Talking Thermodynamics

[derekwarner]

MJM...

We know you are out in the back blocks of OZ somewhere ...& probably with little or poor e-mail conductivity  ......

However just wondering....should we send out a search party?   

Derek

PS....I am in Adelaide for a few weeks...& called into a Hotel called the Thirsty Camel.... I called into the bar...yelled out your name but no one answered so guessed you were not there

[Stuart]

The reference brings a smile to my face

I have posted this before, but I did armature and stator winding as part of my apprentiship

Ok a job came across from the fitters name plate said 5 up so I looked in my book for Royce 5hp not in there but a 25 hp was so of we go take all the details , they were the same as the 25hp so I called over my mentor to check , his reply was yes they are all the same in that frame size they are all 25hp but if it 3as ordered as a 5hp the factory just stamped the nameplate as such

So it’s a case of caveat emptor

Keep up the lessons always a good read

[Steam Haulage]

First the one exception, the boiler horsepower.  It is actually defined in terms of raising a defined quantity of steam at 0 psig and 212 deg F, from water at zero psig and 212 deg F.  ? ?

I don't understand.

Jerry

[Stuart]

Jerry
That’s the energy required to change the state eg. From liquid to a gas (steam) without raising the temp.
It works the other way as well when the said gas is converted back to a liquid it gives up that energy.

The same goes to convert liquid to a solid you remove the energy

In simple terms that’s why your fridge gets hot at the back and cold inside , note you do not cool a fridge you just transfer the heat from inside to out side


Here is a couple of horse power to ponder BHP ( brake horse power  ) measured with a dyno on the output shaft note this is not the input power

Then we have nHP.  Notional hp it’s a steam equivalent form the days of yore

Just remembered some more

The old time RAC hp rating for cars this was not the hp (power ) but the engine size. Eg 12hp was a 1200cc engine

Then you had the hp of a stationary/portable  steam engine this how many good horses it took to move it to its place of work again back in time

But for this old person  ( yes I do look like that inc stick ) 1hp = 746 Watts period

Soory to muddy the waters

[MJM460]

On the outer Barcoo-

Back in real internet territory with regular capacity, so it is obviously time for a bit more thermodynamics!

Thanks everyone for the continuing interest.  Derek, I haven't seen any camels, thought you might have guessed a bit closer.  I am holding off the explanation until the story is complete, but I will get there.  Thanks Stewart for both your contributions.  You may be interested to know that modern outboard engines are rated in a similar way to your 5.  If you order the lower power you get a lower price, but I believe the only difference is the jet in the carburettor.  Welcome aboard, Jerry, Stewart has explained that the definition is only about latent heat, it does not even include the heat necessary to raise the water temperature to the boiling point.  It was a definition introduced in about 1876 and refined by 1884 to give buyers an idea of what size engine the boiler was suitable for.  A little more on that below.  Stuart, I suspect your definition of the horse power of those stationary engines is a little tongue in cheek, but it reminds us of how many tones of fire breathing monster was necessary to replace a horse, and the numbers were possibly similar.  I guess wood was cheaper and more readily available than hay.  I also prefer to confine my use of the term horse power to the precise technical definition of 746 watts, or even better 746 J/s, remembering that one J is one Newton metre, so directly related to the rate of doing work.

Last time we were talking about power, and units for its measurement.  And we introduced the term efficiency as the topic for this time.  Efficiency is a term which, in addition to many common uses has a specific technical meaning.  Well, it should have a specific meaning, but in the way of those wanting to sell products, even the technical meaning gets modified so there are several variations.

The technical definition I prefer is more specifically called the overall thermal efficiency.  This refers to the output power of the plant or machine as a fraction, or more commonly, a percentage of the energy in.  For a steam plant for example, we can measure the power output of the engine, or at least we can in full size practice with a well equipped test stand, and we can measure the fuel consumption, and use the fuel calorific value the calculate the energy content of the fuel.  Then simply divide the engine power output by the energy in the fuel, using consistent units of course.  We can then multiply by 100 if we want to express this fraction as a percentage.

