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Edward Thompson: Wartime C.M.E. Discussion 2012 - 2022

Discussion in 'Steam Traction' started by S.A.C. Martin, May 2, 2012.

  1. Spamcan81

    Spamcan81 Nat Pres stalwart

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    On all the official drawings I’ve seen, all of the Light Pacifics were referred to as WC class. I think the BB designation was a marketing rather than a technical thing.
     
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  2. 30854

    30854 Resident of Nat Pres

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    True. There were more than a few folks (and their squadrons) due for .... at the very least .... a spot of grateful thanks and recognition in 1945.
     
  3. Allegheny

    Allegheny Member

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    I don't totally buy into this. I've been curious about the reason why the Bulleid boiler had its ability to produce steam, and I've not been able to find anything particularly clever about the proportions. My belief is that the Bulleid pacifics are just overdraughted, which obviously didn't do any favours for the fuel consumption. You could do the same trick on anything else with a 50 sqft grate (i.e. reduce the blastpipe size) such as a Duchess or Thompson/Peppercorn pacific or Duke, and get the same steam production as the Bulleid, but with obviously increased coal consumption.

    The draughting on General Steam Navigation is being looked at in line with modern knowledge, and I'm looking forward to seeing what the outcome is.
     
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  4. 30854

    30854 Resident of Nat Pres

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    Never considered that before. Were any tests on consumption conducted whilst 21C1 was still innocent of dampers?

    That observation also opens the question of GP grate tech. After all, the dark proportion of any steam loco's ejecta ain't hot gases.
     
  5. Jamessquared

    Jamessquared Nat Pres stalwart

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    I'm not sure quite what you are expecting of a gas producer system, but it isn't inherently there to control the proper combustion of the volatiles in coal - the fireman already has the tools to do that, combined in some cases with features of boiler design such as a combustion chamber.

    It also has to be noted that a Gas Producer system is inherently less thermodynamically efficient than a boiler without it. That's not huge, and in some instances the operational benefits of a Gas Producer (primarily reduced degradation of the fireweed through clinker) more than outweighs the thermodynamic loss. But the thermodynamic loss is inherent, essentially because for a given stoichiometric combustion of coal, you increase the mass-flow through the fire which gets lost at several hundred degrees through the chimney. Even if the steam you use in your gas producer is essentially "free" (i.e. waste exhaust steam, so you don't get the latent heat loss) you still heat it up by several hundred degrees between inlet under the grate and exhaust; and then throw it all away.

    Tom
     
  6. Allegheny

    Allegheny Member

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    I agree with all of this - but - one molecule of oxygen reacts with one atom of carbon.
    C + O2 > CO2
    In the grate, one molecule of steam reacts with one atom of carbon.
    H2O + C > H2 + CO
    So, if you think about it purely in terms of consuming coal, injecting steam into the grate has a similar effect to injecting pure oxygen.
     
  7. Jamessquared

    Jamessquared Nat Pres stalwart

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    But you’ve forgotten the subsequent reaction, in which the H2 and CO are consumed by more oxygen. Injecting steam into the grate is definitively not the same as injecting oxygen: if it were, you could burn coal with water.

    In a non gas producer system:

    C + O2 = CO2

    In a gas producer:

    C + H2O = CO + H2
    CO + H2 + O2 = CO2 + H2O
    ————-
    C + O2 + H2O = CO2 + H2O

    So stochiometrically the reactions are identical and therefore the combined heat output from the gas producer is the same as burning the carbon directly: the H2O is neither produced nor consumed in the reaction. The issue though is that mass of water passes through from cold to hot. The more water you push through, the greater that loss.

    Tom
     
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  8. Jimc

    Jimc Part of the furniture

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    I've heard this said elsewhere by an engineer trained in steam. It's not a surprise I suppose, every design is a compromise, so it ought to be unsurprising that greater steam production can be managed by taking a hit on economy. What's interesting, perhaps is the concomitant gains and losses in boiler life, ability to run the services etc. But whether sufficiently accurate comparisons can be made to evaluate the compromises is another matter.
     
  9. Allegheny

    Allegheny Member

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    I hadn't forgotten the subsequent reaction.
    I think I should have added that the coal consumption per square foot of grate could be higher with a GPCS system. Porta's GPCS system also had large air intakes for the secondary air for the combustion of the H2 and CO.

    I agree that if steam is injected at 100 deg C and goes up the chimney in excess of 200 deg C it will represent an additional loss.
     
  10. Spamcan81

    Spamcan81 Nat Pres stalwart

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    Word of the day. I will endeavour to drop it into the next conversation I have. Excuse me madam but are you interested in the stoichiometry involved during the cooking of your haddock?
     
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  11. 30854

    30854 Resident of Nat Pres

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    "Sorry young man, do you mean barbecued or steamed haddock?
     
  12. Jamessquared

    Jamessquared Nat Pres stalwart

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    In my defence, “stochiometric” was the precise word to express the concept intended!

    Tom (it’s a chemist thing …)
     
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  13. Eightpot

    Eightpot Resident of Nat Pres Friend

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    I've seen somewhere that the Lemaitre system was originally designed for the 'Lord Nelson' class and this design without alteration was used on the 'Schools' for which the blast nozzles were really too large, and also on the Bulleid Pacifics where they were too small. Hence the good steaming results with the Pacifics.
     
