Author Topic: Talking Thermodynamics  (Read 154540 times)

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #120 on: July 06, 2017, 12:31:47 PM »
Exhaust ports for oscillating engines

Hi Willy, Bob and Zephryn, glad to hear from you all again.  Also received a PM from Derek for me to think about. 

Now, adiabatic enthalpy!  That is a mouthful.  You probably can construct a sentence with the two words together, but without a little more context, they are perhaps not as complete or clear in their meaning as you might like.  I suspect those who nod in wise agreement did not really understand what you said, just as you implied.

To get away from the thermodynamics, a student of English might class "adiabatic" as an adverb, while "enthalpy" is a noun, so you can see the problem.

Enthalpy is a property, just like pressure and temperature (though we have to calculate its value, as we don't have a direct reading gauge), perhaps refer back to Post #113 for more detail.  It's value does not depend on the process involved in getting the fluid, in this case steam, to that condition, just final temperature, pressure and density.

On the other hand, Adiabatic is the description of a specific ideal process, meaning a process that occurs without heat transfer.  Expansion in a well insulated cylinder is a process that closely approximates adiabatic, as is throttling through a valve or nozzle.  An adiabatic process can be analysed using basic thermodynamics, and the work output can be predicted.  Unfortunately most real processes, such as the expansion of steam after admission is cut off, cannot be analysed, so we can't predict the amount of power our engine will produce.  Not very useful if your brief is to design an engine for the Queen Mary.  But we can compare the power produced by a real engine on test with an ideal adiabatic engine, and define an efficiency accordingly.  Just one of many definitions of efficiency, but more about efficiency another time.

So your favourite term needs to be used in the context of an enthalpy change.  An adiabatic process involves a change of enthalpy from which we can calculate the work produced.  A real engine involves a different enthalpy change from that which occurs in a true adiabatic engine, always less, due to thermodynamic losses classed as irreversibility.  Now you are prepared for the one who asks for further explanation, and you can safely refer to "adiabatic enthalpy change" as opposed to real engine enthalpy change.

Bob has very clearly answered your question about feed water entry.  We don't need quite such a complex arrangement in a model boiler, but the principle is still to minimise the temperature difference at the entry point.   Always below the surface to minimise splashing and carryover, and you could extend your inlet pipe a little to get the worst temperature gradient further from the shell.  You could think about other ways of reducing the temperature difference in preparation for a future topic.

Your final question, and Zephryn's comment about whether an oval or straight sided port would help the oscillating engine, very neatly introduces what I was thinking of today.

K. N. Harris, in his excellent book on engines, looked at this problem and suggested modifying the ports to have straight sides.  Many of us have a copy of this book, but I have attached a sketch to show what he described.  It always struck me as a great theoretical solution to the problem, but I did not have the practical knowledge to be able to implement it.  I made a test block and tried with fine files, and tried the traditional " chippies" cold chisel to get the required port shape, both without success, probably due to inadequate skill level in both cases.

Florian might be able to comment on whether his excellent thread on making a broach could be adapted to produce the required shape.  I suspect the real problem for broaching in this application is that the required holes are very shallow.  The cylinder port for example, the broach could only protrude one cylinder diameter before it hit the wall on the far side.  In the base port block, holes do not go right through, and I am not sure if there is space to drill and tap the back face and insert a plug after broaching.

Now, having a mill and rotary table, and having learned many new skills by following the experts on this forum, I have also come to wonder, as you have, about making straight sided oval ports.  I even have a 2.5 mm milling cutter, and could buy a 2.0 mm one, though the bigger ports are fine on my 12 mm bore model.  I suspect that this is what you mean, but again, I have attached a sketch to show what I propose.  Now that I have sketched out, it looks very like Mr Harris' design.  Should work.  As Zephryn says, still not a perfect solution, but surely better than round ports.   Later this year I may get a chance to give it a try.  But not with my present schedule.

Mr. Harris also presents another solution to the exhaust opening conundrum.  This one may be even more effective, and is certainly more interesting from the thermodynamics point of view.  It was even the subject of a patent in the early days of steam, so on to that next time.

