A1 (Peppercorn) and A4 boiler dimensions

Please will someone post the boiler barrel and firebox lengths of the A1 and A4 boilers for me? Curiously the otherwise very comprehensive LNER.info site lists the boiler width and all the heating areas, but not those lengths.

 

Dia 107 (A4) Firebox length -external 10' 5.75" overall length of boiler 29' 1.375". Dia 118 (A1) Firebox length - external 11' 4.125" as to overall length 1' 11.75" shorter than the boiler fitted to the P2 - really usefull, sorry. You can find out a great deal about LNE Pacific boilers quite easily, but you are right, some details are curiously, well, obscure.

 

Thanks, I was looking at GWR boiler dimensions and noted that Churchward's pacific, notoriously a poor steamer, was massively different in dimensions to the others, and wondered if that held any obvious clues to its doggy nature... It seems the proportions were odd: loads of tube surface, relatively small firebox. Interestingly it also had a proportionally big superheater by GW standards - it was all tube and superheater and no firebox.

 

Just goes to show Churchward wasnt Perfect. The fallout from this 'experiment' affected GW policy on superheating for years...

 

Don't jump to conclusions too fast... I suspect that when Great Bear first went out, it was fired as per the locomotive inspectors' observations:- and they were probably based on many years firing on narrow firebox type locomotives.

The southern had the same problems, going over to Bulleid type boilers, from Maunsell & other designs.

Its all too easy to criticise, sat in front of a computer. I would relish the chance of firing the Big 'un. Alas, born too late.....

Finally.... Remember Duke of Gloucester? Written off by BR as a dissapointing design. Would you still level that charge against 71000 today?

Regards,
Ian

 

Ah but the conclusion im jumping too is not that the loco sucked, but ironically that the conclusion that it did was jumped too.
Had there been any need for improvement (on Churchwards already massive improvements) or even for an engine that big on the GW then there surely would have been some persistance...
GW (and some swindon types might say british) Locomotive development peaked with the castle class if you are talking firing a heap in the middle that shakes itself into the corners....just perfect if there s a lot of firing to be done. Only a tweaking in the draughting and more superheat we're needed to get even more out of it though

Might you get a chance to fire Lizzie or even a Duchess ...would it be so different ? ...

As for 71000 Its designers didnt right it off, they new that, like the Jubilees, that it was almost there and a couple of changes here and there and it would go. but the BR Exec. lost the power to authorize any steam development beyond those designed at effeciency a year or so after it was built

Many locos which we're driven and fired in the same fashion as the loco's they replaced didn't perform to potential as a result and its this intransigence which was particular the case on the GW that im having a pick at. Gresleys wide fireboxes would have been a non starter there.

Still if id had to shovel tons of coal for hours everyday for a living i think id be pretty hostile to something that didnt work unless it was shovelled in a new and peculiar way....and these guys werent given instructions, just expected to work it out...

 

You wouldn't get much steam out of a Castle doing that!

 

1 stand corrected

 

The problem The Great Bear had was arguably the First World War. There was evidence that the GWR were going to tweak the design till they got something near optimum, but then the war broke out and all and sundry were volunteering or trying hard just to keep the railway (and war work going). Im not really sure how it got the bad reputation. Yes, it didnt provide anything spectacular compared to the Stars, but then it was a new design, and its still not very clear firing techniques as used on previous types really worked on it. Its forgotten how much development the Great Western put into the Stars and the Saints before they got something they were entirely happy with.

That said, there have been items ive read in various books that suggest when fired well, The Great Bear did provide pretty good service between London and Bristol. And there certainly isnt any record of it being unreliable.

Of course, the drawings for the boiler turned up at the NRM last year, so if anyone wants to have a go at a replica, Ill put a fiver in to get you all started.

 

I'm no expert on this stuff, but having spent some time looking at drawings and graphing some dimensions I suspect that the drawing office didn't spend that much time on it. Quite probably I'm doing Churchward and Hawkesworth an injustice, but to me it looks suspiciously as if they took the front three quarters of a Star, moved the last pair of drivers forward to make some room for a grate about the same size as a No1 (which was proven to be an adequate area to feed Star cylinders) put as big a firebox as possible round the grate and then just joined up the smokebox and the firebox... The heating surface of the firebox proportional to the rest of the boiler is *way* out of line with all the other GWR boilers and with the LMS and LNER Pacifics. From what I can work out the other Pacifics all seem to have a big combustion chamber, which brings the heating surface of the box up to the same sort of proportions as the narow box designs. Ironically Churchward had, some years before, built a series of locomotives with wide fireboxes and combustion chambers, the notorious Krugers, which were very much less than successful, so maybe that was the reason they didn't do the same thing with the Bear. I would guess the big superheater was to try and compensate for the small firebox area.
What exactly did Stanier do with his early Pacifics when they steamed poorly? I understand that the tubes ended up shorter. Was that a case of moving the tube plate forward and adding a combustion chamber?

