|201 users online
Steam & Excursion > Compounding, cont’d.
Date: 06/06/23 06:08
( from the discussion under subject of "the Big Boy"..below..)
The second use of steam from the HP cylinders to the LP cylinders has
drawbacks at the HP cylinders.
When running in "compound" mode, the steam from the HP cylinders
is directed via a 'receiver pipe' to the forward cylinders that power the
wheels of the 'front engine'.
As exhaust ports are opened by the spool valves, while the drive wheels
are rotating, the exiting HP steam, is reduced in pressure by about 33%.
the individual exhaust pulses accouulate in, and pressurize, the
receiver pipe leading to the front, LP cylinders.
The pressure in the receiver pipe also acts directly against the pistons
of the HP engine. So, that the rotative forces applied to the rear driver-set
are reduced by the back-pressure building in the receiver pipe.
The power of the rear pistons is reduced by the differential pressures
on each side of the piston: say, a boiler pressure of 250 psi is sent
to the HP cylinders, the pressure in the receiver pipe could be
130 psi.... to power the front-engine's pistons.
The HP cylinders are "pushing" the front engine's pistons...
while running in compound. The net effect is that the rear engin's
tractive effort is reduced when running in 'compound''.
However, when starting a heavy train, all four cylinders can be
sent steam at full boiler pressure. Such use is called running "in Simple"
mode. The HP cylinders' exhaust is re-directed to an exhaust-ring
at the tip of the blast pipe, directly under the base of the smokestack.
So, in 'simple', the exhsust pressure against the HP pistons is MUCH lower,
than when running compound. The pressure across a piston's 'face'
will be like: 250 psi from the boiler, and 15 to 18 psi to the stack.
The increase in pressure differential gives the rear engine much greater
starting-effort, than when running in compound.
The front cylinders receive live-steam from the boiler, through a pressure-limiting
valve that limits the pressure in the receiver pipe to approximately 130 psi...
(as if it was running on compound steam exhausted from the rear engine).
The engineer can select between the two modes of operation: simple or compound.
But, running in simple uses greater quantities of steam, so 'simple' operation
has limited upper-end speed account of the greater steam demand can exceed
the available grate area. You run out-of firebox capacity to produce enough steam.
Simple is for starting use only, with a heavy train --- but can also be of help, if
fighting on a steep grade --- the engineer can shift to 'simple' and achieve a
booster-like effect while fighting the hill..
With light trains, like WMS #1309 pulls, the engineer can just leave the engine
in compound ---- because the receiver pipe can build pressue in about 3 engine lengths,
so that the rear cylinders seeing less 'back pressure' have plenty of power
to get the train underway.... the engine will gradually "get comfortable" in compound
when under-way, and the receiver pipe gets pressurized.
It's a sophisticated concept, mostly made possible by the really effective use of the
superheater. The once-used steam to the front, LP, cylinder still is receiving
superheated steam, because it is still pressurized in the receiver pipe.
The temperature of the superheated steam will be lower, with the lowered
The degree of superheat is reduced; but, still WELL ABOVE what would be
the commensurate "condensing-temperatures", if at a lower pressure in the
It's an important aspect of running in compound..... after the boiler/firebox
combo rises to be hot enough to generate about 300-degrees of superheat,
over the 'heat-saturated' temperatures of boiler steam.
The 'hot' superheat's first use is at the pistons of the HP cylinders.
Superheating does NOT raise raise boiler pressure --- but it increases
the "molecular vibration amplitude" so that there is greater separation of the
over-excited steam molecules in the superheated steam-flow.... more volume of
steam exits the superheater, than enters the superheater at the boiler inlet-ends.
Superheating generally doesn't occur until the brick-work in the firebox gets
to incandescent state --- about 15 or 20 minutes of steady, strong drafts
through the firebed... and 3,000 F of flame-tip temperatures..
But, when running at high superheat, all four cylinders are receiving steam
at superheated temperatures..
Thank you, Anatole Mallet, the inventor who perfected the concept.
(not proofed, yet)
Edited 4 time(s). Last edit at 06/06/23 19:10 by wcamp1472.
Date: 06/06/23 07:39
Re: Compounding, cont’d.
Don't forget about Baldwins concept of reheating, rather than superheating, steam on their early Mallets?
Date: 06/06/23 08:30
Re: Compounding, cont’d.
The hot gasses from boiling the water get cooled down to the
boiler water's temps --- about 300F, in the first use.
And the grate area is too small to boil water in two separate cantainers.
That 300F ( residual heat) is too low to be of any benefit to the 'second use'
It's total B.S. on either the SantaFe's part, or Baldwin's mischievous
advertising. dept. As useless as those 'hinged boilers' that they tried.
The stack of accordion-like plates, between the boiler's sections,
soon filled with soot, sand and cinders.
The boiler lost it's ability to flex ...stuff got torn, strained or derailed account
of the compressed grit accumulation.
Never repeated by Baldwin or others.
Edited 1 time(s). Last edit at 06/06/23 09:43 by wcamp1472.
Date: 06/06/23 11:54
Re: Compounding, cont’d.
Only a few of the Baldwin separable boilers, something like a half dozen or less, had the accordion pleats Wes describes. I think only one or two had a huge ball jointed boiler. The rest had a straight boiler that looked just like any other boiler, unless you knew what you were looking at and traced the steam and water piping feeding it. The firebox and the rear portion (over the rear drivers set) was a conventional boiler, suitable for a large 2-8-0 or 2-10-0, rigidly attached to the rear cylinder saddle, with its front flue flush with the end of the boiler barrel, and no smokebox.
On hinged boilers, the ball joint or according pleats were affixed. The front of this assembly was bolted to the front "seperable" portion of the boiler (containing a feedwater heater an a reheater) and front smokebox and stack. This assembly was affixed to the frame of the front engine, and turned with that engine. There was no steam pressure in the hinge itself.
Other locos without the boiler hinge had a rigidly smokebox between the two portions. The front portion rode on sliding plates over the front engines frame, as with a conventional Mallets.
Most of these separable engines, not surprisingly, did not serve this way for long.
HOWEVER, DM&IR 2-8-8-2 No. 205 with its original rigid separable boiler wasn't scrapped until 1950, still clearly showing the location of the joint in its boiler jacket.
The guts of 205's feedwater heater and reheater had been disabled and removed decades earlier, replaced by tons of scrap iron to give traction to the front drivers. It took days to cool off when needing shopwork.
Three pictures of 205 in Frank Kings book Locomotives of the Duluth Missabe and Iron Range show this.
Edited 1 time(s). Last edit at 06/07/23 15:01 by LarryDoyle.
Date: 06/06/23 18:26
Re: Compounding, cont’d.
Learning and enjoying this information