Thinking back to the old definition of a boiler horse power, which was the horse power of the engine the boiler might be expected to drive.  One boiler horse power was defined as the energy input required to evaporate 34.5 lb of water at 212 F to steam, equal to 9811 watts.  Now if 9811 watts input is required for each horsepower of engine output, that is each 746 watts of engine output, we can divide 746 by 9811 and multiply by 100 to get an assumed plant efficiency of 7.6%.  Not very impressive really, but such was the technology of the time.  We must remember that in the way of the legal profession, this boiler horse power was not a guaranteed engine output, simply and expected output from some other manufacturers engine, and almost certainly defined the boiler operation and cleanliness.

What does 7.6% mean in terms of fuel consumption?  Well, we need to know the calorific value of the fuel.   Various standards and Google pages give the calorific value of coal as about 25.46 MJ/kg, or 25,460 kJ/kg.  The energy input of 9811 watts equals 9811 J/sec equals 35,319,000 J/hr or 35,319 kJ/hr.  Finally a simple division 35319/25460 yields 1.38 kg of coal per hour for each horse power.

I don't know how that compares with Sabino or modern coal fired merchant ships, but as the definition was finally agreed in 1884, I suggest it is nearer James Watts engines than the modern counterparts.  Does anyone have any figures for the performance of modern coal fired engines in terms of coal consumption and horsepower?

Next time I will look at what efficiency means for our little oscillating engine.

Now it's hard to think of a suitable picture, for this thread, but in the effort to make sure that we don't all turn into thermodynamics nerds, I am going to suggest a little culture, just to balance the technical content.  How about a little poetry?  If you went to the right school you will remember it, but a couple of verses should jog some memories.

On the outer Barcoo,
Where the churches are few,
And men of religion are scanty.
On a road never crossed,
'cept by folk who are lost,
one Michael Magee had a shanty.

Now this Mike was the dad
of a ten year old lad,
plump, healthy and stoutly conditioned
............

Thanks for looking in again,

MJM460

[Stuart]

Yes I have suspected that the definition of the how many horses etc is suspect but I bet it was on someone’s catalog sales pitch at some time , but in the real world it would have given the farmer a idea how big that fire belching monster was

Stuart

[Maryak]

James Watt decided that is was a load of 550lbf that a "good" horse could raise 1 ft in 1 sec. It was also acknowledged that it was deliberately set high!!

For me it is interesting how our measurement systems relate to the world  around those who defined them at a particular point in history. The only one which is universal or perhaps earthieversal is time and at the risk of inciting a riot it's one hell of a stretch to confer on it anything relating to increments of 10's.

 

[MJM460]

More on efficiency

Hi Stuart, I think the industry was still talking about water pumping for mines when the term horsepower was coined as a unit of power, but when portable engines first came in, the ratings were not very high.  I suspect to set up a pair of ploughing engines required enough horses to ensure that they would continue to receive their hay for the foreseeable future.

Hi Maryak, it's good to have you still looking in, your contributions are always appreciated.  I did understand that James Watt's definition of a horse power implied a very strong horse.  I did not realise that it was deliberately so.  Quite the opposite to the normal sales hype which tends to exaggerate the engine output.  However it would tend to attract happy repeat customers who would be delighted to find that their 4 HP engine could actually do what they previously required five to do, and keep it up all day without needing a rest, even when the tubes were a bit sooted up.

I suspect that the earthieverse standard for time probably would be either a day or a year, though the day at least does vary a bit.  Twelve months makes sense in terms of dividing it into four seasons as in moderate climates or two for the tropics.  It can also be divided into three or even six, but the moon phases occurring roughly 13 times per year mucks up the logic a bit.  As you say, trying for ten months in a year, days in a month, or hours in a day does not seem to result in a nice logical system.  It does seem necessary to accommodate the one to three hundred and sixty five and a quarter ratio of orbits around the sun to rotations of the earth, which is not easy with base 10.  But we could have ten "hours" per day, each of 100 "minutes", again each of 100 "seconds" by adjusting the length of the second.  It would then involve a different strange number of vibrations of those caesium atoms.

