A Coarse Guide to the Steam Locomotive for ‘N’ Gauge Modellers

[martyn]

A wonderful explanation, John. You've explained a complicated concept very clearly.

As ever, looking forward to the next part.

Martyn

[Train Waiting]

This ex-physics teacher awards you 10/10, and you get an early lunch mark too 

Thank you, George.

Blimey! 10/10.  I'll spend the rest of the day attempting to convert that to centigrade.

By the way, is it possible to be an 'ex-physics teacher'?  I should have thought that the teaching continues to be done, but in a different environment.

Sadly, I'm not brave enough to subject my theory to rigorous scientific testing by painting the water in Port Poppy harbour on the SuperSilly Train Set Layout blue.

I'd better get back to writing the next postington about superheated steam.  This will involve lots 'N' lots of exciting+++ ^oF to ^oC calculations.  One takes something off and then divides by something else.

*

A wonderful explanation, John. You've explained a complicated concept very clearly.

As ever, looking forward to the next part.

Martyn

Many thanks, Martyn.  The next bit gets even more complicated which caused my brain cell to superheat, so I enlisted some expert help.

*

Thanks again, chums, and all good wishes.

John

[Bealman]

I used to carry that conversion formula around in me head. Yeah, I remember you have to divide by something. Unfortunately, these days, it's gone out of the window, like Monty Python's biny little turds. 

[Ed]

It's is stuck in my head, C to F is: x 9 / 5 + 32 (times nine, divide by 5 and add 32).

Sad ain't I 



Ed

[Bealman]

No, not at all. Jogged me memory! Yep, I knew 9 was in there somewhere.

Anyway, back to John's brilliant thread.

[Jim Easterbrook]

If you can never remember whether to add or subtract 32, or to do it at the start or the end, try this version:

C to F: add 40, times 9, divide 5, subtract 40.
F to C: add 40, times 5, divide 9, subtract 40.

All this talk of superheating reminds me of the thermodynamics I partly understood at university. IIRC an idealised steam engine is a Carnot cycle, whose maximum efficiency is (T2 - T1) / T2, where T2 and T1 are the high and low temperatures in Kelvin. Adding superheating for the pragmatic reason of avoiding condensation in the cylinders increases T2 and so makes the engine potentially more efficient.

Many years ago my parents were on holiday an France and my dad noticed the town square was called "place Carnot". He went into the tourist information office to ask if it was named after the engineer. They didn't know off hand, but had a national biographical dictionary in which they looked up all the Carnots to find one with a local connection. It wasn't the engineer, to my dad's disappointment.

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 39


Hello Chums

How many times do we read something a bit like, 'The GWR stuck to low degree superheating until Mr Hawksworth started fitting three row and four row superheaters'?

Which sort of begs the question, 'What, in the name of ten types of coupling rod, is low degree superheating or three row and four row superheaters?'

Let's see if we can find out.

But first, full disclosure.  I got stuck drafting this postington.  The more I tried to unstick myself, the worse it got.  I simply could not get what I was attempting to say clear enough in my head.  I sought assistance from Friend of Poppingham @Hailstone, and Alex and his friend Ted conferred.  Alex then sent me a message of resounding clarity and followed up with a telephone call.  I am hugely indebted to these two expert enginemen for their kind help.

However, and importantly, any errors or omissions in what follows are mine and mine alone.

***

Right-oh - back to 'Hot Steam Willy'.

You might recall my contention (controversial?) that with one or two exceptions, Great Britain lost its early lead in steam locomotive development around the mid-eighteen-sixties.  What, to me, is one of the most important improvements to the basic Stephensonian locomotive was the practical realisation of an old idea.  And it happened East of Lowestoft.  In Prussia.

After some years of experimentation, initially with smokebox superheaters (please see Part 38 to find out about these), Wilhelm Schmidt designed a practical fire tube superheater.  The first was fitted to a Prussian State Railways' (KPEV) 'G8' 0-8-0 in 1902.  KPEV went in for standardisation and quantity production in a big way and several thousand more of these superheated locomotives were built.

This was transformative.

I propose we start off by discussing what a fire tube superheater is and then end this section on boilers and fireboxes with a brief outline of their adoption in Great Britain.

Dr Schmidt's insight was that, rather than relying on some contraption in the smokebox to attempt to superheat the steam, using the hot gasses from the fire, which had travelled through the boiler tubes, at their coolest, it would be better to pass the steam through the boiler tubes themselves.  Or, to be exact, some of them.

To do this he installed a vessel, consisting of two parts, in the smokebox, at the top of the tubeplate.  This is called the 'superheater header'.  Steam from the boiler passes though the regulator valve, normally located in the steam dome, and flows to one side of the superheater header.  From there it passes through the 'superheater elements' which are located inside the 'flue tubes'.  The flue tubes are like large diameter boiler tubes, typically 5 in. - 5 1/2 in. diameter, rather than the 2 in. or so of 'normal' boiler tubes.

The superheater elements are normally about 1 3/8 in. outside diameter steel tubes and make four passes through a flue tube - with two 'u-bends' at the firebox end and one at the smokebox end, returning to the separate side of the superheater header where the now-superheated steam is available to do its work.

