Steam loco fuel consumption

[BR60103]

After the interesting discussion about coal vs oil as fuel,
can anyone tell me how much coal or oil had to be burned to boil an amount of water?
(I expect there will be a wide range of answers, depending on everything from efficiency to boiler pressure.)

[Anonymous]

I don't know about fuel for water, but here are three statistics:
SP 9, a narrow gauge 4-6-0, burned 5 gallons of oil per mile
A Big Boy burns between 9 and 12 tons of coal per hour when running
I have heard (take with a particle of NaCl) that 3985 is 4% thermally efficient

See you around the forums,
Daniel Parks

[Overmod]

If you know the drawbar hp (for a particular speed and load), you multiply by 42.44 to get BTU/min... that figure being 3% to 6% proportionally of the total BTU/min that has to be burned to make the specified horsepower. Calculate the total BTU/min and divide by the calorific value of the fuel you're using (which will be a value in BTU for a given mass, weight, or volume of fuel -- as a starting point you can assume about 13,000BTU/lb for coal, 18,000BTU/lb oil). From this you can figure out an approximation of how much fuel the engine is likely to consume when producing that horsepower... of course, YMMV ;-}

Naturally, the assumptions here are fairly wild. You can model much more precisely with appropriate equations (cf. the first few chapters of the B&W "Steam: Its Generation and Use" book) but I'll spare the details here. In general, remember that the steam-generation efficiency of 'classical' locomotive boilers was generally pitiful even at best efficiency... but robustness and cheapness were more important than pure thermodynamic efficiency, and the packaging considerations for practical standard-gauge locomotives make many of the 'typical' Rankine-cycle regenerative economies difficult or impossible to achieve in a practical system there.

[jchnhtfd]

Dan -- your figure for the 3985 sounds about right, unfortunately. As Overmod said, locomotive boilers were and are, in general, woefully inefficient. Which didn't matter that much when fuel was cheap and plentiful, but it sure does now! Some of the later (and much more complex) high pressure compound engines had better efficiency.

[M636C]

It appears that high pressure wasn't necessarily the answer. The UP steam turbines which worked at 1500 lbf/sq in, #1 and 2 appear to have been particularly heavy on oil, and it was this, rather than unreliability, that stopped work on them. They were more powerful than contemporary diesels, but it was clear they had no future even in 1939.

Each of the Royal Australian Navy's DDG-2 class destroyers (1200 lbf sq in, 75000 SHP, 35 knots) burned more fuel per year than the fleet of six diesel electric submarines, which individually travelled further (although with much less publicity - intentionally) than the destroyers.

Peter

[jchnhtfd]

Overmod will probably kill me for over simplifying...

It isn't high pressure all by itself which is related to efficiency; it's maximum temperature. Theoretical efficiency is closely related to the maximum temperatures reached (and minimum) in any heat engine, although the relationship is different for various kinds of engines. In principle, one could do almost as well with a very high level of superheat, or multiple reheat.

Compounding can also improve efficiency considerably.

But... as the Delaware & Hudson found out, neither one improved efficiency enough to offset the maintenance, at least at the time when they were doing their experiments.

[Overmod]

No kill, you're going good. BUT...

It isn't maximum temperature per se, it's the temperature DIFFERENCE between the intake and exhaust that makes for best engine efficiency. That's one reason why compounding works well in ships, but not so well on locomotives -- ships have a very cold, virtually infinite source of condenser coolant, which makes both the temperature drop and the pressure drop at the exhaust effective.

With regard to pressure -- again, the absolute presence of high pressure is no guarantee of either high speed or high power. Indeed, the gains to be achieved at the low end of expansion are usually better than those related to high pressure, as pressure goes up or down fairly dramatically with relatively small changes in enthalpy. The center-shaft Parsons turbine on the liner Titanic operated entirely below atmospheric pressure (using the exhaust from two multiple-expansion piston engines) yet produced a considerable amount of the ship's propulsion power. And, of course, the 'official' world steam speed record is held by a locomotive with 250psi pressure, with the second nominally fastest being about 20atm or 290psi -- a key point being that both these locomotives had intentionally free exhaust and good front-end design.

