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I have just returned from a happy weekend spent with my pet project. A group of chums are scratch-building a 12"/foot A1, based on the design by the late Arthur Peppercorn. Although some detail changes have been needed this model has an amazing level of detail (although they stay heads on the boiler look different - but then we had to go to Germany for an off the shelf item!) and is faithful to the original - right down to roller bearings throughout......
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The windows dont look right! lol.

seriously its a great project. when i was on work experience about 7 years ago at birmingham railway museum i did some work on the footplate. (drilled holes. deburred it and painted it. and i also spilt some black paint on one of the steps but luckely nobody noticed and i went over it again with green the next day!)
I only did 2 days of my week as i came down with the worst case of the flu in living memory.
i spent my second week at AJ Reeves and had a great time.

Peter
 

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The engineering skills required to build this loco must have caused some headaches? I bet when she start moving you wont be able to stop those guys from smiling with pride.
 

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"....seriously its a great project. when i was on work experience about 7 years ago at birmingham railway museum i did some work on the footplate. (drilled holes. deburred it and painted it. and i also spilt some black paint on one of the steps but luckely nobody noticed and i went over it again with green the next day!)"

Pedro, good for you! We were wondering who to blame for that....


"The engineering skills required to build this loco must have caused some headaches? I bet when she start moving you wont be able to stop those guys from smiling with pride"

Thunder, we have had to make quite a few changes to method of manufacture in order to gain main line derogation - remember the target is a 90mph loco, which means she'll have to be tested up to 10% over.... 99mph!

The boiler is significantly different from those of the 1960s. Germany has continued to refine the design and Tornado's is fully welded, has no longitudinal (horizontal) stays, a welded U channel foundation ring, a steel firebox and welded, domed flexible stays in the firebox. The smokebox tubeplate is a fabricated ring butt-welded to the boiler and is almost twice as thick as the UK equivalent (hence no need for horzontal stays!).

The best part of the weekend was being able to sit in the driver's bucket seat and dream a little....

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I have to admit that she really looks impressive, even at this stage of her construction, I remember reading some time back, that of all the energy produced by a steam locomotive, only 12% of it actually reached the driving wheels, i don't know if this is true or not, but bearing this in mind, and the difference in the boiler construction of TORNADO, will she be more efficient than the locos built 60 years or so ago?
 

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I wish I knew the answer for sure but with Kylchap exhausts, long travel valves and roller bearings she is going to make good use of the energy she generates - to do better you'd have to have a Porta gas producer firebox and Caprotti valve gear....

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So you are actually building this from scratch rather than renovating an old one?


Thats quite a feat.
It looks really good.


What do you intend to do with it once built? Museum or day trips?


Certainly a new angle on large scale modelling.
 

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Thunder,

With regard to the name chosen, this project was conceived just after the first Gulf War, when RAF Tornadoes were performing so well. Some would argue about this. Perhaps another racehorse name would have been more PC. How about Red Rum?

As for the seemingly abysmal 12% efficiency of steam locomotives, it is all to do the work that is available to be done in the cylinders in expanding superheated steam from boiler pressure down to saturated steam at just a little above atmospheric pressure.

You have asked a difficult question that I am fortunately able answer, but I don't want to blind anyone with science, so those who don't want to know can look away now. If you know any physics and a fair bit about steam locomotives, you should be able to follow this worked example.

My engineering thermodynamics were taught to me in Imperial units, not modern metric units, but if you will bear with me, the answer is still the same. Imagine 1 lb of water being evaporated in a boiler, that is 1/10th of a gallon. I shall base my calculation on a superheated locomotive boiler steaming at 250 psi absolute, or 235 psi gauge pressure, in round figures.

From my little book of steam tables, the enthalpy of superheated steam at 250 psi absolute and 100 degrees Farenheit of superheat is 1371 Btu/lb of steam. If you expand this to saturated steam at atmospheric pressure, the enthalpy becomes 1150.4 Btu/lb. That means that the only energy in the steam that can be usefully used is 1371-1150.4 = 220.6 Btu/lb, or (220.6/1150.4) x 100% = 19%.

No machines are ever 100% efficient and so you have to take into account the pressure drop across the regulator and the pipes and passages that feed the cylinders. Remember about Sir Nigel Gresley's internal streamlining in the A4s? Then there has to be another pressure drop between the cylinders and the blast pipe. So loco engineers usually assume that only 85% of the energy is available, so the efficiency drops to 85% of the 19%, to say about 16%.

A boiler efficiency of 75% in raising the steam, would be very good, with a single pass boiler and superheaters. For comparison, a modern static coal fired boilers has three passes and gets about 85% thermal efficiency. That is 15% of the heat in the coal is lost up the chimney. So really 75% evaporation efficiency is good for a loco.

All this means that the best overall efficiency that you can hope for is 0.75 x 16% = 12%. However there will a further loss of efficiency in the mechanical transmission of that energy to the wheels. Tornado being a 21st Century steam locomotive will have roller bearings to reduce friction losses, however in a steam age locomotive with superheat and a clean boiler, 8% overall would be a safer bet.