Incidentally, the 'u-bends' at the firebox end of the flue tubes are a short distance before the tubeplate, to prevent them from coming into contact with flames from the fire.

Bearing in mind that a picture is worth a thousand words - especially my words - time for a picturingham:





Here, thanks to the patience and courtesy of the wonderful people at the Lakeside & Haverthwaite Railway who, amazingly, don't take exception to my examining their locomotives' innards, we can see the smokebox tubeplate, the superheater header, the boiler tubes, flue tubes and the superheater elements. Even with my course photography, you might just be able to see some of the superheater elements' smokebox end 'u-bends'  By the way, my head was inside the smokebox and the steampipes from the superheater header to the cylinders have been removed.  As have some other things that don't concern us at present.

The boiler is from Fairburn 2-6-4T No. 42085.

Compared to a non-superheated locomotive, a superheated one has fewer 'normal' boiler tubes.  As an example, a BR 'Britannia' 4-6-2 has only 136 boiler tubes but has 40 flue tubes.

The Fairburn tank engine, with a smaller diameter boiler, has 21 flue tubes - I haven't counted the boiler tubes.

Now for that ''x' row superheater' term that we often encounter.  As you can see from the picturingham, the flue tubes are arranged in rows - in this example, three rows of seven flue tubes.  That's a 21 element, three row superheater.

A 'Britannia' has five rows of eight flue tubes, giving it a 40 element, five row superheater.

Put simply, the more superheater elements there are, the higher the steam temperature that will be achieved.

Dr Schmidt recommended that the area though the superheater flues and the boiler tubes should be the same - a 50/50 ratio.  He advocated a target steam temperature of 650^oF / 343^oC, hence his becoming known as 'Hot Steam Willy'.  Please remember that a typical non-superheated locomotive of the time, with a boiler pressure of 175 psi, would have a steam temperature of 377^oF / 192^oC.

Here, possibly of interest, are some superheated steam temperatures recorded at the BR Rugby Test Plant, with the engines working a full power.  The figures in brackets are the number of superheater elements:

LNER 'B1' 4-6-0 (24) - 650^oF / 343^oC
LMS '5MT' 4-6-0 (28) - 680^oF / 360^oC
SR 'MN'  4-6-2 (40) -  700^oF / 371^oC
LMS '8P'  4-6-2 (40) - 710^oF / 376^oC
BR '7MT'  4-6-2 (40) - 730^oF / 387^oC
LNER 'V2' 2-6-2 (43) - 740^oF / 393^oC

Hopefully, Dr Schmidt would have been impressed.

Happily, superheated steam increases in volume compared with saturated steam.  The increase is, on average, in the order of 30 per cent.  This means the boiler has to supply less steam to the cylinders with a resultant saving in coal and water consumption.  Magic!

On a practical level, figures prepared by several railway companies that were adopting superheating in 1906-1913 period showed superheated locomotives achieved an average economy of around 17 per cent based on coal consumption per train mile.  Superheated locomotives also used less water and had an improved haulage capacity.

All in all - a huge step forward.  More to follow.

Thanks, again, to Alex and Ted .


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Cheerie-Bye

John

[Nbodger]

Excellent read and easily understood even by a Bodger.

I worked frequently with a colleague who if he was in a meeting and didn't comprehend the discussion he would ask would you please repeat that but in Janet and John language. Interestingly I am having lunch with him on Thursday.

[chrism]

The superheater elements are normally about 1 3/8 in. outside diameter steel tubes and make three passes through a flue tube - with two 'u-bends' at the firebox end and one at the smokebox end, returning to the separate side of the superheater header where the now-superheated steam is available to do its work.

Actually, and I suspect this is only a typo, it's four passes through the flue tube - back, forward, back and forward again.

Put simply, the more superheater elements there are, the higher the steam temperature that will be achieved.

I don't completely agree with this statement. Each element only has four passes through the flue so it cannot absorb more heat if it has 39 "partners" than if it only has 20.

The main advantage of a greater number of elements is that they can deliver superheated steam at a greater rate than a smaller number can.
In most cases, once the designers had mastered it, the number of elements was chosen to give the required steam flow for the number and size of the cylinders and the speed at which the loco was intended to run - i.e. to ensure that the required volume of steam was available. It's no good delivering steam 100-200 degrees hotter if it can't get to the cylinders quickly enough.

There would be a increase in temperature if the speed at which the steam passed through the elements was reduced by increasing the number of elements (e.g. putting 40 elements in instead of only 20 in the same loco and boiler) but that would have been a secondary consideration, IMO.

[Train Waiting]

Many thanks for this helpful contribution, Chris.

Yes, there was a typo - thank you so much for mentioning it - and is now corrected.  I proof read the post numerous times and still missed that glaring error.  Thank you.

With regard to your second point, I wonder if higher superheat temperatures and, therefore, drier steam was achieved by increasing the number of superheater elements which could then deal more effectively with high volume throughput of steam.