Thanks for following,

MJM460

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

Offline steam guy willy

  • Full Member
  • *****
  • Posts: 2772
Re: Talking Thermodynamics
« Reply #121 on: July 06, 2017, 02:37:12 PM »
Hi, more interesting stuff...i am thinking about a wobbler engine with triangular shaped ports with a 90 degrees offset wobbler part operating at the back of the port plates to achieve a cut off sequence.....!! or would this be clueless rather than corliss ?? !!!! might produce a drawing  !! Also i seem to remember an article in Model  Engineer some time ago with someone doing some experiments along these lines, the person might have been from Australia if i recall..Thanks for all this info,  it will help when i actually meet someone in person that also knows all about T.....
« Last Edit: July 06, 2017, 03:16:41 PM by steam guy willy »

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #122 on: July 07, 2017, 07:15:04 AM »
An alternative solution for oscillating engines.

Hi Willy, that second plate looks like an interesting concept to explore, it would be good to have some way of adjusting valve events.  Definitely not clueless.  You might need an eccentric to drive the plate to allow you to extend the shaft through the two fixed parts.  Also to allow timing adjustments.  I am not sure what the triangular port is about.  To me, ports are a source of irreversible loss, they all  turn good enthalpy into wasted heat in the exhaust.  Since we must have them, they should be large as practical, and always fully open or closed, with opening/closing times minimised.  And timing is the important thing.  But perhaps there is something I have not thought of.  Well worth sketching out and exploring the valve events.  There is also that very interesting design with the slide valve on the side of the cylinder, that has been the subject of some recent build logs. 

The background article in your picture is also  interesting, but I think it is not very helpful to talk about scaling nature.  We are building real engines, just small in size.  So some things might not work quite the way they do in full size, such as viscosity and surface tension.  However, mostly these are more obvious in ships for displacement and wave making, and aeroplanes  for wing lift and drag.  Almost certainly a few factors in making a miniature violin as well if we want the notes to be in the audible range.  It is more useful to think about which particular bit of nature you are trying to scale, than lumping all nature into one catchall term.

To round out my discussion of valve opening, and for oscillating engines in particular, I would like to apply some thermodynamics to the engine Mr Harris called a Uniflow engine.  It was not really an original idea, there was an early patent on the principle that I uncovered in a simple Google search, though it did not seem to have made its owners any great fortune.

The idea was to place an additional exhaust port low down in the cylinder, so that it was only uncovered when the piston was close to the bottom of its travel.  The idea was that the steam, instead of having to reverse its travel to be exhausted at the top, could continue moving down and out, thus avoiding the force on the piston necessary reduce the momentum of the steam and then reverse its direction.  He concludes by inviting his readers to try it, and see the extra output from the engine.  I would certainly encourage anyone interested to give it a try by adding the Uniflow port to their next engine.  And certainly a class of young people about to embark on engineering studies by building a small oscillating engine.  It will help introduce them to some basic thermodynamic principles of heat engines.  I hate to admit that I have not yet done it myself, but I have no doubt about the claimed increase in performance being real.

Observations usually are real, and when they depart from theory, my observation is that it is usually the wrong theory being applied, or often a different theory, also applicable, has more influence on the outcome.  The published explanation in the Uniflow patent appropriately describes momentum, but then alarm bells ring and I find myself skeptical about the explanation for the function of the port.  And I do not wish to be in any way disparaging about Mr Harris' effort, he has certainly made a great contribution to our hobby with his books on engines and boilers.

First, we have to deal with the fact that it is the momentum of the molecules being changed at the piston face which causes the force on the piston that drives the engine.  The crank mechanism means that the piston velocity is reduced to zero at bottom dead centre, so it matters not whether it then accelerates up, down or probably sideways to exit the port.  Of course the torque is minimal near the bottom of the stroke, so loss of torque due to opening the port is not much.

Then we note that the overall downward velocity of the steam is a very small nett velocity, superimposed on the quite high velocity of the molecules in otherwise random directions.  Remember way back, I looked up the typical molecular velocity of about 500 m/s, while the average piston speed is only about 5 m/s.  So the momentum due to the average downward velocity of only 1% of the momentum of the typical molecule, surely does not make that much difference.

I suspect that it is only when we appreciate the difficulty of opening the exhaust port enough to exhaust the pressure at the beginning of the exhaust stroke, that we see the more likely reason for the observed performance improvement.  It provides additional exhaust port area just when it is most needed.  In any case, on a single acting engine, drilling the port, but keeping it separate from the top exhaust, with some provision to block and thus disable it, would surely be an interesting experiment.  Not so easy on a double acting engine.  The piston length would have to be carefully proportioned for the port to work on both up and down strokes.  It would also be more difficult to keep it separate from the other exhaust ports so the engine could be reversed.  Still good food for thought, and a potentially interesting experiment.

