Dryness of steam from model boilers

[grumpyoldman]

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

[uuu]

A similar figure (0.88) is quoted by Tubal Cain in his "model engineer's handbook", as the dryness of a "normal" boiler. But with no further explanation (except to note it means 88% gas and 12% liquid). He does have calculations for the ongoing qualities of the steam during superheating, or after exhaust. I've not got the Martin Evans book.

Wilf

[grumpyoldman]

Thanks, I've got TC's book

[RGR 60130]

Nov 24, 2022 14:37:39 GMT grumpyoldman said:

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

I'm intrigued as to where the energy to dry the steam before the superheater is coming from. He must have a pre-superheater.

Reg

[uuu]

11-12% liquid is not really "dry" though, is it?

Wilf

[RGR 60130]

Nov 24, 2022 18:52:19 GMT uuu said:

11-12% liquid is not really "dry" though, is it?

Wilf

It would be good for an up and downer but I’d hate to think of the repercussions of putting it through a turbine. I found the article and in doing a very quick scan it’s not the greatest. ‘Superheat Temperature’ is mentioned early on. That can be confusing. I think he should be saying ‘Degrees of Superheat’. I will try to have another look over the weekend.

Reg

[springcrocus]

If you don't mind me asking a numpty question, why does knowledge of the dryness of the steam matter? I understand that re-heating steam in superheater tubes adds extra energy to the steam which can usefully be used in the cylinders but I don't see the relevance of how wet or dry the steam is.

Can anyone provide a "dummies" style explanation, please?

Regards, Steve

[uuu]

We'd need to look at the "Steam tables" and do some maths to quantify the position - Tubal Cain has this in the handbook - but for the same volume of dryer steam you'll be carrying more energy (the gas has the latent energy required to evaporate it), and containing less mass (since the liquid part is heavier). So you get more power from the engine, and use less water.

Wilf

PS - You're building an excellent experimental engine, that might provide proof. If the taller inner dome on your 02 collects dryer steam than the lower one, you might see an improved performance.

[springcrocus]

OK, but surely the steam in a model boiler is always going to be at the same level of saturation?

The boiler has a fixed volume and a fixed pressure (safety valves), therefore the temperature of the steam is always going to be the same when the safety valves are feathering. Any extra energy going into the steam will try to increase the pressure and will be released by the safety valves. With these three things fixed, is not the saturation level also be fixed? The only extra energy that can be added must be after the regulator.

If one is considering superheated steam, would the temperature of the steam not be directly proportional to the saturation? I'm obviously missing something here.

Regards, Steve

[uuu]

The wetness in the steam within the boiler is caused by minute droplets of water constantly being ejected from, and rejoining, the surface of the water. It becomes a question of whether their concentration is higher close to the water surface - and whether, in a tall boiler, there's a point above which the concentration of drifting water droplets remains constant. Do different designs have better/worse dryness?

You're right that adding more energy into a boiler at working pressure is not going to improve things- it might even make it worse if your steam outlet is poorly positioned and collects splashes from the bubbling surface.

I don't think there's such a thing as a perfect boiler - one that delivers dry steam. I've seen figures of 0.95 being quoted as attainable.

If you take a quantity of wet steam from a boiler into a mild superheater, you might not increase the temperature, you may just get a drying effect, which would be beneficial.

Wilf

[simplyloco]

Nov 25, 2022 9:41:25 GMT springcrocus said:

OK, but surely the steam in a model boiler is always going to be at the same level of saturation?

SNIP

If one is considering superheated steam, would the temperature of the steam not be directly proportional to the saturation? I'm obviously missing something here.

Regards, Steve

Hi Steve.
I learnt about steam when I was in the military, and I know that steam can't be superheated unless it is dry, but it's easier to quote Dr. Google these days!
John

When steam is more than 100% dry it is called superheated steam. This type of steam is created by adding heat above the saturated steam threshold. The added heat raises the steam's temperature higher than its saturation point, allowing the amount of superheat to be easily determined by simply measuring its temperature.

