Author Topic: Talking Thermodynamics  (Read 123643 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1260 on: September 02, 2019, 12:00:14 PM »
Hi Captain Jerry,  your questions are always welcome, I would mostly rather think about how engines work than read a ďwho done itĒ, or watch evening TV.  Obviously I donít have access to my whole library, but I always find space for my favourite reference book and a calculator.  I enjoy keeping up to date with everyoneís projects and even the chatter on the forum, and I also receive our home daily paper on the iPad.  And keep us touch with our well and truly middle age children, and our grand children.  It all keeps our minds engaged.

Neither my wife or I are much into yearning for the good old days, we both like our creature comforts.  Itís good to have the windows shut and a clean filter in the A/C when the dust is blowing and itís over 30 C.  Navigation is an interesting issue.  We do have a GPS, with lifetime map updates, but it is mainly used to make sure I turn the right way out of the caravan park gate after an overnight stop in an unfamiliar town.  Too easy to get your head turned around by the time you have done three turns around the campground in an unfamiliar town.  And finding your way across a range of hills can occasionally be a challenge.  But the distance to the next turn shown on the GPS is often  500 or more km, as it does not assume you will turn off the highway on to a dirt track.  Electronic tyre pressure monitors are also right up there for importance.  Not to mention engine management.  Itís hardly worth lifting the bonnet (hood?) these days, there is almost nothing you can do without a computer tend the required software.

It is not like the previous generations experienced.  The main routes (donít really deserve to be called highways) are all sealed, though they are awfully narrow when you have a road train coming towards you.  And all the main centres have the major supermarkets with reasonable prices.  We go off road for short distances, the final bit into some worthwhile camp grounds, and sometimes free camp, as we have solar power, along with refrigeration and water.  So a quite civilised way to see the countryside, and see how people live and use the land.  Did some mine tours and a cotton industry tour, some gorge boat rides and a boat around a major fishing port to learn how the industry operates.  But most days we are just enjoying the different environment and the warmer weather.  It is pretty cold at this time of year in Melbourne which is well south.

Oh, and we have to get back to go sailing.  That might be another interest in common, though we are flat water sailors.

I hope that you are well battened down for the hurricane and are able to avoid the worst of the damage to come through safely.

Hi Willy, heat engines convert energy in the form of the random motion of molecules in to mechanical work.  We sense that motion as temperature.  So yes, engines running on air are definitely heat engines.  Heat is a relative term, it is relative to absolute zero, or -273 C, so there is plenty of heat in atmospheric air.  In fact to produce liquid air or LNG, the lower temperature stages of the necessary refrigeration usually include an expansion turbine doing work by driving a compressor rather than using a throttle valve to reduce the fluid temperature.

(Oh, and that turbine was a purchased item from a major manufacturer.  I donít have the equipment or knowledge to make that one.  It was driving the lubricating oil pump for the bearings of a very expensive compressor.)

The energy is delivered by the compressor, but a compressor actually loses some energy, it does not produce it.  A compressor can be driven by an internal combustion engine or a steam turbine, but for most of is they are driven by electricity which is generated in a power station.  Apart from coal, there are power stations using nuclear energy, hydro or even wind power.  The energy for hydro power comes from the sun which evaporated water from the seas, which rises to form clouds and falls as rain into the dams in the highlands, as well as falling on the low lands.  So it is heat from the sun.  Wind power also comes from the heat of the sun which causes the air pressure patterns that create the winds that drive the turbines.  And photo voltaic is obviously harnessing the sunís heat.  So they are all heat engines, if we go far enough in understanding the source.  Each of the devices in the chain merely convert the energy from one form to another, and each involves some losses in the process.  And if you are really into tracing back to the source, coal is energy from the sun trapped a long time ago.

You mentioned a candle half in jest I suspect, when we were talking about the air getting cold, and I missed commenting.  I suggest a candle would not be anywhere near enough, but it you used it to heat the supply air, you could start at a high enough temperature to have an exhaust temperature above freezing, so the idea was not silly.

