Author Topic: Talking Thermodynamics  (Read 16915 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #450 on: November 12, 2017, 02:23:36 AM »
HI MJM ...when i enlarged the photo all the scratch marks disappeared ?!!!!!!Also in your book it looks like it was given away with compliments ..so no problem with reselling borrowing or copying then ?
« Last Edit: November 12, 2017, 02:33:23 AM by steam guy willy »

Offline Steam Haulage

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Re: Talking Thermodynamics
« Reply #451 on: November 12, 2017, 10:12:20 AM »
Hi MJM et al.

I have been intrigued by this thread from the very beginning. Being currently forced to relax and take it easy  I have found time to think more clearly about the work which is being done by the contributors.

As someone who has built up a healthy scepticism about theory unsupported by practical experiment I am pleased you are undertaking some work to support your assertions. On many occasions I have worked to attempt to confirm academic conclusions at the lab. bench. As is often the case Madame Curie's experience has been replicated. I might add that I have worked in some of the best commercial labs available and with persons much more qualified than myself. Some times the simple question 'Have you considered all the variables?' can be an eye opener.

However returning to practical application has anybody here looked at studied the papers read at the I Mech E by George Jackson Churchward? He carefully worked to produce some of the best free steaming boilers available.

Jerry
Not at the Coalface anymore

Offline MJM460

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Re: Talking Thermodynamics
« Reply #452 on: November 12, 2017, 11:27:38 AM »
Hi Admiral DK, I am not sure of the mechanism in the negative coefficient materials.  Almost certainly much expensive research, probably supported by some high level physics theory somewhere along the line.  I wonder if they are semiconductors or something.  At least the mechanism for the normal positive coefficient was well enough understood long enough ago to be included in my 1961 text book, so I was able to look it up to check my memory.  It's not really necessary to have a constant resistance in the heating elements, the purpose of the calculations was simply to see if the change in resistance would explain any significant amount of the apparent heat losses in the boiler test.  It certainly helped, but not enough, so there are obviously other discrepancies to be tracked down as well.

Hi Willy, I am glad you liked the picture, I will get back to that in a moment.  To help you picture 1 lb of steam, for any pressure and temperature, the steam tables will tell you the volume.  A bit harder if you have wet steam, then you also need to know the dryness or another property.  It will depend on how the experiment is proposed.  How did they measure it for the tables?  Well they need a well insulated apparatus, so steam does not condense while they are measuring.  One way of achieving this is to submerge it all in a water bath, with the temperature controlled to the required value, then there is no heat transfer to or from the apparatus.  Possibly a piston and cylinder device so the pressure can be controlled to the required value.  Personally I just leave it to the authors of the steam tables, and have a copy handy.  I find that more convenient than an online copy.  Though that spreadsheet from Ohio that I referred you to recently might come in useful, especially when I need some values in a spreadsheet.

Glad you are finding the thread informative and fascinating, in the end, that is what it is all about.

Hi Steam Haulage, glad to have you following along.  I found your reply just as I was about to post, the the rest was already written before I added this.  The theory is not new to me, it was a basic tool for my job as a mechanical engineer in the oil industry.  The theory works very well there, with calibrated instruments, full sized turbo machines and sophisticated computer programs, but I have long been interested to know how far I could apply this well known theory to our model engines.  When it appears that I am feeling my way along, it is because I am exploring how to apply the theory as I go along. I don't actually have all the answers before I start.  Sometimes this shows!  Churchward and others are the pioneers who laid the foundations for both the science and the  practice, and one can only admire the intellect and ability they applied to their work, and what they achieved without the benefit of all the information and calculating power we now take for granted.  His papers would be most interesting to read, but there is not time for everything.  So my models have thermowells for temperature measurement, and my multimeters have thermocouples as well as voltage probes.  I have a non contact tachometer, and need  a suitable pressure gauge or two and to build a suitable torque measuring device.  It's on the list.  Oh, and a digital scale from the kitchen, and a watch for timing.  But these simple tests provide practical figures to use in the calculations to keep them well grounded.  I tried on my own to just do the calculations, but it did not seem satisfying, but I am really enjoying sharing my journey through the theory, and it is even more interesting and useful when it is guided by the questions readers ask.  They make a huge contribution to the direction of the thread, to my enjoyment, and I hope to theirs.  The questions have taken me in interesting directions I never would have thought of.  And they are all very welcome.  Just a little background to help you see where I am coming from.

