Steam: where does all the unused energy actually go?

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70014IronDuke

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Otherwise known as the thermodynamics of a conventional steam locomotive

Having lunch with friends yesterday, the lady-wife commented that it was "such a shame" that steam could not have survived, whereupon the man-hub asked if, with the very low price of oil on the market, was it possible that there might be an economic case for a steam comeback.

My main line of argument against was that, apart from the dirty jobs involved that few would take on (and I never mentioned the environmental issues), the steam engine was inherently of such low efficiency that even with oil prices at rock bottom, it would never be viable again, at least in the world as we know it. (after all, low oil prices feed into low diesel and low electricity generation prices too.)

I seem to remember reading that steam locomotives at best were about 10% efficient, even theoretically. But that got me thinking: where does all the 90% of energy actually end up?

Obviously, an awful lot of heat leaves the firebox and starts flying down the tubes - but it comes out still pretty hot over the blast-pipe, and then up the chimney.

Does anyone know the typical temperatures on entry into the tubes from a hard working locomotive? And the temperatures at the 'cool' end of the boiler, as the gases enter the smokebox?

I have little idea, but I assume the temperatures in the firebox would be in the region of 200-300C - just guessing. I presume these temperature differences could be measured to some degree in the days of working steam - and this could give a picture of the efficiency of the thermal transfer process in the boiler. But I don't know for sure.

Then, of course, the exhaust steam from the cylinders itself contains energy, whis is expired going up the blast-pipe and drawing flames through the boiler. Plus you have heat losses from the boiler and firebox, plus you have some coal that flies unburned straight through the tubes, out the chimney and into lineside vegetation, whereupon it prmptly starts a fire :)

Then of course, you have the mechanical losses from friction in the piston/cylinder and motion/bearings.

My personal bet would be that the worst losses are heat and unburned coal that flies straight up the chimney - but does anyone have a proper scientific estimation of all the losses?
 
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Peter Mugridge

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I seem to remember reading that steam locomotives at best were about 10% efficient, even theoretically. But that got me thinking: where does all the 90% of energy actually end up?

Obviously, an awful lot of heat leaves the firebox and starts flying down the tubes - but it comes out still pretty hot over the blast-pipe, and then up the chimney.


I think you've basically answered your own question there; most of the unused energy does indeed go out of the smokey end of the thing. There's quite a bit lost through friction within the working parts and also radiated directly out of the locomotive - if you've ever stood next to even the best insulated steam locomotive, you'll feel an awful lot of warmth coming off it.
 

DerekC

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A lot of energy is wasted in the exhaust steam which could be condensed and the energy recycled in the feed water. The trouble is that the kit to do that is big and heavy (as in a power station). Higher pressure and compounding would also help. There were lots of trials of all these things back in the steam day but none of them really took off (at least, not in the UK).
 

John Webb

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The temperature in the gases in the fire box is up to 3,000 deg F and they leave the chimney between 700-750 deg F ("Handbook for Railway Steam Locomotive Enginemen" - BR 1957, reprinted by Ian Allan 1998 onwards.)
This book points out that 4/5ths of the air is Nitrogen which plays no part in the combustion process, yet has to be heated up in the firebox. The temperature at which the Nitrogen and other gases leave the chimney represents a major loss of heat. It was this heat which the "Crosti" boiler tried to reuse to preheat water before it was fed to the main boiler, but it recovered little more heat for a great deal more complexity in operation and maintenance, so it was eventually dropped.
 

thejuggler

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I've also never come across a quiet steam loco. Energy for that noise needs generating.
 

broadgage

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Decades ago our economy was largely fuelled by domestically produced and cheap coal.
Efficiency was often of lesser importance than capital costs and fuel availability. Coal burning steamers were the best choice under the then prevailing conditions.

The only realistic alternatives were diesel and electric power.

Diesel locomotives were much more expensive to build, and required considerable technical expertise to service and maintain.
Prior to north sea oil, oil was an expensive and imported fuel. The government opposed excessive reliance on oil both to conserve foreign currency and to prevent enemies cutting off our fuel supplies.
I doubt that we could have won the last war with a largely diesel railway and the enemy doing their best to sink tanker ships (which are inherently very vulnerable to enemy action) Providing enough oil fuel for basic road transport was bad enough.

Electric traction involved huge capital expense and of course an ample supply of electricity. Before the days of the national grid, providing traction current was a huge problem outside of major cities.
Back in the old days, electricity was also expensive. Power stations were only about 20 to 25% efficient.
Losses in transmission to and within the railway and conversion to DC might reduce the overall efficiency to 15% or less. Not a great improvement over burning the coal in a steamer rather in a power station.
 

70014IronDuke

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I've also never come across a quiet steam loco. Energy for that noise needs generating.

ITYWF energy in sound is totally insignificant. you could shout at a glass of water all your life and not notice the difference in temperature.

