The business about thermal efficiency is one that I find interesting because I believe modest improvements in percent efficiency could have brought big reductions in coal and perhaps water usage and maybe kept steam around a bit longer.
J Parker Lamb's Perfecting the American Steam Locomotive talked about the Big Boy consuming 20 tons of coal/hour, evaporating 100,000 lbs of water/hour, and producing 6000 HP at 40 MPH. I have seen similar evaporation rates for other Super Power steam evaporating 80-100,000 lbs/hr to produce peak HP's of 5000-6000, although the Big Boy seems to take the prize on firing rate.
While it might be hard to figure out where all of the energy goes and how to account for the difference between theoretical thermal efficiency and what you get in real life, what is interesting about steam is that people will tell you for certain historical locomotives what the boiler could do for evaporation rate and what the locomotive could turn out on a shop dynamometer or on the drawbar of a dynamomoter car out on the road. That clue gives some important insight into steam locomotive efficiency and performance.
To get 6000 HP out of 100,000 lbs water/hr evaporated, that means you need to convert 150 BTUs of heat energy per pound of steam into mechanical output -- there is no other way around that. Turning to my handy-dandy steam tables, assuming a boiler pressure of 300 PSI gauge, 315 PSI absolute, superheat temperature of 200 degrees, 15 PSI drop in the steam inlet passages, 15 PSI backpressure on the backstroke of the piston when exhausting steam, I get 60 percent cutoff (there is full steam pressure until 60 percent of the piston stroke after which the valve cuts off and lets the steam expand for the remaining 40 percent of the piston stroke before the exhaust event happens), 154 PSI absolute cylinder pressure at the end of expansion, steam cycle efficiency of 13.2% with feedwater heat, 11.4 without feedwater heat.
Under these 60 percent cutoff conditions, you are getting 80 percent of the power you could get with 100 percent cutoff and full steam pressure over the entire piston stroke with no expansive working, but you are using only 60 percent of full steam so the boiler can keep up. So it seems that the peak HP of these machines was rated at a pretty high percent cutoff -- i.e. pretty close to the peak tractive effort. You are also exhausting steam under pretty high pressure -- about 135 PSI gauge -- this, by the way is far from the back pressure of the blastpipe; this is the initial blowdown pressure when the exhaust port cracks open, which quickly tails off to the exhaust back pressure. This kind of exhaust blowdown is a feature of Otto and Diesel cycle engines as well, unless you are doing something exotic like the super-expansion cycle of the Toyota Prius engine or the exhaust blowdown turbines of the Curtis-Wright turbo-compound engines on the DC-7 airliner.
Here is the interesting part. Suppose you are evaporating water at 315 PSIA and adding 200 deg-F of superheat -- that takes 1335 BTU/lb, disregarding the 180 BTU/lb of heat recovery you could get from feedwater heating. I am assuming now that the UP Big Boy used some kind of low-BTU Western coal -- say 8000 BTU/lb -- there are Eastern coals that have twice that heat content. Dividing the heat content of the evaporated water out of the boiler by the coal stoked into the firebox, the combustion efficiency of the Big Boy is 42 percent -- lower if there is feedwater heat recovery, lower yet if they used higher rank coal.
Poking around some more in the book of steam tables, 30 percent cutoff would extract 195 BTU/lb water as mechanical power, produce about 50 percent of full power, exhaust steam at 47 PSIG (62 PSIA), and give steam cycle efficiencies of 17%/14.5% with/without feedwater heat. Is 30 percent the limit with simple-expansion and piston valve gear? If you went to 15 percent cutoff, either by poppet valves or by compound expansion, you would need an extra 100 deg-F of superheat to avoid cylinder condensation, you would extract 226 BTU/lb water as mechanical power, produce about 30 percent of full power, exhaust at 28 PSIA (13 PSIG -- this is probably where you start needing the fancy ejector nozzles), and operate at 18.7%/16.3% efficiency.