[springcrocus]

Thanks, chaps. I thought the steam would be of uniform density throughout, I didn't realise there was a gradient. I still don't see, though, why the upper steam with less droplets in it has more energy than the lower steam and I'm surprised that turbulence doesn't mix it more thoroughly anyway. I don't see how you can get a temperature or pressure gradient in a closed vessel. Maybe that's not important.

Regards, Steve

[RGR 60130]

Interested parties may find it useful to look at this old thread:

modeleng.proboards.com/thread/12942/concentric-superheaters

Reg

[grumpyoldman]

I've been busy on other jobs for a few days, but thanks for the extra input.
You can add more energy to wet steam without increasing its pressure, it just gets drier. Limit is 100%, after that you're into superheating and that has to be done away from the liquid water. Drying out steam takes a lot of heat compared with superheating it, so if you are trying to do calculations on superheaters knowing the dryness of the steam at entry is important. I'll look at RGRs link ASAP

[tony9f]

The only way I know of to ascertain the dryness fraction of steam is to use some form of calorimeter which I dabbled with many years ago. If I recall correctly the type of calorimeter used was a combining calorimeter which has an orifice for the wet steam to pass through the throttling of which causes it to become superheated briefly. Various temperature and pressure readings are taken and the dryness worked out from the steam tables.
It may be possible to calculate from the temperature of the boiler shell and steam temperature after the superheater but the time you've done all that you'll have probably figured out whether the steam is in superheat from its' temperature anyway because in theory the steam temperature shouldn't be able to increase beyond the given stated in the steam tables until all the water has become 100% saturated. The biggest problem with all this is being able to get the boiler to maintain a constant pressure over the duration of the test.

[tenor]

Nov 25, 2022 8:19:11 GMT springcrocus said:

If you don't mind me asking a numpty question, why does knowledge of the dryness of the steam matter? I understand that re-heating steam in superheater tubes adds extra energy to the steam which can usefully be used in the cylinders but I don't see the relevance of how wet or dry the steam is.

Can anyone provide a "dummies" style explanation, please?

Regards, Steve

I don't very often visit here these days, but grumpyoldman is not very grumpy at all and has been trying to help me out on this topic for some time.

The reason for the interest is that I have been developing a mathematical model of locomotive boiler performance for some years. Yes I know Jim Ewins did a version but I am very unimpressed with that as it doesn't really consider superheater performance. According to Bill Hall's tests, superheat or lack thereof can easily change steam demand for an engine by a factor of 2 or more, so superheat is IMPORTANT.  Bill Hall reckoned about 100 C of superheat would get most of the available benefit.

However, as noted by simplyloco, you can't start to superheat steam until you have dried it.  So if you are taking very wet steam from the boiler the "superheater" may actually be a drier and not increase the temperature at all.

On the other hand, steam temperatures beyond 300 C will start to degrade even superheated cylinder oil quite quickly.  (Main line practice was to atomise the oil with saturated steam to get round some of that problem.)  But in model work it is clear that too much superheat could be a bad thing for engine life.  So in fact, we are trying to aim for quite a narrow band of steam temperature to get good efficiency without losing lubrication.

I shan't bore you all with how to calculate the heat transfer between flue gases and steam in a typical superheater set up, but suffice to say it is do-able.  But it rather helps if you have some idea of what condition the steam is in before you start.  Which we don't!

At present, I am trying to reconcile some tests on a LBSC Brit boiler done at Capetown University (ME 1/8/64) with tests done on a quarry Hunslet by Don Broadley and Alan Green (ME 4466).  They seem to imply very different values for wetness the Brit tests would imply quite dry steam (0.98) to start.  The Quarry Hunslet tests give superheat values too good to be true.  Tubal Cain quites 0.88.  Depending on what value you choose would change the required number of superheater flues by a factor of 2!

Some folk might be interested to see what actually happens in a boiler and why steam comes off wet.  Try these:
www.youtube.com/watch?v=a0xZPl_bwHI

www.youtube.com/watch?v=tI3tWuSsX7c

Yes, it is all a bit nerdy but it stops me from running amok.  I wrote up the boiler calculation program starting in ME 4584 if anybody is interested.

Martin