What does this pipe do on a steam loco?

[Trainman440]

Really dumb question lol but I cannot seem to find the answer anywhere on the internet.

What's the purpose of the bundle of pipes often seen on the engineers side of US steam locos? The ones which is mounted on brackets underneath the runningboard, running nearly the full length of the running board to the cab. Often a bundle of 2 to 4 pipes. 

PSC brass parts catalog just calls the brackets which hold them "air pipe bracket under running board" which doesnt help.

I always called them aftercooler pipes after watching someone call it that in a video years ago but Im beginning to question if that is actually it.

Thanks,

Charles

[gmpullman]

Trainman440

I always called them aftercooler pipes after watching someone call it that in a video

Cooling pipe is the correct answer. These pipes follow the immediate output of the air compressor(s) and allow the air to cool a bit before heading into the reservoirs.

Some modern locos with dual, pilot-mounted compressors use an 'aftercooler' that somewhat resembles a finned radiator to more efficiently remove heat from the compressed air.

Good Luck, Ed

[wrench567]

  Air pipes. When you compress air your also compressing the moisture in the air. That moisture/ water will condenss in the piping and move along to the wet tank so it can collect and get drained. Then the air will be sent to other individual tanks. Moisture in the air system is bad. Not just for icing up when cold, but corrosion too. If the water is not drained the less air you have compressed. A common complaint from a new truck driver is running out of air too quickly. I would ask them if they drained their air tanks. Then I would show them the gallons of water running out.

     Pete.

[Trainman440]

Ah gotcha, makes sense. Thanks! 

[gmpullman]

I have to take a step back and add a caveat to my reply in particular to the photo of the K4 1361. The pipes in question, just below the running board are not the air compressor cooling lines. Here is a closer look at the front end of these pipes:

PRR, Altoona, Pennsylvania, 1987 by Center for Railroad Photography & Art, on Flickr

Note the size of the top pipe, it is 1-1/4" which is the 'trainline' air brake pipe and the 1" pipe below it is the signal line. Below that is the air line that supplies air brakes to the pilot wheels from the brake valve in the cab.

Pennsylvania Railroad - PRR Class K4 4-6-2 steam locomotive Nr. 5436 (Baldwin Locomotive Works, 1927) by Historical Railway Images, on Flickr

Yes, many locomotives have cooling pipes for the air compressor but you should be able to easily see that these are essentially a length of pipe that is 'folded' back on itself much like radiator heat pipes would be. Some even use a 'return bend' (180°) U shaped fitting at each end as evident on this ERIE 2-10-2:

ERIE, Avoca, Pennsylvania, 1947 by Center for Railroad Photography & Art, on Flickr

Good Luck, Ed.

 

[Overmod]

It's important to cool compressed brake air quickly, for some of the same reasons it's valuable to have a coil of copper line at the outlet of an airbrush compressor.

A locomotive compressor handles a large volume of ambient air and compresses it to 140psi, imparting a great deal of heat to it.  This would tend to keep humidity 'in suspension' in the air (to ice out during subsequent expansion through parts of the brake system) and also produce higher pressure in parts of the system that will not be recoverable as effective brake pressure when needed.  So some method of removing compression heat cheaply but effectively needs to be provided, in a way that will survive the bashing and dirty environment that a steam engine must survive.  A good way to do this is via conduction in a long length of uninsulated heavy pipe -- but to get this to fit effectively on a locomotive, this is achieved by multiple parallel runs connected by return bends.  These were usually slung under part of the length of a running board, as it was easy to 'bracket'-support them there and any valves and traps would be easy to inspect and maintain.  The usual place for air compressors in the 'early days' was hanging off the boiler or smokebox, adjacent to this area, and the main reservoir was likewise easy to locate at the 'distal end'

When it became more standard to put compressors up by the pilot beam, and suppliers saw an opportunity to advertise better 'radiators' or 'aftercoolers' that could ride up front in relatively cool and stone-free air circulation...see the '40s and early-'50s Locomotive Cyclopedias for examples.

N&W as an experiment tried locating these up on the curve of the boiler barrel -- again, Ed will have pictures and perhaps some of the discussion that led to it.  This apparently worked well enough to leave in place, but not so well as to start converting other locomotives.

(Note that if you model these, pay attention to the arrangements made to drain liquid condensate water from the brake system.  An improved version of this is the 'spitter' system you hear on diesel-electrics idling in humid weather -- lots of little strategically-located traps that may be operating frequently to sneeze out moisture as soon as it gathers...)

ED: I don't know if you can fix this, but Erie 4129 (pictured at Avoca) is well-known to be a 2-10-2 (R-2 I believe) and the original caption data for that photograph need to be revised accordingly.