Now that picture Willy commented on.  There are no scratches on the ruler, nor in the picture on my iPad that I used as the attached file for my post.  Nor is it on the forum post when I open it on my iPad.  But when I look back to the computer where I used photoshop to reduce the file size, some copies also have that scratch like appearance.  I am sure it is not your computer, I wonder if it is some interference pattern between the lines on the ruler and the compacting algorithm, somewhere between the jpg algorithm and the file size reduction procedure.  Perhaps even the ruler or camera were not quite vertical.  The ice cube appearance is interesting.  The camera shutter speed was 1/60 sec, possibly too slow to freeze the motion, perhaps I should try the flash.  I suspect the ice cube effect is a result of the bubble boundary movement while the shutter was open.  An illustration of how the experimental method can interfere with the experimental results.  I will try a few more pictures.

I hoped the picture would give an idea of the extreme turbulence in the boiler when it is generating steam.  The heating plate is at 63 mm on the ruler so the still water depth is about 30 mm.  You can see liquid rising over 25 mm from the surface, pushed up by the rising vapour.  As it falls back, more rises.  When I look through from opposite the handle, the bubbles appear to rise up each side and collapse back in the middle.  No surprise that the liquid and vapour are well mixed and can be assumed close enough to equilibrium temperature.

Now if you imagine this in your horizontal cylindrical boiler, it's not hard to see why you do not want to start the boiler with the level too high.  Our engines do not run too well on liquid, and the steam needs space to allow the liquid and vapour to properly separate, or disengage.

When we first open steam to the engine, we can expect some condensation as the initial steam heats up the mass of material in the engine cylinder and steam tubing.  If you weigh the cylinder, preferably during the build, you don't want to disassemble a completed engine for this purpose, you can calculate how much heat is required to heat the cylinder to steam temperature.  The steam tables will allow you to calculate how much steam has to condense to provide that much heat.  When the engine is already assembled, very normal if you did not have this idea in mind before the engine was complete, you can still make a reasonable estimate of the cylinder mass from the dimensions or as a proportion of the total engine weight if that is more practical.  You have a fair idea how little it really is if you collect the condensate from the cylinder drains and exhaust.  With a simple superheater in a fired boiler, the condensate normally stops as you would expect, quite quickly,  and the engine continues to run on dry steam.  Of course, if you have no superheater, even dry saturated steam from the boiler, when it is expanded to atmospheric pressure in an adiabatic engine becomes a wet exhaust steam, as we saw back in post #363 (on page 25 of 31 with my forum settings).  Perhaps it would be a good time to return to that point, and see what it means for a real engine.  How much liquid could we expect from the expansion of dry saturated steam to wet exhaust in a real engine?  That will require a whole post, at least.  So I will have a go at starting that next time.

Thanks for your interest,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #453 on: November 13, 2017, 01:43:14 AM »
Hi MJM do you get 1 Lb of steam from boiling dry 1 Lb of water ? as i am still a bit confused about this ?? Thanks.....

Offline MJM460

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Re: Talking Thermodynamics
« Reply #454 on: November 13, 2017, 06:24:34 AM »
Conservation of Mass? -

Hi Willy, definitely yes, a pound of water, when completely boiled produces a pound of steam.  That is the implication of the principal of conservation of mass.  I should strictly be saying pound mass, or lbm. to clearly differentiate between mass and force in the lb.-ft-second unit system, but we are not talking about forces here, only mass, so I hope no confusion.

