Steam Question

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Wed Dec 1 07:40:36 EST 2021

 Yes, there is a quality of steam, generally indicated by total steam temperature reflecting degrees of superheat.  The higher the steam temp, the more work  that can be extracted from each pound of steam.  Generally, a modern steam locomotive will have a saturated steam temp of about 400 degrees and a superheat of 250-300 degrees, for a total temp of 650-700 degrees.  On the N&W with 300 psi boiler pressure, the numbers are saturated steam 421 degrees and superheat around 300 degrees for a total temp of 721 degrees. Superheat temp can vary depending on how hard the loco is being worked.
Others may have a better explanation here.

Dave Stephenson

    On Wednesday, December 1, 2021, 06:15:22 AM EST, NW Mailing List <nw-mailing-list at> wrote:  
 Thank you Matt!  That was more than I hoped for, especially the water additive explanation!  I know how density altitude affects aircraft performance,  and I thought  the larger  temperature difference between steam and the outside air temperature in the winter may cause an increase in power!  Another thing that just popped into my mind is that steam engines are not compression engines, they are expansion engines!!  
The analogy on the decrease in oxygen at higher altitudes and the reduced temperature because of makes a lot of sense too...
When creating steam, in theory is there an operant temperature to get the most out of steam or is it the hotter the steam the greater the expansion.  I understand a hotter fire will increase the quantity of the steam, but is there a quality of steam?

Thank you very much for taking the time to explain!!!!

Christopher J. BunseySent from my iPhone

On Nov 30, 2021, at 8:33 PM, NW Mailing List via NW-Mailing-List <nw-mailing-list at> wrote:

Hi Christopher. The altitude question came up a few months ago. 
My educated guess is that higher elevations probably did not affective starting tractive force as the steam pressure inside the boiler would be constant (subject to safety valve settings), regardless of the external air pressure. Potentially the ability to generate steam rapidly under high loadings (i.e., power) could be reduced since there isn’t as much oxygen for the fuel to interact with and would therefore generate less heat for given amount of fuel. Alternatively, the fueling might have to be forced (i.e., more fuel) to get the same amount of heat.  Dad’s technical books - specifically  “The Steam Locomotive” by M.E. Johnson (Baldwin’s Chief Engineer) - doesn’t mention the topic at all. It could be that steam locomotives were so inefficient that any performance changes / fuel consumption rates related to altitude just weren’t noticeable. 
Along those lines, the book “The Steam Locomotive Energy Story” covers some of the efficiency questions, and is available at the N&WHS Commissary. 
Additives for boiler water and their ratios depended on the water source being used. External methods (softening plants) mixed chemicals into the raw water, then piped it to a settling tank to get various solids out of suspension before the water ever got to a tender. The materials used were lime, soda ash, and sodium aluminate. In another process (called the Zeolite Method) that exchanged salts that tended to cause scale with those that didn’t used aluminum, sodium, calcium and potassium. The downside to external plants was their expense in operation, so they were most often used in heavy traffic areas. 
In locations that didn’t warrant a water softening plant, an internal treatment  was used. This used soda ash, sodium aluminate and tannin to get the solids to precipitate out in the boiler. This was not a preferred method since the chemical ratios were harder to control (especially if applied by the engine crew) and by it’s nature allowed all the solids to get into the boiler, creating more sludge, foaming and other downsides. 
All of this (and much more) from the Johnson book noted above. 
Matt GoodmanColumbus, Ohio, US

On Nov 28, 2021, at 1:27 PM, NW Mailing List <nw-mailing-list at> wrote:

Does the ambient outside temperature and or pressure affect the power produced by a steam locomotive?  I know the less the ambient pressure the lower the boiling temperature of water.  Did the engineers have to adjust their operating procedures in winter vs. summer?
Additionally, what additives did they add to boiler water to avoid scaling, and approximately when did railroads start doing this?

Christopher J. Bunsey

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