That is not all that can be said about thermodynamics in relation to engines, however I believe that clears the decks enough to move on tomorrow to exhaust systems, which should help Derek solve his current issues.  (Derek, I have replied to your message). And I will return to some other engine topics as the need arises. 

I hope you have found this explanation of how an engine changes heat contained in the random motion of tiny molecules, to mechanical work, interesting and informative.

Thanks for dropping in,

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

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #123 on: July 08, 2017, 11:26:26 AM »
Exhaust systems.

Thinking still about a vertical engine, first single acting then adding any additional comments on the difference for double acting, lets now have a look at the exhaust system.

 Keep in mind that the work output of the engine is always reduced by the back pressure on the piston during the exhaust stroke.  It is obvious then, that we can increase the output of the engine by reducing the pressure in the exhaust system.  Now this approach is limited in extent, as we can only at best get to full vacuum, which gives only 101 kPa (14.7 psi) additional difference in pressure across the piston.  In practice, we are unlikely to achieve anywhere near this.  However, in most cases our models are operating at quite low pressure, so even minimal vacuum is a significant proportion.

There are four basic exhaust concepts, so let's look at each in turn.

First, there is the simple engine outlet to atmosphere.  This is seen on some of the beginner engines with a simple piston valve and no obvious way to collect the exhaust into a simple pipe.  Also on simple Mamod engines.  Sometimes, the exhaust is directed up the boiler stack.

This system is fine for simple engines and for engines we only intend to run on air.  It's simplicity is the big advantage.  The exhaust pressure is fixed at atmospheric at the pipe outlet, and the back pressure at the piston depends on how adequately the piping and port is sized.  However, the back pressure on the piston during the exhaust stroke will always be something above atmospheric.

As soon as we add a lubricator, whether using air or steam, the problems begin.  We will have a thin film of oil everywhere near the engine, even in our hair, if we run for very long.  If we are running a boat, a tiny film of oil on the water surface scatters light in a way that makes it very obvious, and that is quite rightly frowned upon.  We start thinking about adding an oil collector to our exhaust system, the second approach to exhaust system design.

The temperature at which oil boils is much higher than the exhaust temperature of a steam engine.  Even at 100 deg C, the vapour pressure of oil is very low, and can reasonably be ignored.  Most of the oil ends up as very tiny droplets, which are entrained in the exhaust, not even enough of them to make a visible fog.  Alternatively, they end up as part of an emulsion with the fine droplets of water in the exhaust which are what we actually see when the exhaust hits the air.  Even the tiny water droplets in a fog do have mass and density, enough to make it possible to separate them from the  un-condensed portion of the water vapour.  They do not settle rapidly due to the air viscosity (you can look up Stokes Law for more information on this). However, if the exhaust stream has some velocity, and we make it go though some rapid changes of direction, we can separate the droplets from our exhaust, and essentially all the oil with them.

The first picture below shows two separators I have built.  The one on the right works very well.  When the engine first starts, the cold metal condenses some of the steam, and water droplets run out the little gooseneck drain pipe pretty freely.  As the metal heats up, the condensing slows, and there is clear vapour out the top which only becomes visible some distance from the stack as the steam cools in the air and condenses to form some fog.  I collect the water from the drain in a little tin, and after a run, the tin contains a few ml. of oily water emulsion.  Not much compared with around 200 ml of water turned to steam in the boiler.  I have not been able to usefully compare the oil collected with how much is lost from the lubricator.  This would be interesting, but somewhat difficult due to small quantities.  Clearly some oil remains smeared on the cylinder walls and piston, and ends up in the drip tray under the engine, and some is clearly carried away, but the problem seems pretty well solved.