  14. 8126

    8126 Member

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    While I agree with the chemistry, I think you may be overplaying the importance of the steam reaction in terms of comparing the GPCS with conventional combustion. As per @JJG Koopmans we can conventionally assume about 1.8kg of combustion gases (including nitrogen and other inerts), to produce 1kg of steam, compared to about 1.6:1 as a theoretical minimum. I believe the GPCS could use up to about 10% of the exhaust steam mass, so we're now assuming 1.9kg of exhaust gas per kg of steam generated, although steam at 1000K has approx twice the specific heat capacity of nitrogen (since we can ignore the latent heat) so we could call it the equivalent of 2kg of exhaust gas per kg steam. Let's say for the sake of argument it's a 10% increase in heat capacity of the exhaust gas and it leaves at the same temperature. If the conventional boiler is running at 80% efficiency, we're down to 78%. Not a great start, but well within the bounds of experimental error.

    But the steam reaction is not a like-for-like comparison. If you have the sort of Welsh coal the GWR used to revel in, capable of building huge deep fires without clinkering, you don't need it. You can still run a gas producer combustion system without steam, with its greater provision for secondary air and explicit goal of producing CO from the firebed and burning that in secondary air above the grate. At that point, the reduced primary air allows reduced carry over of unburnt fuel from the grate, which is a straight gain in efficiency terms. What the steam reaction in the GPCS permits is the use of thick, heavy firebeds (with their attendant benefits in maximum power output and reduced carry-over losses) with coals that would otherwise not be suitable. That was why Wardale was able to legitimately make extravagant efficiency improvement claims for his Class 26; at comparable power outputs the Class 25NC was throwing half the fire up the chimney because it couldn't run such a thick fire. He admitted that it was not a straight comparison of the thermodynamic efficiency of the engine (although the 26 was more efficient as converter of steam to work), but the really big numbers came from increasing the grate limit such that the modified engine was working well within itself where the unmodified engine was not.

    From Wardale's writing, the managagement of such thick firebeds is not without issues, especially as the option of driving a lot of primary air through to get it heated up in the first place was not there to the same extent as with conventional fireboxes. They also require over-draughting to make back the greater pumping losses inherent to the system. Other than the Rio Turbio 2-10-2s, I don't believe there's ever been a large enough fleet run with the system for wide crew familiarity; from the writing about 2644 and 3450, it appears that crews who understood the knack of managing them greatly appreciated the system, but unfamiliar crews could get in trouble.

    I did wonder, given some of the recent descriptions of ovoids filling the smokeboxes in short order, whether the GPCS wouldn't have been rather suitable for them.
     
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  15. Jamessquared

    Jamessquared Nat Pres stalwart

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    I'm not sure I agree with that last bit.

    Firstly, the ovoids were very low in volatiles - it was practically impossible to get any colour from the chimney at all. They were, as I understand, basically anthracite and a binder, so they needed pretty much all primary air through the firebed to burn, and basically no secondary air - at least, in a conventional firebox.

    As for what they then did on the grate: I never experienced any really serious problem with clinker from them. Instead, the issue seemed to be that firstly they produced large quantities of fine ash; secondly the ovoids were themselves very friable, and tended to break up on the fire bed into fine dust. The effect of both was simply to choke the fire.

    The best way I found to get anything out of them was maximum bottom air, keep the fire thin, and always maintain a blast (even if that meant blower on when coasting downhill - it was the periods of no steam demand that caused the fire to degrade). That way, you had a fighting chance of keeping the some kind of free passage of air through the fire. You really wanted to keep the firebed as mobile as possible, and stop dust and ash working its way down through cracks to block everything up. Using them in gas producer conditions would strike me as fraught with problems: you'd solve a problem (clinkering) that in my experience wasn't really significant, but by minimising the blast through the fire, you'd allow it to get blocked with fine particles.

    Tom
     
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  16. 8126

    8126 Member

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    That's a good point, I was only considering half the problem (heavy carry over of unburnt fuel, for which you might wish to reduce primary air flow), while not considering that it was due to the nature of the fuel rather than extremely heavy working, and reducing the primary air even further would lead to it choking.
     
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  17. Allegheny

    Allegheny Member

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    How did they perform, in terms of producing steam?
     
  18. Jamessquared

    Jamessquared Nat Pres stalwart

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    At the risk of turning this into another “how did the ovoids perform” thread: Our line is basically largely uphill in one direction for 11 miles; largely a downhill coast back. Rinse and repeat.

    So first trip starting with a clean fire and with a strong blast, they were fine provided you got the fire made up well in advance - as much as thirty minutes. Problems then started on the return; the fire degraded and if you did nothing, your second trip was likely to be problematic.

    Working practice for most crews was to come into the yard after every trip and do a thorough clean of the fire, empty the ash pan etc; essentially try to start every trip fresh. Which, needless to say, was not popular.

    Tom
     
  19. Allegheny

    Allegheny Member

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    Is there a thread for this discussion? I had a look through the first four or five pages of "Steam traction" and couldn't find anything obvious.
     
  20. Jamessquared

    Jamessquared Nat Pres stalwart

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