Brighton Atlantic: 32424 Beachy Head

Thanks for the time you put into that lengthy explanation! Very illuminating.

Noel

 

And I'll bet the liquid form is far better at cooling the furnace walls, now that I think about it.

Thanks for the enlightenment!

Noel

 

Several locos did run fairly successfully in the UK with feed preheaters. I’m thinking of the GER and the P2. But as has been said the extra maintenance probably did for them in general.

The one device that did stand the test of time was the exhaust steam injector, which still sees service on many mainline locos today, and iirc gives around 10% saving on coal. It always strikes me that the ESI takes the black magic of the injector to new heights


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I'm dubious about that bit of an otherwise excellent exposition.

Without getting into the detailed thermodynamics of the Rankine cycle, surely the work that you get out of the cylinders is related to the pressure and volume of the steam (details depending on the cut-off and hence how much work from steam close to boiler pressure and how much from expansion); whereas far less energy is needed to get the water into the boiler because of its much lower volume. Thus it is largely the latent heat of vaporisation that you put in which provides the work that you get out.

 

Not sure if we are talking at cross purposes, but I was considering the whole system efficiency, not just the cylinder efficiency. My very simplistic view is that:

• Your starting point is cold water, and a heat source (the fire)
• Your end point, at which you lose any further ability to extract useful work, is exhaust steam emerging from the chimney, still at some residual pressure (and therefore at a temperature somewhere over 100C; and still steam, not water).

Therefore, however much energy is needed to raise the temperature of the water from cold to boiling point; evaporate it; and then put more energy in to raise the pressure slightly, is wasted. That includes all the latent heat of evaporation, which is the major component of that loss.

As for boiler feed: you have to take water at atmospheric pressure and pressurise it to boiler pressure to feed it in. That requires work: there are pros and cons of crosshead pumps, mechanical pumps or injectors, but all of them will consume energy. A neat feature of an exhaust steam injector is that it enables some energy recovery that would otherwise go to waste, whereas pumps and live steam injectors consume energy that is thereby not available for productive use.

(I’m reminded of Frankie Howerd: “Entropy, entropy, they’ve all got it entropy”).

Tom

 

Do you mean temperature there, Tom? My understanding of superheat is that by raising the temperature of the steam and water vapour, the liquid content is converted to steam and so enhances the expansive properties, but there is no rise in pressure. That's what I was taught when training to be a fireman, anyway.

 

Either I'm not understanding what you are trying to say or you are not understanding what Tom is trying to say and why steam locos are so inefficient. Hopefully, the diagram below will better explain.

Tom is correct in what he is saying but so are you if talking about superheat. In a steam loco, there are four stages in the steam raising process.

1) Raise the water tempewrature from cold to boiling point
2) Boil the water to turn it into steam
3) Raise the pressure of the steam from 0 psi to working pressure (saturated steam)
4) Raise the temperature of the steam without further raising the pressure (superheated steam)

Again, the diagram should make it more understandable.

It also shows why the use of condensers can be so important in recovering energy in the steam cycle.

 

Your picture is worth a thousand of my words ...

Tom

 

Just a quibble Steve, but the water in the boiler even under full pressure, is still water. It is only when released that it appears as steam, meaning water vapour or a gas even, because the water is at a temperature above the boiling point of water. If the boiler only contained steam one couldn't see what the level was in the water gauges.

 

However, as the water is boiled off you inject/pump in more water which you have to bring up to the boil. If you didn't you wouldn't be able to see the water in the gauge glass and you would also almost certainly have a boiler explosion!

 

Kenneth Williams.

 

Not sure if it's been posted on here, or if it needs its own thread but according to Facebook it's been announced that once 32424 is complete the next new build is to be a SECR E class 4-4-0

 

Also mentioned in the latest SR, who state it still requires ratification from the Bluebell Railway trustees.

 

In marine practice the use of a condenser could sometimes result in fairly drastic negative pressures. I recently read a very good series of articles on turbine theory which noted, almost as an aside, that the low pressure turbine on the Olympic class liners was fed at 10 psi absolute, so -4.7psi gauge, and generated a similar power output to each of the two triple-expansion reciprocating engines which fed it, which in turn took their steam at about 230 psi. This gives a fair idea of the amount of wasted energy in a typical railway locomotive, which certainly will not have been as efficient as a triple expansion marine engine.

Regarding the Class 25 condensers, I was inspired to look up the weight of their tenders, since it suddenly struck me that they're carried on two six wheel bogies, same as the non-condensers tender. It turns out the a Class 25NC tender weighed 105 tons fully loaded (~4o tons tare), whereas a condensing tender weighed 113 tons (~90 tons tare), so despite their vastness the weight penalty isn't as great as it might seem, except that a larger proportion of said weight is dragged around for more of the time. The condensing gear did apparently allow a conscientious crew to fine-tune the feedwater temperature, so running steadily at high power it could be kept quite hot, whereas if blowing off looked imminent the condensing gear could be run harder to give a cooler feed.

 

[QUOTE="


(I’m reminded of Frankie Howerd: “Entropy, entropy, they’ve all got it entropy”).

Tom[/QUOTE]

It was Kenneth Williams who said "Infamy! Infamy!, they've all got it infamy!" in Carry on Cleo.

Apart from those two things your quote is correct.

 

Indeed Kenneth Williams. Clearly the pun about entropy went over certain heads...

Tom

 

Its such a bore when ones esoteric jokes go right over the proles heads.

Down at Mensa this had them in fits.

 

Entropy is only an O-level concept, Mensa's admission criteria have clearly dropped!

 

A notice board at the open day a few weeks ago gave the reasons:

 

Sorry for delay in pursuing this. I was out almost all day yesterday and quite a bit of today.

I'm dubious about some features of that diagram.

It seems to show the water turning to steam at 212 ºF, and therefore at normal armonspheric pressure, not the much higher temperature and pressure in a boiler.

What is shown as energy that can be used is almost all from the superheat, implying that without superheat the efficiency would be extremely low.

HOWEVER this is all a serious distraction from discussing the Brighton Atlantic.