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American steam locomotive efficiency- the effect of blastpipe size and superheat levels.

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Posted by Firelock76 on Saturday, May 14, 2011 9:12 PM

Zu Burgard540:  Ach du lieber!  Ein poster mit ein "Pour le Merite" avatar!  Either you iss a real romantik or der last Fokker Dreidekker pilot livink!   Seriously, welcome back from Iraq, glad you're home safe, and thank you and God bless you for your service!  

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Posted by Dreyfusshudson on Tuesday, May 17, 2011 4:50 PM

Dear Joe,

Many thanks for this, and firstly let me reinforce Firelock76’s comments about your work in Iraq. I travelled a lot in the 1980s with a colleague who had had the Middle East as his patch in the 1970s. I remember him saying that Baghdad (and Tehran for that matter) used to be a beautiful place and I am always haunted by this when I see images of there now.

I would be delighted to share the programme with you; I think I will need to mail it you with supporting documentation, since you can’t send .exe files by Outlook, so if you send your name and address privately, I will pull a package together and send it. One of its great virtues is that it’s incredibly easy to use, and you can do 10s of calculations in an hour to explore all sorts of hypotheses. Likewise I would be very interested in your work on the Big Boys. I now have Kratville’s book on loan, along with King on the As and Farrington on the Santa Fes Big Three. I’m giving full attention to the latter, since it is such a treasure trove of the kind of information I am after, not paid much attention to Kratville yet, though I do intend to ‘build’ the line Ogden to Evanston, to see if I can simulate the test runs of 4014/16 in terms of efficiency coal consumption  and work done, a significant project some way off until I sort the Santa Fe. This will be a great test of how good the models I have of engine efficiency, boiler efficiency and locomotive and train resistance actually are, way outside the range of conditions I’ve tested them out in until now. Maybe you’ve gone a long way towards this?

Thanks for the dimensions of the UP front ends. The FEF2-3 have an area equivalent to a single blastpipe of just over 8¾ ”. However, looking at the pictures on pp 28 and 29 of Kratville, I am wondering what the discharge coefficient through these openings would be. As I understand it, each of these four orifices sit atop a single wide blastpipe? I ask because over here, single blastpipes were normally used; comparing the programme’s output with backpressures measured on test, shows that good fits are obtained, as expected, with a blastpipe discharge coefficient of 0.99. Late in the day, some designs adopted what I believed was an American idea, namely to sit an orifice plate across the nozzle, giving a sharp edged aperture; this not only reduced the diameter, but also decreased the discharge coefficient to about 0.85-0.90, thereby further increasing backpressure. It could be that this UP set up had a discharge coefficient of less than 0.99, hence in effect narrower diameters. (I am not clear what the rationale for the orifice plate was; if you say that the way to increase draught at a given exhaust choke is to increase the difference between the momentum of the exhaust jet and that at the top of the chimney, you can increase draught by increasing jet momentum, achieved by the orifice plate, but at the expense of even higher back pressure than normal; the purpose of the chimney is to get the minimum exit momentum for a given inlet momentum, so maybe the orifice plate (or perhaps also the cross wires found on certain nozzles) was to increase turbulence in the jet, which made the chimney more able to achieve a flatter exhaust profile hence lower momentum, hence better draught? It is the inability to easily calculate the momentum minimising ability of the chimney set up which seems to me to have hobbled exhaust design).

The dimensions of the Big Boy and Challenger exhausts, even if discharge coefficient is 0.99 are smaller than the figure I have for the C&O H8 (roughly 2 *7” blastpipes).

I am bit like a kid in candy store with the Farrington material, but among the things that strike me are a) that the Santa Fe was far more interested in the temperature of the motion bearings than that of the inlet steam! Clearly running hundreds of miles from A to B without running hot was a higher priority than saving a bit of oil. Strange people. In fact, though the data is not entirely clear, the superheat on the 4-8-4s is mediocre, certainly much worse than the 4-6-4s, and this seems to be related to very low gas temperatures at the end of the 21’ long tubes. b) I was amazed that the maximum cut of was fixed at only about 60%, reducing tractive effort from about 80000lbs, as given by the normal formula, to about 70000lbs. And yet they were sent up Cajon pass, with gross train weights of 1200 long tons i.e. about 60000lbs worth of gravity to deal with unassisted!  Keeping the boiler pressure at 100% must have been absolutely essential.  c) There is to British eyes a very surprising increase in the pressure drop between the regulator and steam chest as steam rate increases, much larger than found here, although of course here maximum steam flows were far less. d) At speeds above 60 mph, the cylinder pressure does not even reach steam chest pressure at the beginning of the stroke (the programme reproduces this effect on its predicted indicator cards for the 4-8-4 well) though this is not the disadvantage it might seem and finally, and sadly e) the speeds claimed for 2900s climbing from Dodge City to La Junta in excess of 90mph (96mph in one case) are absurd, though going eastwards on the slight downgrades would be easily possible. In fact, having now built the line all the way from San Bernadino to Barstow, if the regulator on the 3766 had been closed at Cajon summit when speed reached ca 40 mph, it could have remained closed all the way to Barstow, maximum about 123mph just before Victorville, still 60 mph at the West end of D yard(!) I hadn’t realised that the line goes back down nearly to sea level at Needles. The eastbound grades from there are awesome; next challenge is to see how 3776 coped with the 80 or so miles of 1/70 grade on the way to Wampai, and I will be ‘building’ this line next.

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Posted by Firelock76 on Thursday, May 26, 2011 6:51 PM

Just have to add one more thing to "The Tread That Will Not Die!"  I was consulting my copy of " The Steam Locomotive In America"  (1952) and lo and behold there was a comment on exhaust nozzles and backdraft.  Seems the locomotive builders and railroads themselves were always aware of back pressure problems, but they never found a practical solution to the problem.  They DID know about adjustable exhaust nozzles that were in use in other countries but decided that the constant attention they needed to get the best out of them, plus the increased maintanance costs made them just not worth the trouble.  It was easier to live with the inefficiency, which really wasn't all that bad. 

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Posted by Dreyfusshudson on Friday, May 27, 2011 9:39 AM

Dear Firelock,

Many thanks- in a separate stream of correspondence I commented that 'it was a small world' even in the 1920s- the latest developments in US steam technology were avidly looked for in Europe, and I guess the same would be true in reverse. So, very interesting to have confirmation that US designers were aware of European efforts in exhaust design, but with hindsight not too surprising.

I'm busy trying to focus everything folk have sent me into an article along the lines of 'would Chapelon's initiatives have made any difference to latter day US steam', taking the J3a, Niagara, T1 F7, 3460 and 3765 classes as representative. (Sorry N&W and UP fans, data lacking). If anyone would like to critique this prior to submission for publicaiton, I'd be more than happy to have your comments- it'll be a couple more weeks in gestation.

If there's anyone still out there, I have come across something that's in the 'I don't believe this' category. In my analyses of 'Normal' F7 running on the Afternoon Hiawatha, I conclude that the locos were worked close to maximum NTE accelerating from rest up to about 30 mph, when it's the boiler rather than engine limit that comes into play, and worked at just less than 4000IHP to accelerate to 100mph, after which you could cruise at this speed at 2500- 3000IHP. There are several runs that fit this pattern, including a run from Milwaukee to Chicago in about 68 minutes net with 9 cars - the fastest the author knew of. ( See Hiawatha Story). This is comfortably within the computed performance envelope for an F7, but well ahead of what was expected from a J3a it would seem.

