Great American articulated steam engines

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Great American articulated steam engines
Posted by Valeriy on Friday, April 30, 2010 11:15 PM

Hello!

I am interested in great american articulated steam engines such as:

2-10-10-2 Virginian class AE

2-8-8-8-2 Erie class P-1

2-8-8-8-4 Virginian class XA

2-8-8-4 NP class Z-5

2-8-8-2 D&RGW class L-131, L-132

2-8-8-2 C&O class H-7, H-7-A

What issues of Railway Mechanical Engineer (or another magazines and books) has information about this great locomotives?

 

Valeriy.

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Posted by CSSHEGEWISCH on Saturday, May 01, 2010 6:58 AM

I would hardly consider the Triplexes to be "great" articulated locomotives.  Large, to be sure, but hardly great.  Both suffered from too much machinery and not enough boiler.  The Virginian XA was an unqualified failure and wound up having its running gear used for two new locomotives.

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 Valeriy on Saturday, May 01, 2010 7:52 AM

Erie class P-1 and Virginian class XA has unique Triplex system. P-1 has 160 000 lbs tractive effort (see p. 197 Articulated Steam Locomotives Of North America by Robert A. LeMassena, Vol.2). I think it is enough to say this is a great powerful steam engine.

Valeriy.

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Posted by switch7frg on Thursday, May 06, 2010 11:20 AM

Smile,Wink, & Grin  Valeriy; maybe ~ Train Shed Cyclopedia No. 47  from Gregg would help you for the info you want.  Possibly your LHS could get this for you.

                              Respectfully, Cannonball

Y6bs evergreen in my mind

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Posted by UP 4-12-2 on Thursday, May 06, 2010 11:27 AM

Valeriy--

There is a huge difference between calculated theoretical maximum tractive effort and the actual real world performance of articulated steam locomotives.

Some of the engines you listed couldn't even produce enough steam to keep the engine running down the track at a respectable speed with a train, and thus were pretty much total failures and were scrapped early.

Exceptions:

Virginian AE Class 2-10-10-2.  176,000 pounds tractive effort.  It actually did have lots of usable tractive effort, and had a long productive service career, but they just weren't very fast at all.

If you want to discuss the best of the American articulateds, I think your list would benefit from some additions or modifications.  I would suggest adding the following engines:

1. N&W Y-6B 2-8-8-2.  152,206 pounds starting tractive effort, simple, without booster.  Allegedly 170,000 pounds in the final, upgraded version with booster, but records are questionnable.  The ultimate compound articulated!  Efficient and productive.  Two of them could start a 10,000 ton train on a 2% grade in the rain!  Also produced a very serious 5500 actual drawbar horsepower at about 25 mph.

2. UP Big Boy 4-8-8-4.  135,000 pounds starting tractive effort, calculated.  Actual performance was measured to exceed that figure.  The last UP test with a dynamometer car yielded an astounding 6290 drawbar horsepower after starting a heavy train on a grade.

3. N&W A Class 2-6-6-4.  Very respectable starting tractive effort combined with outstanding 6300 drawbar horsepower at reasonably high speed.  Some consider it the greatest all around running articulated when speed is factored into the equation.

4. C&O H-8 2-6-6-6/Virginian 2-6-6-6.  What can I say:  7,498 drawbar horsepower.  Was designed and built to outperform the N&W A Class, but maybe wasn't as effectively used in service as the N&W A Class.

I just read Dr. Eugene Huddleston's World's Greatest Steam Locomotives.  Some of the engines Valeriy listed did indeed make the top 10, but they did not make the top 3.

Huddleston's top 3 were the Big Boy, A-Class, and H-8 2-6-6-6, with an "honorable mention" pretty much going to the Y-6B (he included a special tribute chapter to the ultimate compound mallet).

Respectfully submitted--

John

 

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Posted by selector on Thursday, May 06, 2010 1:53 PM

Adjectives such as "best", "most", "biggest", and "great(est)" really could do with some negotiated agreement as to their meaning.