The confusion is common enough, when we add sugar to our tea, does either the volume or the mass stay constant?  The change in either quantity is not enough to clearly see.  But in turning liquid to vapour, the volume expansion is clear to all, so it is a better example to illustrate the principal.  It is important to remember that volume is definitely not conserved in most processes, it may increase or decrease in any particular case, while, for all practical purposes, mass is conserved in any process.

  Early scientists had conservation of mass as a fundamental principal, but then along came Einstein, and all that had to go out the window.    He was a real spoilsport to the science community of the day, but it did explain some very puzzling results being observed in experiments at the time.  The problem is that famous equation, E = m x c^2, where E is energy, m is mass and c is the speed of light.  Everyone has seen that equation, it is the first one turned to when a film director wants to imply that there is some science going on, but what does it actually mean?  Putting it very simply it means mass can be turned into energy, and energy into mass.  Now the speed of light is a very large number.  And c^2 much larger again, so in a process where this equation applies, it involves a very large amount of energy for a very small amount of mass.  A deceptively simple looking theory, but controlling such processes is another matter entirely.  I know you like banging and crashing, but we would all rather you did not play around with that one.

There is a bit more to the theory, and it tells us that when objects move at any speed, you should make a correction to the mass dependent on the velocity, so the total energy remains constant, because energy really is conserved, so you have to correct the mass in accordance with its velocity.  Similarly, momentum is conserved, and again requires you to make that same correction to the resting mass.  And time comes into the mix somewhere.

In practice, we don't need to do this unless we are dealing with very high velocities.  For practical low velocities, say less than a few times the speed of sound, yes, the speed of sound is a slow dawdle compared with the speed of light, so for any velocity you are likely to deal with in your engine making, the correction is way smaller than anything you can practically measure.  I have looked out the formula for you, so you can calculate the change in your mass for a few speeds you might consider travelling.  The formula is

m = m0/sqrt(1-v^2/c^2)

In case the equation is a bit obtuse to some, I can put it in words -  mass equals mass at zero velocity divided by the square root of (1 minus v squared divided by c squared).

You will know the speed of light of course, about 3 x 10^5 km/sec, or 186,000 miles per second, and you must use the same units for v and c, then that ratio has no units and works with any units for mass, mass will always be in the same units as the rest mass, m0.

You will soon see that the increase for reasonable velocities, is very small.  Go ahead and try it, a good calculator or a spreadsheet can handle the numbers.  But I think any further treatment of the theory of special relativity should probably belong in a different thread, with a different author, and probably a different forum, along with general relativity.  But that much is enough to understand what I mean when I say conservation of mass is no longer considered a fundamental law of physics, but it is a useful enough approximation to help us solve problems.  You will have noticed that the analysis of most thermodynamic problems relies on the basic laws, conservation of energy, conservation of momentum, and conservation of angular momentum, and usually utilises the approximation of conservation of mass to lead to a useful solution.  Definitely worth knowing the limitations of this approach, so you have confidence in the results.  There are further conservation laws that apply instead of conservation of mass, I can look them up of anyone would like to know them.

Hmm!  Doesn't leave much room for a big post on exhaust steam, and besides, the theory of relativity might be enough to think about for a day or two, so I will give myself an early night and talk about exhaust steam tomorrow.

Thanks for following along,

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #455 on: November 14, 2017, 07:13:21 AM »
Engine exhaust steam-

Obviously everyone so still thinking about Special Relativity, certainly a direction I did not anticipate when I started this thread.  I hope it was adequate to convince you that conservation of mass is a very good assumption.  Or perhaps everyone is anxious for me to get back to exhaust steam.

I need to refer you back to diagrams I posted in #268 and #363.  My word they are a long way back, shows how long I have been trying to get back to the subject.  For convenience I have attached them again below, providing of course the meet the new size limit.  If they are not there, it will be because they were just to big, and I will reduce them further, but it may be tomorrow.