 However, there is a claim in a book by Baron Vuillet of something of a different order. It is said that Milwaukee to Chicago was run in 63 minutes with sixteeen cars! This involved covering the first '12miles in 12 minutes' which is truly impossible with this load; I think even 13 minutes would be sensational, and would require e.g. crossing the Kinnikinnic drawbridge at 50+mph. The 'next 62 miles were covered in 37 minutes at an average of 100.7 mph'. Again, I think this is more or less impossible, unless there was a howling northerly gale blowing. That all gets you to Edgebrook in 49minutes, leaving 14 minutes for the last 11.5 miles; whether this is possible depends on what the speed limits at Mayfair, Pacific Junction and Westen Avenue were at that time (1943- Vuillet said some limits had been relaxed by then), but with a slow run in round the bend at Canal Street, this also sounds unlikely to me.

Does anyone have chapter and verse on this claim?

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Posted by CSSHEGEWISCH on Friday, May 27, 2011 10:00 AM

Vuillet's claim comes out to an average speed of 80.95 MPH based on 85 miles for Milwaukee to Chicago.  Without even considering the locomotive's limitations, various speed restrictions in the terminal areas would preclude such a high-speed run.

The daily commute is part of everyday life but I get two rides a day out of it. Paul
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Posted by Dreyfusshudson on Saturday, May 28, 2011 5:05 AM

Dear CS,

 

Thanks for this.

Vuillet also claims that an F7+ 14 got from Milwaukee to Chicago in 65 minutes, which I thought might be possible, but looking more closely, some features e.g. the passing time to Sturtevant and cruising at 107mph are impossible and very unlikely respectively.

This is odd because Vuillet is a very respected and knowledgable source, so he must be quoting another source, perhaps word of mouth.

I don't wish to appear to be knocking the F7s, they would probably get my vote for the gold medal; accelerate from rest at the limit of TE, get up to 100mph and stay there. Take a few extra coaches if needed. No other steam locomotive I know of did that day in day out. But such gold medals are very subjective of course.

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Posted by Dreyfusshudson on Thursday, June 2, 2011 1:16 PM

It’s maybe time to draw this all to a close, so let me try and summarise what I’ve concluded so far, thanks in great part to your inputs.

You shaped my question into ‘Could Chapelon have improved US Steam Passenger Locomotives?’

Chapelon is revered primarily for his efforts to improve efficiency.  In essence his approach had five main elements; high superheat; compounding to maximise expansion ratio; generous steam circuit to ensure maximum steam flow to the cylinders; feedwater heating to recycle otherwise wasted steam heat; and low exhaust pressure through better front end design, all factors together allowing more power to be extracted from smaller, lighter designs. Now US designs had feedwater heaters.  So, of particular interest are US front ends, the level of superheat achieved and steam circuits. In as much as there are differences in TE between US locomotives, those with higher TE will tend to work at better expansion ratios than those with lower ones at a given power output. Also critical is the Adhesion weight, for this is a key determinant of how fast you can start, and when you stall on a steep climb. For high speed working  aerodynamic resistance (frontal cross sectional area and drag coefficient) is also important- speed was limited to 75mph in France so this was not so much of an issue there.

The Table compares the designs in my survey on these criteria.  I have included the Challenger and H-8 for reference purposes. The first point to note is that I have not been able to locate the critical data for UP types, a most fundamental point to which I will return.

Class

J3a

F7

3460

S1b

T1

3765

FEF

Challenger

H8

 

NYC

MILW

ATSF

NYC

PRR

ATSF

UP

UP

C&O

TE, lbs

41680*

50295

49300

61570

64700

79700+

63800

97350

110200

Inlet steam max F

750?

??

750-800

??

800

680

???

???

730

Inlet steam Typical, F

??

700

670-690

680

700-750

630

???

???

700

Valve size “

14

12

13

14

12

15

???

???

12

Boiler- valve chest pressure drop, psi

??

10

Up to 30

??

10

Ca 30 ‘too much’

???

???

12-18

Blastpipe diameter,”

6.5

7

7.5

7.5

7.5?

8

???

???

9.6

Adhesion wt, lbs

210500

216000

213000

274000

273000

287000

267500

404000

471000

 

Frontal area, sqft

155 **

155**

167

155

155**

188

???

???

???

Grate Area, sqft

82

96.5

99

100

96.5

108

100

132

125

*12400lb booster    + in practice TE ca 70000lbs on starting, because maximum cut off 60%

** streamlined

To assess the benefits that might have been attained, two criteria are needed; firstly what was the maximum performance these types could deliver on test?  My analyses of the actual running the passenger types achieved says that, with the exception of one circumstance, evaporation rates were normally in the 500-600lbs/sqft/hr range, i.e. even the largest types didn’t evaporate more than about 60000lbs/hr- with a reasonably good engine efficiency around 14lbs steam/ihp-hr this translates to typical maximum IHPs around 4300. This is my definition of ‘typical’ working level. (Note that types burning lower quality coal (F7, UP types) needed larger grates to generate a given amount of steam).

The one exception is what I would call the Cajon test. On this steep grade, and others like it, engineers had no choice but to work flat out, using the maximum TE the engine can deliver. This falls somewhat from its maximum nominal level as speed builds up, and steam rate hence backpressure increase, and the pressure drop between the boiler and valve chest increases. In this circumstance, then, the benefits of Chapelonisation would have immediate impact; there is a direct need to generate maximum possible TE as speed/steam rate rises. This logic also applies in spades to freight types such as the H8 hauling maximum loads over mountain districts. For all types, up to about 30mph maximum output is TE limited. At this point, steam consumption  is roughly 700lbs/sqft/hr. Beyond this point it rises so sharply if you insist on trying to develop maximum TE that a boiler limit comes into play.  Interestingly, on the long adverse grades elsewhere on the Santa Fe, where maximum power was desirable, but not essential- e.g. the long drag from Needles to Kingman, maximum power was not used, just steady steaming at ca 500-600lbs/sqft/hr.

Points to note from the Table are:

TE: the F7 and 3460 are higher than the J3a, and will develop a given power in shorter more efficient cut offs at speed.  The ATSF 3765 will also benefit in this respect over the FEF, T1 and Niagara on account of its higher cylinder capacity, and is the ideal type for the Cajon test- the other three would fall by the wayside here.

Adhesion To keep adhesion factors around 4.2, given maximum axle loadings around 65000-70000lbs means that Maximum TE for three coupled axles is about 50000, for four axles 70000lbs. High TE is critical for certain tasks, so there’s no point in lightweighting the engine too much or you’ll reduce adhesion weight!

Superheat. There appears to be only limited data on this most critical of efficiency determinants. In this country, all tests (A type superheaters) showed that it increased smoothly with evaporation rate/sqft, the exact level depending primarily on the number of flues (more = better), tube plate distance (longer = worse) and firebox area (larger e.g. with thermic syphons = worse); it is however something that it was not possible to estimate from boiler dimensions- though I’m sure a good computational fluid dynamics programme could address this today. Superheater areas by themselves are meaningless. Generally typical working levels seem to have been around 700F- pretty good, but possible to improve, as the T1 test results showed, and Chapelon would have claimed. One notable exception is the ATSF 4-8-4, which is about 50F below their 4-6-4; this led to a measurable increase in water consumption vs the 3460 across the desert districts. This however passed without comment! This fits with the overwhelming impression formed form going through these test data – the need for operational simplicity/reliability/low maintenance was far more important than worrying about a few percentage points efficiency.

Steam circuits

There appear to be inexplicable differences here between types which achieved perfectly respectable boiler/ steam chest pressure drops of ca 10psi at normal working rates to the Santa Fe types that showed up to 30 psi loss, something they commented on their test reports. Clearly if this could be improved, the superiority of the Santa Fe types on the ‘Cajon test’ would become even greater.

Blastpipe pressure.

