Author Topic: Talking Thermodynamics  (Read 12993 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #345 on: October 10, 2017, 12:36:31 AM »
High MJM ,thanks for this explanation and yes there were a few wisps of steam actually. Would it be possible to add accurate  boiler pressure values  to this graph, and what temperature reading would the pressure be at atmospheric to open the filler cap safely ? Perhaps the wood needs to heat up gradually before it starts to radiate the heat ?as it is insulated partly from the copper ? On the side of the boiler is a neon lamp that is connected to the elements and when the potentiometer is full on this lamp is fully on. When the pot is turned down the lamp will flash on and off in time with the pulses of  the current, the lower the pot the slower the flashes, so when it is full on the elements are getting the full currant.


Offline MJM460

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Re: Talking Thermodynamics
« Reply #346 on: October 10, 2017, 12:16:00 PM »
Boiler pressure and air -

Hi Willy,  I am sad to say that we can't plot the pressure directly against temperature to get a really accurate figure.   The pressure predicted from temperature has to be interpreted with an understanding of the effect of the air that is in the boiler when you first seal the plug.  Earlier in this thread, I have said a couple of times that once you start steam production, the air is soon gone.  You have prompted me to actually do the detailed calculations, and I am starting to realise that the air might take more time to become negligible than I thought.  So let's look at what really happens to the air.  First it is important to realise that the air and water vapour both fill the space independently.  So while the water vapour pressure is accurately found from the steam tables, any air remaining in the boiler adds a further component to the pressure seen by the gauge.  We have already seen that at 100 deg C, the air pressure is 130 kPa and the water vapour pressure is 100 kPa, so a total of 230 kPa.  Keep in mind that these are absolute pressures, so we subtract the atmospheric pressure, say 100 kPa, to get 130 kPa gauge or 18 psig, an 18 psi difference between the gauge and the steam tables.  If we continue to say 150 deg C, the calculation gives a gauge pressure of 75 psig compared with the water vapour pressure contribution of 54.3 psig.  The error is now just over 20 psig.  Both cases so far assume all the original air still in the boiler.

Now the air and water vapour in the vapour are fairly well mixed.  I suspect the steam being generated at the surface might mean a bit lower concentration of air near the surface, but the rapid movement of the molecules means and and water are probably reasonably well mixed further from the liquid surface.  When a little vapour escapes through the engine or the safety valve, the mass of each in the escaping steam is proportional to the partial pressures.  The lost steam is quickly replaced while the heat is applied, but the air is not.  But the air lost in the next time period is proportional to the remaining amount.  After a little has escaped with the steam, the remaining partial pressure is reduced, so the proportion of air in the escaping steam is reduced.  The first half, in a certain time, then half of the remaining amount takes a similar time.  Like the temperature of your coffee in the cooling experiment, which approaches the ambient temperature ever more slowly, the air loss happens increasingly slowly and in principal, while it gets close enough for practical purposes, it does take a significant time.  As the air is lost the discrepancy between the gauge reading and the water vapour pressure obtained from the temperature reduces, but the gauge pressure should always be a little higher than the prediction.  Fortunately the safety valve works on gauge pressure, so always protects us.  Bit if we are relying on the temperature for our pressure , we need around 20 psig safety margin between the design pressure and the intended operating pressure.  If the  safety valve is set a bit closer than that, it will lift before our intended operating pressure, the escaping steam takes a portion of the air with it, thus reducing the air pressure and reducing the difference between the steam table value and the actual gauge pressure.  The engine also runs on total pressure and does not care if it is water vapour or air, and I find the engine starts to run quite soon after 100 deg C is reached, but the air at this stage means the actual gauge pressure is a bit higher than I have assumed.