First diagram was posted to show the adiabatic expansion of steam from three pressure Willy mentioned in his electric boiler tests, so no superheater.  Perhaps the clearest to see is if I use the higher pressure example.  So we were considering the adiabatic expansion of dry steam from the boiler to atmospheric pressure, which I assumed was the standard atmospheric pressure of 101.3 kPa.  The second law of thermodynamics says for adiabatic expansion, the change in entropy is zero.  So for the exhaust steam we have the pressure and entropy, which are independent conditions so they are enough to completely define the exhaust steam.  The results are noted below the graph on the diagram.  Then knowing the properties of the exhaust steam, at least when we do the necessary interpolation of the wet steam table, the work done by the steam during the expansion can be calculated using the first law of thermodynamics, which says the work done is equal to the change in enthalpy between the inlet and exhaust.  Again the answer is noted on the diagram, 254 KJ/kg.  Remember all this is for an ideal engine.  Now the second law of thermodynamics says the change of enthalpy for a real engine is greater than zero.  Unfortunately, it does not say how much greater.  So the exhaust condition of a real engine has higher entropy than the ideal engine, but the answer has to be determined by experiment.  An adiabatic efficiency has been defined as the change in enthalpy for the real engine divided by the change in enthalpy of the adiabatic engine.  You cannot use entropy change for this purpose as the change in entropy for an adiabatic engine is zero, so the calculation would involve dividing by zero, an operation which is not defined in mathematics.  If you try it on your calculator or in a spreadsheet you will always get an error message.  But enthalpy works for the purpose.  I have a vague idea that for a full size real engine, the adiabatic efficiency is found to be about 80%.  Not easy to know what it would be for a model engine, but smaller scale usually results in lower efficiency for various reasons, so more because I am lazy than any real justification, I will assume an appropriate answer is around 75%.  That means the enthalpy change, or work done in a real engine would be 75% the work done in the adiabatic engine, 0.75 x 254 = 191 KJ/kg.  Now you can see I am taking a leaf from Robert Hornby's book, that 75 % figure not only looks reasonable, but it results in the same outlet enthalpy and steam conditions as resulted from ideal expansion of 135 degree steam.  Not coincidence, or some subtle theoretical reason, just saves me the trouble or recalculating the figures.  So it is a completely reasonable assumption for our purpose.

So, when the steam expands from 462 kPa to 100 with an efficiency of 75%, the exhaust enthalpy is 2535 KJ/kg, and the work done by the steam is 191 KJ/kg.  But with those figures we can also calculate the exhaust dryness as 93.8 say 94%, somewhat drier than the adiabatic exhaust, but still 6% of the steam mass is contained in the liquid phase mist of that exhaust vapour.

It is difficult to know what that 6% would look like in terms of a water phase in the exhaust or would it be a mist.  It may the answer to Derek's conundrum of where does all the condensate come from in his engine.  A simple boiler test should give an idea of the boiler steam production.  Some collection of the condensate and measuring the quantity in a given time, would give a clue.  Remember that second law of thermodynamics, it says the change of entropy will always be equal(for an ideal engine) or greater than zero (for all real engines).  It cannot be less.  This means the entropy of the exhaust cannot be less than that that from the ideal engine or 92%.  So an expansion of steam cannot result in less than 92% dryness. 

Now you might wonder if heat loss from the cylinder could cause extra condensation, as that is part of the departure from ideal.  Certainly, when the engine is cold, and steam is first admitted, there is a lot of condensation during the warm up time.  So continued heat loss would contribute to the exhaust steam wetness.  Again we have to go back to those earlier heat loss calculations.   While the cylinder block is heating, heat transfer is only dependent on the steam condensing film coefficient on the inside cylinder surface.  Once the cylinder is up to temperature, further heat loss has to involve convection to the surrounding air.  We have seen how little this is over the whole boiler shell area, and even less when some cladding is applied.  The heat loss to air from the relatively small area of the outside of the cylinder will be very small.  The latent heat of steam is high enough, that the mass of steam condensed by heat loss to air will be tiny and very difficult to pick up experimentally.   The first step to explore the possibilities might be to eliminate the possibility of carryover by doing a short run, starting from a relatively low level, to see if the condensate stops after the initial warmup.