The figures for blastpipe diameter above are, for multiple blastpipes, equivalent single blastpipe diameters. It seems that US designers thought that blastpipe pressure up to 12-15 psi were ‘ok’, and they were happy with these levels for their number 1 priority was good draughting/steaming. At speed, at the maximum steam rates operated in practice say 60000lbs/hr, this criterion was normally beaten. At 60000lbs/hr, blastpipe pressure would be around 14 psi with the 6.5” J3 a nozzle, 10.5 with the F7 7” nozzle, 8 psi with the Niagara/3460/T1 7.5” nozzle and 6 psi with the 3765 8” nozzle. Going from 6.5” to 8”, brings a clear gain of about 500IHP, ca 12% at this steam rate. Going to 9.5” brings another 100HP, but diminishing returns are setting in, and there’s really no need to go above this for high speed working. Under the slow speed maximum output ‘Cajon test’ at ca 85000lbs/hr, 66000lbs TE at 25 mph, blastpipe pressure would be about 14 psi with the 3765 Layden set up. Here even a 10” blastpipe would still give about 5.5 psi, but there would be a very valuable gain of about 3000lbs in ITE.

Streamlining

The difference in aerodynamic resistance between the massive, blunt 178 sqft envelope of the 3765, and the streamlined 155sqft front end of the Milw F7 is of the order of 200HP, the benefit going straight through to the drawbar. I’m no aerodynamicist, but the streamlining on the T1 and J3a looks more cosmetic to me. Running at 100mph, there would be a big benefit for the F7; other types running more slowly less so; in fact its seems to me that the ATSF 4-8-4s spent much of the time either working hard but slowly uphill, or at high speed coasting without power and braking downhill, so for them streamlining would have brought virtually no economy benefit.

In general, I conclude that all but the T1 would have benefitted from higher superheat at normal operating speeds- the T1 boiler shows this can be done- perhaps 50 degrees more being a reasonable target. All would have gained significant benefit from blastpipe diameters about 1”-2 “ greater than they had. Together these two benefits together could have given improvements in efficiency of 10-15% at maximum daily working rates.  Whether this would have been worth the maintenance trouble, I cannot say, but had steam had a future, any issues would surely have been solved.

The reason the absence of data for the UP types in general and the FEF in particular is so critical is that Jabelmann was said to be ‘obsessed with back pressure and superheat’.  It is possible therefore, particularly in the absence of thermic syphons, and with a shortish (19’) boiler, that they already achieved superheat approaching 750F at normal steam rates. Kratville gives pictures of a Jabelmann exhaust, but nowhere can I find what the diameter of the eight nozzles were. 8*3” would give the equivalent of 8.5“ single diameter; 8*3.5” almost 10”. So it may be that the ‘Chapelonised’ 4-8-4 already exists, and is heading to St Louis as I type. It would not however be as good as an ATSF 4-8-4 on the ‘Cajon test’. The C&O H8 already ticks all Chapelon’s boxes.

I have a couple of leads I am still following up to try to establish what the Jabelmann nozzle diameters were. I shall wander the planet, a lonely soul until I find out.  Perhaps the simplest thing to do would be to hop on AA 55 tomorrow with a tape measure, and head to St Louis on Friday night, and get into 844’s smokebox. I feel that these days such behaviour would lead to me being incarcerated for my troubles, but prisons can be quite comfortable now.

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Posted by Burgard540 on Tuesday, June 7, 2011 1:04 PM

DryfussHudson,

Just wanted to answer some questions about the use of multiple nozzles instead of a single blastpipe.  The Challengers and Big Boys had two 4 nozzle plates that sat atop a stand that formed a receiver for the engine exhaust pipes.  Also, the pipe for exhaust steam to flow to the exhaust steam injector was connected there.  The downside to this configuration is that the back pressures on of one engines could interfere with another engine during its exhaust stroke.  The April, 1943 tests recorded a back pressure of 18 psig.

Regarding circular nozzles, the flow of steam is simply proportional to its area.  Other shapes of nozzles follow the ratio of area to perimeter, the larger the perimeter for an area reduces flow.  This is known as "mean hydraulic depth".  However, larger perimeters allow greater contact between the exhaust steam jets and the combustion gases which entrains more gases and thus greater vacuum.  Comparing the 8 nozzles to a single large circular nozzle of equivalent area gives:    

8 nozzles at 3" ID - total area 56.55 sq in  - total inside perimeter  75.40 in

1 nozzle at 8.485" ID - total area 56.55 sq in - total inside perimeter 26.66 in

With regards to the equivalent coefficient on the program, I'll have to get back to you.

Cheers,

Joe

"If a nation expects to be ignorant and free, it expects what never was and never will be." Thomas Jefferson

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Posted by Dreyfusshudson on Wednesday, June 8, 2011 7:11 AM

Dear Joe,

Thanks for these comments.

I didn’t have the 18psi pressure for the Big Boy. If you look at the 1943 tests, they covered the 39 miles from Ogden to Echo in about 100 minutes, average speed 23mph, so probably got up to about 30 mph. Loads were about 3750 US tons, 3350 long tons. Average evaporation rate was about 82500lbs/hr, (just over 500 lbs/sqft/hr) blastpipe rate about 75000lbs/hr.  

I have had an initial stab at building the line from Ogden to Echo. I covered the distance in about 100 minutes with 3350 long tons of UK passenger stock (!) by dint of a) accelerating slowly, keeping speed to less than 15mph over the first mile- I assume the train was leaving the yards b) accelerating up to 30 mph to the foot of the 1.14% grade 5 miles out of town, peak of 5000IHP whilst accelerating c) working at 90+% of nominal TE to maintain 18 mph up the 6 miles of 1.14%, about 5500IHP, 115000lbs TE and d) maintaining 26-30 mph on the gentler grades beyond there on around  3600-4400IHP. Average IHP was about 4400 over the middle 37 miles. Cut off would be 60+% on the 1.14%, pretty easy working at 30% or less on the easier grades at 30 mph- playing with the train really. Since much of the work being done is against gravity, the choice of the wrong type of stock won’t be too tragic for the power estimates. With the hardest working, there would be peak backpressures of about 18 psi on the 1.14% if the effective diameter of the blastpipes was between 8.5 and 9.0”. This is a bit less than I was expecting, similar to the PRR Q2, less than the 9.6” of the C&O H8, which would have given less than 10psi at this rate of working. However, these calculations are very much preliminary estimates, and hard data on the Jabelmann systems still needed. A lot can change when you know the facts!

If you raise the load to 4200 short tons, as was practiced, you do need full power working at maximum NTE to clear the summit of the 1.14% at about 15 mph.

I understand the logic of the greater perimeter argument, but think it not particularly helpful. You can analyse the front end using standard turbulent jet theory, which says that entrainment depends only on nozzle diameter and the speed of flow through it; with four interacting jets, the calculations can still be done. However, if you do the sums, this standard entrainment model does not give sensible answers. A more fruitful approach I found was using fluid mechanics/momentum balance. The purpose of the chimney set up is to minimise the outlet momentum. Unfortunately this is difficult to calculate from the front end dimensions. What I did was calculate how successful various chimney set ups tested in the UK were at minimising momentum. Quite significant differences, reasons not obvious, but it’s all a long time ago now.

One thing that is really bugging me about my analyses of US Passenger types is how no one seemed to expect to work them regularly at evaporation rates of more than about 600lbs/sqft/hr, 700lbs/sqft/hr an absolute limi, well inside what they could steam at, and well inside what the engine could consume if pressed. Now this is very sensible from the point of view of both coal economy, and I suspect boiler maintenance. So people were behaving very rationally. How strange! Surely this point of view is documented in contemporary literature?

