Big Boy -- big coal consumption

[Paul Milenkovic]

As a young railfan, I was all about Diesels because they were modern and efficient and steam was of little interest to me, but in my older years, I don't think I am a complete steam fanatic, but I am intrigued by the operation of steam and the what-could-have-been.

On the topic of steam and thermal and operational efficiency, I got my poppa's ancient Keenan and Keyes, Thermodynamic Properties of Steam out to crunch some numbers on, what else, the Big Boy. I have used those "steam tables" to do calculations on dehumidifiers and air conditioners, but it just dawned on me: there is a lot of information relative to steam locomotive thermal efficiency in there.

Based on averages of several sources, I have come up with the numbers that the Big Boy consumed 28 tons of coal and 24,000 gallons of water running at a constant 40 MPH for about 1 hour climbing Sherman Hill. Go ahead and nitpick my numbers, but I believe that I am at least in the general ball park. It had 68" drivers, 24" bore by 32" stroke cylinders in rounded numbers, and if I understand double-acting, double-engined, simple-expansion steam correctly, there are 8 cylinder strokes per revolutions of the 68" wheels 67 cubic feet of swept cylinder volume for every 17.8 linear feet of travel.

OK. 28 tons of coal in an hour. I heard they were using Western coal, so I will assume 9000 BTU/lb or a total of 504 million BTU's to make the hill climb. 24,000 gallons of water in an hour. Assuming 300 PSI, 800 deg F superheat, and making some assumptions about some waste-heat recovery on feedwater heat, I get about 1300 BTU to evaporate a pound of water or a total of 250 million BTU consumed in steam generation. Whoa! They were operating their combustion system at only 50 percent efficiency? Actually, doesn't sound that crazy -- during the hill climb, they may have been drafting like crazy to raise that much steam, and a good measure of the coal could be coming out the stack as cinders (the thick steam and cinders exhaust stack beloved by railfans). Since the Big Boy emphasis was on power instead of thermal efficiency, they may have been burning coal well in excess of what results in good heat transfer in the already oversized firebox and boiler.

OK, the steam tables tell me that 300 PSI 800 deg F superheated steam has a specific volume of 2.4 ft^3/lb. Travelling 40 miles, the cylinders are exhausting 795,000 cubic feet of steam. The boiler is putting out 461,000 cubic feet of steam. I know both sets of numbers because people tell me how much water the thing consumed, and they tell me the size of the cylinders and the diameter of the drivers. This means they are operating at 42 percent cutoff.

At 42 percent cutoff, they are exhausting superheated steam at 580 deg F, 142 PSI. You know, I could be off on the superheat, because why would they apply that much superheat only to discharge superheated steam. But if you have better numbers, I will rerun all my calculations for you. How do I know the exhaust pressure and temperature? The cutoff tells me the specific volume of the steam exhaust, and the entropy of the exhaust is the same as the entropy of the inlet if the piston is a "reversible" expander. Searching for the right combination of entropy and specific volume, I came up with that number.

How much HP is generated with that much steam and at that inlet and exhaust conditions? The steam tables give three numbers: specific volume, entropy (stays constant in an ideal piston), and the enthalpy, the amount of heat available to do work in the piston. The enthalpy difference between inlet and output is 94 BTU/pound. After all the conversion factors, that works out to about 7100 HP. My sources said the Big Boy peaked at 6000 HP -- maybe the 1100 HP difference is effect of my simplifications (didn't account for steam-circuit throttling) or perhaps the HP loss of the pistons, valves, and rods. At least I am in the neighborhood of a reasonable answer.

That works out to about 6 percent thermal efficiency of converting steam to mechanical work or about 3 percent thermal efficiency of converting coal into mechanical work. How can anyone base technology based on 3 percent thermal efficiency? Well actually we do, only we drive SUV's in city traffic instead of operate Big Boys.

But lets talk about that 3 percent efficiency. If you run a steam cycle with 300 PSI, 800 deg F superheated steam inlet, 30 PSI absolute (15 PSI gauge) exhaust, you will operate at 85 percent cutoff and get about 20 percent thermal efficiency. Of course this is not taking into account the thermal efficiency of combustion, heat loss, steam inlet throttling, mechanical losses, etc, but it gives you an upper bound and an explanation why Champelon claimed 10 percent and Porta and ACE thought they could do yet better. But why the 3 percent? This analysis suggests that 1) they were throughing half their energy away in their combustion and flue system, and 2) they were operating at very low cutoffs to climb the hill. Between the two factors, the 3 percent efficiency is right in line.