A further factor in all of this is that as the steam is used, the water level reduces, so the remaining vapour space is larger.  The steam needed to fill this volume evaporates from the water, but there is no more air, so the air pressure is further reduced as the level falls, and the error due to the air content is further reduced.  So I think that up to 150 deg C, the error is a maximum of about 20 psi which should be noticeable on the gauge, but the error reduces as steam production goes on, and the reduction in error should be noticeable.  After a few runs, you should have a good idea of how long it takes to get down to an acceptable error, then you will always have a good idea whether your pressure gauge calibration is still correct.

I must admit the error is a bit greater than I was expecting, and the time for the air to effectively be gone is certainly longer than I thought.  It looks like I had better add pressure gauges to my two small boilers which currently only have temperature measurement, plus a carefully checked safety valve.

Then, what temperature can you safely remove the plug?   In industry, you can open piping or equipment only when you have proved the pressure is zero by opening a vent valve.  Even then, sometimes the vent valve is blocked by rust or dirt, and there is still pressure inside.  That is another place where Murphy interferes with the best laid plans.  On our models if I wanted to be sure, I would add a whistle, though a vent valve would also do.  Personally I normally let it cool down overnight.  In order to refill for another run as soon as possible, I suggest even with a gauge, zero is really hard to confirm.  It clearly has to be less than 100 deg C, sixty degrees is normally considered a safe temperature to touch, though if you do not have tough skin it will still feel pretty hot.  Then I would leave the regulator open and help the engine turn over while any pressure was obvious.  At 60 deg C, the water vapour pressure is about 20 kPa absolute, so needs a lot of air to get to 100 kPa absolute or close to zero gauge.  With experience and suitable precautions against getting scolded, you may find a little higher Ok.  If it is really desirable to to get started sooner, probably the safest course is to use a hand pump to refill the boiler to the appropriate level gauge indication.  Of course, once you pump in a bit of cold water, the temperature will soon fall, so you can remove the plug if necessary to check the level.

You have mentioned that your level gauge is not very reliable.  I believe small level gauges are notorious, and the vigorous boiling of water makes the level bounce around.  Also, when boiling is occurring, part of the vapour is below the water level, and sudden changes of pressure change the effective density of the liquid phase which then upsets the level gauge as it does in a large boiler.  When the boiler is cooling for refilling, the density should be more steady, but you still have to deal with surface tension issues and the meniscus which both change the apparent level.  It may help to take the gauge apart and clean the paths into the boiler in case they are salted up, but these are practical issues and I don't have a lot of experience with gauges in model sizes.  Industrial level gauges have much larger passages and different problems.  Your colleagues in the club should be able to make more useful suggestions.

That neon light sounds like a nifty way to tell if the boiler is lightly or heavily loaded.  I like that feature, but it makes it hard to determine how much time the element is on, and hence how much heat is actually being generated.  I assume it is fully on during the heat up phase, a bit of flashing as you near the safety valve setting would be an additional cause of a curve in the heat up curve, however, I suspect that that wisp of steam is a sufficient explanation of a very slight curvature.

I hope that clarifies the issues sufficiently,

MJM460

« Last Edit: October 10, 2017, 12:24:42 PM by MJM460 »
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #347 on: October 10, 2017, 03:34:45 PM »
Hi MJM, Thanks for your further info, and leaving things to cool down provides a good excuse to go to a cafe and drink coffee etc etc . I have done a test with my thermometer and putting it in a boiling kettle only shows a reading of 93 C actually so %7 out, i have yet to get some ice but that will be interesting soon.! So all these calculations in fact are quite hypothetical, and need actual laboratory conditions to get an accurate reading and the formulae should be more like  Air pressure (at x humidity, at x height above sea level ,at x wind speed  ,at x etc etc etc)  X the rest of a formula should bring in all the variables necessary but do need to be known.....

Willbert....