And with travel, we also find a highlight is speaking to people who live in the remote areas and coming to understand the issues that dictate their lives and attitudes.  As you say, most of us only want to leave in peace.

Thanks for looking in,

MJM460
« Last Edit: September 02, 2019, 12:07:50 PM by MJM460 »
The more I learn, the more I find that I still have to learn!

Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1261 on: September 05, 2019, 02:34:50 AM »
Hi MJM , thinking about boiling /evaporating water...H2O  plus everything else in it.....can you use non potable water to cook and drink without any thing else contaminating it ?? Dose the limescale component get carried into the rest of the pipework in an engine , however minuscule ??

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1262 on: September 05, 2019, 08:38:45 AM »
Hi Willy, I have seen many signs warning that the water is not fit to drink.  I assume that lawyers are responsible for most of them, otherwise I am not sure how previous generations ever found water to drink.

Of course their immune systems may have been better than ours, and we have to be realistic and appreciate that not many reached the average age of members on this forum.

But my wife grew up on a farm where only tank water was available.  It always tasted good, possibly because of, rather than despite the odd possum or bird that inevitably found their way in through the top opening and drowned!  Eventually the water would start to smell and the tank had to be cleaned out.  Quite a difficult decision on a remote farm in a drought.

More seriously, I suspect that most of the water with such signs was ok to drink, especially if you boil it first, but nobody wants to be sued if you get sick, and nobody has actually tested the water.  It is easier to put up a sign.

When you cook in such water, the cooking process is a step towards sterilising the water, and certainly extends the range of quality that can be consumed.  However, any contaminants in the water that survive the temperature will contaminate the cooked food.  If the water contains inorganic compounds and heavy metals they will be in the cooked food.  And sometimes we add those contaminants deliberately, for example salt or sugar to flavour the food.

When you boil the water, in principle most contaminants stay in the water, but in practice there are always small droplets of liquid water carried over with the steam, despite cyclonic separators and crinkle plate separators that are installed in modern full size boilers.  Even more is carried over from our small boilers without those separators.  Those droplets carry their share of the contaminants in the boiler water, though it is a very small proportion of the total water involved.

In a full size plant, operating 24/7, this carryover eventually results in turbine blades and even piping being fouled by those compounds from the water.  It is interesting that even high speed turbines can become quite badly fouled over time so their performance is affected, without ever getting significantly out of balance.

In our models, I am not sure how many operate enough hours for these deposits to be visible, and the biggest problem tends to be the deposits in the boiler, as most of the contaminants stay behind. 

I will be interested to hear from those who operate their engines for much longer periods to see how long it takes in a model or small craft environment for significant deposits to build up.

MJM460



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Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1263 on: September 06, 2019, 02:42:49 AM »
HI MJM, If one uses brackish ..even seawater to "steam" vegetables is that still safe......When i was young 60 years ago we had a not very deep tidal well. !! after a few years we actually had a hundred foot well bored ...when the water came out it was sparkling clear !! however  after a few hours it turned a rusty red colour and stained everything red  !!!  so all the sheets and underwear were stained !  we then had to get a filter to remove all the rust.....

Willy

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1264 on: September 06, 2019, 03:42:26 AM »
Hi Willy.....when I was young  :old:......[also some 60 years ago] I seem to remember we were taught that rust was the product of oxidation of ferrous [iron or steel] material

So later on in life, also reminded that the rate of rust produced on those iron & steel materials on the floor of the ocean was greatly reduced due to only oxygen available being that en-trained in the water itself

So unless the 100ft deep bore was made with pipes that were internally pre-rusted, I wonder if the rust colour could have been from a mineral deposit in the aquifer?...


