Steam & Excursion > Air pump sound

Gang, SP's cab-forwards were noted for the distinctive "choo-choo" sound of their smokebox-mounted air pumps as they exhaust steam into the atmosphere. Were these engines unique in their air pumps operating this way? For example, I see that Chessie 1309's front-mounted pumps don't appear to exhaust the steam the same way, or make that sound. 



Edited 1 time(s). Last edit at 09/07/23 14:13 by kurtarmbruster.

It depends.....

The compressors run on heat-saturated steam...its steam at the same 
Temperature as the boiling water that produced it.  Steam moves 
to areas of lower pressure,  as the pressure drops, so does a percentage 
of 'steam', so when the pressure drops, the temperature drops, and
a portion of water vapor condenses-out.

Cross-compound air compressors expand steam steam twice --- thus,
the water content of the exhausted steam is considerable ---
its a very 'wet' vapor trail.

There are many ways to pipe the exhaust path to get the wet steam skyward.
The most common way was to pipe the compressor exhaust into the smoke box,
and feeding the main exhaust nozzle, beneath the base of the smokestack.....

You will see this arrangement most common to compressors that are
mounted mid-way along the boiler.  They are heavy, so some designers
will mount two compressors, one on either side of tte boilers, with the exhaust
piped to the base of the 'stack.

Or, you may have two cross-compound compressors mounted 
on one side of the boiler.  In such arrangement, you can expect 
an equalizing weight to be found on the other side of the boiler, like 
a power-reverse cylinder.

in later years it became common to mount compressors at the front
of the engine: either hung on the smoke-box front, or on locos with 
cast, one-piece main frames, there will be cast air compressor support 
brackets at the front of the loco frames,  ahead of the cylinder castings.

Some locos you may see one feedwater pump and one air compressor, 
mounted on the two front mounted, support brackets.

Most commonly with smokebox, front mounted compressors you will find 
the exhaust piped directly, straight upward.  Some piping is arranged to 
release the exhausted steam near the top of the smoke stack.
C&O, NKP and some others used the water-separators on their 
front-mounted air compressor' exhaust piping.

On NKP 2-8-4 'Berks', the exhaust pipes are fed to a 'separator' 
mounted near the 'stack.  The separator has a baffle inside, so that the water 
content of the exhaust steam is collected and fed down to track level,
and the vapor content is released, skywards, near the stack.

Now, they all can make differtent exhaust noises, but it's how it's piped
and directed, that gives each arrangement its unique exhaust sounds.
But, even two similar arrangements, on sister locos, can produce 
different sounds.

Those exhausts piped into the smokebox and up the stack,
will be hard to hear, and well muffled.

​W.

( The air compressors on 1309 (both) exhaust into the vapor/water separator 
   which you can find on top of the 1309's smokebox, between the bell bracket,
   and the 'stack ---- if you can find a close-up.  The sound will be combined
   according to varying compressed air demands, train-handling, etc.)

 



Edited 9 time(s). Last edit at 09/07/23 20:10 by wcamp1472.

kurtarmbruster Wrote:
-------------------------------------------------------
> Gang, SP's cab-forwards were noted for the
> distinctive "choo-choo" sound of their
> smokebox-mounted air pumps as they exhaust steam
> into the atmosphere. Were these engines unique in
> their air pumps operating this way?

Pretty much, yes. 

For example, I
> see that Chessie 1309's front-mounted pumps don't
> appear to exhaust the steam the same way, or make
> that sound. 

The air pump exhaust is piped into a "muffler", as most other railroads did. However, some roads piped the pump exhaust into the smokebox exhaust nozzle stand.

Thanks for the excellent responses, guys, very interesting. Those SP ACs sure put on a show!

I remember standing next to DM&IR Yellowstones 224 and 225 at Proctor, MN on the morning of July 2, 1961 thinking "These sound just like the whistling airpumps on SP AC's! (Two fantrips that day and their last runs). Both were Baldwin products from 1941 and had smoke box mounted air pumps.

Warren Frost 

Several photographs in Robert Church's book on cab forwards show the air pump exhaust on the later classes being piped from the compressors straight up into the sky.  No mufflers, no condensate separators, nothing.