I asked earlier if anyone has applied or seen applied a steam separator to their boiler outlet.  It is interesting to notice that Thomas has built a steam separator on his square boiler.  I don't know if you are reading this Thomas, I don't expect that your separator, (you have called it a steam trap, but that is a matter of terminology), was installed as a result of this thread, so I expect you are thinking along the same lines for your electric boiler which also does not have a superheater, and you know that the engine expansion results in wet steam.  But also, it may reflect your previous experience with carryover from your electric boilers.

I hope that is all clear enough, if so, perhaps I should look at what happens if we start at at a lower pressure, perhaps more typical of an unloaded run at a show.

Thanks for reading,

MJM460

« Last Edit: November 14, 2017, 07:23:32 AM by MJM460 »
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #456 on: November 15, 2017, 08:46:22 AM »
Must have been having a bit of a brain fade yesterday, in two places even.  My comments about a steam separator were of course talking about a separator on the boiler outlet, or engine steam inlet, while the majority of the post was about engine exhaust.  Obviously the two ideas must be kept separate.  If there is a reason for a separator on the boiler outlet, it has to be about liquid carryover, and is a model scale approach to the installation of steam separators in the steam drum of a full size boiler.  I think they were even shown in one of Maryaks early posts.  It is not about wet exhaust steam.  My apologies for that, I should have focussed on the engine exhaust steam.

Second, I included two photos yesterday, then missed referring to the second one.  The reason it was included is because it included an engine expansion line on the T-s diagram for a real engine, that line from point 3 to point 4.  You will see it has a curve towards higher entropy rather than the vertical, constant entropy line of the adiabatic engine.  The scale of the first diagram makes it difficult to show this curve clearly, but it has the same form as the one shown in the second  diagram.  The higher entropy of the real engine exhaust steam means that the real engine exhaust steam enthalpy when it reaches the exhaust pressure is higher than it would be for an adiabatic engine.  The first law says the work done is equal to the change in enthalpy, so the lower the exhaust enthalpy, the more difference from the engine inlet, so the more work done by the steam on the piston.  The higher exhaust enthalpy from a real engine means the steam does less work in the real engine than in the adiabatic engine.

I hope that post makes more sense now.  So let's continue with that second drawing, at least it is on topic.  That second drawing yesterday was drawn to illustrate the process and test results for my own boiler, Meths fired with a superheater tube wound around inside the furnace.  I introduced this in post #268 back on page 18 of the current 31, and did all the calculations then, so I won't repeat them here.  But I would like to point out the differences between the electric boiler with no superheater and the fired boiler at quite similar pressure but with a superheater.  You can see on the second diagram, the expansion of the adiabatic engine point 3 to point 5, with the superheater outlet steam as engine inlet.  It meets the exhaust pressure very close to the saturated vapour line.  You have to look at constant entropy to say which side of the line it really is.  Now that superheater outlet has an entropy of 7.3018 KJ/kg.  If we look in the steam table entry for 101.3 kPa, sg = 7.3549 which is more than the adiabatic exhaust entropy, so the exhaust is wet steam.  Interpolation of that data for 101.3 gives us dryness = (7.3018 - 1.3069)/(7.3549 - 1.3069) = 0.9912. 

To give you an idea of the effect of atmospheric pressure, previously I did the calculation for 100 kPa which has a boiling point of 99.63 C instead of 101.3 kPa with a boiling point of 100 C.  The answer was 0.9905.  You can see it is not a major influence.  If I do not have a barometer reading, I tend to assume the value which gives the most convenient figure for calculations, that is whether it is more convenient to have a temperature of 100 degrees, or a pressure of 100 kPa, as the difference is not significant compared with other measurement errors.  We are trying to gain understanding, not resolve a legal dispute.  The point is that starting with that superheater outlet temperature, the adiabatic engine exhaust is wet steam but only just, with a dryness of about 0.99.