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Posted by Burgard540 on Sunday, June 12, 2011 12:15 PM

Dreyfusshudson,

A lot of the work moving the train between Ogden and Wahsatch was fighting gravity.  To move 4500 short tons (locomotive, tender & train) up the total climb of 2501 feet in 205 minutes requires about 3300 continuous horsepower.  This neglects of course all the other resistances and horizontal distance covered.

Do you have any textbooks or papers that you found helpful for calculating the fluid momentum balance & turbulent jet theory?  Fluid mechanics was a course that I didn't really pay attention much too at the university....oops.

Lately I've been studying steam flow through nozzles.  The exhaust nozzles (or blast pipe) and smoke stack form the configuration of a convergent nozzle flowing into a convergent-divergent nozzle.  The areas of each are known.  The mass flow of steam and combustion gases can be calculated fairly well.  Any one have references regarding the study of gas velocities through smoke stacks?

Joe

"If a nation expects to be ignorant and free, it expects what never was and never will be." Thomas Jefferson

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Posted by Dreyfusshudson on Monday, June 13, 2011 5:01 AM

Dear Joe,

Thanks- the fact that the majority of the work is being done against gravity is why I was reasonably relaxed that even though the rolling resistances I was using were quite possibly wrong, I wouldn’t be too far out; gravity in Utah is pretty much what it is everywhere else!

I’ve now had a go at simulating the more detailed log of 4016 on p22 of Kratville as far as Echo. The UP gradient profile is wonderfully detailed, but when the train is often strung out over two or three changes of grade, the best you can hope to get from an analysis of train motion is to get into the right ballpark! In fact, even though I’m using what could be a completely inappropriate rolling resistance equation, I get incredibly good agreement with the reported ‘Drawbar horsepower Corrected’ (EDHP) figures, simulating to the reported speeds and overall time.

4016 was only being worked nearly flat out on the 1.14% from ca MP 981-987, easier on the other stretches (if you count 5000IHP as easy). The peak reported EDHPs of over 6000 are a consequence of the easing of the grade at the point in question, not deliberate advancing of cut off to achieve this. The easing of the grade allows the train to accelerate, and cylinder power naturally increases with engine speed at constant cut off, hence EDHP; at these speeds, the parallel increase in locomotive resistance is not as steep.

It’s at this point that my problems begin. I have the average IHP over the whole trip as 4350, about 7500 IHP-hrs. I could easily be 5% out, but given the agreement with the EDHPs, not much more than that.  Now if you look at the reported cylinder consumption, it’s 144000lbs. This implies that the cylinder efficiency (SSC) is just less than 20lbs steam/ihp-hr. Now, making a conservative assumption about superheat (it mightn’t be top notch given the long tubes and huge combustion chamber, N&W A was only 650F), my computer programme, which is a pretty reliable friend says that SSC will indeed be about 20 for the 25 minutes flogging up the 1.14% in near full gear. However, on the easier sections, if the throttle was wide open, and cut off were eased right back to allow efficient expansion, SSC would fall to about 15lbs/ihp-hr, so overall I would not expect average SSC to be much worse than 16, say 120000lbs cylinder consumption. 24000lbs is a huge discrepancy. There will be some auxiliaries e.g. stoker feed of course, but nothing like this.  The only reasonable explanation for this is that the power was eased not by shortening cut off, but by part closing the regulator, sorry, throttle, leaving the locomotive working in a very long, inefficient cut off. There are two bits of support for this idea. Firstly Kratville says of the run of 4014 (and with some pride!) ‘All the way up the throttle was open less than full’. Secondly, on the third run made, which was slightly faster, water consumption comes down by 17000lbs to 127000, pretty much where I would expect it. So maybe on the third run, because they were ‘hustling’, they did use full throttle, shorter cut offs. Even across this enormous distance in time, I have huge respect for the crews, so if inefficient part regulator was being used, there must have been a good reason. Question is, what?

Even worse, the report says that the specific evaporation, lbs steam/lb coal is only 4.3. Now the 144000lbs figure for water requires an hourly evaporation rate of about 85000lbs/hr, say 560lbs/sqft/hr. Under this condition, with 13800BthU/lb coal, you will normally find about 7.3 lbs/steam/ihp-hr everywhere in the world. Mechanical stoking causes higher unburned coal loss, because the screw grinds up the coal; absolute worst case scenario on the data I have, 10% worse, 6.4 lbs steam/lbs coal. Now Wyoming coal, from an authoritative source in another thread is only 11700 BthU/lb, 15% worse, hence 5.4 lbs steam/lb coal. But 4.3- another 20%- that’s terrible. The unburned coal loss must have been amazing- Kratville does talk about the ‘rain of the cinders on the roof’. I am at a loss to explain this- the Big Boys were not terrible- they used the same technology as everyone else. I have two hand waving, not thought through possibilities in mind. Firstly, could this be an altitude effect? Like an athlete in Mexico, the thinner mountain air means the Big Boy had to get a greater volume of air into its lungs to produce a given amount of heat. If there is any truth in this, then surely tonnage ratings for climbing steep banks near sea level (e.g. north from San Luis Obispo) would be greater than the top of Donner Pass. The second is the method of firing. I have a quote from a British observer riding an FEF west from Lincoln that I have never fully understood. ‘The coal used was small and dusty, and a feature of its use was that hardly any of it lay on the firebed. At the start of our trip, the fire was only about 1 ½” thick over the grate, and only intermittently bright, and it returned to the same condition at intermediate stops. The action of starting the stoker was akin to using oil fuel, in that the coal ignited and burned mainly in suspension in the firebox and combustion chamber……..as rate of combustion increased, unburned particles increased in volume, and their shooting forth by the powerful blast was the cause of those wonderful pictures of American trains in action where an almost unbelievable column of black smoke rockets into the stratosphere’.  This way of working, if practiced would seem to me to be likely to lead to very high unburned losses, although there is no evidence from the Big Boy test photos, of ’unbelievable columns of black some’. Any thoughts on this welcome.

I have a very helpful paper on the fluid dynamics of front ends that I’ll send you. It’s not the finished  article, in that at the end of it, you still can’t use the maths to design a blastpipe+ exhaust system that will give optimum entrainment for a given firebed and boiler resistance for reasons I’ll explain. However, I find the conceptual thinking it embodies very powerful.

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Posted by Burgard540 on Tuesday, June 14, 2011 2:35 PM

Dave,

"gravity in Utah is pretty much whait is everywhere else!" - Ha ha.  For everybody's sake I hope so!

Thanks for the papers on Front Ends.  I'll be looking them over quite a bit before I reply...you're ahead of me on this subject.  I've really only been studying the steam locomotive from a design engineer's perspective for a little over a year now. 

In a Railway Mechanical Engineer on the 3rd or 4th series of Challenger locomotives, it said they were designed to burn coal of 11,800 Btu / lb.  I would assume the Big Boys and other UP locomotives would be designed for similar coal heating values, and 11,800 Btu / lb coal is consistent for coal from Rocks Springs, WY. 

As a side note, I never understood why engineers don't use the lower heating value of coal.  For example, the Hydrogen content of the coal could easily cause 500+ Btu's to be lost in the latent heat of the water vapor formed via combustion.  It's only ~4% loss, but not insignificant.  This excludes the moisture content of the coal before firing.

Other references have made comments about stokers causing much of the coal to be burned in suspension, never hitting the grates.  I think "Last of the Giants" makes a claim similar.  I'll try to look up the sources.  The unburned fuel losses of locomotives was quite large, running upwards of 50% at high firing rates (175 + lbs coal per sq ft grate per hour).  Simply not enough firebox volume & time for combustion to take place before the combustion gases enter the flues.  Ralph Johnson states that modern locomotive boilers produced boiler efficiencies of 80% at firing rates of 100 lbs coal per sq foot of gate per hour due to large grates and combustion chambers ("The Steam Locomotive" pg 82-83).  The Big Boys would fall into that modern category, but the low evaporation rate of ~4.5 lbs of water per pound of coal fired does seem low (rate of firing ~125 lbs coal per sq foot grate per hour, not excessive).