Was not a simple Challenger great?  How about the trusty USRA 2-6-6-2 and 2-8-8-2?  Why were they not worthy of being called 'great'?  I would list the A Class as an honourable mention at the very least.

The Yellowstones were 'great' articulated engines surely.  And I would agree that the rather smallish articulated Y series from the N&W were worth inclusion on such a list.

-Crandell

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Posted by UP 4-12-2 on Thursday, May 06, 2010 2:10 PM

According to Huddleston, the USRA 2-6-6-2 was regarded, even during its era, as nearly a "failure" in comparison to the much more successful and powerful USRA heavy mallet 2-8-8-2.  Only 2 railroads bought them, and even though C&O built 10 late copies, they were primarily to replace worn-out locomotives on mine branch line service--where they were ideally suited.

Huddleston, in his book, did attempt to address Crandell's comments.  Also, he was talking about the "World's Greatest"--but he did say that all the engines in his top ten list were great engines.

The top ten engines included the Yellowstones, both DM&IR and NP versions, and the NP Challenger, along with the WP 257 Class 2-8-8-2, and I believe the B&O EM-1 Yellowstone.

The Rio Grande L-131 2-8-8-2 was left out because the list was limited to "engines built after 1930" and it was a bit too early, and not quite as powerful as the WP 2-8-8-2.

I think Huddleston's main emphasis was on Superpower ie horsepower at (more than drag freight era) speed--and engines that had all the most modern equipment--though the engines with high starting tractive effort did all rate well.

Many folks love the UP Challenger, but in this comparison it does not rate quite as highly as some other engines because WM, NP, and DRGW all had challengers that produced more tractive effort and/or more horsepower at speed.  The UP engine was certainly a fine and versatile design, and served quite admirably in just about any service they assigned it to, but Huddleston's point of view is that other Challengers, at least in terms of power output, were superior machines to the UP version.  Rio Grande most certainly preferred their own design (with 8000 pounds more starting tractive effort) over UP's.

The Y-6B was called the ultimate development of the USRA 2-8-8-2--but in reality there were so many improvements over the 30 years after the USRA engines that it is almost a completely different animal, though it looks nearly the same.

We are debating 60 years late, engines that folks like myself never even had the chance to see run (not even one of them).  There's also an awful lot of misinformation and incorrect data online.

Huddleston does make the point that few railroads actually had the kind of terrain that was ideally suited to modern Superpower steam locomotives--Union Pacific most certainly did--but Challengers operating in the east were unable to run at their full potential.  In the "Arms race of Super Steam" it might be argued that some railroads had (or were sold) outstanding locomotive designs that were just not fully suited to their particular track profile, etc.

John

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Posted by JimValle on Thursday, May 06, 2010 4:00 PM

Nobody's going to mention the SP's cab-forwards, AC-4 thru 12?  They were very useful and versitile, perhaps the first articulateds to be equally comfortable running in both passenger and freight service.  Not quite as heavy as some of the others mentioned, they packed a decent amount of tractive effort and you couldn't beat them for visibility.  One development of the type, the AC-9 was actually built to run the conventional way because it was originally a coal burner.  With a neat skyline casing and an all weather cab, it may have been the handsomist of all the articulated types.  And one other one - the American Railroad of Puerto Rico's Baldwin 0-6-6-0's of 1904.  Thiese engines were meter gauge and had the distinction of being the very first articulated mallets ever built in the United States.  B&O's "Old Maude" was just an upscaled version of this engine.  

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Posted by GP40-2 on Sunday, May 09, 2010 8:04 PM
UP 4-12-2

2. UP Big Boy 4-8-8-4.  135,000 pounds starting tractive effort, calculated.  Actual performance was measured to exceed that figure.  The last UP test with a dynamometer car yielded an astounding 6290 drawbar horsepower after starting a heavy train on a grade.

3. N&W A Class 2-6-6-4.  Very respectable starting tractive effort combined with outstanding 6300 drawbar horsepower at reasonably high speed.  Some consider it the greatest all around running articulated when speed is factored into the equation.