However, when I measure the outlet temperature for the real engine, I find 104 degrees at point 4.  The sloping extension of the horizontal line in the wet region is the correct form for a constant pressure line in the superheat region.  This is above the saturation temperature for atmospheric pressure exhaust.  So I had to go to the superheat table for that exhaust steam.  Definitely advantageous to assume 100 kPa here, to minimise effort in interpolation for a trivial difference to the answer.  Temperature and pressure in the superheated range are independent, and sufficient to completely define the steam properties.  Once again, back in mid September, I did the calculations and found the adiabatic efficiency was 72%.    Not so far from that 75% I assumed yesterday.  I also did the calculations again for 105 degrees, to see the effect of a one degree temperature error.  The digital thermometer only displays whole degrees, so potentially at some point, as little as 0.1 degrees would make it switch to the next digit, so the reading must always be considered to have an uncertainty of +/- 1 digit.

Assuming the exhaust temperature was actually 105 C gave an adiabatic efficiency of 64%, clearly 103 degrees would have pushed it the other way, so certainly that 75% I had assumed yesterday is in the right range, and we should not place too much emphasis on the actual figure over that range.  Clearly my test result was not far from the full size figure I remembered of 80% if the measured temperature had been 103 C, and remembering my memory is not totally reliable.   Perhaps I need to add that cladding to the cylinder now, and see if it makes a difference.  The test work list is mounting, will I ever get back to making an engine?  But I am enjoying the imposed discipline of actually doing the calculations with real numbers, and not just knowing how to do it.  So I hope you are all enjoying the reading, and I hope learning something of interest and even useful towards understanding your engines better.

Next time I would like to look at what those test results mean.  Can we now calculate the power output of our engine? Or what does it actually mean?

Thanks for following along

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #457 on: November 16, 2017, 02:18:10 AM »
hi MJM, yet more relevant info  ...great... I have just used my boiler to raise steam for an engine and i have also put lots of rock wool under the boiler in the metal stand which brings the boiler bottom 1.75" above the wooden board. I was surprised to see that the safety valve lifted at 139 C this time rather than 135 c when i put the graph up on a previous post ?? would this be because of the insulation or perhaps the safety valve sticking ?? I did not measure the time however as this was just to raise steam...so i am sure you have an explanation for this ?......Thanks,You can see on the graph the green line where i have put an arrow to show where the safety valve was blowing off
« Last Edit: November 16, 2017, 02:23:47 AM by steam guy willy »

Offline derekwarner_decoy

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Re: Talking Thermodynamics
« Reply #458 on: November 16, 2017, 04:48:08 AM »
Hullo Willy...I have been following along [& a few contributions] with this thread since day 1

From a mechanical point, the additional insulation to the boiler shell should not have any differing level/point to the actual pressure the relief valve functions

Most model steam relief valves are relatively in-expensive simple direct acting in their design and function...variation in actual lifting pressure could be relative to the surface finish on the passive coils on the ends of the spring, the accuracy in seating of the ball in the spring and against the sealing surface within the valve and each of these could be summed as hysteresis and so could cause a perceived variation in the actual lifting pressure

Another aspect to consider is the additional insulation would provide a more gradual or uniform steam pressure increase relative to the heat input, so this should actually provide a more progressive pressure increase within the boiler acting upon the relief valve

Derek
« Last Edit: November 16, 2017, 10:31:19 AM by derekwarner_decoy »
Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline MJM460