But what about the 3 percent efficiency? The story told is that after charging up the hill for one hour at 40 MPH, they had to stop and refill the tender with coal and water. Someone could fill me in if they stopped midway -- you don't want to be just shy of the crest of the hill and run out of either coal or water. So how much time did they save charging Sherman Hill at 40 MPH when they could have taken it slower, used less coal and water, and not had to stop?

I am thinking they could boost combustion efficiency by 50 percent and perhaps double their steam efficiency with expansive working -- they could boost efficiency from 3 percent to 9 percent. And taking the hill a bit slower, they would need maybe one quarter the steam generation rate and wouldn't need the 4-wheel trailing truck. I am starting to think, hmmm, a 2-10-10-2 Mallet like the kind NW operated -- you know, with compound expansion so that you could take hills while generating a lot of tractive effort.

Perhaps they wouldn't have to do anything quite as extreme as the NW 2-10-10-2's with their slow speeds and massive front cylinders. But maybe if they added an extra hour to climbing Sherman Hill, they could make that up with many fewer coal and water stops, not to mention saving money on that much coal.

[feltonhill]

Whew, you're really getting into this stuff!

If your're referring to N&W, they had 2-8-8-2s. The only 2-10-10-2s in the east were on the Virginian.

I assume you have Kratville's book on BB. There are coal and water consumption figures there, IIRC. Sound similar to yours.

The 3% figure is probably only a little low, because the max I've heard is about 4% for a simple loco. According to Wardale's book, The Red Devil and Other Tales..., C&O 614 was only getting maybe 3% during the ACE tests, without benefit of feedwater heater. A Y6 could do considerably better than that, but I don't have the figures handy and I don't want to guess and spread misinformation. Have to get back on that.

Are you sure about the 40 mph speed up Sherman Hill? I thought that it was closer to 25-30 mph on most uphill runs. They could let 'em ramble on the downhill side.

I'm gonna have to re-read your post about 5-6 more times. I've worked on the same stuff for years, but not from the same direction. Interesting stuff.

[dldance]

great analysis - but Sherman Hill is a bottleneck now and was then. One way of reducing the impact of a capacity bottleneck is to increase velocity. From UP's response - it looks to me like coal was cheaper than more tracks and right-of-way.

dd

[Anonymous]

Paul: Wow. You are serious about this. Couple comments.

1. Your BTU figure for "western coal" is actually "Powder River Basin coal," which was used only by Northern Pacific. It's a common mistake these days to confuse one for the other, I know, but it makes me cringe. Try 11,000 BTU and you'll be in the ballpark for the Hanna, Rock Springs, or Kemmerer coal used by UP.

2. I deeply doubt the average freight train ran up Sherman Hill at 40 mph! Try 15-25 mph. Railroads generate cash by generating maximum GTM's at minimum expense, and high speeds uphill are about the biggest blunder they can make. If you go back and look at Kindig's and Perry's notes on their photos, they give speeds in the 15-25 mph range uphill. And if you poke around in the Ehrenberger/Gschwind and Kratville books, you'll find more evidence pointing unerringly to slow speeds uphill. Not fast.

OS

[Paul Milenkovic]

One source of the 40 MPH is http://www.nps.gov/stea/bigboy.htm,
but a closer read suggests that may be average, not over the ruling grade.

If you believe that they used up 24,000 gallons of water in an hour, they had to be doing at least in the high 20's. Otherwise you wouldn't go through that much steam, even at no cutoff.

If you are burning 28 tons of coal to evaporate 24,000 gallons of water, your combustion efficiency has to be low, and if you are burning that much coal climbing a big hill at slower speeds and lower cutoffs than I had indicated, you are not only burning a lot of coal to raise steam, you are going through a lot of steam.

I am wondering if simple-expansion steam is pretty wasteful in climbing mountains. You can operate simple-expansion at high cutoffs, but not when you are climbing a big hill.

[Anonymous]

That NPS claim of 3,800 tons at 40 mph "across the mountains" is preposterous. The physics alone would require at least TWO Big Boys, maybe three or four.

Ruling grade on the Wyoming Division in either direction is 0.82% (not HELPER grade).
Grade resistance for 3800 tons on 0.82% is 62,320 lbs.
Rolling resistance using LS&MS empirical tests would be 16 lbs. per ton at 40 mph, or 60,800 lbs.
Total tractive effort required uphill at 40 mph would be 123,120 lbs.

HP = (speed x tractive effort)/375, or (40 x 123,120)/375, or 13,132 hp!