Derek
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #1265 on: September 06, 2019, 12:38:54 PM »
Hi Willy, I should know the answer to that one as I allís enjoy sailing as an outdoor recreation.  Also, on my recent travels, I saw where salt from the sea is harvested by evaporating the water in shallow ponds in a very low rainfall sunny area.  I know they wash the salt and recrystallise it to remove dirt and such, but am not sure if there is other processing to remove any of the salts other than sodium chloride in the sea.  And I am thinking some of that salt ends up as table salt.

Previous generations used to add lots of salt in cooking, especially potatoes and porridge.  These days, medical advice is that too much salt causes serious health problems so we all reduce our salt intake.  The salt added to the water for cooking does end up in the food.  My wife has taken it so far that she has low sodium levels and has been advised by the doctor to increase her salt intake a little.

I know we canít drink seawater, but that is because so many of our body processes depend on osmosis, and if you have the concentration on the wrong side of the cell walls, it mucks with our system badly.  Sea water is too salty for us to safely drink.  However it is likely that we could tolerate the salt from cooking with sea water, especially short term in an emergency.  However, with all the plastics and other stuff we dispose of out of sight and out of mind by dumping it in the sea these days, it is less safe than it used to be.

My first thought on the well was that the water might have corroded the pipe brining the water to the surface, as some underground waters can be corrosive, but Derek makes a good point about lack of oxygen, so again I am not sure.  It may have been from mineral deposits.  But then, the ground water in the North wast of Australia is not rusty coloured, yet the ground contains as much iron as anywhere, and digging it up is a huge industry.  But it is so full of calcium, that the dregs of your coffee cup are gritty.  It always pays to leave the last little bit in the cup.

And neither the water there, nor sea water are good in a boiler!  (Just to bring it back to thermodynamics and engines.)

MJM460


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Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1266 on: September 14, 2019, 02:48:15 AM »
hi MJM,  I am putting the beam engine together and am fitting the piston for the steam test...i have no drawings for the inside of this engine so am wondering if there are strict parameters for the diameter of the piston and the spaces at the ends of the travel ?? are  there different gaps for different types of engines??   

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1267 on: September 14, 2019, 12:56:04 PM »
Hi Willy, the spaces at the ends of the cylinder when the piston is at top or bottom dead centres determine the clearance volume.

In principle, the clearance volume has an effect on the efficiency, but I donít think it is all that important if it is small.  Interested to hear from others on this.  Basically, the steam in the clearance volume is released to exhaust pressure along with the rest of the steam in the cylinder, and has to be replaced with high pressure steam when the inlet valve opens, so there is increased steam consumption, while the steam only does the same amount of work on the piston.

However, in practice, you never want zero clearance or worse, even at maximum dimensional tolerances or tolerable wear on bearings etc.,you donít want the piston to hit at either end.

On my little engines I aim for about 1 mm each end.  Itís a big proportion for those small engines, but I am not sure that I can reliably achieve less.

However, even on a larger engine 1 mm is probably still achievable without needing to go larger on clearance.  Again it will be interesting to hear what others achieve.

On a large compressor I had purchased for a client, LP cylinders about 30Ē dia, we were aiming for 1 mm and checked it by inserting a lead wire with the valve removed, and squashing it by barring the machine over.  I think the same considerations would apply for a steam engine.

Obviously for a combustion engine, the clearance determines the compression ratio, so different considerations.

I hope that helps.

MJM460

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Offline steamer

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Re: Talking Thermodynamics
« Reply #1268 on: September 14, 2019, 02:18:43 PM »
Condensation is a bigger problem in small engines than cylinder clearance.     Cylinder clearance on big stuff is handled with compression or Exhaust lap, but on the small stuff...it's really not helpful.

Dave
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Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1269 on: September 15, 2019, 12:44:48 AM »
MJM notes......."checked it by inserting a lead wire".......