Now think about an early 20th century locomotive with no superheat, a boiler pressure of 150 psi, and conventional bearings. You are looking at perhaps 6% efficiency, or less.

Colombo
 

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Hi Colombo, Many thanks for taking the time to explain why there is such a large energy loss in steam locos, I found it quite fasinating reading, I have to admit i was supprised that only 15% went up the chimney. Its quite amazing really when you think about it.
 

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Thunder,

In fact at least 25% of the heat in the coal goes up the chimney as hot products of combustion. That is excess air, nitrogen, carbon dioxide, and unburned fuel.

On top of that all the latent heat in the steam also goes the same way via the blast pipe.. That means up to 88% of the heat goes up the chimney. The other losses are friction in the motion and convective heat transfer from the boiler shell.

The Dabeg feed water heater fitted to some B12s and Atlantics by the LNER were an attempt to increase the thermal efficiency by reducing the losses. The Crosti boiler sought to increase the efficiency of the boiler by adding another heat exchange section. Caprotti valve gear set out to reduce the flow resistance of the steam passages.

Colombo
 

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Colombo,

Question 1:
I have a very hazy memory of thermodynamics. The Carnot steam cycle comes into this somewhere doesn't it? Something to do with the maximum theoretical efficiency possible being around 37%??

Question 2:
Is "compounding" an attempt to get two lots of work out of the steam - first in the high pressure cylinders and then again in the low pressure set? From my visit to the French railway museum in Mulhouse, I got the impression that the French persisted with compounding right up to the end of steam design in that country.

David
 

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David,

You just had me diving for my copy of Mooney's "Engineering Thermodynamics" . You are correct in suggesting that the Carnot Cycle is an application of the Second Law of Thermodynamics and applies to Heat Engines. There is a formula for calculating the maximum possible efficiency of a Heat Engine there, but the figure depends upon the temperatures of the heat source and sink.

Regarding compounding, you are quite right about the double expansion process. British engineers eventually decided on the use of a less complex arrangement called long travel valves, as pioneered by the GWR after their trials of the De Glehn compound system. This enabled locomotives to operate with a greater cut off so that more steam is admitted into the cylinder at the beginning of the cylinder stroke and then the valve is rapidly closed, rather than steam being admitted for a longer period during the stroke.

Incidentally, I have just found a mistake in my calculation above. Will anybody else spot it, I wonder?

Colombo
 

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Steam at 235 psig and 100 degreeF?
With water boiling at 212 degF.....
I assume you meant 1000 degF!

Regards,
John Webb (BSc Physics 1977!)
 

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Discussion Starter · #16 ·
With regard to the name, the A1 is associated with two RAF squadrons who flew Tornadoes in Gulf War 1 but the name captures a feeling of energy and unstoppability as well. Red Rum was a National Hunt horse - all the LNER "horses" were flat winners!


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John,

In my example, I was referring to the degree of superheat. At 250 psia water boils at 401 degrees F. With 100 degrees of superheat you have superheated steam at 500 degrees F. Naturally with a higher degree of superheat, a better performance could be acheived, subject the limits imposed by the materials economically available.

In trials in about 1912, Raven tried a higher degree of superheat using a Schmidt superheater in his Class Z Atlantics and acheived a superheated steam temperature of 640 Fahrenheit at a working pressure of 150 psig. That would equate to 274 degrees of superheat.

It would be interesting to know what degree of superheat it is intended that Tornado will employ.

Colombo

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>psia, psig
I'm assuming psi is Pounds per Square Inch and that 'a' has some relationship to atmospheric but the 'g' baffles me and is too far from 'a' on the keyboard to be a typo?

Am I correct in thinking that marine boilers can go a lot higher in terms of psi like 1000+?

David
 

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David,

Sorry I baffled you.

psia = Pounds per square inch absolute
psig = Pounds per square inch gauge

In fact psia = psig + 14.7.

Atmospheric pressure = 14.7 pounds per square inch absolute.

Now just to add to the fun, I have found the design calculations for the proposed class 5AT advanced steam locomotive which have been prepared by Wardale Engineering and Associates.

The web site for the project is at: http://www.5at.co.uk/
For the design calculations see: http://www.5at.co.uk/FDC.1.4.pdf

This locomotive has been designed to have an overall thermal efficiency referred to maximum drawbar power of 11.4%. In comparison, it is stated that a class 7MT Britannia acheived 7.7% and a Standard class 5 acheived 6.8%.

I really have enjoyed finding all this information. I have never designed a steam engine of any kind, and I have had to go back to first principles and 40 odd years is a long time. I was often baffled then. However, I have been working with process boiler plant off and on ever since.

Colombo
 

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>I really have enjoyed finding all this information. I have never designed a steam engine of any kind, and I have had to go back to first principles and 40 odd years is a long time. I was often baffled then. However, I have been working with process boiler plant off and on ever since.

And I have enjoyed reading it. I did not pursue thermodynamics myself, I spent my most of my spare time in the computer lab.

David
 
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