The separator on the left of the first picture is still a work in progress and I still do not consider it successful.  More on that later.  First how does a separator work?  Let's look more closely at the one on the right.  The horizontal steam inlet is not on the centreline of the vertical cylinder, but enters tangentially.  As a result, the steam entering is directed against the cylindrical wall and forced to follow round a circular path.  Centrifugal force results in heavier particles, our oily water fog, hitting the walls, and running down to be collected at the bottom.  Dry vapour, being less dense tends to stay closer to the centre.  Now, the vertical outlet extends down inside the cylinder to below the inlet.  Thus, the vapour whirling around in the cylinder has to make its way downward, then inward before it can exit vertically up the outlet stack.  More sudden changes of direction.  The heavier droplets tend to continue in straight paths due to their momentum.  The second picture shows the outlet removed, and you can see the internal extension.  No complex science to it.  Guided by the principle, I made it out of scrap tubing left over from a household plumbing job.  In industry, these cyclonic separators are carefully designed and velocities determined to remove a calculated percentage of particles above a certain size.  But the simple approach seems to work adequately for my purpose.

I will talk next time about the design on the left in these pictures.

MJM460
« Last Edit: July 09, 2017, 06:25:48 AM by MJM460 »
The more I learn, the more I find that I still have to learn!

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #124 on: July 09, 2017, 12:06:23 PM »
Another simple separator.

 Last time I included a description and pictures of two simple exhaust separators I have made.  Apologies for the typo in the last line yesterday, I have now corrected it.  I noted that the one on the left is still a work in progress, as I am not happy with its performance.  I was trying to achieve a very low profile to go under my paddle engine style joy valve engine posted in my engine showcase thread (currently on page 3 of Engine Showcase).  Very low cylinders are an advantage for stability in a boat, but make collecting exhaust oil difficult as the exhaust should slope down to the collector. 

Initially the larger diameter section of the vertical part was not included.  The inlet was near the top of one end of the horizontal cylinder.  The drain outlet was low on the other end.  Inside, both pies were extended to a point near the opposite end.  The idea was the engine exhaust has to turn 180 deg at the end of the internal pipe and return to near the inlet end before turning 90 deg to exit via the vertical stack.  Seemed worth a try, but the tube I had was really too small for my level of ability.  Despite several tries, the tubes seem to interfere on the inside and did not end up where required.  I did eventually discover the reason, but I decided a larger diameter cylinder would be a better solution.  I then tried making the separator part in the vertical section that you can see in the photos, back to the whirling design, and keeping the horizontal cylinder as a simple collection pot.  The inlet height was just adequate, but I could have made the whole thing vertical.  Both arrangements of this one still carry over oily water and splutter out the stack, so clearly not satisfactory.  The horizontal tube is only 19 mm o.d., I need to get some 25/25.4 mm tube and try again, but I will be lucky to get back to it this year.

The simple exhaust separator as described does quite a good job of removing oil and incidental condensate, but does not really do any useful condensing of the exhaust steam.  In fact the momentum changes in all those changes of direction actually increase the back pressure on the engine.

The next level of exhaust system development is to include a condenser.  A ship crossing the ocean might want to collect and reuse the water for example, or you may just wish to collect all the water from your exhaust to eliminate any possibility of escaping oil.  Unfortunately, a simple condenser will still not give you any vacuum to increase your engine output.  In addition, the collected water needs good oil removal treatment before it is suitable for the boiler.  Those with experience of full size steam plant will know the issue for achieving vacuum is air, but let's look first at simple condensing as a half way step for those with less experience.

In order to design a condenser, we need to know how much heat has to be rejected to condense the water.  Again we apply the first law of thermodynamics or conservation of energy.  A condenser is basically constant pressure, and there is a continuous flow of steam in and water out.  For this case, our text book tells us that the heat transferred is equal to the change in enthalpy, just as in the boiler.  The main difference is we must add heat to the water in the boiler, while we have to take heat away in the condenser.

Of course we must again begin with the starting conditions at the condenser inlet.  We can measure the exhaust temperature, and we know the pressure is fixed by the outlet to the atmosphere.  It is quite likely that the exhaust temperature, once we get steady running conditions, will be very close to  99 or100 deg C, depending on the atmospheric pressure.  However, we will normally have wet steam, that is steam that is partially condensing, and that is where our problems begin.

  If we have achieved some superheat in our boiler, more on that later, the engine exhaust will probably be in the range of 95% dry, meaning about 5% of the steam is condensed, but 95% of the latent heat is still to be removed.  We can look up the steam tables and work out the enthalpy for 95% dryness, but for our present purpose, assuming dry saturated steam will be close enough.  The enthalpy of dry saturated steam at 100 kPa and 99.6 deg C is 2676 kJ/kg, while saturated water is 419 kJ/kg.  So we have to reject 2258 kJ/kg, usually to cooling water.  Now this is a similar magnitude to the heat input to the boiler.  Please don't worry that the figures are not exactly the same as our earlier boiler example, the main difference is the engine performance, which we have not looked at, and that assumption of dry steam.  There is also a further difference of 292 kJ/kg, nearly 13%, depending on whether we just want to condense at 100 deg, or continue to cool it to say 30 deg C.