Offline MJM460

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Re: Talking Thermodynamics
« Reply #348 on: October 11, 2017, 12:36:57 PM »
More on boiler pressure and air-

Hi Willy, I am surprised that the thermocouple is so far out, but I am not yet convinced.  To get an accurate temperature reference at boiling water temperature of 100, you need to get as close as possible to equilibrium, just like in your boiler.  So you need a lid on the kettle, just tilted enough to admit the probe, and perhaps stack a few face washers around to minimise the steam escape, and after a while enough air is gone and you will get much closer to 100.  Careful with those face washers, the steam will make them hot.  If you get 98 per 99, I would accept your meter as reading correctly.  But also check the barometric pressure.  Either your calibrated barometer, or check the weather bureau data for your area.  Standard atmospheric pressure is 101.35 kPa absolute, or 1013.5 hPa, which is the water vapour pressure at 100 deg C.  If there is a low pressure system passing over, 1000hPa will give 99.63.  You can see this in the steam tables.  It takes a bit of interpolation for other pressures.  Similarly for zero degrees, you need water liquid and solid ice in equilibrium.  So lots of ice, minimal water, insulate your jug with a towel, probe deep into the water through the slightly tilted lid, along with a stirring stick so you can keep it well mixed, and some more face washers to minimise heat gain.  It is harder in practice to exclude the air from the zero point than the boiling point as there is not enough vapour generated to displace air.  You would need a vacuum pump to get that triple point condition.  But again if you can get within 1 or 2 degrees of the liquid-solid equilibrium, I would accept the metre as more accurate than the calibration check process.

Now before you dismiss all this as hypothetical, please bear with me while I have a go at a more precise explanation.  A nights sleep, a powerful analysis tool, enabled me to come up with a simpler and more precise calculation of the air pressure contribution to the total pressure over time.  Looking at the boiler in isolation and trying to calculate the loss of air was not very easy, so I took the easy way out and gave only the maximum error, which occurs as you first heat up, and decided that I really did not know how long it took to loose the air.  But if we expand the control volume, to use text book terminology, to include the engine, we have a situation which is much simpler to analyse.  Now it does not make any difference to the boiler whether the engine is a conventional reciprocating engine which takes many strokes per minute, or if it is a very large piston which just accepts all the steam for your total run time in one very long stroke.  Nor does it matter to the boiler what the engine does with the steam next, just as long at it does not gain or lose heat while it takes in all this steam.  Now I have assumed that your run time ends when your 600 ml of water is reduced to 300, in order to stop while the element is still covered.  So you evaporate 300 ml, or 300 grams of water, at a constant temperature of 135 deg.  Now the initial 1.7 grams of water to give saturation vapour pressure at the 15 degrees starting temperature is not very significant compared with the total of 300 grams evaporated.  I assumed an initial vapour space of 0.5 litres.  Now when 300 grams of water evaporated at 135 deg C it makes 175 litres of vapour.  But the initial charge of air at atmospheric pressure is only 0.556 grams.  While the water evaporates into the vapour space as the total volume expands into that very big cylinder, so the mass of water in the vapour increases by evaporation of the liquid, the air mass is constant and in the end, we have expanded the initial air from 0.5 litres to 175 litres.  We had initially calculated that when the temperature reaches 135 deg, the absolute air pressure is 130 kPa.  The expansion of the air volume, remember gases always fill the whole space, the ideal gas law tells us the pressure of air at the larger volume.  Because we are assuming a constant temperature before we start the expansion, the ideal gas law is basically P1 x V1 = P2 x V2.  The condition in the boiler at any time is just a known sample of that total end volume.

I basically calculated the air pressure, total pressure and gauge pressure 10 times for the volume expanding 0.5 l to 175 l.  If the run lasts about 20 minutes, that is roughly every 2 minutes.  Now if that model is clear and accepted, the resulting gauge pressures are the answer to the problem. 