In a slightly different cylinder application, we found ''deformable plastic" cord to be a superior medium when attempting to achieve measuring clearances approaching 0.05mm class of fit between a 200 diameter semi spherical rod end ball, and the 200 semi spherical diameter cup in the 580 diameter piston] ÖÖ

The piston rod was AS1444 Grade 4140 Ė Q&T to Condition T 1000 MPa , the piston forging was of identical material including the 1000 MPa Q&TÖÖ I considered that a design of this  H7/g6 > G7/h6 Class, calculated and to be totally achievable when measured in a conventional shaft to hole format, but the Standard did not specify any method of understanding the average deviation of mating spherical surfaces clearance   

After assembly of the rod into the piston and the piston lower plate bolted, the lead wire on disassembly was extremely fragile & self shredded or tore easily during the vertical lifting of the rod from the piston, with subsequent measurement of the lead near impossible

The plastic cord displaced itself however maintained mechanical stability such that it could be accurately measuredÖ

[yes {to bring back reality of :old: dimensions} we were looking for thicknesses of ~~ 0.002Ē] after the 493kg mass force of the rod weight and the tightening process of the piston bottom end plate had flattened the plastic cord

Derek

« Last Edit: September 15, 2019, 12:58:48 PM by derekwarner »
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Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1270 on: September 15, 2019, 03:07:58 AM »
Hi MJM ,Derek and dave ..thanks for the info ..interesting things to think about there...Thanks ..

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1271 on: September 16, 2019, 01:09:07 AM »
Hi Dave, good point about the condensate, probably a good reason to have more rather than less clearance on a steam engine.  My compressors didnít like any liquid either, it turned them into very dangerous beasts.

Hi Derek, it is possible that we also used that plastic cord, I am not sure, now that you mention it.  But we were looking for about 1 mm, so not as difficult as your application.  Itís amazing what can be done by experienced people to get around difficult practical problems such as that.

MJM460

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Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1272 on: October 28, 2019, 03:52:18 AM »
Hi MJM, I have some more questions for you and they are about the Beeleigh Woolf Compound engine that they are currently restoring. They have cleaned up the engine and replaced missing and broken parts and are now running the engine with an electric motor coupled to a car road wheel that operates against the flywheel periphery. The are thinking about running the engine on compressed air and I have a few questions about the viability of this.  The engines cylinders are within the large casting that serves as a steam jacket. there are no drain cocks to the cylinders themselves but just one at the bottom of the steam jacket. To start the engine the steam is let into the steam jacket to heat up the whole engine block right through to the cylinders. The steam is prevented from entering and condensing into the cylinders by having the valves in the midway position so the inlet ports are closed. once the cylinder block is at the operating temperature that is determined by steam rather than water leaving the drain cock at the bottom of the jacket. So a few questions about this. When the engine has stopped there is still steam in the cylinders and ports that will condense when the engine has cooled down. So how will this water escape from the lower parts of the cylinders.?  when you have water in an enclosed space in an engine and the ports will it evaporate after some time so when the engine is restarted the water wont be compressed and shatter the engine ? this may not happen if the engine is once again brought up to temp with steam . But will running the engine on compressed air be a viable option if water has been allowed  to accumulate.  On a locomotive engine the drain cocks are left open to release any water but this engine does not have these drain cocks. ??  just wondering about this and thinking about the thermodynamic actions  of water in enclosed spaces ?? this is quite a lengthy diatribe and I hope it is a valid query . A pic of the cross section of the cylinders arrangement showing the ports at the top of the engine...

willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1273 on: October 28, 2019, 11:52:39 AM »
Hi Willy, itís never a silly question if you donít know the answer, and it is very important to consider condensate while starting up the engine.  Likewise the condensation of the remaining engine when the engine is shut down.

I can have a go at the theory, but this is another of those areas where nothing beats practical experience, so I hope others will also chip in with their experience of operating reciprocating engines

First the water, the issue is of course that it is practically incompressible, so you have to be careful not to trap it between a moving piston and a stationary head, as stopping the piston suddenly when it hits the water requires a huge pressure force to change the momentum of a heavy piston in such a short time.  And you will get copious quantities of condensate when warming things up. 