Let's have a quick look at the implications of that.  Think about the quantity of heat rejected at the condenser, compared with the boiler heat input.  It means that most of the heat from our fuel goes into evaporating water so it can go up the stack.  That is the prime reason for the very low overall thermal efficiency of a steam plant.  Even the best full size plants with every known heat recovery trick in the book, were only approaching 30% last time I looked.  I am not sure if power station scale plant running supercritical surpass that these days, or by how much, if they do.  If you have read the reports of the locomotive efficiency competitions run by some clubs, you will know the best 5" gauge locomotives are only around 5%.  There would seem to some opportunity and incentive to look at practical heat recovery methods.  A good starting point for next time.

Thanks for looking in,

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

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #125 on: July 10, 2017, 10:46:42 AM »
Waste heat recovery?

We noted last time that most of the heat from the fuel goes into evaporating water that goes out with the exhaust, this is the largest loss or energy wastage in any steam plant.  Close behind, as next largest loss, is the flue gas that goes to atmosphere directly.  Then we have friction, which also ends up as heat, and other relatively small losses.  The large amounts of heat in the flue gas and the engine exhaust surely provide some opportunity for recovery to help generating steam.  We will talk about the boiler later, but can we reuse any of the heat in the exhaust steam?

We know it's temperature is relatively low, around 100 deg.  A fundamental law of heat transfer is that heat will only flow from a higher temperature to a lower temperature.  So exhaust heat is only useful for heating objects up to 100 deg.  In fact, practical limitations on heat transfer area mean that we can possibly get to around 80 or 90 deg, though more likely lower.  In full size practice, steam plant can be integrated with district heating systems to directly replace burning of fuel for heating.  On a model, it's a bit over the top to heat the captains cabin.  Feed water could be pre-heated.  Fuel needs vapourising, a process that absorbs heat that otherwise comes from the fuel.  And exhaust heat can be rejected to cooling water.  In practice, we have plenty of heat in the exhaust, but it's low temperature limits the places it can be usefully recovered.

In the early days, condensing was by direct cold water injection, a process still useful in some situations. And some forum members are interested in historical engines which used direct injection condensing.  The condensed steam and injected water all end up at the same temperature.  How do we analyse this case?  It's not constant volume or constant pressure, but we can still look up enthalpy of the steam and water before and after mixing, and assume it all ends up at the same temperature.

Let's assume the steam is saturated vapour at 100 deg, hg=2676 kJ/kg and we want to add enough water to just condense it, and have it all end up at saturated liquid at 100 deg.  If our water starts at say 15 deg, then hf = 62.99 kJ/kg.  When it is all saturated water at 100 deg, hr = 419 kJ/kg.  The steam has to loose 2676-419=2257 kJ/kg, (it's listed in the tables as hfg, no need for sums this time), while each kg of water gains 419-63= 356 kJ.  Division on your calculator 2257/356=6.3 kg of water for each kg of steam.  To put this into perspective, the tables tell us dry saturated steam at 100 C occupies 1.67 m^3/kg, while 6.3 kg of water occupies only 0.0063 m^3, or just 6.3 litres, so a very small amount by volume.  If we want it all to end at a lower temperature, we can use a higher water flow.  We can look up the saturated water enthalpy at the temperature we choose, and recalculate the water flow requirement.

Apart from direct injection, condensing exhaust steam requires the use of a heat exchanger.  Now understanding a heat exchanger requires a basic understanding of heat transfer.  So let's look at that next time.

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

Offline steam guy willy

  • Full Member
  • *****
  • Posts: 2772
Re: Talking Thermodynamics
« Reply #126 on: July 10, 2017, 11:24:43 AM »
hi, still following along... a useful unit of waste heat at 100 degrees could be cups of tea/coffee per person per hour perhaps..!!! ;D actually i am taking in this info seriously and often wonder about the ideal steam engine configuration . As we all know no energy is lost or created, it has to end up somewhere. Also when calculating E does the digging up the coal, oil,gas and transportation  come into the equation ?

