Off the Rails - Train Stuff

[Jordangbr]

It seems to be in vogue to rebuild lost locomotives at the moment with the Baby Deltic coming along as well as a proposed recreation of HS4000 Kestrel Britain’s most powerful diesel locomotive ever built.

[thunderer]

I beleive it is more to do with "where others have led" combined with a desire to fill the gaps of lost motive power that has led to the "boom" of new-build or re-creation builds in the last few years.

Heck this country led the way, to begin with, with railways so we shold be intertested in preserving the historical stock and motive power. Replacing the irreplaceable at least with something that, in the case of 10000, at least outwardly, represents lost motive power that should have been preserved is an attempt to address that situation. The skills are out there and as the old adage goes "use it or lose it".

My own multi-part question is this: When the enthusiasts of previous and current generations are no longer here, who will carry on the preservation legacy within the UK? how much longer can it last in the face of the political pressure to become as carbon neutral as possible and give-up reliance on fossil or pollutive fuels.

[Renegadenemo]

I watched some guys milling flour using a water wheel a few months back. There'll always be some nutter who wants to mess with old machinery no matter what it is.

[Filtertron]

Trains, eh?

Clap your peepers on that:

[captain sparkle]

How about "the biggest steam train in the world" on the rails again - https://www.youtube.com/watch?v=NuJPVhtQFeQ

[Renegadenemo]

Okay - question for the steam train enthusiasts.

My understanding is that you fill your loco up with coal and water, light a fire and once everything is hot and pressurised away you go. The steam passes through several expansion stages then is blown up the spout to make the fire draw. Is that correct?

I never got into trains for reasons even I don't understand but I have always been interested in marine steam engines having spent many, many hours messing about with them on the bottom of the sea.

What you do with your ship is much the same except that you don't throw the water away. Once you've finished extracting a chunk of the energy you put into it by burning coal what you're left with is pretty hot steam so that goes through a condenser to turn it back into water and from there it goes for another spin through the boiler and thence the cylinders. The point being that you only had to burn enough coal to get your water up to 99.9 degrees ONCE and from there you just need a small amount of coal to top it up again whereas in your steam train you have to bring your water to the boil on an ongoing basis then throw all that energy away.

Now then, the above may be fatally flawed because, as I said, I don't know about trains, but if it's right then why were trains left to be hopelessly inefficient compared to marine engines when the technology was proven?
Yes, you'd have to get your fire drawing properly but surely some sort of scavenge fan wasn't beyond the engineers of the day. And there'd not be a lot of seawater to cool your condenser but there was no shortage of air and surface area on a train so just why? The train would be able to carry less water and less fuel or have greater range for the same fuel load so what am I missing here. Anyone?

[mtskull]

“My understanding is that you fill your loco up with coal and water, light a fire and once everything is hot and pressurised away you go. The steam passes through several expansion stages then is blown up the spout to make the fire draw. Is that correct?”

Indeed it is. The rest, however, is flawed (In attempting to explain why, I’ll leave aside the matter of steam turbines, which rely on condensing the exhaust steam for several reasons not directly associated with heat recovery).

A thermodynamicist could explain this in terms of Joules or Btu’s or whatever but, basically, most of the energy used in creating steam is expended not in raising the temperature of the water to its boiling point, but in effecting the transition between the liquid and the solid state. To be specific, the amount of energy needed to change the state of water from a liquid to gas is 540 times the amount of energy needed to raise the temperature of the same amount of water by 1 degree Celsius. We are not simply talking about the difference between 99.9 and the 100 degrees your kettle boils at either; water boils at a higher temperature the greater the pressure; at the working pressure of a typical steam boiler you would be looking at nearer 200 deg C. Once the steam has done its work by expanding in the cylinders its temperature will be almost halved, so it isn’t just a matter of “topping up” the heat of the water back to its boiling point either; by the time you have condensed the by now unpressurised steam, its temperature must be below 100 deg C, so the energy saved by condensing and re-using the exhaust steam cannot exceed the energy which would have been required to raise the water temperature from ambient to 100 deg C; a relatively small gain which generally wouldn’t justify the additional cost not only of providing, but also maintaining, a suitable condensing arrangement.

Using exhaust steam to heat the feed water has been tried with varying degrees of success over the years but, in an age where coal was cheap, it was generally not considered worth the extra cost. To quote from a Wikipedia article about British Railways 9F class locomotives so fitted: “The Crosti preheaters provided less improvement than had been expected, and were a problem for maintenance, owing to acidic fluegases condensing in the feedwater heater and causing corrosion. All were converted back to a more standard form within a few years...”
If you are really bored, there is an in depth paper on the subject of feed water heating here: https://www.gutenberg.org/files/22657/2 ... ating.html

Back to topic: Without going into the reasons why salt water is undesirable in a steam boiler, suffice to point out that railway locomotives can readily access supplies of fresh water to replenish their boilers whereas ships cannot, hence it is desirable in a marine application to condense the exhaust steam when the alternative would be to carry, or create by evaporating salt water, enough fresh water to supply the boilers for an entire voyage.

Then there are considerations of the size and weight of the condensing apparatus; a locomotive is much more constrained than a ship in this regard, especially as a ship can use a water to water heat exchanger, whereas a locomotive must use a water to air exchanger which is about one quarter as efficient and (trust me on this or look it up if you will), in order to get rid of sufficient heat has to be HUGE. (incidentally, difficulty in making these work efficiently has dogged just about every attempt to build a successful steam turbine locomotive).

After all that, I suppose I really ought to sign off with as a geek.....

[mtskull]

How about "the biggest steam train in the world" on the rails again - https://www.youtube.com/watch?v=NuJPVhtQFeQ

Nice.

[Renegadenemo]

After all that, I suppose I really ought to sign off with as a geek.....

Geeks are good - they fit in around here. I guessed it had to be the condensing problem, though the condensers on marine engines can be pretty compact units. That said, they do have a limitless supply of seawater to pull the heat out. How many expansion stages to steam trains use then? Marine engines seemed optimised at three.
As it happens we have the first ever steam turbine powered vessel, the Turbinia, just up the road here in Newcastle because, as everyone knows, the marine steam turbine is a Geordie invention.

[Filtertron]

It's pretty much directly into the valve chests, then to the cylinders and from there up the blastpipe. Unless the engine is a compound type, the expansion cycle is once only. On a compound engine on a locomotive, the steam is admitted to a small high pressure cylinder. The exhaust steam is then passed into a larger low pressure cylinder to use the low pressure expansion before exhausting up the blastpipe. Compounding isn't as common on locomotives. Vauclain compounds had a common cylinder block for each side where the low pressure cylinder was directly above the high pressure cylinder. The piston rods also shared a common crosshead. Some Garratts also used compounding. The lead engine usually had the high pressure cylinders, with the rear unit carrying the low pressure cylinders. American articulated locomotives also used a similar compounding arrangement to the Garratts.