You will remember that at 135 the pressure from water vapour is 31.3 psig.  With the typical small gauge on a model boiler, with scale marks at 10 or 20 psi intervals, it would be pretty hard to read that more precisely than calling it 30.  The calculation gave 31.87 psig after just the first 30 ml was evaporated.  And 31.5 after 90 ml was evaporated, so a very rapid initial fall which then only slowly trends towards the long term value.  Now if the air remains always well mixed as assumed by the assumptions, it is never all gone.  The pressure after evaporating 300 ml was 31.40, only 0.1 psi high, but in practice it was close enough after about 4 to 6 minutes.  Not just the first blast from the safety valve, but never the less, quite quickly after the engine starts to run.  The error is really only significant during that initial heat up to safety valve set pressure that the maximum error is observed.  And that is readily calculated from the initial vapour space volume. 

While the initial humidity would normally be less than 100%, after a short time in the sealed boiler  it will be made up by evaporation from the liquid, but will make less than1% difference to the pressure contribution of the air, so I think can be ignored. 

I think this is a more rigorous approach to the calculation, and I hope it makes sense.  Like a metal part, a knowledge building block sometimes takes more than one attempt.  The first analysis that comes to mind is not necessarily satisfactory, and sometimes needs some additional work. 

The clear learning that comes from the whole conversation is the influence of the initial air in the boiler.  For pressure testing a calibrated gauge is required, and I am sure that a boiler inspector would insist on a calibrated gauge for the initial safety valve setting.  However, a model gauge calibration and safety valve setting can be checked quite accurately towards the end of a run.  And used with understanding of the influence of air, the temperature gauge is quite adequate for normal monitoring of your boiler operation.

In addition we have learned that attempting to prove zero gauge pressure by subtraction of atmospheric pressure from an absolute pressure of a similar magnitude, inferred by the temperature reading is subject to errors that make it unreliable.  Better to be safe by waiting longer for cooling, or preferably, use a whistle.

I hope that is a more satisfactory ending point than yesterday, thanks for following along.

MJM460
« Last Edit: October 11, 2017, 12:46:07 PM by MJM460 »
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #349 on: October 11, 2017, 02:31:33 PM »
HiMJM, thanks for this  ,more info get my head around !! I have been testing the Thermocouple in melting ice and i think the battery may need changing as it should show 32 on the farenhieght  scale and 0 on the centigrade scale !! Also an account from the 1827 Farey book about removal of the pressure suddenly from the boiler ....very colourful language !! Here are a few pics of the readings ......Also he mentions  'Sensible heat" again !
« Last Edit: October 11, 2017, 02:35:04 PM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #350 on: October 12, 2017, 12:41:32 PM »
Hi Willy,  I think you will find your battery ok, those instruments usually have a low battery indicator, but your open glass with one or two ice blocks just won't cut it as a zero degree reference.  Make a tray of ice blocks in the freezer compartment, many little blocks are best as there is more contact with the water.  Put your glass of water, half full, in the fridge while the ice blocks freeze, (perhaps overnight?). Then fill the glass with as many ice blocks as will fit, fold a towel to the height of your glass and wrap it around, better still leave the towel in the fridge as well, mix the ice and water really well and cover the top.  After a while to allow it to get to equilibrium it will get a lot closer to zero.  I would suggest that with the open glass which may have started at room temperature, the 4 degree reading is possibly quite accurate.  Similar care is needed to get close to equilibrium temperature at the boiling point.

Sensible heat is reflected in a temperature change you can sense with a thermometer, or even your finger if the temperature is appropriate.  You know by now that freezing or melting of ice and evaporation or condensing of water both involve quite large amounts of heat.  However both processes occur at constant temperature, and you cannot sense the heat input with a temperature instrument.  You would actually have to measure the volume of the solid, or weigh it, and measure the volume or mass of liquid, and from this infer the heat change from steam table data.  But it's a wonderful article from your book.  The language makes it hard to read, and demonstrates the difficulty of describing things if you do not have the mathematical tools, quantitative data, and even those more intangible properties of enthalpy and entropy.  A bit like Pythagorus describing his theory for right angled triangles.