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #127 on: July 10, 2017, 12:57:21 PM »
Hi Willy,  glad to find that you are still following along.  And once again you put your finger on right on the important points and the source of much confusion.  And you continue to keep me on track, it is much appreciated, thank you.

I will give some thought to how best to answer  for tomorrow. 

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

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #128 on: July 11, 2017, 12:44:47 PM »
Hi Willy,

Thanks again for the question on units, an omission I had better fix before proceeding.

I have been using kJ as the unit of energy, usually in the context of kJ/kg, or kiloJoules/kilogram.  If you are more familiar with imperial units, you might want to compare this with the BTU (British Thermal Unit.  For all practical purposes, one BTU = 1 kJ/kg.  It is not exact, the exact figure is 1.05505585262 kJ.  Now considering that the BTU depends on the pound mass, while the kJ depends on the kg mass, and that these two mass standards are provided by different relatively arbitrary size lumps of exotic alloy in a measurement laboratory somewhere, there was never going to be a convenient simple conversion factor.  Given the source standards, it is surprising that it is anywhere near so close.  You can see from this that a kJ/kg relates to a Btu/lb by a factor that reflects the ratio of kg to lb.  The factor is 1 Btu/lb = 2.326 kJ/kg.

But in terms of the new world rational standard units of cups of tea, we need some simple calculations.  Obviously you would want to use the heat required to heat the water from 15 deg C to 100 deg C, and of course we have to define the standard cup.

The attached picture shows three cups from our cupboard.  The formal garden party model has a capacity of 150 ml, or 0.15 kg of water.  The centre on 250 ml, and the coffee mug on the right 320 ml.  I am advised by an impeccable source, that the standard metric cup, as used in cooking is 250 ml.  Adopting this standard gives a very convenient conversion factor of 4 cups equals one kg.  None of that 12 significant figure nonsense.  And a good size for coffee as well.

So the direct injection condenser I talked about last time required 6.3 kg of water at 15 deg C for each kg of steam at 100 deg C.  So we multiply by 4, and we find a kg of steam will heat the water for 4 x 6.3 = 25.2 cups of tea, or more realistically 25 cups plus a small top up for the first finished.

Then the context changed, to how many cups per hour?  Once we introduce time, we are no longer just talking about energy, we are talking about rate of energy transfer, better known as power.  I will leave that until a later day, but need to expand on the difference.

In talking about kJ/kg, or cups of tea per kg of steam, we don't know or care about how long it takes, (unless we are next in line for a cup of tea of course).  A big engine could use a kilogram of steam in a few seconds, while my little oscillator will take several hours, they can both make the same number of cups, from each kg of steam, it will just be a very slow tea party with the little oscillator, not a Sarah Palin event at all.  Oops, no more politics!

Digging coal, transport of coal and crude oil and processing all require energy as your question implies.  Most crude oil produced so far comes out of the ground at high pressure, but this is changing as available oil is used, and more pumping is required these days.  I don't have the detailed information to do the calculations, but if you want to know how much of the worlds energy reserve is used to run the car, you would have to add the energy to transport the oil, process it in a refinery, transport it to the gas station, pump it out of the underground tank into your car.  In principal, the cost of all this energy input is included in the cost you pay when you fill up.  And the heat from all this activity is lost to the atmosphere from where part is radiated to the cold of outer space.  The calculation of how much is radiated is complex and is best left to the experts.

Finally, the ideal configuration for a steam plant.  I really don't think there is one ideal.  It is a matter of understanding the energy balance for your steam plant, and knowing exactly where all the heat is going.  Then you can look at applications which could usefully use the heat before it finally is exhausted to atmosphere thus replacing fuel that would otherwise be consumed.

Power stations operate at very  high pressure which gives higher efficiency.  They use feed water heating, air pre-heating, fuel pre-heating, reheat circuits, and every other possible means to raise efficiency.  Power stations in colder climates use district heating schemes to use exhaust heat before it is lost to the atmosphere.  I have worked in industrial plants where power is produced by turbines with exhaust pressure above atmospheric pressure, so the condensing temperature was high enough to drive a process heating operation which otherwise would have required more fuel.  This scheme, often called cogeneration, produces power at a the thermal efficiency of around 80%, based on the extra fuel consumed for power plus process compared with the minimum for the process heating alone.  So it is a matter of knowing the potential, and looking at the opportunities.  Unfortunately, such opportunities do not always exist, or the quantities do not match sufficiently well, or the temperatures just do not allow the necessary heat transfer.