I understand that the second as a unit of time was selected by an ancient civilisation, who used a base 60 number system and hence the 60 seconds to a minute, 60 minutes to an hour.  Base 60 has the advantage of having many factors so you can divide it by 2, 3, 4, 5 and 6, and also 10, 12, 15, 20 and 30 without needing fractions.  They did pretty well to measure seconds, though I seem to remember from physics that a pendulum with about a metre length has a period of 1 second.  But they were working all this out before the metre was defined.  Perhaps they had a unit of length that corresponded with a one second pendulum.  We can certainly measure time pretty accurately these days with the device on our wrist, but the time differences in Formula 1 and Olympic events make a second seem pretty large.  Though I wouldn't be ashamed to come second to Ian Thorpe by one hundredth of a second or so.  Yet no one can remember who came second except the one who did it.  And those atomic vibrations would still be necessary so we can measure the difference in time a signal takes to reach us from different satellites and calculate our position on the earths surface within 5 metres.

I still have to learn not to be too quick to announce the next topic.  A little while after posting, I usually remember other aspects that I had intended to cover.  I believe that in some cultures it is called a stairways moment.  So our little engine will have to wait a bit longer, at least until I am off the stairway.

Last time I referred back to the old definition of boiler horsepower which implied a thermal efficiency of 7.6%, and noted that it was not very impressive.  So what do modern engines achieve?  Last time I looked, it was only the very best large scale plants that were just nudging 50%, and that required all the subtle tricks of heat recovery, together with high pressures and temperature.  No doubt they are a little higher now, but I suspect not much.  Most are much nearer 30%.

At the other end of the scale we have the locomotive efficiency competitions reported quite regularly In Model Engineer Magazine.  To understand the reported results, and how they relate to overall thermal efficiency, we have to note that the rankings are based on drawbar power.  That is, they have a carriage towed by the engine that measures the drawbar pull and the speed over the measured run.  So we should qualify the results as drawbar efficiency.  It is not really fair to compare drawbar efficiency with efficiency based on a stationary engine output horsepower.  It obviously is measured after all the work to drive the locomotive along the track is used, including moving the mass of the boiler, and friction introduced by the bogey cars etc.  Of course it is not an unreasonable measure for a mobile engine.  It does not matter how much power is produced by the actual engine parts, if most of it is used just driving itself and its boiler along the track.  We are definitely more interested in how much load it can pull.  I don't have access to my magazine collection at the moment, but from memory, the best 5 inch gauge locomotives are around 5%, and the smaller gauges nearer 1%.  Perhaps someone has a report handy and can confirm or update these figures.  I think those figures for a 5 in gauge loco are quite impressive and it is obvious that friction leads to proportionally greater losses in smaller engines.

If only somewhere between one and five percent of the heat available from the fuel is converted into work, what happens to the rest?  Is it really lost?  What about that basic law of physics, conservation of energy?

The first law of thermodynamics says that heat energy can be converted to work, but the second basically says not only can we not get more energy out than we put in, we will always loose some in the process.  Perhaps more accurately, we will always get less work out than the energy we put in.  Conservation of energy says we do not actually lose energy, it just goes somewhere other than getting converted into work. 

That is probably enough for one session.  Next time I will look at where the energy goes.

Thanks for following along,

MJM460

[MJM460]

Conservation of energy -

Last time, I started looking at efficiency and how we cannot convert all the energy from fuel into work.  The old definition of a boiler horsepower implies an efficiency of only 7.6%.  The law of conservation of energy says the remaining heat is not lost, it just is not converted to work.  This raises the obvious question of what happens to the rest, 92.4% of the energy from fuel that is not converted to work.  Let's start looking at where it all goes.

If we start by looking at the boiler.  Fuel needs oxygen to burn, and we normally obtain the oxygen from air.  As a part of the combustion process, carbon in the fuel is oxidised to carbon dioxide.  Hydrogen is oxidised to H2O.  Any sulphur in the fuel on oxidised to sulphur dioxide, SO2.   Air is about 80% nitrogen.  In very high temperature processes, there are even oxides of nitrogen produced, but for the most part, nitrogen just goes along for the ride, absorbing heat from combustion, and carrying quite a bit of heat up the stack.  The oxidation reactions all release heat, so the combustion products are hot.  This heat crosses the heat transfer surfaces to boil the water in the boiler.  But all the heat cannot be transferred, heat is only transferred from a hotter substance to a cooler one.  So in the end, after the maximum possible heat transfer, the whole mass of combustion products is hotter than the steam we are producing, and it goes up the stack carrying all that heat with it.  Obviously a significant heat loss from our process.  Now this would be bad enough if we only used enough air to exactly supply the requirement for combustion.  Unfortunately, it is very difficult to mix the fuel and air sufficiently well to achieve this, so even very sophisticated burners require excess air, just to achieve complete combustion.  It is not efficient to allow unturned fuel to go up the stack.  But perhaps more importantly, insufficient air means the burning only partly proceeds.  Rather than getting CO2 plus unburned fuel, we find carbon burns to carbon monoxide.  While CO2 cannot sustain breathing, it is in fact produced in our bodies during breathing, and so long as there is sufficient oxygen available, it is not particularly harmful.  However, carbon monoxide is quite toxic in even small concentrations.  This is why it is necessary to have good ventilation if we have combustion indoors.  Preferably a chimney over the fire place, as there is usually a tiny amount of CO formed in any combustion, and we cannot afford to allow its concentration to accumulate.