 

I don't quite follow that. The firebox is there to boil water, as are the firetubes, whereas the superheater area is there to dry the steam?

 

He's probably arguing that the larger superheater heating surface compensated for the relatively small firebox heating surface in the total heat transfer stakes but superheaters of any size don't contribute to the ability to burn coal and that's what counts in the end.

 

I'm out of my comfort zone here, but I've never understood this "dry the steam" bit. The superheater must increase steam temperature, albeit its doing it downstream of the regulator. Steam temperature in the boiler, *if* I understand things correctly, is mainly a function of the boiler pressure: the higher the boiler pressure the hotter the steam (and thus the more energy in it). The superheater then increases temperature above that point, which can't be done in the boiler because its full of water. Does it increase the pressure too? So that makes me think that a lower boiler pressure and a larger superheater is liable to end up with a similar steam temperature to a higher boiler pressure and a smaller superheater. And we know that some lines reduced boiler pressure when they put superheaters on. Either way isn't is just a question of getting that energy from the coal into the steam so it can be extracted again by the expansion process?... Is there a proper engineer in the house who knows the maths and can correct me? Presumably the designer has, whether by direct measurement or empirical investigation, a target steam pressure and temperature (and rate of flow) he/she wants to achieve in the steam chest given the capabilities of the various components *and* the lubrication challenges... But whatever the mechanism one thing I think I can be reasonably confident in suggesting that hot air going up the chimney is of no use to anyone, somehow the heat generated on/above the grate has got to be transferred to the steam to do anything useful.

 

Trying to keep it simple! In the loco boiler, the heat source (the fire) is surrounded by water so that fire can only boil water and cannot raise the temperature of the steam above it to anything other than the temperature that it is boiling at, which is a function of the boiler pressure. By subjecting the steam taken from the boiler to additional heat input, you can heat it above the saturation temperature. This can be done with another heat source but, on a steam locomotive, is done by passing the steam through the superheater elements, which are surrounded by hot gases. As the only energy that can be recovered (used) in the steam locomotive engine is the difference between the input energy into the cylinders and the outlet energy in the exhaust, it stands to reason that the more energy that can be put into the steam above the exhaust temperature/pressure, the better the efficiency. In simple terms, let us say that the energy in the exhaust steam is E, the energy in the saturated steam is A and the additional energy in the superheat is B. Then the efficiency of a saturated engine is 100 x (A-E)/A and that of the superheated engine is 100 x (A+B-E)/(A+B), which is a greater value. ( I think I've got that right. It's late and the red wine has been flowing and I am trying to keep it simple!)
In terms of hot air (and exhaust steam) going up the chimney, this accounts for something like 80% of the heat energy in the fuel at the best of times. Then you have to add auxiliary usage and radiation losses.........

 

The phenomenon known as wall effect can account for 50% of the steam admitted to the cylinder. It is therefore important that the cylinder temperature is above that of saturation at all times. What you are trying to avoid is condensation so high superheat is vital. In addition cylinder lagging and steam jacketing should also be used.

As to the boiler, it is not the heart of the locomotive, the exhaust system is. A boiler should be able to evaporate 70kg of water per sq metre of evaporative surface per hour and if it is not achieving this kind of figure, it could be very badly proportioned, but your first stop is the exhaust system.

Interestingly Gresley is the only UK locomotive engineer credited with understanding the value of high superheat.

I don't believe that 50 sq ft grate areas are required in the UK, you should be able to achieve outputs in excess of 4000hp with 41.25 sq ft.

 

I think it's not just the exhaust, but the whole route from the regulator to the chimney. Gresley, and others, understood the importance of making the steam lines smooth - the so-called 'streamlining' of the internals of the steam pipes, avoiding sharp changes of direction, then good valve gear, then finally, as above, a good exhaust arrangement which doesn't create inappropriate levels of back pressure, whilst still creating draught to assist in gas flow from the fire to the chimney.

Tortuous steam paths may be tolerable on a plodding goods locomotive, but are not good on an express loco.

An example is the Midland's Lickey Banker - superb at its job of assisting trains up the Lickey, but when it was tried on the open road, to see if the design would be good for eliminating the necessity of double-heading the heavy coal trains that seemed to flow continuously from the Midlands to London, it was found wanting. Primarily it was the steam flow that was a problem. (Undersized axleboxes was another, but 'Big Emma' was not alone amongst Midland locomotives in that regard - that's another story, of course...)