I hope that resolves the question on units of energy, and provides a little useful background for many interesting discussions.

Back to heat transfer next time.

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

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #129 on: July 12, 2017, 01:13:24 PM »
My next step in exhaust system exploration is to add a condenser.  So far in our discussion of energy, we have only looked at steam properties.  These properties are purely a function of temperature, pressure and volume, and can be looked up in the steam tables.  Properties do not depend on previous history, or how the steam came to be at those conditions.  One place where there is difficulty with this approach is when an engine exhaust consists of wet steam.  I would prefer to leave how the exhaust steam condition can be estimated, and proceed on the basis of condensing saturated dry steam.  It turns out that this is normally not too bad an estimate, and besides, wet steam is a mixture, perhaps a fog of saturated water in droplets and dry steam.  We only have to condense the steam, and it is perhaps 90 - 95% of the total exhaust.

If we have hot steam and cool water in proximity, whether separated by perhaps the copper wall of a tube, or closely mixed like our direct injection process, heat will flow from the hot steam to the colder water until it is all at the same temperature, then no further heat will flow.  (Known as the zeroth law of thermodynamics.)  However with our guests waiting for their tea, we need to make sure that the necessary heat transfer occurs in an acceptable time.

A condenser has to be able to transfer heat at the rate necessary to condense all the exhaust steam.  So now we not only need to know the steam properties, listed in units of kJ/kg in the steam tables, we need to know how many kg/hr are being produced.

I have three miniature boilers, and the test measurements I have made are all under 0.6 kg/hr using methylated spirits fuel.  They are quite small boilers for small engines.  A slightly larger boiler for a twin cylinder engine might evaporate 1 kg/hr.  This figure is obviously easy to scale up or down for any boiler.

We have already found that to condense 1 kg of dry steam at 100 deg C involves heat transfer of 2257 kJ, and we want to do this in an hour.  In terms of the new national standard, teacups 25.2 per hour.  We now know how many guests we can invite to the party.  And in ISO Metric units,  where the time unit is seconds, we need a heat exchanger rated for 2257 kJ/hr = 0.627 KJ/s. If we remember that 1 kJ = 1000 J, and 1 J/s = 1 Watt which is the unit for power.  So the heating power of our exchanger will be 627 W, or 0.627 kW which is a bit more than half of a common 1 kW one bar electric radiator.  This conversion of units of mechanical power, J/s, to units of heat, and electrical power with the constant equal to 1 in each case, is another plus for the ISO metric system.

To design a heat exchanger for this rating requires some understanding of heat transfer, so we had better look at that next time.

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

Offline steam guy willy

  • Full Member
  • *****
  • Posts: 2772
Re: Talking Thermodynamics
« Reply #130 on: July 12, 2017, 02:11:01 PM »
Hi, so much info here ....leading to so many more questions !!  Is the weight of the steam proportional to the pressure of the steam  ? Possibly yes. If you have a vertical pipe full of water the pressure at the bottom sends a jet quite far out however at the top of the pipe it will just dribble out !!!???   Also we now don't use Pounds /inches anymore but are the formulas still the same as in MKS units  ?? Looking at one of my books i am waiting for the 37th edition that might have been brought up to date !! Also iv you have an enclosed cylinder full of steam and then at the same temperature you squeeze the piston into it ...what will happen ??.....( my brain is starting to hurt now, sorry) !!
« Last Edit: July 12, 2017, 02:15:07 PM by steam guy willy »

Offline steam guy willy

  • Full Member
  • *****
  • Posts: 2772
Re: Talking Thermodynamics
« Reply #131 on: July 12, 2017, 02:20:15 PM »
Hi, Also in an infernal combustion engine the inlet/exhaust ports are different sizes and the cams can be set differently . so .could a steam engine have four ports  2 for the inlet cycle and two (larger)? for the exhaust cycle ? My brain is really starting to hurt now.!! Is it because i am an autodidact ??................

Offline Dan Rowe

  • Full Member
  • ****
  • Posts: 874
  • Dripping Springs TX USA
Re: Talking Thermodynamics
« Reply #132 on: July 12, 2017, 04:36:26 PM »
so .could a steam engine have four ports  2 for the inlet cycle and two (larger)? for the exhaust cycle ?