You may suggest leaving out the nitrogen and just burning fuel in oxygen to reduce the heat lost by the nitrogen being heated on the way through.  Separating the nitrogen is not practical for two reasons.  First, it takes a huge amount of energy to separate them, so that would not help our efficiency.   Of course in a model, we could cheat a bit, and buy in some liquid oxygen, and not count the energy used "by others" to separate it from air.  That brings up the second reason.  It would be very dangerous.  Even a slight increase in the oxygen content of the atmosphere causes combustible stuff to burn much more vigorously, and makes even stuff we normally consider non combustible burn quite well.  Given a high enough oxygen concentration, even iron, steel and concrete will burn.   I have been told that if there is a fire in an oxygen plant, there is nothing to clean up afterwards, even the concrete foundations are destroyed.  Fortunately, I don't have direct experience of this.  I think we had better just accept that nitrogen in the air will reduce our efficiency.  Perhaps 20 to 30% of the energy in the fuel is lost up the stack in a typical boiler.  Even more in a hand stoked coal fired boiler, as the practicalities of stoking mean that there is usually even more excess air than found in say a gas fired boiler.

Staying with the boiler for the moment, apart from the stack gases, the other obvious source of heat loss is the loss from the furnace or boiler casing.  If we hold our hand near the boiler we can feel the radiant heat.  If we hold our hand above the boiler, we can feel the hot air rising due to the convection losses.  This is one source of heat loss we can do something about.  If we insulate the furnace casing well, we can significantly reduce these losses.  It is even easier in a marine fire tube boiler, where the combustion is completely contained within the boiler.  We can insulate the shell quite well with as much thickness as the installation will allow and really minimise these external losses.

A slightly shorter post this time, but a good place to pause.  Next time, I will follow through the steam cycle and look at where energy is lost around the engine.  There should be some items there that we can attend to, to increase our engine efficiency.

Thanks for looking in,

MJM460

[steam guy willy]

Hi.Good to see you back.....talking about time.......Using a year would be interesting as a leap year would through out the calculations !!  Also on efficiency a couple of pics and text from the Engineer in 1881 using the superheat to heat up the exhaust from the HP to LP cylinders......

[MJM460]

Hi Willy,  while I like the getting away from it all, it is also good to have communication, so good to be back and glad to have you on board again.  I guess the main disadvantage of adopting a year as a time standard is that approximate quarter day that gives rise to the leap years.  Any standard quantity has to have an agreed constant value.  Perhaps the second or the caesium atom vibration is the best after all.

Interesting to see that the reheat cycle was used so long ago, and to see just how it was implemented in those days.  The reheat cycle is used in modern power stations, where the exhaust from a high pressure turbine is piped back to the boiler, and through a secondary superheater coil, before returning to the intermediate pressure turbine.  My old textbook even shows the intermediate turbine exhaust through another reheat coil in the boiler before returning to the lp turbine.  The heat input to the reheat circuit increases the energy available for the next pressure level and so increases the power station output and efficiency without requiring more fuel.

Last time I was looking at where the heat goes in a boiler, this time let's look at the losses in the engine.  We have already looked at how the work is generated during the power stroke of the engine, but where are the "losses" in the engine?  Remember, the law of conservation of energy says the energy is not lost, it just does not get converted to work.  There are basically two quite different loss mechanisms.  First there are the processes that result in heat passing through the engine without being converted to work.  Then there are processes that use the work already created within the engine before there is any excess at the output shaft.