(I stress the 'sound' part - obviously the chuff-chuff represents a lot more in energy in terms of energy in the exhaust steam.)

In any case, if it were significant, the energy-loss from some diesels would be more significant.
--- old post above --- --- new post below ---
I think you've basically answered your own question there; most of the unused energy does indeed go out of the smokey end of the thing.

No, I haven't answered my own question, because I'd like to know scientific, measured estimates, not just my or anyone's 'thoughts' alone. That's what I'm seeking here. In other words, can you back up your "most of the unused energy does indeed go out of the smokey end of the thing"?

... and also radiated directly out of the locomotive - if you've ever stood next to even the best insulated steam locomotive, you'll feel an awful lot of warmth coming off it.

Funny thing, I have, and no I haven't. Perhaps in hot weather, a bit yes, but I haven't noticed really feeling the radiated heat from a steam loco - except on the footplate, and yes, with the firebox doors open.
 
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edwin_m

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ITYWF energy in sound is totally insignificant. you could shout at a glass of water all your life and not notice the difference in temperature.

(I stress the 'sound' part - obviously the chuff-chuff represents a lot more in energy in terms of energy in the exhaust steam.)

In any case, if it were significant, the energy-loss from some diesels would be more significant.

Agreed.

As a comparison the sound system at a concert hall will typically consume hundreds of watts of electrical power, and according to a quick Google only about 2% of that actually comes out as sound with the rest turned into heat. So the sound from a locomotive is also likely to be only a few tens of watts, whereas its total mechanical power output can be a million watts or more (and the energy content of the coal fed in much more again).
 

70014IronDuke

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The temperature in the gases in the fire box is up to 3,000 deg F and they leave the chimney between 700-750 deg F ("Handbook for Railway Steam Locomotive Enginemen" - BR 1957, reprinted by Ian Allan 1998 onwards.)

Ah, now we are getting somewhere. That's the old 'black book' I suppose. (I had one somewhere, many years ago.) Thanks.

This book points out that 4/5ths of the air is Nitrogen which plays no part in the combustion process, yet has to be heated up in the firebox. The temperature at which the Nitrogen and other gases leave the chimney represents a major loss of heat.

Does it give any numbers? Of course, 4/5 of the air pumped into a diesel cylinder also contains Nitrogen, and it ditto plays no part in the combustion process before it's flung out in the exhaust. That's nature for yer.

It was this heat which the "Crosti" boiler tried to reuse to preheat water before it was fed to the main boiler, but it recovered little more heat for a great deal more complexity in operation and maintenance, so it was eventually dropped.

Yes, sure, I am aware of that. But while interesting, I've seen loads of writings on various ways of trying to reduce the steam engine's energy thermal losses (including condensing systems) - what I have not seen is realistic numbers that show the make-up of these losses. That is the point of my starting this thread.
 

coppercapped

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The major heat loss is in the steam which is exhausted, and the reason can be explained by basic physics.

When a material changes phase, that is when a solid turns into a liquid or a liquid changes into a gas, it is necessary to add energy. This energy is considerable. Think about water being heated in a kettle - it absorbs energy from the fire or electric heating element until the water boils, that is the water is converted to steam. Water requires 1 calorie of heat energy to heat 1 gram of it through 1 deg C - this is called the specific heat of water. This means that it takes 80,000 calories to heat 1kg of water from 20 deg C to 100 deg C at which temperature, if the kettle is at sea level, it starts to boil.

'Calories' are no longer used in scientific calculations, the preferred System International (SI) unit is the Joule (named after the Manchester brewer of the same name). 4.18 Joules is the same as 1 calorie, so the answer of calculation above can be restated as 336,000 Joules.

Continuing with the kettle, even if the heating element is left on the water temperature will not exceed 100 deg C - what happens is that the water is being converted to steam at the same temperature as can be seen in the bubbles which form. Experiment shows that 2,260,000 Joules are required to convert 1 kg of water at its boiling-point to steam at the same temperature. This is known as the specific latent heat of steam whereby the word "latent" in this context means hidden or concealed.

These figures show that it needs 2,260,000 divided by 336,000 times as much energy to turn water at 100 deg C to steam at 100 deg C than it does to heat the water from room temperature to 100 deg C. That is 6.7 times as much energy.

As the steam locomotive is an open-cycle machine all this latent heat energy is lost out of the chimney. Closed cycle machines, such as ships' engines or power stations can recover this energy and put it back into the feed water - big power stations can achieve about 40% energy efficiency - a big difference to that possible with locomotives.

(I know that a steam locomotive boiler is pressurised, so the boiling point is higher than I used in my example, but the same effect holds true).