At 20 mph, rolling resistance would be 7 lbs./ton. That gives a HP requirement up the 0.82% grade of 4,742 HP. Now we're in the ballpark. I think an average speed of 15 mph between Evanston and Hermosa Tunnel either way, including start and stop but discounting all meets and temporary speed restrictions, is more likely. Add a helper westward from Cheyenne and eastward from Ogden.

OS

[feltonhill]

If Big Boy evaporated 24,000 gallons of water an hour, I believe that's approximately 200,000 lbs/hr (8.33 lbs/gallon?). Sounds a little high. The PRR Q2 has the highest recorded total evaporation I've heard of (tank water + feedwater heater condensate), about 137,000 lbs per hour. This was sufficient to produce about 7,900 indicated HP at 57.4 mph. Unless the Big Boy was unusually inefficient, and I don't believe it was, I'd say the tank water consumption would be in the 120,000-125,000 lbs per hour neighborhood, or 14,000-15,000 gallons per hour. These figures are probably a bit conservative (high), and it may have done better than that because my setup tends to overstate locomotive resistance unless I make some adjustments. That amount of steam would be sufficient to produce 6,100 dbhp at 30-40 mph, which is what BB actually achieved based on Kratville's book.

You're probably on the right track (pun intended) but some of your sources may be nonspecific or inconsistent.

The NPS figures were not location-specific, and were actually somewhat misleading because of the wording. Too much gee-whiz in the blurb.

[tatans]

Geez, this stuff is facinating, not quite as simple as a kettle boiling on the stove eh? with the power of steam, could it have been saved with the advent of modern insulations? the massive loss of heat was incredible but coal was cheap and today it's looking pretty good compared to oil. Gee, and the fireman only had to shovel 28 tons of coal an hour, how big was his shovel??????

[Anonymous]

QUOTE: Originally posted by tatans

Gee, and the fireman only had to shovel 28 tons of coal an hour, how big was his shovel??????

it's called auto feed, the fireman was just there to make sure it all worked right.

[oltmannd]

Just as a comparison, an SD60's efficiency (duty cycle weighted) making juice for the TMs is about 34%. It costs about another 15% or so getting that power thru to the coupler, so overall thermal efficiency is about 29% - which is about as high as some older coal fired power plants.

For a GP38-2, its 22% at the coupler and I'd imagine the early Fs and GPs were a few % pts worse than this.

It's easy to see, just from this, why RR dieselized.

[Anonymous]

And to say nothing of ridding the property of coaling towers, water tanks, water treatment plants, ash pits, most of the shops, the armies of men who staffed them, and doubling or tripling the tonnage hauled by one train crew.

The question is not whether steam was scrapped too early: it's whether steam was scrapped too late!

OS

[timz]

"I have come up with the numbers that the Big Boy consumed 28 tons of coal and 24,000 gallons of water running at a constant 40 MPH for about 1 hour climbing Sherman Hill."

I haven't been able to find that NPS claim, but I guess you're not actually talking about Sherman Hill? Don't imagine that UP freights (hauled by any engines) typically climbed long grades at 40 mph, on Sherman or elsewhere.

"If you run a steam cycle with 300 PSI, 800 deg F superheated steam inlet, 30 PSI absolute (15 PSI gauge) exhaust, you will operate at 85 percent cutoff and get about 20 percent thermal efficiency [based on the heat in the steam]."

When you say 85% cutoff, you mean steam admission is cut off when the piston stroke is 15% complete?

[Anonymous]

QUOTE: Originally posted by SteamerFan

QUOTE: Originally posted by tatans

Gee, and the fireman only had to shovel 28 tons of coal an hour, how big was his shovel??????

it's called auto feed, the fireman was just there to make sure it all worked right.

Hard to believe the job of the fireman would only get easier. [;)]

[tatans]

Am I right? the engine burned 28 tons of coal an hour, AND 120 TONS of water??? (24,000 gals, x 10 lbs /gal. =240,000 lbs. divided by 2000=120 tons) is this correct??? need some help on this one.

[timz]

He presumably didn't mean Imperial gallons, so 24000 gallons equals 100 tons. So he didn't mean an hour, either.

[Paul Milenkovic]

About "cutoff." I am calling "85 percent cutoff" that steam enters the cylinder for 15 percent of the stroke and expands the remaining 85 percent of the way (the steam is cut off for most of the stroke). I am told this is about the highest expansion you can get typically with simple-expansion and conventional valve gear, but poppet-valve or other systems could do a little better (maybe 90 percent).