I think with yesterday's calculation, I can now provide your requested graph. You will remember yesterday I said that the total pressure approached the equilibrium value quite quickly, and today I did a few more calculations in that first 10% of the run! and I am really back to my original contention that the air is soon effectively gone.  If the total run is 20 minutes, the pressure will be virtually true steam pressure in about 20 seconds.  A short safety valve blast or enough steam to warm up the engine.  This should be evident by comparing the gauge readings with the temperature readings.  So you should first see a pressure around 20 psi high, which should drop back quickly once the safety valve lifts, especially then if you release a little steam to warm up the engine cylinder.  I have two nice graphs, one for the heat up period, and one for the run time.  I am having technology troubles I getting a picture within the size limit, but I should get there tomorrow.

Just a short post tonight, I need the pictures to make it all a bit clearer,

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #351 on: October 12, 2017, 01:28:33 PM »
 Hi MJM , So Sensible heat is not the obverse of silly heat !!! I have wondered about this for some time as all the old books talk about Sensible heat !! Also ice does funny things when cooling/heating as i have been led to believe . I wonder when the words Enthalpy and Adiabatic came into being ?  Like Mr Watt and Mr Farraday,  was there a Mr Enthalpy and a Mr Adiabatic ? !!!sorry just being a bit silly there  But it would make for interesting formulas IE   1 Cholmondly-Smythe divided by 1 Monatgue-Fitzpatric = 1 Dionosius-Ericson.........!!Also from your rethinking on these posts ,if you did pull a vacuum in the boiler would it behave differently ?? or should one try that out ? .Using an electrical element would be a good way of getting uniform readings from the heat input point of view....unless you tried to do it just after the World cup Football game when everybody switched on their own kettles !! Sorry about these ramblings but i have always thought out side the box !! Good to here about your continued interest with my own small contributions

Willy

Offline paul gough

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Re: Talking Thermodynamics
« Reply #352 on: October 12, 2017, 10:53:59 PM »
Hi Willy,  I note you have included a Dionysius in your formula. Caution! If this is of the type 'Lardner' be aware that it may contain a lot of hot air, causing unstable results. When placed under scrutiny it yields surprising behaviour, including 'criminal conversation'! An introduction to its behaviour can be had here;<https://en.wikipedia.org/wiki/Dionysius_Lardner> Regards, Paul Gough.
« Last Edit: October 12, 2017, 11:07:06 PM by paul gough »

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #353 on: October 13, 2017, 01:00:06 AM »
Hi Paul ,found this in one of his books.........says it all really !!! will look your link up...........

willy.

Offline paul gough

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Re: Talking Thermodynamics
« Reply #354 on: October 13, 2017, 08:04:48 AM »
Hi Willy, glad you get a laugh from Dr. Lardner, he is something of an engineering/science Sir Humphry from Yes Minister, a consummate obfuscator, and it seems, at times an incompetent, if not fraudulent, self promoter among other more nefarious activities. When looking at historical sources and it includes engineering historical writings, one has to be as alert to mythology as much as one would when considering the practicality of Icarus's flying equipment or the veracity of the events occurrence. All sources need to be measured against other evidence from the era to determine which is credible, (for the time), and which is to be ignored/avoided. I had two of his books and threw them in the bin. Apologies to MJM for this diversion. Regards Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #355 on: October 13, 2017, 08:07:26 AM »
Hi Willy and Paul, that Lardner character seems to have been quite controversial.  Good to hear from you again Paul.  Willy, the page you included is pretty hard to read, without the previous page anyway.  Print is clear enough, but the language is heavy going.  I am really not sure what he is getting at.  Obviously no friend of Mr Wolfe though.  Looking now at Pauls second post, that probably means I was not far off the mark!  Personally I prefer to go to the more modern books, they tend to be easier to read as well as including more up to date ideas.  Nevertheless, a bit of humour is always appreciated, especially in a thread like this.  And your thinking outside the square, seems always to find the gap in my description that I need to fill in, so I hope everyone else is finding them as helpful as I am.  I don't know where the terms enthalpy and entropy came from.  But just as well those other characters didn't intermarry and combine hyphenated names!