Heat loss from the cylinder, is heat from the steam that does not contribute to the engine output.  We have talked about reducing this loss by cylinder cladding, or even jacketing.  Cladding, being totally simple and passive is probably the most useful approach.  Jacketing is more complex, and also consumes steam that just may be more effectively used inside the cylinder.

Leakage past the piston is another area that steam is consumed without being effectively converted to work.  We can work on the piston to cylinder fit, but generally piston rings seem to be the most commonly applied design to reduce blow-by without introducing too much friction.  I will leave it to those with more expertise to describe how best to make effective rings.  If, like me, you are not up to making good rings, you can do much worse than just machine the groves as though you were going to insert rings.  Sometimes a soft packing is inserted, but the ring grooves are helpful even without it.  The flow past the piston is caused by the pressure difference between the power stroke side and the exhaust side of the piston.  This pressure difference accelerates the steam flow through the gap.  When grooves are machined in the piston, the flow expands into the groove, generally slowing but with lots of turbulence and a minimum of pressure recovery normally seen in a well designed Venturi.  More energy is consumed in accelerating the flow again into the next section of the gap.  The expansion losses occur again at the next groove.  The result of all the lost energy in each groove and the energy to reaccelerate the flow means the resistance to flow past the piston is much higher than for a plain piston, so the flow is much less.  It is called a labyrinth piston.

In case you think this would not work, I can assure you that it is used in labyrinth piston compressors.  These machines are used when the machines must be completely oil free.  The piston is supported by the cross head and piston rod so that it does not touch the cylinder walls.  Helped by a vertical configuration.  Even hydrogen can be compressed to quite high pressure s with these machines, though they do have a very large number of grooves.  Search for labyrinth piston compressors, and perhaps the Sulzer site as a manufacturer of these machines.  If you can't make good piston rings, it can't hurt to include two or three ring grooves anyway.

In a similar way, rod packing leakage reduces the output of the under side of the piston.  We usually insert a soft packing to minimise this leakage.  In a similar way, if you make some packing ring grooves they reduce the loss a little, even without the packing, which can introduce a lot of friction in a small engine if too tight.

It is worth looking at all these sources of steam blow by, but in the end they are not the major component of that 92% we are looking for.

The other main loss in the engine occurs after the energy is successfully converted to work, the work is used within the engine, so not available for doing external work.

I expect others will think of more areas where energy is lost before it is converted to work, otherwise I will look at the work lost within the engine.

Thanks for looking in,

MJM460

[paul gough]

Thanks MJM for re-awakening my brain regarding labyrinth pistons, I shall chase them further on the web. I am currently pondering methods of eliminating friction from my small twin cylinder Gauge 1 Lion loco, 3/8 D pistons and contemplating the viability of replacing the slide valves with outside admission piston valves as the load on each of the tiny slide valves at 60 psi is 2.2 kilos and on a surface area, (underside of valve), of 0.06  sq. inches. Glad the discussion is rolling on again. Trust you enjoyed the dry country down South and got to see the bright lights of Thargomindah. Regards Paul Gough. 

[steam guy willy]

Hi is there optimum shapes for these labyrinth grooves and do they get smaller towards the middle ? There is quite a lot of info in my old books from the 1830's on, and it is surprising the amount of knowledge that was available all those years ago to engine builders .However the pecuniary considerations tended to outweigh the actual finished engines that were produced. What would be the best wood be for cylinder lagging btw and are there tables the same as for metals regarding conduction etc etc ?

[steam guy willy]

I have just read through your posts now and was wondering how efficient is the human body ?? in Asia where the POW men ate a lot of rice and not much else ,they still managed to do a hard days work? Vegans also manage to become successful athletes without any meat etc so how does one man power compare to 1 horsepower ? so are food calories equatable to coal calories ?  just wondering !!