Joe

 

 

 

 

 

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Posted by Dreyfusshudson on Wednesday, June 15, 2011 6:15 AM

Dear Joe,

Thanks. I’m not sure when Locomotive engineers began to come to terms with the fact that they were losing 4% of heat due steam from the combustion of the volatiles in the coal. It may have been when the Orsat apparatus for analysis of smokebox gases became widely available- with this you can begin to have a stab at the overall boiler heat balance. Not sure when this was, may have been when interest in the subject was waning. It may not even be mentioned in Lawford Fry’s book (1924)- haven’t got access to all of it..

In terms of overall boiler heat balance, for a simple boiler you lose about 12% of heat in the waste gases+4% in latent heat of steam (these two largely independent of firing rate)+ unburned coal losses which are about 15% at 100lbsq/sft/hr with careful handfiring, increasing rapidly with firing rate, say 70% overall efficiency at 100lbs/sqft/hr. However, if you recycle 10-15% of the steam through a feedwater heater, then in effect this is an improvement to boiler efficiency, giving you the 80% figure you quote. In other words, Johnson’s figure makes perfect sense to me, and against this the Big Boy figure does seem low. The Big Boys only had exhaust injectors, not feedwater heaters, but this would not explain the difference.

In the paper I sent you there is a graph of mine plotting unburned coal loss/sqft vs gas flow/sqft for handfired types, and the one mechanically stoked type tested in the UK. At the kind of gas flow rates we are talking about, mechanical stoking is about 10lbs/sqft/hr worse. However, I have always assumed (it’s not stated) that what the UK testers did was to use the stoker to feed a normal hand fired bed (say 6-12” deep) so the coal burns in the bed. I’m sure there will be references to stoker firing technique in the US literature, most interested to hear what you come up with.

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Posted by Dreyfusshudson on Thursday, June 16, 2011 7:23 AM

A short supplementary on Big Boy Boiler efficiency.  I have checked the boiler efficiency data I have for the PRR T1, C&O H8 and NYC Niagara against a model of boiler efficiency I have developed.  This is based on the correct first principles, but these principles are expressed as empirical equations derived from UK test data, i.e. are not fundamental scientific expressions, so might not apply outside the range of data on which the empirical equations are based. I was pleasantly surprised to find that the model gets the boiler efficiency on the above three types pretty much spot on. I had to use an expression for unburned coal loss for a mechanically stoked boiler based on the only UK locomotive ever tested this way, and I thought this might be a weakness. Evidently not. Unburned coal loss generally increases with increasing firing rate/sqft (better I think heat produced/sqft grate, which approximates to evaporation rate/sqft). The Table below, derived from the model, shows a smooth increase in unburned coal loss with evaporation rate/sqft for these types.  I have included for comparison what a Niagara might achieve with two firemen manually stoking the grate.

Locomotive

Evaporation rate

 

Unburned coal loss as % fired

Lbs/hr

Lbs/sqft/hr

Hand fired NYC Niagara, maximum service rate

62150

622

11

Mech stoked Niagara, maximum service rate

62150

622

21

C&O H8 High steam rate road tests

96050

712

26

PRR T1, Very high rate plant tests

96050

1044

38

UP Big Boy on Wahsatch

84750

565

47

 

The use of mechanical stoking on the Niagara is predicted to lead to a 10% increase in unburned coal loss. At high in service steam rates, the NYC and C&O types are losing about 20-25% coal unburned. At the exceptionally high rates practised at Altoona, losses were approaching 40% on the T1.

The Big Boy tests show that at a lower specific evaporation rates than the other three types, unburned coal loss was approaching 50% of that fired. In other words, there is something fundamentally different about the nature of the coal fired, or the way it was fired on the UP which caused very high levels of unburned loss. It is possible that the Big Boy front end drew more air through the grate than other types, and this may have added to the effect, but I do not think this can possibly be a standalone explanation of these data.

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Posted by Paul Milenkovic on Sunday, June 19, 2011 12:21 PM

Hand fired Niagara?  Is this a hypothetical or does this represent some test where they had guys working in relays?

With respect to the Big Boy, I believe they were using some kind of Western States low-BTU high-ash coal -- there is a lot of advantage with steam to use local supplies of fuel to save on shipping because you need so much of it.  Perhaps the closest modern equivalent to the Big Boy fuel would be the famous Powder River Basin coal that many of the railroads today ship to power companies.

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by erikem on Sunday, June 19, 2011 6:44 PM

Re: Big Boy vs Allegheny

The biggest difference between the Big Boy boiler and the Allegheny boiler was the firebox. The Big Boy's firebox extended over the rear drivers and thus had to be made shallower than optimal. The Allegheny's firebox was entirely behind the drivers and was therefor deeper. The deeper firebox would lead to better flow of combustion gas plus unburned coal particles and present more direct heating surface. The Allegheny also had overfire jets, where the Big Boy just had ports in the walls of the firebox.

- Erik

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Posted by Dreyfusshudson on Monday, June 20, 2011 5:38 AM

Dear Paul,

The hand fired Niagara is simply the result of a number crunching exercise. There was research done into what the maximum quasi sustained shovelling rate for a reasonably fit human was, and the answer was ca 4000lbs/hr, possibly on terra firma e.g. a building site. In this country, with hand firing, crews are still reguarly trying to get maximum outputs from their locomotives on fan trips, to please the passengers. It is quite clear that the peak sustained performances achievable are about 2400 IHP; a handful have made 2600IHP. If you do the sums, this says that the fireman is able to sustain 4500-5000lbs/hr for 15 minutes or so. This is the bar to higher sustained outputs. In the short term, 3000IHP is the target, but this involves running down the boiler level. Here 's a loco that's still trying to join the elite 3000IHP club- best effort about 2950IHP over 5 minutes hauling 500 tons at 75mph up a 0.5% grade. In this clip, the exhaust sharpens around 0'15" because the crew have been running downhill more or less without steam for the last 35miles, and the crew are preparing to attack the mile of 1% beyond the station.

http://www.youtube.com/watch?v=v2rmtJ9EUBQ

(OK a bit gratuitous, perhaps best filed under 'Why this subject retains its interest'.)

So, two firemen could in principle fire a Niagara to its normal maximum output, were it not for the fact that a) they would have to perform some kind of ballet across the cab floor; the distance between the tender and the firehole door ideally needs to allow a simple one movement swing and b) they would have to have the energy to fling the coal to the back corners of the enormous firebox.

If the Niagara used most of its 42 tons of coal from Croton to Chicago in normal operation, hand firing would save about 6 tons of coal per trip, not enough to pay for an army of additional balletic musclemen, I think.

I'll comment on Wyoming coal in my response to Erik.

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Posted by Dreyfusshudson on Monday, June 20, 2011 6:25 AM

Dear Erik,

Many thanks-I'm in a parallel conversation with local experts who are making exactly the same points about the Big Boy firebox.

However, I remain  sceptical about these arguments; the reason is that the lifetime of a particle of coal flying through the firebox is less than a second, whereas the burn time for particles of more than a millimetre is measured in minutes (Wardale provides the data). Backing this up, I have test results on the PRR Q2 with and without induction tubes, from which they conclude they have no meaningful impact on boiler efficiency.