4. C&O H-8 2-6-6-6/Virginian 2-6-6-6.  What can I say:  7,498 drawbar horsepower.

The 6,290 HP for the Big Boy came from only one locomotive in one test. Collectively in tests, the Big Boy's were more like 5,800 DBHP engines.

To my knowledge, there is no N&W test data or documents to support the 6,300 DBHP claim for the Class A. The N&W never claimed that much HP from an A. In fact, the only people who make that claim are railfans. I have no idea where they got that figure other than out of thin air.

The Allegheny's 7,498 DBHP was from a test where the dyno readings were bouncing around like crazy. The data points were all over the place. A best fit curve shows the actual DBHP to be around 6,500 at that speed, which is still far above any other steam locomotive.

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Posted by UP 4-12-2 on Monday, May 10, 2010 7:53 AM

Do you know where the best fit curve for the Allegheny was published?

Also--steam locomotive test data is only as good as the job the crew was doing of running and firing the engine.

Though the Big Boys evidently performed well, there seems to be some evidence that the UP Challengers steamed better throughout all speed ranges--or rather that on sustained grades the Big Boys tended to slow down a bit and apparently could not produce quite enough steam.  Perhaps the variation in Big Boy numbers has something to do with the challenges posed in firing them?

John

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Posted by daveklepper on Monday, May 10, 2010 8:57 AM

All these large locomotives had automatic stokers, although operating them certainly required skill.  I don't think the Big Boys' slowing down on hills had anything to do with a lack of performance.   They were rated as more powerful locomotives, thus had longer and heavier trains to pull, and the heavier the train of course the slower the uphill climb.  Possibly loading them to their capacity was more routine than with the Challengers.

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Posted by UP 4-12-2 on Monday, May 10, 2010 12:01 PM

Huddleston states in his book that the Big Boys were more difficult to fire than the Challengers, and it was Huddleston who suggested it seemed the Big Boys were unable to produce quite enough steam at speed on heavy grades.  According to Huddleston, UP's design goal was to achieve 25 mph unassisted on the Eastbound average 1.14%, 24 mile grade over the Wasatch--which they apparently did not quite achieve.  Eventually, two Challengers with about 70 car train lengths proved to be the ideal choice over that division--till the end of steam.

Even though all the big articulateds had stokers, some were easier than others to fire.

Kratville also says in his books the Big Boys were more difficult to fire than the Challengers.  Also, when converted to oil, the Big Boy (#4005) did not perform very well at all, whereas the Challengers converted easily to oil and steamed quite well.

John

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Posted by timz on Monday, May 10, 2010 12:23 PM

GP40-2
The N&W never claimed that much HP [6300 dbhp] from an A.

They did claim it-- as I recall they claimed it for the 275-psi engine. You remember they also claimed it pulled 7500 tons at 64 mph on the level, which would be even more impressive.

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Posted by feltonhill on Monday, May 10, 2010 2:43 PM

 I found a lecture presented by Robert M. Pilcher 3/12/53, at that time Assistant Engineer of Tests for N&W.  He had this in his script:

 "Under unusual spot conditions dynamometer records a maximum sustained horsepower of 6300 at 45 mph."  [He did not specify boiler pressure for this reading, but other sources indicate that it was 275 psi.  No test report has been found yet]

 "In usual day to day operation, the dynamometer record indicates drawbar horsepower between 5200 and 5400 over long distances at speeds between 35 and 40 mph while handling 175 loaded trains over almost level track."

These figures occur frequently in N&W public statements and articles written in the 1940s-50s.

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Posted by UP 4-12-2 on Monday, May 10, 2010 3:45 PM

My copy of Huddleston's book is loaned out to a friend, so I can't find the source he may or may not have cited for the 6,300 dbhp figure for the A Class.

Regarding the SP Cab Forwards mentioned above:  They were surely great engines, and I'm not sure if the starting tractive effort was high enough to get them onto Huddleston's "top 10".  It may have been the engine I left out. 