High superheat is essential for longer expansion -- i.e. using a given metered charge of steam "longer" in a given stroke without getting into the condensation region (which kills the pressure efficiency with the phase change). Note that this says little or nothing about the maximum power available from the engine, or per se about the maximum speed the engine could achieve; in practice there were very firm limits on the degree of superheat that valve and cylinder lubricants could tolerate, and on the amount and metallurgical type of superheater element surface that could be cost-effectively packaged in a locomotive boiler. A certain degree of superheat is essential to effective locomotive operation at short cutoff, of course, but heat transfer from combustion gas to steam is far less effective both in terms of density and temperature differential than transfer from gas to water, e.g. in the firebox sheets and boiler. Draw your conclusions accordingly, gentlemen...

Reheat is highly valuable in steam-turbine Rankine cycles where packaging is not a major concern. I know of no historical locomotive with a reasonable service life which utilized steam reheat cost-effectively, however. The added cost, weight, and complexity usually don't give proportionate reductions in operating cost -- to say nothing of the loss of flexibility inherent in having to balance the steam flows across a wide range of potential loads and speeds!

Jamie's point about the D&H is sure true, though, and ought to be one of the great 'take-home messages' of this discussion. If a locomotive is a tool for generating the maximum number of ton-miles a month at minimum total cost, then anything that reduces the available in-service time or the number of trains actually operated may well outweigh the value of lower fuel cost... and any increased capital or maintenance cost associated with the designs comes right off the bottom line. That was even true, in the '50s, for otherwise-essential features like feedwater heaters -- on some roads, C&O being an example that comes to mind.

[dinwitty]

I forget where I saw it, I saw a comparison for N&W steam vs Diesel, and it clearly shows Diesels were more efficient, but at that time, coal was far cheaper, and it was a matter of money the N&W kept steam going longer.
As far as oil to steam use factor, boilers may well be less efficient.

Whats sad is steam loco development ceased when the diesels took over, perhaps there would be efficient steamers today.

gotta love the steamers tho

[Anonymous]

It should be very apparent steam locomotives were hungry beasts. They ran thru coal like crazy and drank endless amounts of water. Water plugs were everywhere and online fueling stations were a necessity. Fuel and water alone required vast amounts of manpower and resources. And then there was the disposal of the ashes.
The Sante Fe had great problems with water in Arizona. It was just darn hard to find. So tanker trains were used to refill the tanks between Needles and Ash Fork. In cold country the water towers were heated to keep things flowing. As Mark said before the quality of coal determined what kind of firebox your loco had. Sort of like sizing the furnace for your home. Every railroad designed their locos around the quality of coal available. And the locomotive & tender length was a compromise on turntable length, and coal/water capacity. This determined how far you could run a given locomotive on a given division before fuel and water were needed. Woe to the crew who ran low on water. Diesels eliminated a big headache!

[BR60103]

Maybe I should restate my question:
How much water can I boil with one ton of coal?
But, boy, do you guys get into it!

[arbfbe]

For an update on the steam vs diesel debate we can compare the ACE modern steamer that Ross Rowland was trying to get off the ground in the early 1980's. I was working in Alliance, NE when the Railfan article came out. We were spotting a dozen or so tank cars of diesel each day for the diesels out of the PRB but would have to spot two 100 car train of coal daily if the diesels were replaced with the ACE style steamers with state of the art technology. Is it any wonder the project died.

[feltonhill]

As a rule of thumb (and how big is the thumb??):

Light working conditions, about 50 lbs coal/sq.ft. of grate/hr - 9.0 lbs of water per lb coal, 18, 000 lbs of water per ton coal

Moderate working conditions, 100 lbs coal/sq.ft. grate/hr - 6.5 lbs water per lb coal, 13,000 lbs water per ton coal

Heavy working conditions, more than 170 lbs coal/sq.ft. grate/hr - 5.5 lbs water per lb coal, 11,000 lbs water per ton coal

This is based on data from a wide range of "modern" steam locomotives (250-300 psi., 2-8-4 to 2-6-6-6.., wherever I could find adequate, consistent info). The figures look more scientific than they really are. There are large deviations from these averages, and the data range is from about 5 to 11 lbs of water/lb coal, "depending......." But since you asked, this is the best I have. Ralph Johnson's book, The Steam Locomotive, also has information on this subject, although it has to be assembled from several different pages of tables and graphs.