Now there is UK data that clearly shows the benefits for overfire air with hand firing of lump coal- significantly reduced coal loss. When firing lump coal, there has to be a process which produces particles that are small enough to be swept away by the air flow. This is I think the production of fine coal char of a few mm in size from the coal surface as it burns rapidly- the stuff that lands in your eye when you look out of the coach window. With more overfire air, and less primary firebed air, there will be a cooler firebed which leads to less char production at a given rate of carbon transfer to the gas phase - the carbon is only partially oxidised to CO in the firebed. This is subsequently oxidised to CO2 by the overfire air. This is the basis of the GPCS system also.

However, with mechanical stoking, the majority of the unburned loss is from the stoker feed. I would bet these fines never reach the firebed, but  are just swept away, perhaps fragmenting further in the heat of the firebox.  A small increase in oxygen partial pressure from overfire is not going to influence this. Hence over fire air can influence loss from the bed, but not I suspect from the much larger stoker feed. Higher firebox oxygen can influence the rate of oxidation of volatiles, so reduce black smoke. This is consistent with Wardale.

So, my hunch is that the Big Boy's low boiler efficiency is indeed a consequence of the Wyoming coal used, and that its problem is that it either crushes more easily in the stoker screw or is more prone to fragementing in the heat of the firebox.  

If you were to fire a Big Boy with Appalachian coal, (much more like UK coal) my bet would be it would behave pretty much like an H8. This very intersting experment would be a rather expensive to organise in 2011, but I'm sure quite a few people would turn up to Cheyenne to watch.

Somewhere in the memory banks is I think a quote by a UK CME, Sir Jasper Spock visiting Wyoming in the late 19th Century 'It's coal, Isambard, but not as we know it'.

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Posted by Burgard540 on Tuesday, June 21, 2011 8:48 AM

The firebox of the Big Boy locomotives, by loading gauge limitations, had to be long & shallow since it was above the rear 2 driving wheels.  Kratville makes the statement that the firebox should have been about 2 feet deeper for proper proportion.  For me the question is for the same firebox volume, what are the effects of a shallower firebox with long path length of combustion gases vs. deeper firebox with shorter path length of combustion gases?  Intuitively, you would want a longer length for the combustion gases so that they have a greater time to complete the combustion process.  As designed & built, the Big Boys & Challengers had over fire air openings on the sides of the firebox.  These secondary air openings were later removed about 1948-1949, why I don't know considering the now well known advantages of introducing large amounts of air over the firebed.   (This should probably be in a new thread about firebox design...)

Regarding the coal used on the Union Pacific, I've been trying to track down good information on this subject, ie the percentage break down of its elemental components, ash contents, typical moisture content, etc.  The coal UP used for its steam locomotives came from Rock Springs or Hanna.  It was not the Powder River Basin coal mostly mined now out of Wyoming (Btu value of ~8500 per lb)  From various sources (like Babcock & Wilcox), Rock Springs coal has a heat value of about 11000-12500 Btu/lb.  It's friability is a good question, since the small pieces would be sucked right out the stack in the stream.  Also, what was the typical size of coal particles after passing through the stoker? 

Regarding getting the Alleghany's and Big Boys fired up to compare...there's nothing 20 million dollars can't solve!  

 

"If a nation expects to be ignorant and free, it expects what never was and never will be." Thomas Jefferson

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Posted by Dreyfusshudson on Wednesday, June 22, 2011 9:18 AM

Dear Joe,

Thanks for this. The kinetics of combustion of CO to CO2 seem to be pretty fast at firebox temperatures, at the kind of excess airs present-that's why you can use GPCS, or fire a thicker bed with less primary air and reduce unburned losses from the firebed. As far as I know, firebox design isn't a constraint here. Some goes for combustion of volatile compnenets of coal I think, more secondary air allows combustion to complete, less black smoke. My suspicion remains that firebox design won't have too much effect on the combustion of dust from the stoker- the coal burning process much too low.

Just had information that the French used graded coal-1- 2" pieces to reduce coal losses with mechanical stoking, so this could be another variable in the 21st Century Sherman Hill trials. For completeness, a DMIR Yellowstone and SP cab forward need to be added, I think.

My understanding is that the path of the Big Boys et al in such locations is still marked by a layer of coal char on either side of the tracks, something for the 31st century archaeologist to ponder.

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Posted by Dreyfusshudson on Thursday, July 7, 2011 12:11 PM

A correspondent has kindly provided me with a detailed drawing of a Big Boy front end, which shows the external diameter of the 8 blastpipe nozzles is about 4 1/8". Usung Kratville's picture of same, this puts the internal diameter of the nozzle at just less than 3 1/2". This gives an area equivalent to about a 9.5" single blastpipe- similar to the C&0 H-8. This means that even at a cylinder flow of of 100000lbs/hr ( say 5800 IHP at 20mph, 7000IHP at 40 mph) the back pressure will be less than 10psi- nothing too desperate, and under many operating conditions quite low. I imagine the FEF might be a bit  narrower, but if like other 4-8-4s they were normally worked at no more 65000lbs/hr, the blastpipe pressures would also be low.

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Posted by Burgard540 on Friday, July 8, 2011 9:19 AM

Dave,

Which drawing did he send you?  UPdrawing 443 CA 33736 (Jan 16, 1951)  is where I got the dimensions for the nozzle tips (3" ID & 4-5/8" OD press fit into nozzle plate).  It also lists the nozzle sizes for most other UP steam locos.  I'll send it and other pertinent front end dwg's via emal.

Joe

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Posted by Dreyfusshudson on Friday, July 8, 2011 1:04 PM

Thanks- I  was sent a copy of fig 2.112 on p171 of an unknown book- page titled Steam Locomotives- 4-8-8-4 Articulated types. I blew the image up, and measured the 33" shown across one of the chimney cowls as 63.5 mm. On this scale, the outside diameter of the nozzle is ca 9mm - about 4 5/8" as you say, but there is a taper at the top, leaving only 8mm", 4 1/6" though I quoted 4 1/8" ( margin of error).

I then looked at the pictures on p29 of Kratville; on the lower one the taper at the top is quite visible. I measured the outside width across the top of the taper as 50mm, the internal diameter about 41 mm, giving me about 3 3/8".

If the diameter is indeed only 3" as you say, that's equivalent to about an 8.5" blastpipe ( assuming 0.99 discharge coefficient)- not brilliant for something intended to steam at 100000+lbs/hr.

Very keen to get the nozzle diameters for the FEF also- can't get the relavant books that might have it on interlibrary loan here.

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Posted by Dreyfusshudson on Wednesday, July 13, 2011 6:34 AM

Have now got more data from Joe and other sources on UP front ends. The question of front end dimensions is not trivial, since the UP was constantly experimenting with and developing their locomotives. What is clear is that by 1951 the nozzles on the Big Boy and FEF were 3", on the Challenger 3 1/8". There is a 1942 drawing showing clearly 3.5" nozzles on the Big Boy, but this is marked 'not used' and was replaced by another one less than a month afterwards, when other features of the front end had been changed. A reference to nozzle spec is given, but this drawing is not to hand. In these and other diagrams up to 1946, the profile of the nozzles is similar, about 4 3/4" external diameter, with a taper to about 4 1/8" at the top, which would give a lip of about 1/4" if the nozzle internal diameter was indeed 3.5", as per my measurement of Kratville's photo.

Another UK correspondent says that the nozzles were down to 3" by 1947 as per the Locomotive cyclopedia, reduced from the original. Joe suggests this change might have been connected with the removal of over fire air at that time.

This all confirms the idea that a fractional reduction in engine efficiency was quite acceptable against other prioirties.