Mark

 

Thanks Steve, 242A1, that helps.
The list of fundamental inefficiencies in the steam engine is pretty intimidating really...

So, lets review my understanding.

Firebox and boiler produce saturated steam, which has a temperature (and energy) limited by pressure. For a given pressure the heating capabilities of the boiler primarily define the volume of steam is produced: the energy of each litre of steam only varies if the boiler/crew can't maintain the intended working pressure. One suspects that crews would have described a loco that couldn't maintain a satisfactory boiler pressure as a poor steamer.

The superheater, because it's not working in the presence of boiling water, can increase the amount of energy stored per litre of steam to a higher figure than the boiler can as superheated steam pressure is not limited by pressure.

<strikethrough>The location of the superheater puzzles me now I think of it. Presumably there are engineering limitations, but as the combustion gases must be reducing in temperature as they pass down the tubes, the superheater would seem to be located in the coolest part of the boiler, which must limit its capability to superheat the steam. The fact that it operates satisfactorily in the coolest part of the boiler suggests that the exhaust gases are still extremely hot as they leave the tubes, which is a big efficiency loss... </strikethrough>

I now understand the desirability of devices to preheat the boiler feed water - is the name "economiser"? - if the remaining heat in the combustion gases can be utilised its almost analagous to a exhaust turbo-supercharger in an internal combustion engine - previously wasted energy harnessed.

I'm not quite convinced about the efficiency equation - at least as I understand efficiency - because it only seems to be part of the story. The same amount of energy burned at the grate could presumably be transmitted either by a larger volume of steam at lower heat or a smaller volume at greater heat, which suggests that a locomotive with a larger cylinder capacity might deliver the same power at a lower steam temperature. Of course there are about ninety dozen tradeoffs in every direction on that balance, from greater frictional losses in larger cylinders to frying the cylinder lubrication oil. I suspect its probably a little simplistic to say that other designers didn't understand the value of high superheat. It seems at least possible that there were other factors which also affected the cost of running the locomotive. Its after all pretty well documented that Churchward used moderate superheat because the oils available to him couldn't cope with higher steam temperature, with the result that other running costs increased. As I understand it from reading Cook the costs of overhauls were at least as big a factor in the costs of the railway as the cost of fuel, so a saving in fuel/efficiency which resulted in a reduction of the mileage between heavy overhauls might well have been a false economy.

Going back to the Bear though, it appears to me that
- because the boiler had a small firebox heating surface and a very large tube area it would probably not have boiled steam as well as either a narrow firebox or a wide box with a combustion chamber, as the other Pacifics had. Its easy to imagine the crews having much more trouble keeping the boiler up to the working pressure than they would have done with a No1 boiler on a Star. On the other hand the large superheater would have tended to compensate for lower boiler pressure by increasing the energy in the steam, but would that have been obvious to the crews?
- I may be doing Churchward and Hawkesworth an injustice, but when I look at the drawings and dimensions of the Bear it doesn't look as if they spent very much time on the project. Oversimplification is always dangerous, but it looks exactly as if they took the front three quarters of a Star, moved the rear drivers forward and drew in a grate the same sort of area as fed the Star cylinders adequately, drew a firebox on top of it and then joined up the firebox and the smokebox with tubes.
- Churchward had designed a locomotive with a wide firebox and a combustion chamber, the features the later Pacifics have, but it had been a failure (although the boilers were kept running at low pressure in Swindon works for 50 plus years). Its easy to understand that he wouldn't have been keen to try another one.

 

Jimc, I haven't time to respond to all your points right now and I'm sure Steve will do a better job (I started on the Sat/Superheated query but gave up in the end as too complex). Anyway - your comment about the superheater being in the coolest part of the boiler. It sounds as if you think the superheater is that 'lump' inside the smokebox? If so, that's just the header - the superheater elements extend from there back along the large diameter flue tubes almost into the firebox, at which point the gases are at their hottest. Apologies if I'm teaching you to suck eggs here. There are lots of boiler cross-sectional drgs on the interweb that would show this. Iain

 

Fine: that makes sense. For some reason I had a picture in my head of the elements only going about half way down the large tubes...

 

Glad to help. So refreshing to see someone wanting to understand the finer points of these wonderful beasts. Here's a link to a reasonable diagram showing the superheater elements. http://www.greatwestern.org.uk/basic1.htm Even better visit the NRM at York is possible and see the sectioned Merchant Navy. Iain