What I get out of this exercise is that if Porta's Argentinia ran an 18.7% efficient steam cycle at an effective 15 percent cutoff (that machine was conventionally-valved but compounded) and measured a 12 percent thermal efficiency, they were running that boiler at about 65 percent combustion efficiency.
Perhaps the going from 30 percent cutoff to 15 percent cutoff by some fancier engine -- compounding, poppet valves, geared steam engines, what have you -- is barking up the wrong tree if it gets you a 10 percent reduction in coal consumption. Increasing boiler effiency from 42 to 65 percent would get you a whopping 35 percent reduction in coal consumption.
So maybe the focus on the steam cycle and the drive is all the wrong emphasis. The improved draft ejectors (Klychap, Giesel, Lempor, and so on) reduced back pressure, yes, but maybe the big deal was increasing the draft rate or the ability to pull boiler draft through more surface area of boiler tubes or something, anything, to increase boiler efficiency. I was also reading a back-issue of Trains with a "Confessions of a railroad fireman" article, and the experience of (hand) firing many different locomotives suggested that locomotives were all over the map in "steaming ability" -- you could work yourself into a sweat on some engines while on others, throwing in a couple shovel fulls of coal would bring steam pressure up nicely -- the "steaming ability" experienced by the degree of back soreness of the tallowpot may directly correlate with wide variations in boiler efficiency across the fleet.
With that in mind, does anyone out there have insights into the boiler/heat-transfer side to locomotive efficiency? I always thought that Super Power was a brute-force approach to power at the expense of efficiency, but the trailing truck and the huge fire box maybe meant improved combustion efficiency over earlier designs instead of mere raw pounds of steam evaporated. Are there some reasonable limits on boiler efficiency? Was boiler efficiency on the Big Boy so low because they were overfiring coal (and blowing a lot out the stack as partly-burnt cinders) to get more power out of it to make it over the ruling grade?
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Right interesting post there Paul.
I seem to recall seeing figures of 50 to 60% for locomotive boiler efficiency and would guess that size and weight limits imposed by locomotive service adversely affect efficiency. Most of the heat transfer in power plant boilers is done by radiation which is made possible by the large furnace volume. In a steam locomotive, the radiative heat transfer would occur in the firebox and combustion chamber, with heat transfer in the tubes and flues primarily convective. The article on the D&H high pressure experimentals in the June 1967 issue of Trains touched on the relative heat transfer in the firebox versus the tubes and flues.
At least one midwestern road was thinking about either a 4-6-6 or 4-8-6 to get a firebox large enough to get combustion rates and heat transfer similar to a power plant.
I also seem to recall the the Allegheny's steam pressure dropped by almost half by the time it got to the pistons when running at speed. I don't recall seeing any data on where the pressure drops were occuring, but there were plenty of suspects.
I don't think incomplete combustion was the problem with low boiler efficiency, it wasn't that difficult to keep a clean stack (especially after overfire jets were added). One caveat is that I don't know how much CO was in the exhaust - most of the energy released in going from C + O2 to CO2 is in the CO + O reaction.
One final comment in my ratherdisjointed reply. The data on coal consumption in Kratville's book on the Big Boy works out to 4.4% thermal efficiency. A coal plant with minimal emissions reduction equipment can achieve about 40% thermal efficiency, but most now are lucky to get 35%.
Paul Milenkovic wrote: Lamb's Perfecting the American Steam Locomotive talked about the Big Boy consuming 20 tons of coal/hour, evaporating 100,000 lbs of water/hour, and producing 6000 HP at 40 MPH.
Paul Milenkovic wrote: I have seen similar evaporation rates for other Super Power steam evaporating 80-100,000 lbs/hr to produce peak HP's of 5000-6000
Coal quality varies quite a bit by seam and by mining method (and whether it's washed or not), and for all I know Union Pacific Railroad bought the dirtiest, lowest heating value coal that the Union Pacific Coal Company mined. That said, average values are:
Hanna Basin, 10,000 BTU/lb.