Again--my copy of the book is loaned out, or I could look it up.

Sorry for the confusion.

John

 

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Posted by UP 4-12-2 on Monday, May 10, 2010 3:51 PM

To the original poster, Valeriy--

If you check the used book market, there are some fine books regarding the various engines you mentioned.

There is a book devoted entirely to the H-7 Class.

There is a book called Northern Pacific Super Steam Era.

There are books by Robert LeMassena.  I think the titles are:  Articulated Steam Locomotives of North America, Volumes 1 and 2. 

Respectfully submitted--

John

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Posted by GP40-2 on Monday, May 10, 2010 10:41 PM
feltonhill

 I found a lecture presented by Robert M. Pilcher 3/12/53, at that time Assistant Engineer of Tests for N&W.  He had this in his script:

 "Under unusual spot conditions dynamometer records a maximum sustained horsepower of 6300 at 45 mph."  [He did not specify boiler pressure for this reading, but other sources indicate that it was 275 psi.  No test report has been found yet]

 "In usual day to day operation, the dynamometer record indicates drawbar horsepower between 5200 and 5400 over long distances at speeds between 35 and 40 mph while handling 175 loaded trains over almost level track."

These figures occur frequently in N&W public statements and articles written in the 1940s-50s.

Timz,

Three bits of information stick out here: "under unusual conditions", "no test report has been found", "in usual operation dynamometer records indicates drawbar horsepower between 5200 and 5400"

That's why I made the statement, to my knowledge, no records exist of a Class A producing 6,300 HP. The "under unusual conditions" and "no test report found" raises more questions. There are plenty of N&W sources stating the 5200 to 5400 HP figure however. Who knows, maybe under the right conditions, the A could produce 6000 HP, but I have found no records to indicate it.

What I find interesting, if my data is right, is the Class A didn't have any more direct heating surface than the Class J (580 sq.ft. vs 575 sq.ft) and lacked thermic syphons and circulators. By comparison, the two other large Eastern simple articulated locomotives, the H8 Allegheny and EM1 both had 760 sq.ft. direct heating surface, thermic syphons and circulators. Perhaps feltonhill can comment on this aspect of the A's design.

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Posted by feltonhill on Tuesday, May 11, 2010 7:30 AM

The N&W Class A had 530+57 SF (firebox+circulators) of direct heating surface  vs the J's 518+60 SF of DHS, relatively close.  These are as-built figures for 1200-1237.  The A relied more on its indirect HS, which was significantly  larger than the J (6,063 vs 4,693 SF).  The final order of A's were equipped with six circulators as built instead of arch tubes.  Based on photos, at least 15 of the older A's were retrofitted with circulators. There may have been more.

Something about circulators, arch tubes and syphons has always bothered me.  By industry standards, they are considered part of direct heating surface and are used to estimate DHS evaporation.  Yet I've always wondered if they have the same heat transfer capability as the firebox sheets.  It seems that the water velocity through these additions would be very rapid, much more so than the water surrounding the firebox.  Could it be that arch tubes, circulators and syphons are "less equal" than firebox sheets in terms of heat transfer per unit time?  Circulators and arch tubes have a relatively small contribution to DHS, but syphons have a much larger effect.  If so, locomotives equipped with syphons would seem to have a falsely large advantage over those equipped with circulators as far as estimated evaporative capacity is concerned.  Anyone know if this is the case?

Be back later.

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Posted by Anonymous on Tuesday, May 11, 2010 8:21 AM

feltonhill
Something about circulators, arch tubes and syphons has always bothered me.  By industry standards, they are considered part of direct heating surface and are used to estimate DHS evaporation.  Yet I've always wondered if they have the same heat transfer capability as the firebox sheets. 

Didn't the UP steam-crew removed some of the T-circulaters of 3985 firebox without any known power loss? Hard to say for me, if these are anyway valuable in a oil-burning firebox.