The temperatures I quoted were measured at the time - I think the Rugby Test Plant examples I included are interesting as, even allowing for different designs and covering power classes 5-8, the steam temperature achieved at maximum power correlates to the number of elements from the 'B1' with 24 to the 'V2' with 43.

I'm sorry for being so shockingly untechnical, but might the answer to this lie in the easier 'throughput' of steam with more superheater elements?

The next part will look at the introduction of fire tube superheaters in Great Britain.  I have included some points from a three-way discussion which occurred after Henry Fowler read a paper on superheating to the Institution of Civil Engineers in 1913.

The BTC's Handbook for Steam Locomotive Enginemen of 1957 states on page 36:

'The temperature of superheated steam at working pressure ranges from about 600^oF to 750^oF depending on the design of the superheater and the way in which the locomotive is being worked.'

The Rugby data has steam temperatures taken at 25%, 50%, 75% and full power.  All the locomotives show the degree of superheat being achieved rising as the power output increases. In the 'B1's' case, with 24 elements, from 520^oF at 25% to 650^oF at full power.

I hope we get further helpful contributions to this fascinating discussion.

Thanks again, Chris.

With all good wishes.

John

[chrism]

With regard to your second point, I wonder if higher superheat temperatures and, therefore, drier steam was achieved by increasing the number of superheater elements which could then deal more effectively with high volume throughput of steam.

Yes, that's right.

The faster that the steam is passing through a single element, the less superheating it will gain. Therefore, for a desired rate of throughput, increasing the number of elements means that the steam is going through each one slower and therefore getting more superheat.
Also, more elements will reduce the risk of the power output being "throttled" if the steam can't flow through the superheater fast enough to keep up with the demand from the cylinders.

As with everything, it was a fine balancing act to design a boiler to have sufficient main heating surface area and sufficient superheating surface area for the intended work. Too many superheating flues could result in too few small boiler tubes and, hence, a reduced overall heating surface area so the boiler wouldn't produce enough steam in the first place.

That's why the designers didn't just make all the boiler tubes flue tubes and run superheating elements through the lot.  A superheating flue has a smaller surface area than the four (possibly five) small tubes that it replaces, plus it loses some heat to the steam in the element when the loco's running. Therefore the overall heat and surface area available for making the saturated steam would be too small.

[martyn]

Regarding superheat temperatures, my earlier post on the 'E special' superheater tried on the A1s may have some relevance. The larger heating area did not correspond to the expected gain in superheat temperature.

Also, without going through all my 'green bibles', I'm pretty sure that it was realised that the LNER boiler designs were approaching the theoretical maximum temperatures. Again, I think that shortening one design's superheater loops actually resulted in higher temperatures-I'm not sure.

And also, the later LNER boiler designs were critised because of the ratio of small to superheater tubes and also firebox size; it is stated in one of the books that too much 'wet steam' was being generated and that the superheater was consequently working below optimum.

I think, and will check later.

And regarding overall design, the main steam pipes, regulator, and steam collector , as well as the superheater header and tubes had to be capable of handling the quantity of steam being generated and used; the A4s had large diameter steam pipes, a reverse bend in the A1/A3 steam pipes/cylinder block eliminated, and the pipe joints machined internally to avoid restrictions and obstructions.


Since reading this wonderful thread, I've also had an 'I wonder?' As the superheater raised temperatures and hence increased steam volume, did the increased heat energy mean that more energy was available to be changed to mechanical energy by the cylinders? I haven't a clue.



Martyn

[acook]

Hi John

It was my understainding the GWR preferred a low degree of superheat due to limitations with the cylinder lubrication, the oils available at the time carbonised at higher superheat temperatures.
It was not until the introduction of synthetic lubricants that higher superheat temperatures were attainable.

Alan

[chrism]

Since reading this wonderful thread, I've also had an 'I wonder?' As the superheater raised temperatures and hence increased steam volume, did the increased heat energy mean that more energy was available to be changed to mechanical energy by the cylinders? I haven't a clue.

Happy to be corrected but I would have said not - or not much.

A cylinder can only ingest a fixed volume of steam and, if that's superheated, it will be more expanded and contain fewer molecules than if it's saturated steam. There will be a pressure increase due to the higher temperature but whether or not that offsets the reduced mass and does cause greater power I'm not sure.

The principle advantages of superheating are a reduced tendency for the steam to condense back to water in the steam passages and cylinders and reduced consumption of water and coal.

There are disadvantages too, mainly the cost of fitting and maintenance but, in general, the advantages outweighed the disadvantages - for higher speed working at least. It wasn't found of much use for shunting locos nor for slow traffic.

For example, the North Eastern Railway fitted superheaters to some of its Class P mineral locomotives but later began to remove them.

[Hiawatha]

And I would say, yes ... 

On the above mentioned Prussian G8 the superheater enabled the increase of the cylinders compared to their "wet steam" predecessors, the G7.1.
On the G7.1 the cylinders had a bore and stroke of 520x600 mm which was increased to 550x660 for the first few G8s. Within three years the G8 cylinder diameter was enlarged three times to 575, 590 and 600 mm, enabled by the improvements in superheater technology between 1902 and 1906.