describes the Santa Fe as doing just that where there were long expanses of flat suitable for fast running punctuated by severe grades. The UP, however, though they could realize operational benefit from not needing the pusher. I guess the design trade was, especially with the low-grade coal used, to burn coal inefficiently if you had to in balancing the two requirements.
So maybe the calculus was to burn a tenderful of cheap coal to climb the Wasatch grade on an engine that could run fast beyond the grade, and that saved on the cost of a helper district on their main line?
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Overmod I'd think for the 'levitated combustion' described in the literature the secondary-air vents would be valuable, but might also 'spoil' take up from the stoked bed; if there were problems in keeping the forward part of the grate correctly 'populated' (as Kratville relates there was at startup firing) the secondary flow might interfere with stoker ability to disperse coal to the grates fully enough to avoid clinkering or burn through under heavy firing.
I'd think for the 'levitated combustion' described in the literature the secondary-air vents would be valuable, but might also 'spoil' take up from the stoked bed; if there were problems in keeping the forward part of the grate correctly 'populated' (as Kratville relates there was at startup firing) the secondary flow might interfere with stoker ability to disperse coal to the grates fully enough to avoid clinkering or burn through under heavy firing.
The usual response to levitated combustion of fines in a locomotive firebox is along the lines of, "As if that is ever going to happen."
But happen it does if the data on the firebox extensions called "combustion chambers" are to be believed.
As discussed on another Forum thread, Wardale was deeply skeptical of combustion chambers contributing anything while at the same time he posts a chart from a long-ago published paper on the unique talent of the Pennsy M1 firebox for achieving high firing rates as it approaches its grate limit.
Alfred Bruce praises combustion chambers as representing progress in steam locomotive design although conceding that the they were a maintenance headache until welded construction came to be allowed, at least in the firebox.
The advantage of the combustion chamber appears to be about a 50 percent increase in allowed firing rate before the grate limit is reached? Not as dramatic as enthusiasts claim for the GPCS, but even Wardale concedes that the small, no more than 20 percent reduction in heat demand for a given steam production from a feedwater heating system makes a difference of where on the firing rate curve you operate and hence boiler efficiency. It is curious then how Wardale overlooks this effect when he reproduces the M1 firing rate chart where it is staring him in the face?
So, if a combustion chamber or a deeper firebox increasing the firebox volume, and Alfred Bruce suggests that firebox volume rather than grate area is the proper figure-of-merit for combustion capacity, results in more complete combustion, something must be burning elsewhere than on the firebed?
Thanks for your detailed reponse.
All of the possibilites definitely seem plausible. Maybe there is someone out there that can give the definite reasoning behind the change.
Below is a link to the picture I mentioned previously, in case you are interested.
https://flic.kr/p/2jrw9c9
One very immediate thing to consider is how the original secondary-air ports were implemented. Remember these are hollow tubes between the inner firebox and the wrapper in the legs, and would either have to be full-pen welded (as in all the little ones in Bulleid's Leader boiler) or made with coordinated OD threads like a staybolt and driven in with a tool... I think it relatively clear from construction and detail photos they were supposed to be welded. Now every location you have what is essentially a more rigid fixed staybolt, where you 'could have had' a flexible, or a conventional bolt easier to drive and service. These would moreover be relatively cool compared to 'other' staybolts and it is at least possible that vertical circulation in the legs might be affected -- I doubt this is very significant, but increasing effective circulation in a radiant section is perhaps the quickest way to improve non-forced steam generation in a Stephenson boiler of this kind.
We certainly see over time that UP found it desirable to control the actual influx of secondary air, and in a way involving very careful fabrication of sliding plates with handles. As the engines came to be used in wartime, I can see why elimination of the whole complicated fabrication, both in 'new' power and as fire boxes came due for maintenance or replacement, might have been pursued. I hope someone can find or contribute an account of these progressive changes ... as the changes certainly seem to be.
Now, there is certainly a 'thing' involving too much air admitted close to the grate, which in cold weather can lead to early quench, poorer relative economy, and worse smoking. [Let me observe in passing that antismoke overfire air arrangements are a different thing technically.] I suspect this to be the case in these engines, particularly in the reported winter gale conditions, and over time the somewhat theoretical 'combustion' advantages of better secondary admission but poor ore heat might have become less important than simple maintenance, more reliable on-the-road performance, or both.
It seems it didn't like my photo. I will work on getting it uploaded
Good Evening Overmod,
Firstly, thank you for your response to my question. Secondly, I'll say from the start that although I'm very interested in steam locomotives and their fuel combustion, I am far from an expert!
