Dryness

[duncan webster 1Participant @duncanwebster1]

I've found an article in ME 4466 from 2013 which quotes figures for the dryness of steam produced by model boilers (before the superheater). It quotes a figure of 0.89 and references Tubal Cain (the English one, not the American upstart) in a book by Martin Evans called The Model Steam Locomotive. I've not got the book. If anyone has could they see if there are any more details, in particular how did TC get that figure. Any other concrete info appreciated

[duncan webster 1Participant @duncanwebster1]

[ChrisLHParticipant @chrislh]

TC has a chapter in his "Model Engineer's Handbook" on the subject. He doesn't say he has measured steam conditions in a particular case. He does say however that "a normal boiler will provide steam at about 0.88 dryness fraction"

[Roderick JenkinsParticipant @roderickjenkins93242]

I've had a skim through "The Model Steam Locomotives" and I couldn't see a mention. Nor in "Model Locomotive Boilers".

Rod

[Tom SheppardParticipant @tomsheppard60052]

Point of order. The English Tubal-Cain is just as much an upstart. As the great-great-great-great-great grandson of Adam and Eve, the real one was an Israelite!

[Anonymous]

Dryness fraction is not easy to measure. If the steam is reasonably dry the normal method is to use a throttling calorimeter. This only works if the steam becomes superheated after throttling. For wetter steam one can combine a seperating and a throttling calorimeter. I doubt that all model boilers are the similar, so quoting one figure is most likely an estimate.

Andrew

[OldironParticipant @oldiron]

Posted by Tom Sheppard on 24/11/2022 18:33:52:

Point of order. The English Tubal-Cain is just as much an upstart. As the great-great-great-great-great grandson of Adam and Eve, the real one was an Israelite!

And of course as is usually the case the American one spells his name differently.

regards

[Grindstone CowboyParticipant @grindstonecowboy]

I think he pre-dated Israel (son of Moses) by a considerable margin. Could he have been a Canaanite (or, being an artificer in metals, a Tin-Canaanite)?

Rob

[duncan webster 1Participant @duncanwebster1]

Thanks, I never thought of looking in TC's handbook. As TC was not a loco builder I suspect this is a typical industrial figure. I'd measure it by condensing the steam into a known mass of water, then from temperature rise and weight gain you can calculate dryness. Unfortunately the 2 locos I have are too heavy for me to get out of the house now, or I'd have a go.

[Howard LewisParticipant @howardlewis46836]

Throttling steam does dry it, but how the dryness is measured is a mystery

The Tubal Cains of model engineering took their pseudonyms, since they were, like many of us, "Articifers in Metal"

A Freemason can tell us much more about the first one, his activities and his fate.f

Howard

[Martin Johnson 1Participant @martinjohnson1]

I would like to thank Duncan for starting this thread, which seems to be largely about chasing down data for me.

A bit of context might be useful.

Back in ME 4584 of 13 April 2018, I published a write up of a boiler performance prediction spreadsheet I developed. This started from a similar point to Bill Hall's program but has developed along slightly different lines. It became pretty clear that superheater design was fundamental to locomotive performance – as Bill Hall proved beyond doubt. However, the Ewins ratio method takes virtually no account of superheaters, yet an effective superheater according to Bill Hall's work can cut steam demand by around 50%.

Like all good programs, I have endeavoured to calibrate my program against tests of which there are very few for anything less than full size. Duncan has put me on to work by Busbridge (ME 1/8/64) and Ewins (Testing of locomotive boilers in Martin Evans' book) and more recently an ME article by Don Broadley and Alan Green (ME 4466 perhaps, but not sure). A key question is how dry is the steam at the boiler dome? Working back from Busbridge's and Ewin's results suggests it is quite dry – albeit with considerable mathematical work to calculate the heat transfer in the superheat flues and elements. Broadley and Green's results seem just a bit too good to be true, even assuming perfectly dry steam before the superheaters. There are also reports that Bill Hall measured temperature rise across the superheater on a Speedy under track conditions and found no temperature rise at all – suggesting that the superheater was making the steam drier, but not superheating it.

Why does all this matter? Well it is clear that plenty of superheat is a recipe for success. However too much superheat with steam temperature well over 300C will rapidly degrade cylinder oil leading to rapid wear. However, one would expect steam coming off a tiny boiler with minimal height between steam offtake and water surface to be pretty wet – in that case you need enough superheater surface to first dry the steam and then superheat it.

And that is the design conundrum. So if anybody can point us toward measured data or even custom and practice then it would be really useful.

Thanks,

Martin

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