The specification in nozzle diameters to an 1/8" is interesting- variations of this magnitude shown for other types between coal and oil firing, winter and summer. We did the same in this country, but a random sample of locomotives brought in for testing in 'typical' condition revealed most of them had significant deposits in the blastpipe- some up to 1/2".(!). So much for drawings 

 

 

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Posted by Burgard540 on Saturday, July 16, 2011 1:23 PM

Some other ideas:

The gas area through the tubes and flues should be as high as possible to avoid the draft loss (especially at higher firing rates).   Samller area (and longer tubes) requires greater back pressure in the cylinders, due to higher velocities in the tubes.  The higher velocity has the advantage that since for convection the heat transfer is largely dependpent on the amount of gas flow, doubling the gas flow per tube increases the evaporation 90%.  Total gas flow area is limited by the diameter of the boiler due to loading gauge. 

So intuitively it seems that there's a trade off for greater heat transfer & evaporation due to higher gas velocities in the tubes, at the cost of higher back pressure in the cylinders.  This could explain the vast disparity between the heating surface for the locomotives below.      

I calculated the gas flow are for the high powered articulated loco's: Big Boys, Allegheny's, and N&W "A"s.  My calculated answer for Big Boy (first series #4000-4019) was within less than 1% than the given figure of 1631 sq in or 11.33 sq ft.  I assumed that the 2.25" tubes were all 11 BWG (0.120") wall thickness.

Big Boys (4000-4019) - 11.43 sq ft (Flues & Tubes 22 ft long); Total HS 8,355 sq ft

Allegheny (1600-1644) - 12.49 sq ft (Flues & Tubes 23 ft long); Total HS 10,426 sq ft

Allegheny (1645-1659) - 13.00 sq ft (Flues & Tubes 23 ft long); Total HS 9,717 sq ft

Class "A" - 11.04 sq ft (Flues & Tubes 24'1" long); Total HS 9,353 sq ft

Curious what the back pressures were for these locomotives under full power, especially the "A" since it had only 3.5" flues at over 24 ft long.  Big Boys had a back pressure of 18 psi. 

 

Going back to Big Boys lbs of water evaporated per of coal, the figures given for the April, 1943 tests were actual numbers not equivalent evaporation.  The 4.44 lbs water evaporated would be increase to 6.40 lbs equivalent evaporation per lb of coal fired.  Equivalent evaporation factor about 1.44.  I thought of this after seeing a few old reference books rating the boilers' evaporation per pound of fuel this way.

Other thoughts?

Joe

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Posted by Dreyfusshudson on Sunday, July 17, 2011 3:08 PM

Dear Joe,

Thanks- not sure if I understand the beginning of what you write properly. Are you saying that if you increase boiler resistance by either narrowing the tubes or making the boiler longer to improve heat transfer, you have to improve draughting, which means that you have to narrow the blastpipe, which means back pressure goes up, so in effect you are trading boiler efficiency for engine efficiency?

If so, I would say the following. Firstly, back then, they did have to make this trade off, because the understanding of front end design we have now was not available.  However, Chapelon said, only half in jest I think, that you should actually start locomotive design with the front end exhaust system; with the kind of systems he was developing, you could up the draught if need be without increasing backpressure, so the trade off was not needed.

More fundamentally I would say this. According to the computer model I have, heat transfer in the last few feet of the boiler tubes is negligible; every little bit of extra evaporation/efficiency is worth having if there is no cost associated with it of course, but longer tubes do cost both in materials, maintenance, and quite probably reduced superheat- a far greater loss to engine efficiency than the very small gain in boiler efficiency the longer tubes give. So, the proper solution if faced with this would be to cut the tubes down- 18’ is more than enough I would guess. (Though it wouldn’t do much for aesthetics if the Big Boy/Challenger boilers were 5’ shorter!)

So, for this reason, I don’t think the variation in heating surface areas you quote will be of much consequence to boiler efficiency, though they may indirectly influence superheat.

I have just rediscovered some stuff that adds weight to the idea that optimising boiler tube heat transfer is not that big a target. One of our testing stations looked at their data and tried to calculate what firebox temperature ought to have been from first principles- Stefan Bolztmann and the like. The approach really only needs you to know radiant surface area (which they assumed was just the firebox), and the tube gas flow, and the amount of coal fully burned (as opposed to fired, given some is lost unburned). You get these latter two via Orsat smokebox gas analyses.

Here are some figures they quote for where the burned coal’s heat goes; firebox radiation 38.3%, firebox convection  5.5%, superheating steam 9.5%, evaporation via  convection in tubes 28.1%, 18.7% lost in flue gases.   Trouble is that these calculations indicate that firebox temperatures were about 300F higher than they actually measured. Now they knew enough to know that what the pyrometer said was not necessarily accurate; even so, 300F is a big difference. Since it is the T4 Stefan Bolztmann radiant contribution that has the biggest effect on the estimate of T, I turned the calculations on their head and asked ‘What is the radiant surface area if the measured temperatures are correct?’ The answer is roughly twice the actual firebox area, which implies there is a radiant section in the firetubes just less than 10% of their length- about 1.5’ on UK designs close to the firebox. Now with this kind of approach there is a huge uncertainty on what this radiant length would actually be, but I think the data are good enough to confirm that the tubes do have a radiant section, hence the radiant heat transfer is significantly greater than the 38.3% above, hence the convective tube contribution significantly less than 28.1%, and since gas temperature decreases exponentially along the tubes, no surprise that losing this small piece of convection is a minor contributor. This also supports the idea I floated earlier based on computer models, that, were it not for a radiant section of the superheater close to the firebox, superheaters would be pretty ineffective, and there is a trade off between having elements too close to the firebox and getting burned, and amount of superheat you get.

If only there was, or we could do, proper experimentation to sort all these things out!

With regard to blastpipe pressures, then at equal steam rate, assuming (not necessarily correctly) that all blastpipe discharge coefficients were the same and close to 1, and that the earlier analysis of Big Boy blastpipe diameters over time is correct, then it would be 1942 Big Boy= H-8<A< 1946 Big Boy, with the Blast going sonic (> 12.3 psig) at about 110000 lbs/hr cylinder rate on the first two, around 90000lbs/hr or less on the latter.

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Posted by Burgard540 on Tuesday, July 19, 2011 12:30 PM

I could have written my previous post more clearly.  It was more born out of the questions regarding locomotive boiler capacities and performance given their design parameters such as direct heating surface (firebox & combustion chamber), heating surface of tubes & flues, gas flow area, etc.  The ALCO built steam locomotives for UP (FEF's, Challengers, and Big Boys) had significantly less heating surface compared to locomotives of similar size.  However, their performance was comparable.  Going just by total amount of HS doesn't explain the boiler's capacity, assuming similar heat energy inputs. 

Theoretically, the greater diameter of tube require less draft & less back pressure, but as the diameter increases the less heat is transferred.  The length of the tube should not be beyond 100-120 internal diameters.  Most of the temperature difference in the combustion gases is needed to overcome the stationary gas film along the tube's walls.  The best way to reduce the films effects is to increase the gas velocity.  The greater velocity of flow also causes the gases to be exposed to more tube surface in a given time, thus increasing the heat transfer of the gas.  Stationary gases in the tube can only give heat via radiation which as you mentioned only occurs for a brief initial length of tubes.

So, the ideal tubes and flues should be as small and long as possible with high velocity of gas flow.  Of course the there is diminishing returns, where smaller diameters and longer flues lead to greater soot build-up and increasing back pressures.  

The question then becomes for a given quantity of coal burned and combustion gas produced, does a the boiler perform better with relatively equal gas flow area and higher gas velocities through the tubes of less area vs. larger amounts of tube heating surface and slower gas velocity?  And at what point does increasing velocity for a given tube length lead to too high exit temperatures, greater draft & back pressure, and thus loss of efficiency?  Not to forget the front end design....