Found the limitation of the Numbers spreadsheet on iPad today.  I was having trouble getting the file size of my graphs down, and of course Apple is quite sure you don't need to know file size, so it takes some trickery.  Finally decided to send it all to the desk top machine, which then would not open a Numbers spreadsheet properly.  In the end, got the figures across, re constructed the graphs, printed on a better scanner and so on.  Numbers is really great for reviewing a spreadsheet, but is more limited when it comes to building one, particularly on graphs.  However, all done now, so back on subject.

Attached are the two graphs which show the results of the calculations relevant to boiler pressure and temperature readings, I hope pretty much exactly what you wanted.  The heat up one shows the relationship of the total gauge pressure, seen on the pressure gauge to the equilibrium water vapour pressure.  The lines slowly diverge due to the increasing contribution of the air pressure as the boiler heats up.  The air pressure increase is a linear function of absolute temperature.  It has a very tiny dependence on the starting temperature, due to the very low water vapour pressure at ambient temperatures, but you can see what happens.  Unfortunately, if you let some air out by lifting the safety valve or a vent early, you would change the mass of air and need a new total pressure curve a bit lower than the one I have shown.

Once you start releasing steam, whether to the atmosphere via a safety valve, or through an engine, the Run Time graph is the one that applies.  You can see that the air contribution reduces very quickly in 1 - 2 % of the total run time to near enough to zero, considering what can be read on a small gauge.  That would be less than 20 seconds of a twenty minute run.  But the error does not totally disappear, even after 300 ml of water has been evaporated.  It still contributes 0.1 psi to the gauge pressure and that is still reducing but at a very slow rate.  You would have to evaporate as much water again to half that error.  The graph is not very dependent on the steaming temperature, but is determined mainly by the initial quantity of air in the vapour space, which was assumed to be 0.5 litres.

So, while I would reiterate the point that you always need a safety valve, and a gauge is good check that the valve is working at the correct pressure, your temperature gauge is probably much easier to read accurately for general run purposes than a tiny gauge, so long as it is used with some understanding.  After only a few minutes of engine running, the gauge and the vapour pressure from your temperature reading should match.

In principal, you could reduce the mass of air in the boiler with a vacuum pump, and this would reduce the associated error during heat up, as well as that long tail on the run time graph, so long as you have a tight shut off regulator that stays shut under vacuum.  I only have a direct line to the engine, and low boiler pressure would lift the slide valve and draw in air.  The oscillating engine should be ok though.  However I am not convinced it is worth the effort.  The graphs are easier to use than I was expecting.  So here they are.

MJM460
« Last Edit: October 13, 2017, 08:10:38 AM by MJM460 »
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #356 on: October 13, 2017, 08:42:32 AM »
MJM, Willy, This air/vapour, temp/pressure exercise has indeed been an interesting phenomena to have explored and explained. I have only ever considered the negative 'oxygen' aspect of air in the boiler and where practicable raised steam with a relatively 'high' water level and brought it down appropriately as it heats and expands all the while having an open vent from the steam space so that over the course of steam raising there is an evacuation of air with vapour and also the vent gives a good indicator of that point when you just start to get pressure, often a gauge does not. With a loco boiler this vent is often, for convenience, via the steamway cock and blow through cock on the gauge glass or the blower in the smokebox. Regards Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #357 on: October 13, 2017, 12:54:32 PM »
Hi Paul, your note is a perfect addition, not out of context at all.  You see it is another example of where theory, no matter how complete, is not all that must be considered for any particular case. 

I assume that you are talking about  steel boilers, where of course a lot of effort goes into feed water treatment to remove oxygen down to minute levels in order to minimise corrosion throughout the system.  Even quite basic steam plants generally have a feed water de-aerator and a chemical treatment plan that includes a chemical oxygen scavenger.  There is no point in doing this if the oxygen scavenging system is then loaded up with oxygen introduced as part of the start up procedure.  And your procedure is a good way to eliminate that initial air by sweeping it out with steam during the warm up.  In addition, in a full size steel boiler, it is normally important to heat up slowly to minimise temperature differences which introduce thermal stresses that can be quite undesirable.