 

For Valeriy:

here you may find some notes about the Virginian AE and more about its railroad ops... 

http://www.catskillarchive.com/rrextra/ngstory.Html

See page 366 ( picture of an AE blasting out of a tunnel )

P. 370 Virginian hauling 17.500t record train in 1921

P. 378 Pic. of the largest iron horse

The AE class set many records:

- most weight on drivers

- largest boiler diameter

- largest low pressure cyl.

- highest starting TE of two coupled artic. engines

Cheers

lars

 

 

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Posted by Anonymous on Tuesday, May 11, 2010 9:55 AM

UP 4-12-2

Kratville also says in his books the Big Boys were more difficult to fire than the Challengers.  Also, when converted to oil, the Big Boy (#4005) did not perform very well at all, whereas the Challengers converted easily to oil and steamed quite well.

John

When converted to oil, Kratville says they were steaming best. The leaking problem was another one, but not the steaming capabilities.

lars

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Posted by UP 4-12-2 on Tuesday, May 11, 2010 11:58 AM

I'm confused.

Challengers leaking or Big Boys leaking?  I read the Big Boy did not perform well when converted to oil, and was quickly converted back to coal, with no further attempts to convert a big boy to oil.

What was the leaking problem? (I only have read 3 of Kratville's books: the two 4-12-2 books and The Challengers, which makes reference to earlier works on the Big Boy and the 4-8-4, but doesn't repeat any of that material, instead assuming the reader will have read them).

If I recall correctly, Huddleston said something about the size and shape of the Big Boy firebox not being as conducive to oil burning as the Challenger.  No author is perfect...was that just opinion and not true?

Help me out here, please.

Thanks.

John

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Posted by timz on Tuesday, May 11, 2010 5:09 PM

GP40-2
to my knowledge, no records exist of a Class A producing 6,300 HP.

Everyone's agreed on that.

N&W first made the claim in 1936; in 1941 Railway Age, Pond said the A's maximum "sustained horsepower at the drawbar" was 6300 at 45 mph. As always, we haven't the faintest idea what "sustained" means.

 (And yes, he said the engine was 275 psi.)

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Posted by timz on Tuesday, May 11, 2010 5:13 PM

UP 4-12-2
According to Huddleston, UP's design goal was to achieve 25 mph unassisted on the Eastbound average 1.14%, 24 mile grade over the Wasatch

I can't find where Huddleston said that.

Obviously they could make 25 mph with some sort of reduced tonnage, but no reason to think the UP was hoping for 25 mph on 1.14% with 3600 tons or 4450 tons or anything like that. What did Huddleston actually say?

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Posted by UP 4-12-2 on Tuesday, May 11, 2010 5:32 PM

He wrote in World's Greatest Steam Locomotives, in one of the sections where he discusses the Big Boy and the dynamometer tests on it, that UP's own publicity stated they wanted to take an entire train (tonnage not specifically given) over the Wasatch with a Big Boy unassisted, and he added that they wanted to do that at 25 mph.  Apparently during the various tests, they found the best a Big Boy could actually do was about 17 or 18 mph, and that to haul the typical train length they wanted (70 cars or more) at the speed they wanted, that they ended up needing two Big Boys, which was viewed as unacceptable.

I'm assuming 70 cars is about 3600 tons?

He states they eventually found that two Challengers, one front and one on the rear, was the ideal motive power solution for 70 car trains eastbound over the Wasatch.  Kratville corroborates the use of two Challengers (at least one an earlier 3800 class) as being the preferred power in The Challengers, but didn't specifically discuss the preferred train length/weight anywhere in that book that I am able to find.  Kratville did comment that Union Pacific men were very sorry to see the Challengers go from the Wasatch because they did a terrific job.

Perhaps from the time the Big Boy was introduced until the time of the tests later in the 1940's, UP's idea of a "typical" train length over the Wasatch actually increased quite significantly?

Perhaps I misunderstood/misquoted Dr. Huddleston's work.

As I said above, the book is currently loaned out to a friend.  I'm unable to look up the page right now.

John

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Posted by Anonymous on Tuesday, May 11, 2010 6:44 PM

UP 4-12-2
I'm confused.