Fortunately I do have a copy of Kratville's book, and I have enjoyed it thoroughly( I have the '72 printing). That is kind of where this question began. The secondary air vents are clearly visible in several photographs, and you are absolutely cprrect that Kratville indicates they worked very well on the locomotives they were installed on.
I became intrigued when I had a chance to visit the National Railroad Museum in Green Bay, WI.
On the 4017's firebox they were conspicuously absent. The cleanouts for the circulators are clearly vislble. Where the secondary air should be you can see where plate was welded in and stay bolts were added. I've attached a picture for reference. The blue arrows point to a couple of the welds. This shows the left side or firebox
Maybe I'm missing something, or there is a mystery to dig into.
Thanks again for responding!
Paul MilenkovicWardale suggested that the Chinese somehow achieved near-smokeless combustion with what he regarded as simply awful coal from the standpoint of "rice-sized" lumps, large amount of slack, high ash content and problematically low ash fusion temperature. He theorized that they were getting a large amount of secondary air on account of the delay built in to the closing of the firedoor.
Books could be written on this... and have been. There used to be a site which had some early-20th-Century 'railroader's magazine' articles, one of which described when it was more practical for a locomotive to make smoke whatever the college-boy thermodynamicist-wannabe thought. It takes some reflection about what's involved in variable-condition firing into varying load, almost radically unlike most people's idea of operation and definitely unlike how power boilers are fired, to arrive at all the things that have to be attended to, often with multiple rapid interactive changes required, to do satisfactory coal firing in a typucal 'reasonably-advanced' Stephenson boiler.
Some of the Chinese combustion 'efficiency' is certainly due to levitated combustion of fines in 'delayed shutoff' primary air -- look at some of the places Porta puts defined secondary air to assist this general idea. The first problem is that unless the secondary air is preheated it may tend to quench luminance rather than increase it with oxygenation; the second is that if you do preheat it the density goes down and the stoich volume changes for equivalent mass flow.
Part of the fun is that you're balancing what is basically a dual-fuel setup with one set of firing controls (which are usually only accidentally designed to work with, let alone deliver, pulverized or 'fine' fuel). There are for example Australian locomotives that stoke a fire on a grate, and then have some form of fuel oil firing over the grate (using it as a very effective flame holder) for short-term power excursions or higher power. Fines will do the latter as long as (1) their TOF is sufficient to take them from lightoff to full mass combustion 'out of luminance' by the time they get to quench at the rear tubesheet, and (2) there isn't too much ash, or not enough rank, and luminous combustion starts back up again in the exhaust gas -- think Sandaoling effects.
The book also chronicles his frustration with getting the Gas Producer Combustion System (GPCS) to work at all with the Chinese coal, although the ability to maintain GPCS in South Africa appeared to be a sometime thing without the most highly motivated fireman. He theorizes that how the Chinese crews were able to burn their coal at all had to do with wetting it down to keep it from being scattered out the stack, shoveling it on to portions of firebed that already had molten ash, and getting it to stick to that slag. He reasoned that this style of operation was a non-starter as his contract specified implementing a GPCS, which the higher-ups in China insisted upon, but maintaining a deep, GPCS firebed in that state would rapidly end up with a clinkered mess.
It will probably not be lost on you that 'gas firing' loses much of the advantage of luminous flame in producing actual radiant heat transfer -- we can gauge approximately how much by reference to Besler tubes, which absorb selective radiation at transparent-gas emission peaks and then re-emit as blackbodies oriented substantially normal to inside tube-wall surface.
Wardale writes how pulverized coal would have been the answer only no one let him implement it.
Besides all of the problems of either handling or making pulverized coal on the tender, I read that for the low ash fusion temperature he describes, slag would quickly coat and plug the firetubes.
What Wardale was describing has the name "wet bottom" in electric power boilers.
Personally, and since my early technical experience was in modifying financial accounting software, one can take the view that everything the customer is doing is wrong and it is your job to change everything to the correct, improved process. The other point of view is to observe the process that is operational and benefiting the customer and look to see what incremental improvements could be made. I would have been curious to learn more about the Chinese firing practices and see if one could build upon that, but again, the higher-ups were insisting on GPCS because they had heard good things about it.
You may have noticed there is a near-100% incidence of GPCS being removed, sometimes with great haste, where it has been tried in general service. If it were as magically great or easy to run as Porta thought, you'd see much more utilization. I do think there are ways to do it, but not in the ways current proponents are touting.
Wardale suggested that the Chinese somehow achieved near-smokeless combustion with what he regarded as simply awful coal from the standpoint of "rice-sized" lumps, large amount of slack, high ash content and problematically low ash fusion temperature.
He theorized that they were getting a large amount of secondary air on account of the delay built in to the closing of the firedoor.