If Willy followed your procedure, he could also drive the air out of the boiler during startup, and then the temperature measurement would give a more accurate boiler pressure almost from the start, as there would not be air to add to the water vapour pressure.  However, he has a copper boiler where the corrosion issue are not very significant.  I am not sure how much oxygen is involved in scale formation, but on model run times, it is generally easily handled with a descaling procedure.  His electric heater arrangement produces very little in the way of uneven expansion as the sheath is free to expand and the water then carries heat to the shell quite evenly.  In addition, you will remember that Willy wanted to raise steam more quickly with the limited heat input from his heating element.  In that case, any early steam loss absorbs a disproportionate amount of energy due to the latent heat it carries away, and so increases the time until steam is up to pressure for running the engine.  He is also dealing with the issues of trying to read pressure accurately on a tiny model pressure gauge.  In the hobby world, accurate temperature measurement is easier these days due to readily available thermocouple instruments.

So in one case corrosion is the biggest concern, thermal expansion stresses undesirable,  accurate pressure gauges readily available, startup time not really important, it is very short as a proportion of the normal operating time of a full size machine, and these factors determine the development of the startup procedure.  Though the ships captain probably doesn't see it that way.

In the other case, corrosion not really an issue, difficult to measure pressure with small gauges, desire to reduce startup time.  In this case, understanding how the thermodynamics affects pressure inferred from temperature measurement, and starting the boiler completely sealed to minimise heat loss through vented steam may become an appropriate procedure.  However, to get rid of air through steam venting when the pressure is fixed by the open vent, then to seal and raise pressure could be in the long term, the best procedure.  It allows a little more time for examining the intricate details of the engine while the boiler heats up.  Once the engine is running, the details move too fast.

I am glad you are finding it an interesting aspect to explore,

Thanks for following along

MJM460
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #358 on: October 13, 2017, 05:55:49 PM »
I guess I should have prefaced my comments by indicating I was principally addressing steel boilers from larger 'model' sizes i.e. 12 inch gauge locos to full size. Though our codes generally demand steel from anything over 8 inch dia. barrels, which is smallish, and would I presume capture a lot of 71/4 gauge models. I thought the 'scavenging' of  air via venting during steam up might have been useful knowledge/technique to somebody especially if they don't have full size experience or are isolated enthusiasts, like me, who don't have the luxury of contact with like minded peers. I agree with all your commentary. A few of years ago I visited Mr. Jaycar, (electronics parts shop), when on a trip down south looking for temp. probes and a digital readout. At the time what was on offer was a too cumbersome to incorporate into my tiny G1 Lion loco. However there was a reasonably smallish probe/digital readout combo for temp. that one could reconfigure the connections so as to calibrate it for pressure, thus achieving a digital pressure readout/gauge. The instructions with the little screen had a wiring diagram showing the particular connections that achieved any numeral or combination of numerals on the display, so a bit of patience and diligence with a small soldering iron was all it would have taken to "make" an electronic pressure gauge from the temp. probe & readout unit and which would probably be suitable for a stationary set up, e.g. Willy's. However I'm not up to date with what might be available now, maybe micro sized digital pressure sensing units can be had readily?? Hope there is something useful in all my babble. Regards Paul Gough.
« Last Edit: October 13, 2017, 06:02:05 PM by paul gough »

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #359 on: October 14, 2017, 12:50:00 AM »
Hi MJM ,thanks for that ,i have been looking for a temp/preasure table on the web but could not find any. Do these graphs depend on the ratio of air (compressible ) to water (non compressible) ? Interesting about the steel boiler water treatment ! I notice that the heat up graph is non linear but this may be because the time part is missing ? Would it be possible to make a 3D model including a time element?.....
Willy