BR did a lot of testing of steam locomotives in its early days, both on the road and in the Rugby test plant. Many of the results were published in Bulletins which the public could buy. As rule of thumb the boilers, at steam rates in the middle of the boiler's design range, were about 80% efficient. That is about 80% of the potential heat energy in the coal was converted to energy in the steam. The boiler efficiency dropped off at low steam rates, the 'stand-by' losses, and also fell off at high steam rates as when the engine was working hard the draught tended to disrupt the fire. This was one of the reasons there were so many different sized locomotives - the designers tried to provide a grate size optimised for the duty.
 
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John Webb

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Ah, now we are getting somewhere. That's the old 'black book' I suppose. (I had one somewhere, many years ago.) Thanks.
Does it give any numbers? Of course, 4/5 of the air pumped into a diesel cylinder also contains Nitrogen, and it ditto plays no part in the combustion process before it's flung out in the exhaust. That's nature for yer.

Yes, this is the old 'black book', although I have one of the reprints.

The book makes no attempt to completely detail all the losses; it concentrates on the correct way to fire a loco to maximise efficiency, but nothing else. I'm sure that the Research People at Derby in BR days and others before them produced detailed calculations. Possibly a trawl through the 'Search Engine' at the NRM might turn up something?
 

70014IronDuke

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The major heat loss is in the steam which is exhausted, and the reason can be explained by basic physics.

When a material changes phase, that is when a solid turns into a liquid or a liquid changes into a gas, it is necessary to add energy. This energy is considerable. Think about water being heated in a kettle - it absorbs energy from the fire or electric heating element until the water boils, that is the water is converted to steam. Water requires 1 calorie of heat energy to heat 1 gram of it through 1 deg C - this is called the specific heat of water. This means that it takes 80,000 calories to heat 1kg of water from 20 deg C to 100 deg C at which temperature, if the kettle is at sea level, it starts to boil.

'Calories' are no longer used in scientific calculations, the preferred System International (SI) unit is the Joule (named after the Manchester brewer of the same name). 4.18 Joules is the same as 1 calorie, so the answer of calculation above can be restated as 336,000 Joules.

Continuing with the kettle, even if the heating element is left on the water temperature will not exceed 100 deg C - what happens is that the water is being converted to steam at the same temperature as can be seen in the bubbles which form. Experiment shows that 2,260,000 Joules are required to convert 1 kg of water at its boiling-point to steam at the same temperature. This is known as the specific latent heat of steam whereby the word "latent" in this context means hidden or concealed.

These figures show that it needs 2,260,000 divided by 336,000 times as much energy to turn water at 100 deg C to steam at 100 deg C than it does to heat the water from room temperature to 100 deg C. That is 6.7 times as much energy......

(Apologies for late reply - just too busy to think about this.) Yes, good point on the latent heat of steam. But of course, if the steam itself is ejected at a high temperature (higher than 100C) it will also contain wasted heat over and above the latent heat loss (and the water-heat loss).
That will depend on the specific heat of steam and the exhaust temperature.
--- old post above --- --- new post below ---

Yes indeed. Thank you. Needs time and a clear head to get into this.
 

Requeststop

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(Apologies for late reply - just too busy to think about this.) Yes, good point on the latent heat of steam. But of course, if the steam itself is ejected at a high temperature (higher than 100C) it will also contain wasted heat over and above the latent heat loss (and the water-heat loss).
That will depend on the specific heat of steam and the exhaust temperature.
--- old post above --- --- new post below ---


Yes indeed. Thank you. Needs time and a clear head to get into this.

The law of conservation of energy, a fundamental concept of physics, states that the total amount of energy remains constant in an isolated system. It implies that energy can neither be created nor destroyed, but can be change from one form to another. So it is possible that each form of energy can be changed from one to another. These forms can be:
Chemical Energy,
Light Energy,
Electrical Energy,
Mechanical Energy,
Mechanical Wave,
Nuclear Energy,
Potential Energy,
Kinetic Energy,
Thermal Energy,
Gravitational Energy,
Magnetic Energy,
Radiant Energy,
Energy of Ionisation
Elastic Energy,
Energy of Rest
and finally and most importantly
Heat Energy

Heat Energy is where most energy is lost as it passed into the environment. As more and more energy is used then a lot of energy is lost into the environment as so we get warmer and warmer. They say it's about carbon usage, well yes, but we are getting warmer because of energy use.

For your steam engine, you will understand where the unused energy goes to.
 

edwin_m

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Heat Energy is where most energy is lost as it passed into the environment. As more and more energy is used then a lot of energy is lost into the environment as so we get warmer and warmer. They say it's about carbon usage, well yes, but we are getting warmer because of energy use.


This is indeed true but not really relevant.

The amount of heat generated by human activity is tiny compared with the amount that arrives on the planet by way of sunlight. Without the effect of CO2, the fractional increase in temperature from waste heat would be self-correcting because a bit more heat would be radiated out into space.

However CO2 in the atmosphere interferes with this re-radiation and traps some of the heat, whether from the sun or from other sources. Hence the colloquial term "greenhouse effect".
 
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