Clearly, an undersized locomotive boiler could be forced and produce the same quantity of steam as a larger boiler at the sacrifice of fuel efficiency.  However, I don't think steam loco designers did this on purpose, nor would a railroad like UP (which had an excellent engineering department) accept this flaw (i.e. losing money on extra needed fuel for the same train hauling capacity). 

No additional comments needed regarding the lack of heat transfer the last few feet of tube length, that's been well documented through various tests. 

I agree that radiative heat transfer occurs in the tubes & flues briefly.  That idea had occurred to me a while ago after studying the radiation of luminous and non-luminous gases and flames.   The combustion gases entering the flues and tubes would have significant amounts of CO2 and H2O (good radiators) and be at high temperatures approaching 2000 degrees F.

Regarding superheater elements too close to the firebox, the Big Boys originally had the superheater elements end 15” from the firebox tube plate.  However, that was increased to 24” in March, 1943, and later increased again to 36” in August, 1950.  It would be interesting to see if there were any noted performance differences or tests done to measure the effect on the final superheat temperature (or possible less pressure drop) from the shortened elements.

Sadly a lot of these questions would require full scale testing, and each locomotive would be its own special case.  Needless to say, some good parameters would be found. 

 Cheers

Addendum - Of course not to be neglected in the design is the type of coal or fuel used.

"If a nation expects to be ignorant and free, it expects what never was and never will be." Thomas Jefferson

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Posted by GP40-2 on Tuesday, July 19, 2011 5:44 PM

Burgard540

I could have written my previous post more clearly.  It was more born out of the questions regarding locomotive boiler capacities and performance given their design parameters such as direct heating surface (firebox & combustion chamber), heating surface of tubes & flues, gas flow area, etc.  The ALCO built steam locomotives for UP (FEF's, Challengers, and Big Boys) had significantly less heating surface compared to locomotives of similar size.  However, their performance was comparable.  Going just by total amount of HS doesn't explain the boiler's capacity, assuming similar heat energy inputs. 

But, were they really "undersized" compared to other steam locomotives? Or, was it just a case of a different ratio of direct heating surface to indirect heating surface compared to other locomotives?

As Dreyfusshudson correctly pointed out, a locomotive with many small diameter, long tubes did not get any additional heating benefit out of the last 4 or 5 feet of those tubes. However, what the additional length of the smaller tubes did was to give the locomotive the appearance of having a larger heating surface than a locomotive with shorter, larger diameter fire tubes.

Case in point. The N&W Class A, designed in 1935-36, used 24 feet long, 3.5" diameter tubes and had an evaporative heating surface of 6,639 sq.ft. However, its firebox area (which by far produces the majority of the steam) was 587 sq.ft.

Contrast that to the B&O EM-1, which was designed in 1944. The EM-1 used 4" diameter tubes just 20 feet long for a total evaporative heating surface of 5,300 sq.ft. What that doesn't tell you is that the EM-1 had a  firebox area of 758 sq.ft. which was equal to the H8 Allegheny in size. The firebox and combustion chamber on an EM-1 was 27 feet long, 7 feet longer than its fire tubes. The UP Big Boy had similar construction with a 704 sq.ft. firebox area.

Comparing a locomotive's total heating surface doesn't tell the full story about the steam production capacity of a boiler and must be used with caution when comparing steam locomotives.

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Posted by Burgard540 on Wednesday, July 20, 2011 1:19 PM

GP40-2

Comparing a locomotive's total heating surface doesn't tell the full story about the steam production capacity of a boiler and must be used with caution when comparing steam locomotives.

Thanks - that's pretty much my point in plain language. 

  Simple Articulated Locos
Railroad UP SP DM&IR C&O C&O N&W UP
Locomotive Big Boy AC-10 M-3 H-8  H-8 A Challenger
Year Built 1941 1942 1941 1941 1948 1936 1942
Wheel Arrangment 4-8-8-4 4-8-8-2 2-8-8-4 2-6-6-6 2-6-6-6 2-6-6-4 4-6-6-4
Builder ALCO Baldwin  Baldwin Lima Lima N&W ALCO
Flue Length, ft 22'-0"     23'-0" 23'-0" 24'-1" 20'-0"
Gas Flow Area, ft² 11.43   12.49 13.00 11.04  
Grate Area, ft² 150.3 139 125 135 135 122 132.2
Indirect HS, ft² 5185 5992 6032 6478 6033 6063 4215
Direct HS, ft² 704 513 750 762 762 587 602
Superheater, ft² 2466 2616 2770 3186 2922 2703 2355
Total HS, ft² 8355 9121 9552 10426 9717 9353 7172
SH % of Total HS 29.52 28.68 29.00 30.56 30.07 28.90 32.84
Direct HS % of Total HS 8.43 5.62 7.85 7.31 7.84 6.28 8.39
Indirect HS % of Total HS 62.06 65.69 63.15 62.13 62.09 64.82 58.77
               
Boiler Pressure, psi 300 250 240 260 260 300 280
Cylinder Dia, in 23.75 24 26 22.5 22.5 24 21
Stroke, in 32 32 32 33 33 30 32
Driver Dia, in 68 63.5 63 67 67 70 69
Tractive Effort, lbs 135375 123364 140093 110211 110211 114000 97350
Factor of Adhesion 3.99 4.31 4.00 4.61 4.57 3.79 4.16

  Northern Locomotives
Railroad Mil. Rd N&W NYC SP UP
Locomotive Northern J Niagara GS-4 FEF-2
Year Built 1937 1943 1945 1941 1939
Wheel Arrangment 4-8-4 4-8-4 4-8-4 4-8-4 4-8-4
Builder Baldwin N&W ALCO Lima ALCO
Flue Length, ft         19'-0"
Gas Flow Area, ft²      
Grate Area, ft² 106 107.7 101 90.4 100.2
Indirect HS, ft² 4931 4693 4302 4502 3971
Direct HS, ft² 578 578 517 385 499
Superheater, ft² 2336 2177 2073 2086 1900
Total HS, ft² 7845 7448 6892 6973 6370
SH % of Total HS 29.78 29.23 30.08 29.92 29.83
Direct HS % of Total HS 7.37 7.76 7.50 5.52 7.83
Indirect HS % of Total HS 62.86 63.01 62.42 64.56 62.34
           
Boiler Pressure, psi 285 275 275 300 300
Cylinder Dia, in 26 27 25.5 25.5 25
Stroke, in 32 32 32 32 32
Driver Dia, in 74 70 79 80 80
Tractive Effort, lbs 70816 77899 61568 66326 63800
Factor of Adhesion 3.99 3.70 4.47 4.16 4.23

If you look at the chart, notice that the proportions of heating surface are nearly identical regardless of the type of locomotive.  Of the total heating surface: 63% is tubes & flues, 7-8% is firebox, and 29-30% is superheater.  Notice that the ALCO built UP locos had the highest proportion of firebox heating surface at 8.4% for the articulated's and 7.8% for the 4-8-4's. 

Regarding tube length, the example Ralph Johnson uses of a USRA loco where increasing the flue length from 19 to 25 ft long increased the heating surface 29% (sounds good) but only increased the heat absorption by 2%.  

Again, my point is that tubes and flues are more effective heat absorbers per unit if they are of moderate length and the velocity of the combustion gases is high.  However, to achieve the better convective heat absorption requires decreasing the gas flow area, thus increasing the draft and back pressure.  The trade-off of slightly less engine efficieny for greater boiler efficiency.  Although to me, the focus should be increasing combustion efficiency.

"If a nation expects to be ignorant and free, it expects what never was and never will be." Thomas Jefferson

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Posted by Dreyfusshudson on Wednesday, July 20, 2011 5:17 PM

Thanks for these additional inputs- I'm heading off for two weeks vacation now, and will add my thoughts when I'm back

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