Rock Springs, 10,900 BTU/lb.
I have no idea where NKP or NYC or C&O or anyone else got their locomotive fuel and even when it came from company mines, which seam they were in and how dirty they mined it can make a big difference. Some nominal average values for current-day washed coal are:
Central Appalachia, 12,500 BTU/lb.
Northern Appalachia, 13,500 BTU/lb.
Illinois Basin, 11,800 BTU/lb.
timz wrote: Paul Milenkovic wrote: Lamb's Perfecting the American Steam Locomotive talked about the Big Boy consuming 20 tons of coal/hour, evaporating 100,000 lbs of water/hour, and producing 6000 HP at 40 MPH.40000 lb of coal to evaporate 100000 lb of water? Surely Wyoming coal wasn't that inert? Paul Milenkovic wrote: I have seen similar evaporation rates for other Super Power steam evaporating 80-100,000 lbs/hr to produce peak HP's of 5000-6000And they were burning less than 20000 lb of fuel per hour, weren't they?
The 1943 test data in Kratville's book on the Big Boy shows evaporation rates of 76,000 to 85,000 pounds per hour with coal consumption ranging from 16,300 to 19,600 pounds per hour. The ratio of water evaporated to coal consumed varied from 4.15 to 4.72. Assuming a bit under 200F degrees of superheat, the enthalpy of the steam would be 1315 BTU/lbm-H2O or 6200 BTU/lbm-coal (best case). Taking RWM's value of 10,000 BTU/lbm for the coal, this gives a boiler efficiency of 62% at best.
The 62 percent boiler efficiency and firing rates a bit below 10 tons coal/ hour seem more plausible. That 20 tons/hour may be the capacity of the stoker feed for some such thing and seemed a bit high.
The other thing, if they were able to evaporate 76,000 lb water at 16,300 lbs coal at a speed where they could operate at 30 percent cutoff, the thermal efficiency would work out to 9 percent, and at that level not far off the mark achieved by Porta and Chapelon, one could understand where the UP thought the Big Boy worth operating well into the 1950s. Of course the thermal efficiency went downward for less optimal operating conditions (your mileage may vary as they say).
erikem wrote:Taking RWM's value of 10,000 BTU/lbm for the coal, this gives a [4-8+8-4] boiler efficiency of 62% at best.
Other locomotives, using Eastern coal, would evaporate maybe 7 lb of water per pound of coal. SFe's oil burners evaporated maybe 11 lb/lb (with 20000 BTU/lb? oil). If Big Boy only evaporated 4 to 4.5 lb/lb, seems likely they were using cooler coal than that.
timz wrote:Other locomotives, using Eastern coal, would evaporate maybe 7 lb of water per pound of coal. SFe's oil burners evaporated maybe 11 lb/lb (with 20000 BTU/lb? oil). If Big Boy only evaporated 4 to 4.5 lb/lb, seems likely they were using cooler coal than that.
Since the best Eastern coal had 14,000 BTU/lbm, that would scale pretty well with the Big Boy (7/14=5/10) - I may be w-a--y-y-y off base assuming 14,000 BTU/lbm. If the figures for SFe's oil burners are the likes of their 4-6-4's, 4-8-4's and 2-10-4's, bear in mind that their firebox design was not as constrained as the Big Boy's (i.e. the firebox was supported by the trailing truck and not extending over the rear drivers as in the Big Boy).
What I don't have access to is steaming and firing rates for the Big Boy at a lighter load and would strongly expect that the water/coal ratio would be better at lighter loads (better heat transfer). Yet another point to consider with the Big Boy is most of its operational life was above 4,500' MSL.
I happen to be home tonight so pulling from the shelf the obscure book "Railway Fuel," Eugene McAuliffe (President UP Coal Company), 1927, we learn among other useful facts:
UP in 1925 produced 23,707,454,000 gross ton-miles in freight and mixed-train service, consuming 1,649,165 tons of coal, for an average 139.13 pounds of coal to produce 1,000 ton-miles.
