double vs. single stack question

[steamandbonsai]

I'm curious as to why the U.P., for example, preferred a double stack on some of their locos (Northerns and challengers). What were the benefits? Some Roads clearly did not prefer them--wasn't it Rio Grande or Clinchfield that acquired some of the U.P./Alco Challengers and one of the roads converted them back to single stack. Anybody?

[steamandbonsai]

Ok, how about an educated WAG.... somebody?

[mjahn]

Page 28 of "Big Boy" by William W. Kratville talks of nozzle wear vs smoke lift from the stacks. Perhaps the nozzles and petticoats were being chewed away too quickly for UP's liking so they spread the wear out the lengthen replacement intervals? They certainly didn't burn all the coal that went into the firebox so some of it had to go out the stacks and sand blast them in the process.

On a side note, the book also claims on pg. 28 that UP experimented with a single stack on a Big Boy but was unsatisfied with it and went back to double stacks.

Sorry I don't have a better answer...

[steamandbonsai]

Thanks, mjahn. that is one possible factor.

Like I said, the railroads did have some strong opinions about single vs double, as redesigning a smoke box to single stack when it had been a double stack loco (UP challengers) and a proven performer as a double....One would imagine they felt it would be worth the effort and labor......

[Rwilliams]

In one long our of print UP book, I remember reading that the UP actually experimented with a triple stack on one of the FEF-3 class Northerns. There were rumors that a FEF-4 class of Northerns might have followed after the end of WWII. The testing of the triple stack was research for the super FEF-4 class that never came to pass. Diesels proved themselves during the war and some wonderful golden age of steam designs never became reality.

The SP actually experimented with roller bearings on a few of their big SP class 4-10-2 locomotives in order to reduce maintenance and increase availability. Again, the invasion of the diesel ended the experiment with roller bearings.

[Fender]

Pure speculation on my part.... but could be that the UP believed the needed draft could be generated with less back pressure in the exhaust steam using two blast nozzles and separate stacks. Somewhat like the Lempor (sp?) exhaust.

[FriscoJim]

That would be my guess, too. Similar to the Giesl Ejector which used an oblong stack to provide enough exhaust capacity... since the diameter of the stack is limited by necessity, the only other option is multiple stacks to provide the needed area. After all, this was the era of developing higher horsepower at speed by reducing backpressure while increasing draughting ability...

[srrl5]

One of the reason for double stacks is the loading gauge. A single stack would have to be much taller to achieve proper seal. Two smaller diameter stacks will provide the same volume as one larger stack, but not have to be as tall to achieve proper seal.

On the UP loco you'll notice the boilers are so large in diameter there is not much clearance above them. Sand and steam dome barely protrude above the boiler.

David

[Doug_Edwards]

Dan,

The UP did some experimenting on stacks I think in the 30's. They converted some of their two cylinder engines to a huge "Sweeny" stack that was large enough to drop a 55 gal drum down. They were down right homely imho, but must have worked, as a few locos on display still have them. I would guess that they had multiple nozzles, or some sort of spreader on the exhaust. The UP USRA Mike in Walla walla has one of these stacks, but I am not sure I have an image of it.

This does not answer your question on the dual stacks, but might be related. I'm sure someone in the UP historical society would have a definitive answer.

Fwiw.

Regards,

Doug

[JJG Koopmans]

Hi,
It looks like you missed the earlier prolific discussion on smoke stack dimensions.
In order to be functional a stack must be at least twice its diameter long. The diameter itself
is determined by the ratio between steam used and combustion products to be exhausted.
In a large boiler the height of the stack is limited by the loading gauge which does not
allow a proper chimney length. Since splitting the exhaust orifice area in two gives linear
dimensions of square root (2) or 0.71 the relative length of the chimney goes up allowing
a longer road for the redistribution of momentum. This is also the rationale for any multiple
chimney or -orifice. It does not matter whether this is a double, quadruple, Giesl, Kylchap
or Lempor.
Going back from double to single must have been a poor design and misunderstanding why
it worked in the first place.
Regards
Jos Koopmans

[steamandbonsai]

Thanks, all, for those replies--good info.

[Boiler Builder]

Union Pacific RR Nozzle stats