 

UP 4-12-2, @all

here is the original text...

page 39:
From a steaming standpoint, the 4005 steamed better
than any oil burning power UP men had
seen on the road. However, the single burner
caused spot heating on the huge crown sheet
which in turn, caused it to leak. Every trip
was the same - when you lookes in the firebox
it was just like a rainstorm, with water poring
down so fastthat it almost exstinguished the
fire!

A standard Thomas oil-burner was installed...

Of course they leaked, but I believe that problem could have be technically resolved.

The most likely reason was, the single oil burner did not fit with the distances of oil refueling service points along the line. These points were more accommodated and suitable for passenger trains than  for freight trains with a 4000 class. Therefore, further investigations were not followed....

 

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Posted by Anonymous on Tuesday, May 11, 2010 6:47 PM

timz
no reason to think the UP was hoping for 25 mph on 1.14% with 3600 tons or 4450 tons or anything like that. What did Huddleston actually say?

 

...Do not know what he would say, I say 15mph%1.14 with 4000tons, 1hr sustained...

lars

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Posted by feltonhill on Tuesday, May 11, 2010 8:15 PM

 I wanted to see under what conditions an as-built Class A (275 psi, no circulators) could develop 6,300 DBHP.  Here's what I did:

Unit evaporation, 92 lbs/SF direct heating surface, not at all out of reach.  Johnson considered 125 lbs/SF DHS high

116,055 lbs/hr total evaporation (actual figure)

8% to auxiliaries

107,000 lbs steam/hr to engines at 300 deg superheat

6,900 lbs total locomotive resistance at 45 mph

6,300 dbhp at 45 mph (purportedly an actual figure)

So 6,300 may have been possible, as Pilcher said, under unusual conditions, but none of the above numbers are outrageous.  N&W normally didn't push its locomotives above a unit evaporation of about 80 lbs/SF DHS/hr even on tests.  N&W's rated DBHP readings were generally daily achievable in-service figures, very conservative compared to some other roads.  Maybe that's why Pilcher used the word "unusual".

Just my guess.......

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Posted by timz on Tuesday, May 11, 2010 8:43 PM

UP 4-12-2
He [Huddleston] wrote in World's Greatest Steam Locomotives, in one of the sections where he discusses the Big Boy and the dynamometer tests on it, that UP's own publicity stated they wanted to take an entire train (tonnage not specifically given) over the Wasatch with a Big Boy unassisted,

No argument there.
UP 4-12-2
and he added that they wanted to do that at 25 mph.
I looked again; still can't find any indication they hoped for that, and still can't find where he said they hoped for that. Anybody else find it?

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Posted by erikem on Tuesday, May 11, 2010 11:09 PM

feltonhill

Something about circulators, arch tubes and syphons has always bothered me.  By industry standards, they are considered part of direct heating surface and are used to estimate DHS evaporation.  Yet I've always wondered if they have the same heat transfer capability as the firebox sheets.  It seems that the water velocity through these additions would be very rapid, much more so than the water surrounding the firebox.  Could it be that arch tubes, circulators and syphons are "less equal" than firebox sheets in terms of heat transfer per unit time?  Circulators and arch tubes have a relatively small contribution to DHS, but syphons have a much larger effect.  If so, locomotives equipped with syphons would seem to have a falsely large advantage over those equipped with circulators as far as estimated evaporative capacity is concerned.  Anyone know if this is the case?

 

Rapidly flowing water is better for heat transfer than relatively stagnant water, so any loss in effectiveness with respect to the sheets would have more to do with the amount of heat transferred from the combustion products to the syphon. Heat transfer to the "Direct Heating Surface" is largely radiative, so the parts of the syphon facing the sheets would be in sort of a shadow and possibly getting less heat than the parts pacing the interior of the firebox.