ATSF in 1923 produced 25,292,987,000 gross ton-miles coal-fired in all services, consuming 4,080,449,500 pounds of coal, for an average of 161 pounds of coal per 1,000 ton-miles; and 26,264,374 gross ton-miles oil-fired in all services, consuming 2,073,043,700 pounds of oil for an average of 103 pounds of pil per 1,000 ton-miles.
All railroads in 1925 averaged 159 pounds of coal per 1,000 gross ton-miles.
UP coal mines averaged on an as-received basis:
Hanna Basin sub-bituminous 10,890 BTU/lb.
Rock Springs Field bituminous 12,580 BTU/lb.
Kemmerer District bituminous 12,580 BTU/lb.
Summarizing some other coals of interest (however, I don't know the proportion of railway use of these fields, which I have picked as sort-of representative):
West Virginia, Iaeger District, 14,110 BTU/lb.
Virginia Tazewell County bituminous, non-coking, 14,860 BTU/lb.
Tennessee Grundy County bituminous, 13,300 BTU/lb.
Ohio Hocking County bituminous, 12,510 BTU/lb.
New Mexico Raton Field, 12,970 BTU/lb.
New Mexico Gallup District sub-bituminous 11,230 BTU/lb.
Illinois La Salle County bituminous, 10,990 BTU/lb.
Pennsylvania Clearfield County bituminous, 14,020 BTU/lb.
Northern Pacific did some interesting tests in early 1926 with the same locomotive, a W-3 superheated Mikado (180 psi boiler pressure) with four different coals, averaging the results of six different round trips with each coal. The BTU average on an as-received basis (11 tender samples):
Eastern coal (source not stated) 13,255 BTU/lb.
Rosebud (Montana) sub-bituminous 8,487 BTU/lb.
Red Lodge (Montana) bituminous 10,159 BTU/lb.
Roslyn (Washington) bituminous 11,161 BTU/lb.
Unfortunately test results are reported only for Red Lodge and Rosebud:
One pound of Rosebud coal evaporated 4.33 pounds of water, 136 pounds generated 1,000 gross ton-miles, and 6.77 pounds generated one draw-bar horsepower-hour.
One pound of Red Lodge coal evaporated 5.81 pounds water, 104 lbs generated 1,000 gross ton-miles, and 4.72 pounds generated one draw-bar horsepower-hour.
Back to Mr. Big Boy, we don't know which coal it was burning but I see no reason to not think it wasn't burning Rock Springs coal.
RWM
Railway Man wrote:Red Lodge (Montana) bituminous 10,159 BTU/lb.One pound of Red Lodge coal evaporated 5.81 pounds water, 104 lbs generated 1,000 gross ton-miles, and 4.72 pounds generated one draw-bar horsepower-hour.
Thanks for the info - I highlighted the Red Lodge figures as I was in Red Lodge the last week of June (my uncle has a cabin just south of there). The mine mouth is still up on the bluff on the eastern side of town, the NP branch was torn up ca 1980. Bear Creek coal was mined about 3 miles east of Red Lodge.
I assume you're thinking of what power plants often do today in order to meet emissions limits while still buying the least expensive coal possible. Coal plants rarely blended prior to the advent of emissions regulations in 1971 because coal blending costs quite a bit of money. They would never do it now if they didn't have emissions regulations to meet.
Railways never blended coal as a focused practice on a broad, long-term basis, to my knowledge. I've never seen any mention of it. I would not be suprised if someone appears with information that a specific railroad did it at a specific terminal, but I would be very surprised if the practice was not due to some unusual set of circumstances.
Coal blending did happen as an ad hoc practice, for example, when a terminal received some carloads of known poor-quality coal. In order to not have locomotives fall down with trains and be charged with train delays, the terminal would sit on the carloads of poor quality coal and slowly blend them in with the good stuff, for example for every carload of good coal it emptied into the coaling tower pit it would open one door of a car with the poor stuff. Or some variation thereof.
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