[MJM460]

More on efficiency -

Hi Paul, good to have you back.  Unfortunately I missed Thargomindah, passed by a bit north of there.  Was probably closer some years ago when I did some work in SA.  Wife says we had to bypass it as the coffee might not be as good as at Gregory.  With your little valves, I think the normal force is 60 x 0.06 = 3.6 lbf or about 1.6 kgf for each valve, so 3.2 kgf for two valves, it seems quite large in proportion to such a small engine, but it is a well known disadvantage of slide valves.  But the friction force which is at right angles to the normal force is smaller.  Steel on steel has a friction coefficient of about 0.3, but a well lubricated bronze surface should be significantly less.  Search for friction coefficient for various surfaces.  Multiply the normal force by the friction coefficient to get the force necessary to move the valves.  The point will be whether you can make piston valves in such a small size which seal well enough to be more efficient than the more simple slide valve.  I would also ask if inside admission piston valve would be better balanced for forces, involve less sealing issues with the gland packing and less rod end force influence.  I never got to see a real labyrinth piston up close, but the principle is sound, so I always put a groove or three in my small pistons on the grounds that it should be better than plain pistons.  It should be possible to make a spare piston for an engine and test both ways, but I feel the difference would be masked by the inevitable difference in diameter that I would almost certainly produce.

Hi Willy, my Heat transfer text book has a table of thermal properties including conductivity for metals and a separate table for insulating materials.  Pine, fir and spruce are all listed as 0.15 W/(m.K), while oak is listed as 0.19, and cork 0.042.  This suggests that any timber will do, if you compare with figures of 80 to 110 for copper and brass.  Just use something that will take a nice finish if the wood will be visible, but use cork if it will be covered by a metal sheet, and make it as thick as aesthetics will allow.  You are quite right to notice that the human body is in another league for efficiency, but it is still subject to the same laws of thermodynamics.  Not all the energy in the food is converted into work.  Some goes out as heat in breath temperature, breath humidity, or perspiration, or conduction to air and quite a bit is used just to breath, think and circulate blood, and digest the food.  Meat is a good source of protein necessary to build muscle, (vegans must get this from other foods), and is also good for sustained energy release.  But energy is more quickly released from carbohydrate foods such as rice.  And the body is incredibly good at conserving energy particularly in time of drought, no doubt why food restriction dieting is so difficult, our body just says why, we might be hungry tomorrow?  And then closes down to conserve as much energy as possible until more food is available.  But those POWs also paid a terrible price in terms of weight loss, far beyond what was compatible with good health.  I have a conversion Ap on my iPad that says one food calorie is equivalent to 1000 calories.  It makes sense in terms of the published calorie values of foods and the energy we produce.  My heat transfer book has quite comprehensive conversion tables but does not include food calories.  It does however consistently include in brackets (thermochemical) after calories and Btu in each description of energy units in the table, which is perhaps a pointed reference to the "nonstandard" energy unit used in those other disciplines.

I don't have a reference to a standard manpower, though I know it varies in a wide range over the population.  I was in Fremantle when the Americas cup was sailed there, (for work, really) and a university professor and his team attempted to answer the question by building a machine with a similar action to the coffee grinder winches on the boats.  They carted the machine around various pubs and challenged willing guys as to who could produce the most power.  The machine successfully gathered data until the races were over and the real winch grinders joined in.  The machine had to be rebuilt several times before it was strong enough to absorb the power of those guys.  They just blew it apart at every attempt.  Similarly, cyclists are known to have a good power to weight ratio, not the power of winch grinders, but not as heavy and so were usually favoured in the early experiments on man powered flight.  But I don't know the actual figures of power produced.  Of course, I am in another league, and don't expect to be called up as a winch grinder or for man powered flight anytime soon.

Regarding the labyrinth groves, as far as I know they are simply square groves like empty piston ring grooves.  The idea is that there is no Venturi effect with the associated pressure recovery when the gas slows and expands into the groove, the velocity energy is mostly lost in turbulence, which of course heats the gas a little, increasing the volume that has to accelerate onto the next space.  The energy lost adds up with each groove, so reduces the bypass flow.  Besides those labyrinth piston compressors, the similar labyrinth configuration is used on the shaft of steam turbines, gas turbines and on centrifugal and axial compressors, however for a really effective seal modern manufacturing techniques allow dry, non-contacting mechanical seals with really low leakage, and low power loss, so the labyrinth seals tend to be more of a first stage, especially for flammable or hazardous gases.

Didn't make much progress with the losses in engines today, but a little discussion around these interesting questions is helpful, so I really appreciate the questions.  Perhaps tomorrow.

Thanks for looking in,

MJM460