In this case, the engine is vertical, so condensate will tend to settle under gravity at the bottom for the underside of the piston and on the piston for the top side.  With the cylinder ports as you have drawn them, if you bar over the engine slowly for a couple of revs, quite a job if you only have a bar and holes in the flywheel, any water at the bottom of the cylinder will be forced up the  passage and will escape to the exhaust, if necessary using the little valve lift that is normally provided for just this reason.  There will be no great pressure generated to accelerate the water if the piston is moving slowly.  Similarly for the top of the piston, when the piston reaches the top, the water will be able to run down the passage to the exhaust port, and again it will lift the valve sufficiently off the face if it is not already partly open to the exhaust.  So, while I think we would all like to see a drain cock at the bottom of the cylinder, the engine has lasted for quite a long time without it, so I suspect it will be ok, especially if it is barred through that first bottom dead centre.  Of course, in a horizontal engine, any water is more likely to be trapped, unless the valves are at the bottom of the cylinder.

Normally you get a lot of condensate when you first admit steam to a cold cylinder.  Using that jacket to preheat the cylinder means the condensate in the jacket is the main quantity to be dealt with, and you said that drains are provided for that.  It is likely that no one ever preheats for long enough to get the cylinder right up to steam temperature, so there will probably be a little condensate formed when steam first enters the cylinder, but the quantity should be manageable especially with a slow start.

When the engine is shutdown, the steam space in the cylinder will still have some steam, although only at atmospheric pressure, or even a bit lower if there is a condenser.  When the engine cools this will of course condense, and with no drains at the bottom, it will remain in the cylinder.  On top of the piston, some condensate will probably remain on the piston and between the piston and cylinder on top of the rings.

Any water remaining in the cylinder will tend to evaporate but only until the vapour pressure reaches equilibrium vapour pressure for water at the temperature of the cylinder.  If some air is blown through the cylinder, it is possible to reduce the humidity and keep evaporating the water until there is insufficient remaining for any condensation.

If I was developing the operating procedure, I would leave the jacket heating on for a while after the engine is shut down, and bar over the engine a few times or blow dry air through to carry out the remaining water vapour.  Then I would be looking for tricks to absorb any remaking water and keep the engine dry until restarted.  Even admitting atmospheric air through the steam line while the engine is kept warm by steam in the jacket and preferably turned over a few times will get it quite dry.  But some of our marine engineers may well have a much better idea.

By the way, that steam jacket would be left open to steam possibly with a steam trap on the outlet, during normal engine operation as condensation on the cylinder walls reduces the efficiency of the engine.  They clearly knew about this at the time that engine was built, and considered the jacket worth the complexity of the cylinder casting.

When the restoration is complete, I assume that there will be no water left in the cylinder, so should not be a problem when first running on air.  In any case any water should be carried out through the cylinder passages to the exhaust as described above.

With running it on air, as a compound engine, the main issue will be having enough load on the engine that the inlet pressure does not result in negative pressure in the lp cylinder at the end of the expansion.  However there may be a simpling valve or similar device to admit high pressure air to the lp cylinder, especially for startup, (including starting on steam) so leaving that open should avoid any problem.

I hope that answers your concerns, but donít hesitate to ask more questions, or discuss it further if something does not make sense.

Exciting time as the project gets near time to see the engine running after all the work.

MJM460





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Online steam guy willy

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Re: Talking Thermodynamics
« Reply #1274 on: October 29, 2019, 12:12:51 AM »
Hi MJM, thanks for the reply.    So the water would not be such a problem with a slide valve ..but what about piston valves. ? however locos do have automatic drain cocks.  If you had a sealed tube with some water in it and left it in the Sahara desert with its large chang in day and night temperatures , would the water evaporate and condensate daily  ?  Will using compressed air in the Beeleigh engine actually introduce moisture into the engine ?  On my numerous visits to this engine i have always noticed that it was very. cold . if the use of warmish ambient temperature in the compressor is used will there be condensation building up in the engine ??

Willy