"Red Devil" chronicals his frustration with what Wardale perceived as passive aggression of factory people towards the improvements he was under contract with the Chinese government to implement. Whereas he was treated this way by higher ups in South Africa, the shop people along with some highly motivated locomotive crews he worked with had a "can do" and a "get 'er done" attitude that he didn't have in China.
The book also chronicles his frustration with getting the Gas Producer Combustion System (GPCS) to work at all with the Chinese coal, although the ability to maintain GPCS in South Africa appeared to be a sometime thing without the most highly motivated fireman.
He theorizes that how the Chinese crews were able to burn their coal at all had to do with wetting it down to keep it from being scattered out the stack, shoveling it on to portions of firebed that already had molten ash, and getting it to stick to that slag. He reasoned that this style of operation was a non-starter as his contract specified implementing a GPCS, which the higher-ups in China insisted upon, but maintaining a deep, GPCS firebed in that state would rapidly end up with a clinkered mess.
Wardale writes how pulverized coal would have been the answer only no one let him implement it. Besides all of the problems of either handling or making pulverized coal on the tender, I read that for the low ash fusion temperature he describes, slag would quickly coat and plug the firetubes.
What Wardale was describing has the name "wet bottom" in electric power boilers. Personally, and since my early technical experience was in modifying financial accounting software, one can take the view that everything the customer is doing is wrong and it is your job to change everything to the correct, improved process. The other point of view is to observe the process that is operational and benefiting the customer and look to see what incremental improvements could be made. I would have been curious to learn more about the Chinese firing practices and see if one could build upon that, but again, the higher-ups were insisting on GPCS because they had heard good things about it.
Whatever they were doing in China, there are a lot of photographs of steam locomotives with a clean stack.
railracerRegarding the Big Boys firebox, as designed it had overfire or secondary air vents, In pictures from the 1950's you can see that they have been removed. I was curious as to why?
Kratville's book on the Big Boys (originally printed 1963, with editions in 1972 and 2004) discusses the secondary-air arrangements. These consisted in the original versions of a double row (unequal length) of comparatively small ports, which acquired adjustable slides in later years to permit reduction or blanking of at least some, probably all of the orifices. Kratville noted that the arrangement was first tried on 4-8-2 7006 in 1939, and he does not mention that the secondary-air arrangement was removed or blanked off at any point -- something that I think he would have mentioned had it been found important, as he goes into extensive detail on other details such as ashpans and fire suppression.
Do not make the mistake of confusing the secondary-air ports with the external cleanout plug arrangements for the circulators, which superficially resemble the larger openings in deep fireboxes for antismoke overfire jets or guns. If you look at the online version of the book, pp.35-39 to start
https://babel.hathitrust.org/cgi/pt?id=uiug.30112097454620&view=1up&seq=40&size=125
you will see illustrations of the circulators, inside and outside the firebox, and the arrangement of secondary-air holes.
Did this help with drafting, or combustion efficiency? Or since the Big Boys burned most of the coal above the grates did they simply not produce enough benefit to justify the maintenance cost?
i hink it's logical to conclude that levitated combustion of fines would have been impossibly smoky in that long firebox without a properly-distributed source of secondary air; I simply do not know if UP and Alco intended for the engine to be fired that way, and no discussion of stoking practice indicated that suspended combustion as opposed to 'kicking' coal to the forward corners of the relatively low firebox space. What is certain is that large amounts of suspended fines would produce enormous levels of acrid black smoke proportional to the amount of luminous-flame carbon if not given proper secondary air ... and some pictures of operating 4000s show just this sort of effect.
Remember the extremes of weather that these engines operated under. When air-operated cylinder cocks can freeze and have to be thawed with torches because the superheated steam venting through them does not, and when 'gale force' crosswinds blow ashes straight out of the pan as they fall, it may not be surprising that control of the effective 'quench' of cold air through the secondary holes came to be recognized as desirable. There is certainly a long and sometimes comical history of problems with Porta's arrangements for secondary combustion air in various places of GPCS arrangements over the years. 4014, with the Dickens-Barker burner, relies on very careful primary-air admission and it would be valuable to see if any of the secondary air porting 'as built' is still open; I don't believe it is fully used, or the interesting ability of the engine to maintain operating pressure at 'idle' on natural draft would not work.
Hello,
Regarding the Big Boys firebox, as designed it had overfire or secondary air vents, In pictures from the 1950's you can see that they have been removed. I was curious as to why? Did this help with drafting, or combustion efficiency? Or since the Big Boys burned most of the coal above the grates did they simply not produce enough benefit to justify the maintenance cost?
Any information would be appreciated!
Thank you.
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