What makes the compressed air in steam locos??

[Anonymous]

No wonder that diesel loco has shaft directly connected to the air compressor to generate the compressed air, but when it comes to steam locos, what source of machine used to do that function while the shaft from the prime mower is not presented???

Karn[:)]

[Randy Stahl]

Steam driven air compressors of the recprocating variety.
Randy

[Overmod]

Steam air pumps were an interesting subject.

"Traditional" steam power used steam drives for most of the ancillary stuff (electric being much more expensive as well as somewhat less 'efficient' at the required scale). This is why feedwater pumps, stoker engines, etc. are usually steam driven rather than using electric motors (as on most stationary boilers).

Turns out, though, that steam-driven air pumps are somewhat more efficient than a shaft-driven pump, because expansion is used to generate compression and these are 'balancing' pressure effects. This is particularly true with the Westinghouse-type cross-compound air pumps. I dimly recall an excellent animated graphic on the Web showing how these operate, but can't take the time now to hunt up the URL. Compressing sizable quantities of air up to brake-reservoir pressure is not a trivial exercise, but the compound pump was good at doing 'both halves' of that particular job while using a minimum of steam and heat energy.

Karn's original post seems to indicate the belief that steam locomotives use air pumping from the wheels or cylinder drive once they are in motion. This isn't the case on any locomotive I know of, large or small; the reciprocating pump has steam valved to it whenever air compression is desired.

Randy: Are all diesel compressors single-stage, multicylinder compressors driven at high shaft rpm?

[csxengineer98]

diesel locomotive air compressers are driven 1 of 2 ways... shaft driven (shaft from the prime mover hooked directly to the compresser)...or as in the dash 8s and up GE models...they are an electric motor driven...
the shaft driven ones have a maget valve that will stop the compresser from loading (makeing air) when a preset main air resoear PSI is reached... but the shaft will continue to spin but no air is bening compressed....when the PSI drops to below another preset PSI...the magnet valve allows the compresser to load agin..thus making compressed air
now the motor driven ones will only compress air when the motor is turned on... this is done automaticly when the PSI of the main resovear drops below the preset PSI..and will shut off automaticly also...
csx engineer

[Randy Stahl]

Mostly locomotive air compressors are 2 stage. The SD75I has 6 cylinders, 4 low pressure and 2 high pressure. The SD45 has 3 cylinder, 2 LP 1 HP
Randy

[Overmod]

Now that I see Randy is 'live' --

Years ago, I noted that the air compressor in one of the D&H's PAs (ex-Santa Fe) resembled a small radial engine, complete with little air-cooling fins on the cylinders. Don't remember seeing any difference in size, or unusual valving. Was, or is, this a common design?

[Randy Stahl]

WABCO sells air compressors either air cooled(like the one you described) or water cooled. As far as know there is no difference in CFM out put. ALCO PA!!!!!!!!!!!!!!
Randy