Author Topic: Talking Thermodynamics  (Read 122310 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!

Offline 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



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

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #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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Illawarra Live Steamers Co-op - Australia
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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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Offline 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

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

Online 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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Offline 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

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