Steam & Excursion > Steam for this Saturday: Cass Scenic

A 2007 photo run on the Cass Scenic. Crew looks like they're taking a break. 

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Notice the superheater pipe coming out of the smokebox,
near the stack base.  Its the large, horizontal pipe feeding the cylinders.

On steamers, it's an important asset because you use less molecules 
of water, if the steam temperature is raised.... like to over-600F.
Some superheaters deliver 700F steam to the pistons.  

On cooling, Steam doesn't get to the water-state until it's below saturated* temperature,
at the lower steam pressure.   On rod-engines, it's common to have superheated 
steam a coming to the base of the smoke-stack.... even at a drop in steam pressure.

Steam, in the presence of boiling water will have the same temperature
as the water that boils it.  A hotter fire only boils the water more quickly.

350F water Temp wil produce 350F steam... steam at the same temperature as 
the water is said to be heat-saturated.  "Saturated" has nothing to do with liquid 
content, saturated steam refers to the gas-state of water, at the temperature of the
water that boils it.

As you know steam only move towards the lower-pressure volume.
And, yes dropping pressure lowers the steam temperature, already at 
'saturated' levels...so, some of the steam forms a cloud of cooler water vapor.

Superheated steam comes from a system of steam pipes leading away from the 
firebox area, and the flowing steam is routed, in the superheater piping,
to very near the flames from the firebox, inside the flues heading to
the smokestack.

The hot flames enter the flues which are surrounded by "cold" boiler water.
Boiling water is typically 375F to 400F, and its steam is also the same 
temperatures.  

Flame tips from carbon fuels are 3,000F, while being fired with a strong draft.  
The 3,000 F flames quickly get cooled, when surrounded  by the much lower
temperature boiler-water.   The hottest flame temperatures are very quickly
cooled in a short distance ---- about 30" from the firebox, and the combustion gasses are much colder.... by being surrounded by the cold boiler water.

So, superheater units have many 180- degree bends, so that they can have
their piping-ends heated to the hottest temps.... hottest temps are closer to the firebox.

Steam is mainly superheated by consistent air-drafting through the
firebox. Dwell-time of the steam
traveling through the units is crucial
to higher superheat temps.

You can have a light train, and flat
track, while cruising with a partial throttle opening… if the steam to the cylinders stays at low flow-velocity rates, that Steam can become superheated, even with a low-throttle! The longer Steam flow
stays in ‘hot’ units, it can get to very
high superheat temps.

On Shays, the superheated steam is piped, like in the photo.
Most Shays were built as non-superheated, so their steam pipes 
come directly out of the steam dome, down to the cylinders.

Superteated steam theoretically enlarges the water capacity of 
the tenders by 30%.  But, with few cars, superheated steam does not 
occur as easily as with heavier trains, so "water-savings" are not achieved.

I like superheated Shays!
But saturated Shays are most common.

​W.


​(* 'Saturated' Steam is steam temperature at the same temperature as the
     water that boils it.  You can add more heat to the boilerwater, but because
      of the pressure-limiting safety valves, pressure is controlled.... so, hotter
      fire does not raise the boiling point, so you'll only boil the water faster.   

      The steam [water converted to the gas-state] is the same temperature 
       as the water that boiled it, and the steam is said to be "heat-saturated"...
         it can be bone-dry --- no water droplets---  Until it begins to flow towards
          the lower pressure.   As steam flows to the lower pressure,  its pressure
           lowers proportionately,  and more liquid water droplets form. ) 



Edited 5 time(s). Last edit at 01/11/25 07:46 by wcamp1472.

Ha!   Same Shay- as seen on my home road: The Skeena River Railway.
SR

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Thanks as always for the wonderful information.
—Mac

wcamp1472 Wrote:
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> I like superheated Shays!
> But saturated Shays are most common.
>
> ​W.

"You can have a light train, and flat track, while cruising with a partial throttle opening… if the steam to the cylinders stays at low flow-velocity rates, that Steam can become superheated, even with a low-throttle! The longer Steam flow stays in ‘hot’ units, it can get to very high superheat temps."

So, after hearing so many stories about 4014 not steaming well, then I read this comment, just above.  4014 has been pulling light trains, probably cruising with a partial throttle opening.  So why doesn't its steam reach superheated state, based on the above comment by Wes??