Does anyone know what the mechanical advantages are of longer boilers and larger fireboxes? I know they both have to do with speed, power, etc. I remember reading somewhere when I was a kid that one influenced potential power output while another had something to do with speed, but I can't remember which does what. The all-knowing google has been....less than helpful with this topic, so I figured I'd ask you guys. Even if you could point me in the right direction, that would be much appreciated. Thanks!
Currently dreaming in the parents' basement...
The boiler proper (those of us infected with Porta-itis call this the 'convection section) is the relative easiest part of this: it's never much less than 20' and never much more than 22' for most effective use of combustion heat. Any additional shell length is better used in a radiant 'combustion chamber' between the firebox structure and the physical 'rear tubesheet' location. Chapelon proposed a 'sectional boiler' that used the front of the convection section as the last stage of feedwater heating... but for modern American power this is quite short, just over 3' on a PRR T1 boiler.
The great thing that was discovered over the years was that radiant uptake in the firebox and chamber represented a significant part of actual steam generation. In fact the heat available goes up as the fourth power of temperature (particularly effective if the flame is luminous) and therefore increasing both the combustion-plume length and the area exposed to it can remarkably increase steam generation. (There is an associated concern that much enhanced circulation to the water legs of the firebox is an advantage, in part to preclude a nasty effect called DNB, and a device called a Cunningham circulator is effective in this regard, preferential to Nicholson syphons for a number of reasons.
The 'better' design for a boiler is to have the grate as 'low' as possible and the barrel diameter as large as possible. That is usually precluded by the 'prevailing' loading gage; what was perhaps the 'ultimate' Stephenson-style boiler was the Lima 'double-Belpaire' which had an enlarged chamber heating surface but at the cost of limiting drivers underneath to about 76". The ability to carry a MUCH larger boiler along this principle is one of the salient advantages of a Garratt in North American proportions --those were not built, but it would have been fun to see them tried.
In late practice the idea of 'forcing' the fire, close to the measured grate limit, proved a very false economy when maintenance was factored into costs. So the design 'result' was, a bit naively, to use a (much!) larger and wider grate, carry the fire thinner, and benefit from lower induced-draft from more efficient low-backpressure front ends by carrying a less intensely 'drawn' fire over a greater area. The problem is that with higher pressure, even with better alloy steels, comes added plate thickness and hence greater weight, and this gets worse with fancy circulators, syphons, smile overfire systems, etc. so not only is there more weight, it is slung off the back end of the equalized driver wheelbase and requires more and more carrying axles for the necessary balance of guiding integrity and axle load. Hence the C&O Allegheny, which might be thought of as a 'Berk-and-a-half', wound up with a firebox needing a three-axle truck -- on a railroad famous for really, really high permissible axle load! -- even though it produced lower indicated and dbhp than a PRR Q2 that made do with a two-axle trailer... and Lima was actively trying to peddle a follow-on poppet-valve eight-coupled locomotive with a big double-Belpaire that would likewise require a six-wheel trailing truck. (Lima could not find any takers for this 'innovative' Long Compression design, bit not for want of trying!)
Meanwhile, for fun, some of the optimization of steam gets dramatically interesting when you have stack-train loading gage available. Steam separation alone becomes a cinch instead of a perpetual nightmare... and real, practical economization and Rankine heat recovery from the exhaust become easy as well as merely demonstrated.
Short answer re firebox...the bigger the firebox, the bigger the fire you can build, and you can burn more fuel. That means a hotter fire, that can convert more water into steam, which ultimately means more power.
When a single axle was added under steam engines, (like a 2-8-2 instead of a 2-8-0) it meant the firebox didn't need to be squeezed between the drivers, so could be wider. The change to two axles under the firebox (like a 4-6-4 instead of a 4-6-2) meant you could have a wider and longer firebox - even more power, known as "Superpower".
wjstix Short answer re firebox...the bigger the firebox, the bigger the fire you can build, and you can burn more fuel. That means a hotter fire, that can convert more water into steam, which ultimately means more power.
There are also questions of operating steam at other than its maximum sustained output. This sometimes goes unrecognized in discussions of steam like those in 1947 for Niagara vs. E7, or Brown's in 1961 for the Nickel Plate, or 2-10-2s on Dona Cristina. If you have a huge GA to go with your heavy firebox you have to keep it covered, free of holes, free of clinkering, etc. and you are still hauling around the huge volume of steel and water representing the 'rest of the firebox' that holds the huge grate. If you have substantial slow-ordering -- more critical to a big reciprocating locomotive -- or any particular waiting in sidings or at other points ... the bigger fire can become a misery, or an excuse to 'economize' with cheaper or mine-run coal.
Note that this is not to say interesting part-load economy was not achievable with modern power ... although not all the long-term consequences may be properly considered. Tuplin (from England) reported seeing a crew on a NYC Niagara using sliding-pressure firing to do the work of a smaller H10 on the fuel and water budget of the smaller locomotive. Whether there was increased thermal damage to the boiler structure or increased risk of water carryover from this he did not say. But it does indicate in principle that a light fire on appropriate grate area represents the 'best' use of combustion to make steam (as opposed to the best capitalization of a locomotive by having it drag as much rated load per hour as possible regardless of marginal costs) and a number of trends in postwar steam indicated, distinct from Lima's stillborn desire for large and heavy radiant optimization, that you could in effect use smaller firing arrangements with better use of the Rankine cycle for efficiency. At least theoretically you could build a large 2-8-2 with a good front end, 3-axis lead truck, air preheat, Cunningham circulators and exhaust economization with the same effective performance at the cylinders and drivers of a fairly large conventional 4-8-4 ... the 'catch' being that, as in France, you'd need careful,attentive and mindful control over running it. Which was somewhat unlikely in a context of seniority, slapdash training on new technologies, and wildly decreasing profitability of steam as it had to be supported.
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