This sounds a lot like Corliss valve gear, see:
https://en.wikipedia.org/wiki/Corliss_steam_engine

Dan
ShaylocoDan

Offline steam guy willy

  • Full Member
  • *****
  • Posts: 2772
Re: Talking Thermodynamics
« Reply #133 on: July 12, 2017, 05:11:16 PM »
Actually ,yes you are right ...i was thinking about a slide valve engine though !! I have never really looked at corliss steam engine dynamics ,only heard the annoying tic, click,clunk, etc etc... Ok thats dealt with !!  Ok  so steam pressure works in all directions yes? so if you convert a certain weight of water into steam and introduce it into a closed container, will the steam working in all directions cancel out the extra weight until it condenses and becomes water at the bottom of the container ??? or is weight and gravity not compatible and is that why rockets are not powered by steam engines ??  Sorry i am not trying to be flippant just curious and asking the same questions that Watts  pupils might have asked all those years ago !!

Offline MJM460

  • Full Member
  • ****
  • Posts: 1434
  • Melbourne, Australia
Re: Talking Thermodynamics
« Reply #134 on: July 13, 2017, 01:21:43 PM »
I had intended to talk about heat transfer this time but Willy has more questions of the type that prompted me to start this thread.  They are not frivolous questions, Willy,  I suspect that you have very eloquently put into words what others are also wondering.  So I will look at your questions before continuing with heat transfer.

Now after my last post, I had better add a small point before you ask.  One horse power is equal to 746 watts, so 1 kW = 1.34 HP.

So is weight proportional to pressure?  Back to basics for this one.  Weight is the force due to the action of gravity on a mass, so weight is proportional to mass, but requires a value for g, roughly 9.8 in metric units, or 32 in Imperial units unless you really need to be super accurate. 

If you know the pressure, volume and temperature, then the steam tables will tell you the specific volume, reciprocal of density, so you can work out the mass and hence the force due to gravity.  So pressure is a factor determining the force, but it is only one of three factors involved.

Now your column of water.  So far, in each of the problems involving the first law of thermodynamics, or conservation of energy, I have assumed there is no change of elevation.  In each case there should be a term for change of elevation, however the pressure change is also easily derived from density considerations.  The formula is P = density x g x height.  It is a case where the gravitational constant is required in an SI calculation.  You can check the formula is dimensionally correct by looking at the dimensions on each side of the equation.

[P] = [F] / [A] = [m x acceleration] / [area] = kg. m / s^2 / m^2 = kg / (m.s^2)

Similarly, for the [density x g x height] = (kg / m^3) x (m/s^2) x (m) = kg.m.m/(m^3.s^2)

So [density x g x h] = kg/(m.s^2) = [P]

This analysis does not prove the formula, that comes mostly from definitions of the various factors, but it shows that the dimensions of acceleration are required, which gives the clue to the student that g is required, a very powerful cheat if you like.

That might seem a bit off the track, but it shows that height and density combine to make pressure.  In your water column, the difference in the pressure at the hole near the top and the pressure at the hole near the bottom is proportional to the density and the vertical height between the holes.

For a water column, the density is 1000 kg/ m^3, while for steam, lets assume atmospheric pressure and at 100 deg C, the steam tables tell us the specific volume is 1.67 m^3/kg, so the density is about 0.6 kg/m^3.  It is clear the difference in pressure for every meter in height difference will be nearly 2000:1.  You can see why the difference in height for steam is not very significant for steam in a model.

The energy equation also should include a velocity term, like height difference, the difference in velocity at the inlet and outlet is usually very small for a model, and so omitted.  But when a more complete energy equation is used, there is also a velocity term.  If the equation is written for your column experiment, we should include both terms, and we see the pressure due to height can be directly related to the velocity of discharge at each hole.  As you note, the velocity from the lower hole is much higher than the velocity from the top hole.  With steam there would still be a difference, but the difference would be considerably less, and for practical purposes we would normally ignore it.

That is pretty heavy going, I hope I have provided at least a little clarity. It is also getting late here, so I will look at the mks system, and your steam cylinder and isothermal compression next time.

Thank you also to Dan for answering the valve question, I will add a few comments, but Dan is the expert on valve gear, so I am delighted to see his contribution. Preferably, "talking" should involve more conversation and participants, and be less like a lecture.

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