About the steam-table calculations. From the superheat tables, steam at 300 PSI (absolute, 285 PSI gauge), 800 deg F, has a specific volume v of 2.4 cu ft/lb steam, enthalpy h of 1420 BTU/lb, and entropy s of 1.7165. The specific volume is how much space is taken up by a pound of steam; as it expands, the specific volume gets bigger, and the ratio of initial to final specific volume quantifies expansive working and the valve cutoff ratio. The enthalpy is the energy in the steam available for doing work. The entropy is a number that stays constant in a reversible-adiabatic process (i.e. a piston expander).

I assume that an efficient steam expander takes the high pressure steam at high cutoff and exhausts at 30 PSI (15 PSI gauge) back pressure -- I take this as an upper bound of what can be achieved with really good steam-circuit streamlining and a really good exhaust ejector. I go to the 30 PSI superheat steam table entry and look for an entry where the entropy is close to 1.7165 -- I find that 30 PSI, 270 deg F gives v = 14.2, h = 1174, s = 1.7095.

The enthalpy difference 1420-1174 = 246 BTU/lb is the energy converted to work by the piston, if there was no heat added or lost and no piston friction or other losses. If we assume a heat-recovery feedwater arrangement where near boiling, atmospheric-pressure water is pumped into the boiler at 180 BTU/lb, the enthalpy input of the boiler is 1420-180 = 1240 BTU/lb, and the thermal efficiency is 256/1240 = 19.8 percent. Oh, and the ratio of initial or final steam volume is 2.4/14.2 = .169 so the cutoff is 1-.169 or 83 percent.

This type of calculation starts to break down at low cutoffs where you have "blow down" of high exhaust steam pressure into a somewhat lower back pressure and get irreversible expansion -- at 0 cutoff, this calculation would give zero power and 0 efficiency because it assumes cylinder back pressure the same as steam inlet. The advantage of "enthalpy balance" (method best for steam turbines where you have constant pressure inlet and outlet) is the easy use of tables rather than having to do integrations on expansion curves.

Back to the Big Boy! I guess railfan Web numbers need to be run through a filter, but if the Big Boy really used up a tender-load of coal and water in one hour during a hard pull, and if it was going 20 MPH, does that mean they had fuel and water stops every 20 miles? I guess I got to look for Kratville's book for better numbers.

[adrianspeeder]

Now this is a cool thread. Just straight up engineering back in the day on how to get things done.

In a tug of war could the big boy whoop up on a dash nine or something?

Adrianspeeder

[timz]

"I am calling "85 percent cutoff" that steam enters the cylinder for 15 percent of the stroke and expands the remaining 85 percent of the way"

Just remember when you say "85% cutoff" to railfans we'll think you mean steam being admitted for 85% of the stroke. I certainly thought that was the usual meaning among railroaders.

[tatans]

Timz: You're right, I used Imperial gallons, his quote was "it burned 28 tons of coal & 24,000 gals.(American gallons) water at 40 mph for about 1 hour.(up Sherman Hill) This is an enormous amount of fuel & water, and I knew more water was used than coal, but never knew just how much, also 40 mph seems pretty fast for a giant load of cars ripping up a big hill. (now I know how much a U.S. gallon weighs-thanks) It would seem the coal industry did nothing but supply coal for steam engines at this rate.
Does anyone out there think there is a scientific organization that is still exploring or improving steam power as a method of power????

[tree68]

It's fascinating to realize how much water those beasts really used. You never see the water - it's inside the tank. The coal you can see, so you know it's disappearing into the firebox.

[Train Guy 3]

QUOTE: Originally posted by SteamerFan

QUOTE: Originally posted by tatans

Gee, and the fireman only had to shovel 28 tons of coal an hour, how big was his shovel??????

it's called auto feed, the fireman was just there to make sure it all worked right.

When the USRA was around, wasn't there a requirement for all steam locomotives with a firebox over 80 sq. ft. to have an automatic stoker?

[Anonymous]

Really interesting thread!

25 tons/hour uphill is a number I recall reading. Speeds varied quite a bit in videos I have of operation on the hill. Manifest trains ran quite a bit faster, drags as slow as 15 mph. Double-heading was also common, sometimes with Gas Turbines in later years.

I recently read about some tests NYC did on 2 Niagra's, one with 75" drivers, the other with 79". As expected the smaller drivers moved the peak HP to lower speeds, however the larger drivered loco pullled everything NYC wanted it to and did so on less coal and water. Some have suggested that super-steam locos should have been built with smaller drivers, but I wonder what happens to the Big Boy's steam, water, and fuel requirements if you redid the calculations with 60" drivers instead of 68".