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Re: Talking Thermodynamics
« Reply #459 on: November 16, 2017, 11:22:15 AM »
Hi Willy, good to have you back again.  Along with the extra rockwool under the boiler, did you put some on the ends?  Derek has very well explained some of the reasons your safety valve might lift at a different pressure.  At one time, boiler safety valves were fitted with easing gear, a lever fitted so so the valve stem could be lifted as part of the operating procedure to ensure they were not stuck.  Not sure if they still are, as there are problems with them resealing tightly.  However on my models I normally lift the valve and drop it back on the seat a few times before starting up to ensure it is not corroded up since the last time.  I suspect this might also help reduce the uncertainty of the set pressure, but there will always be some variation for the reasons Derek has described.  I understand why you did not record all the times again, you put a lot of effort into that last time.  Certainly put me to shame, so I will now have to do it, at least for a time or two.  However, I do normally record the time that I light the burner and the times steaming starts and finishes.  If you do this much, you will see clearly of your insulation changes have had a useful effect.  Then from the start and end of steam production, if you also measure the remaining water in the boiler after it has cooled down, you can also calculate the rate of steam production, which will help confirm the heat input and losses, apart from being interesting for looking at the engine in more detail.  In view of our discovery of a slight non-linearity in the first few minutes, it might be worth recording the temperature at about 2 minutes. Considering our earlier discussion, even better might be to record the time as closely as possible immediately the temperature reaches 40 degrees or some other suitable temperature, and another time as it reaches 130 degrees, so you have a more precise time for a given number of whole degrees as an alternative to reading temperatures to 0.1. I can tell you from experience that even with a meter with the facility, it is quite hard to consistently measure a rising temperature to 0.1 deg.

Hi Derek, good to have you checking in again.  I have not heard from you on your condensate issue since we both returned from our travels.  Have you looked at any further?  Or are you, like me, still trying to cut back the backlog of jobs that just wait for your return?  Does the current discussion about exhaust steam point to a possible answer?  Or do you have much more condensate?

Thanks for your very complete answer to Willy's question.  The detail of reasons for the variability is quite interesting.   I hope others will come in with other factors they are aware of, particularly the ones relevant to our model safety valves.  My full size experience is much more with machinery than boilers, and anyway, during commissioning, everything is new and clean and freshly calibrated, so I don't have much to contribute on that.  We have different issues to deal with.

 Before we look at what the calculated work from the steam expansion means, it is worth doing the calculations for the lowest boiler pressure from Willy's test.  A boiler temperature of 118 degrees has a saturation pressure of 187 kPa.  This is about 12.7 psig, so in the range you might use to run a model unloaded at a show.  The steam tables give us enthalpy of 2703.4, entropy of 7.1511.

Expanded to 101.3 kPa, we have hf = 419.04, hfg=2676.1, sf = 1.3069, sg = 7.3549 all as the previous examples.  In an adiabatic engine, the entropy is the same as the supply to the engine, 7.1511.  We can immediately see that for an adiabatic expansion, se = 7.1511, so the exhaust is wet steam.  We can calculate the exhaust steam dryness, 0.966, the enthalpy, 2600 and the enthalpy change 104 KJ/kg for the adiabatic engine.

If we assume that for a real engine, the adiabatic efficiency is again 75%, so we can compare it with the previous example.  Then we calculate the real engine enthalpy change = 0.75 x 104 = 76kJ/kg.  From this, the real exhaust enthalpy is 2676.1- 76 = 2627.4.  Again we have two independent properties, pressure and enthalpy, so we can calculate all the properties of the steam.

Checking that 101.3 kPa data, we can calculate the exhaust dryness 0.978, and even the entropy to see how much it increased if you like.  But even at this low boiler pressure, the exhaust is still wet, though at 98%, this means that only 2% of the steam flow is condensed in the engine, but is this anymore than a vapour mist?  Unfortunately in the tests from a boiler without a superheater, because the exhaust steam from our real engine is wet, we do not have enough information to calculate all the properties of the steam, as temperature and pressure are not independent in the wet range, in particular we cannot calculate the enthalpy and hence the work out from the real engine, but in any case, it cannot be more than from an adiabatic engine.

The boiler test results we have looked at so far have illustrated how we can calculate the work done in adiabatic expansion, which is a real upper limit to the work that can be obtained from any real engine operating between the same boiler and exhaust pressure.