- Erik

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Posted by GP40-2 on Saturday, May 15, 2010 10:11 PM
erikem

feltonhill
The N&W Class A had 530+57 SF (firebox+circulators) of direct heating surface vs the J's 518+60 SF of DHS, relatively close. These are as-built figures for 1200-1237. The A relied more on its indirect HS, which was significantly larger than the J (6,063 vs 4,693 SF). The final order of A's were equipped with six circulators as built instead of arch tubes. Based on photos, at least 15 of the older A's were retrofitted with circulators. There may have been more.

Something about circulators, arch tubes and syphons has always bothered me.  By industry standards, they are considered part of direct heating surface and are used to estimate DHS evaporation.  Yet I've always wondered if they have the same heat transfer capability as the firebox sheets.  It seems that the water velocity through these additions would be very rapid, much more so than the water surrounding the firebox.  Could it be that arch tubes, circulators and syphons are "less equal" than firebox sheets in terms of heat transfer per unit time?  Circulators and arch tubes have a relatively small contribution to DHS, but syphons have a much larger effect.  If so, locomotives equipped with syphons would seem to have a falsely large advantage over those equipped with circulators as far as estimated evaporative capacity is concerned.  Anyone know if this is the case?

 

Rapidly flowing water is better for heat transfer than relatively stagnant water, so any loss in effectiveness with respect to the sheets would have more to do with the amount of heat transferred from the combustion products to the syphon. Heat transfer to the "Direct Heating Surface" is largely radiative, so the parts of the syphon facing the sheets would be in sort of a shadow and possibly getting less heat than the parts pacing the interior of the firebox.

- Erik

Feltonhill, As Erik pointed out, rapidly flowing water is much better for heat transfer. This is why the water pump in a car engine runs as such high speed. I am still quite surprised the Class A's relatively small direct heating surface. Heat transfer is exponential in nature for radiative (direct) heating surfaces vs. linear for convective (indirect) heating surface. A modest increase in direct heating surface will pay huge dividends in steam production, while a large increase in indirect heating surface pays little.

Relying on large amounts of indirect heating surface (IHS) has other issues as well. A locomotive boiler can only be so big and still fit into a railroad's loading gauge. The way to increase IHS is to use longer, smaller diameter tubes. The only way to increase the length of the tubes, and still keep the overall boiler length the same, is to decrease the length of the combustion chamber (which is direct heating surface). The increased length of the tubes would also decrease the efficiency of exhaust gas flow to the stacks. The smaller diameter of the tubes presents problems of maintaining enough oxygen to allow complete combustion. The temperature gradient across the tubes from back to front will encourage condensation of combustion gasses in the tubes which is very bad from an efficiency point, plus it allows corrosive byproduct to accumulate in the tubes. It is much better for the gasses to fully burn in a larger combustion chamber that will absorb the radiative heat, and have shorter, larger diameter tubes to exhaust the spent combustion gas as quickly as possible

This design change can clearly be seen in the late steam era. ALCO did it with the NYC 4-8-4 Niagara, Lima did it with the C&O 4-8-4 J3a. Lima not only changed the tubes to 4" diameter, but they shortened the tube section and correspondingly lengthened the combustion chamber compared to the earlier J3. Baldwin did it with the B&O 2-8-8-4 EM1, which may be the most dramatic example. For a locomotive with the boiler length of the EM1, the 4" diameter fire tube section was barely 20' long. In comparison, the firebox / combustion chamber ran from the firebox doors in the cab, clear up to the rear axle of the front (!) set of drivers. The EM1's direct heating surface was supported by 7 axles -- the 2 rear trailing axles, the 4 rear drive axles, and the 4th drive axle of the front engine. Back in the 1970's, old B&O engineers told me that an EM1 may run out of adhesion on a hard pull, but they never, ever ran out of steam at speed. No wonder, they were a giant combustion chamber on wheels!

I'm sure this wasn't lost on the N&W in the late steam era. I suspect by the time the last A's were constructed, the "handwriting was already on the wall" as far as the future of steam was concerned. Probably made no economic sense to totally redesign the Class A to late steam specification, so they did some upgrades (additional circulators) and called it a day.

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