[feltonhill]

It looks like we're getting glazed by large numbers. If BB did burn 28 tons of coal per hour, it would have been a hugely uneconomical endeavor.

To give some perspective to this figure, it indicates that the firing rate would be 368 lbs of coal per sq.ft. of grate/hour. This is would be appropriate only for a full-flog, let's-see-what-it-can-do type of test, if that. Most locomotive tests I've worked with do not get over a firing rate of 250 lbs/SF/hr. at maximum output. Over the road is usually under 150 lbs/SF/hr. The lower the better, because combustion and heat transfer take time. The law of diminishing returns is alive and well when boilers are pushed too far up the evaporation and firing scale. You may get more steam (up to the grate limit or evaporation limit), but you will pay for it!

Kratville's book indicates that sustained over the road performance between Ogden, Echo and Evanston was more like 16,000-19,000 lbs or coal/hr and 70,000-77,000 lbs tank water/hr. How accurate they are, or under what exact conditions they were derived, I don't know. Nonetheless, this is a far cry from the figures posted on the NPS site and other sources that tout 28 tons/one tank of water per hour. I'm not saying it never happened (never say never), but it sounds like a theoretical value. BB could generate the published 6,100-6,200 DBHP figure (depends on how you read the graph) without going to extreme measures.

[nanaimo73]

Paul-
Thank you for starting this thread.
I hope that during 2006 you find the time to rejoin the forum.

Overmod, RLHainey, I would like to see you guys come back as well.

[Anonymous]

Good thread - glad to see you guys getting into it.

If you want figures for N&W's Y6, Louis Newton has them in volume 3 of his Rails Remembered. He was present on dynamometer tests working eastbound out of Williamson to Bluefield. This includes 25+MPH running with 7200 tons up the Tug River from Williamson to Farm, where the train was halved for the trip up Elkhorn Mountain's 1.4%, where dragging speeds in the 15MPH range were the norm.

In this volume Newton also has data for the three trips between Williamson and Portsmouth with Class A 2-6-6-4 1239 and trains grossing 16,000 tons made in 1952 after the F7s were tested.

Newton makes no calculations or efficiency claims, but his data are precise and very, very interesting. On the dynamometer car runs, coal was weighed in the tenders during the runs (in 200-pound increments) and the water was measured.

All the runs were with the auxiliary tenders, giving both classes of engines 40,000 gallons of water to start out with (22,000 gallons main tender, 18,000 gallons in the auxiliary tender), and no water or coal stops had to be made en route with either the A or the Y6. I don't have the book in front of me, but as I recall, the Y6 used 35,000 gallons of water on the Williamson-Bluefield run. So if you want to get the book, his data will certainly give you some fresh stuff to chew on.

Old Timer

[Anonymous]

Asketh Train Guy 3:

"When the USRA was around, wasn't there a requirement for all steam locomotives with a firebox over 80 sq. ft. to have an automatic stoker?"

I don't think it was a grate area requirement; it was probably a combination of grate area and weight on drivers. The only USRA designs available without stoker were the two switchers and the light Pacific, which was equipped with a coal pusher. The light Mikes had a grate area of 66+ sq. ft. and they were stoker-fired (the Southern got duplicates in 1923 that were shovel fired). Everything else (heavy Pacifics, both Mountains, heavy Mikado, both Santa Fes), was in the 70-80 sq. ft. range except the Mallets which were, of course, larger.

Old Timer

[beaulieu]

The stoker requirement was from the ICC in the pre-FRA days. Lenoir Loree of the D&H was miffed by this regulation.

[switch7frg]

Hi,Oldtimer;; For years I have been reading about N&W "s Super steamers of 1952.Never could figure out why their steam figures were so kept secret. However, your post has revealed a very good insight on the subject of the A & Y6B. Thank you very much. Switch8frg.

[espeefoamer]

QUOTE: Originally posted by O.S.

And to say nothing of ridding the property of coaling towers, water tanks, water treatment plants, ash pits, most of the shops, the armies of men who staffed them, and doubling or tripling the tonnage hauled by one train crew.

The question is not whether steam was scrapped too early: it's whether steam was scrapped too late!

OS

Dieselization would have been completed around 5-8 years sooner if WW 2 had not intervened.

[Kurn]

The scary part is I have absolutely no backround or training in this type of physics,yet I understand it.
All I know is how a steam loco works.Fascinating stuff.