We have seen that for a real engine, an experimental result is needed, but then we can calculate the work from the real expansion, and that it is always less than that from a real engine.  A reasonable guess for a model is about 75%, but we need many more modellers to test their own engines to make a more accurate estimate of typical values.  So far I have data from only one test, so the standard deviation could be quite large.  I will eventually do more tests which will help, but similar independent tests by other modellers is better again.

Then we looked at engine potential performance, starting with dry saturated steam from a boiler and found over a range of pressures the engine exhaust was always in the wet range, though well above 90% dry for 50 psig and higher nearer 99% for lower pressures.  With a superheater, from a similar pressure, the exhaust steam was found on test to be slightly superheated still, and hence gives dry exhaust.  The temperature and pressure are then independent, we can calculate all the steam properties.

So what does all this calculation mean for our engine power output?  You will have noticed that even though we appear to have calculated our engine power output, I have always refrained from calling it that.  Sometimes I have called it the work done by the steam on the piston face, and I think this is the most accurate description.  Now of course there is a long way from work on the piston face to shaft output.

I think that is a good place to take a break.  Tomorrow I will talk about what we can learn from those simple boiler tests.

Thanks for following along,

MJM460
« Last Edit: November 16, 2017, 11:25:35 AM by MJM460 »
The more I learn, the more I find that I still have to learn!

Offline derekwarner_decoy

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Re: Talking Thermodynamics
« Reply #460 on: November 16, 2017, 01:40:28 PM »
Certainly progressing MJM....just slowly......

The revised 1/4" steam exhaust trunking spools have been manufactured, lagged & painted [last coat today] & loosely assembled  :hammerbash: to show the differences

I will confirm condensate outcome at the earliest time....[as I also want to understand the expected differences]

Derek
Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #461 on: November 16, 2017, 02:46:26 PM »
Hi MJM and Derek, thanks for that explanation , I thought it might have been the safety valve sticking at first but it did continue to blow off at 139 c a few times later. Also i have managed to find the  "bible" Tome by Dalby all 740 pages with lots of info and drawings and tables !!! enjoy !! This also shows the steam trap mentioned elsewhere attached to the steam chest. There is also a table with the steam velocity !!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #462 on: November 16, 2017, 09:49:19 PM »
Hi MJM, I have now completed a new boiler test with lots of insulation and was surprised that there was not much difference in the time taken to reach the same temp/ preasures in fact the temp was only 1 degree centigrade higher !! all the other parameters were all the same however and followed exactly the same temperature differences . This either shows the instruments behaved the same or they are not wildly inaccurate !! so we can say that your comments about 4 times the insulation is needed for any appreciable difference in insulation is correct . The blowoff temp also went back to 135 !! although the temp continued to rise with the valve leaking slightly, so over to you ........The starting temp was 18 degrees this time and i measured the weight of water in my jug and at the 600ml graduation the weight was 589 grams ....
« Last Edit: November 16, 2017, 10:05:51 PM by steam guy willy »

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #463 on: November 17, 2017, 02:42:17 AM »
HI MGM, It is now about 2.30AM and just checked the temp of the boiler and it is 43 C    the boiler was switched off about 9.10 PM so all that insulation is doing some good !!..............I am about to go to bed but may have to get up in a bit as the 'waterworks' dictate so will keep you posted !!!  Now 3 AM ...40 C,,,, So 1 degree every 10 mins...should be back to 18 c about 6.40 am......!!
« Last Edit: November 17, 2017, 03:10:31 AM by steam guy willy »

Offline derekwarner_decoy

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Re: Talking Thermodynamics
« Reply #464 on: November 17, 2017, 04:42:31 AM »
Willy......from the steam tables.....we see........

135 degrees C = 30.72 PSI
139 degrees C = 36.26 PSI

So the 4 degrees C increase represents a ~~ a 3% increase in temperature, however the same temperature increase provides an ~~ 18% increase in pressure

Your comment that the valve later returned to the lower lifting point suggests that the valve is faulty :zap:...so understanding this, it should really be stripped & checked

Derek
Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au