Author Topic: Talking Thermodynamics  (Read 143407 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1320 on: April 25, 2020, 01:33:31 PM »
Hi Terry, thanks for that.  I donít like to keep using the wrong term, so it is always helpful to have confirmation when these things arise. 

Hi Willy, It all depends on the structure of the building.  If you had something a bit out of balance on the lathe, and the lathe was on a wooden floor with some sort of connection the studs supporting the floor above, there could be some transmission, though if a timber structure, timber has some natural damping.

The lathe would have to be a long way out of balance to cause a forced vibration through the whole structure, and you would surely know it.  But if the speed of the lathe coincided with a natural frequency of the floor above, or the wave frequency of the bath, and some transmission occurred, an amplified resonant frequency could occur.  It seems intuitively to be a bit too much of a coincidence, but I do know from my working life that things only have to coincide with one of the harmonics to get resonance, and often there are so many harmonics that it is hard to get sufficiently far away from them to avoid a degree of resonance.

But you are right, there is always a reason, it just takes careful observation and sometimes sophisticated instrumentation to identify the cause.  But I would not recommend running up stairs to check while you leave the lathe running any time, but especially if it is badly out of balance.  You would need a second person as observer!

MJM460

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 #1321 on: April 26, 2020, 02:02:26 AM »
Hi MJM , ok thanks for that...Lets see if anybody else has any ideas ??  ..... in the meantime  I've been thinking about using microwave energy to use in a steam boiler ??  with modern plastics  and non conductive fittings is this possible and is it used ??  they use induction heating in furnaces so it might be similar ...?  I haven't a clue how one might do it   so just wondering  ?? I have been using my microwave as a means of sterilising my home made compost  before I use it for seedling propagation . I do this so I don't get lots of other seeds  growing in the pots !!...

Willy


Offline MJM460

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Re: Talking Thermodynamics
« Reply #1322 on: April 26, 2020, 01:16:06 PM »
Hi Willy, I am also hoping some others have some ideas, so am waiting with interest.

Microwaves can certainly boil water, I do it frequently when heating soup for lunch.  Not that I set out to boil the soup, the jug is better for that, but I notice that by the time the soup is hot, it is often boiling around the edge at the liquid surface.  No problem with a Pyrex glass container, but in the plastic ones, it tends to damage the plastic around the edge of the liquid surface. 

That much energy into a closed pressure vessel will raise the temperature in the same way as any other heat source.  But I am not keen to experiment with making boilers out of plastic!  There may be suitable plastics, but I donít know of any that have a sufficient safety margins of strength or temperature stability for even relatively low pressure steam.  I am very conservative in that area, silver soldered copper for home construction, and steel only where properly qualified welders, weld procedures and quality control are used.  It is possible to go with less, but I am not advocating it.

Good idea for sterilising soil.  Do you have a specific procedure?  Do you need a cup of water in the microwave in addition to the soil to protect the appliance?  Or is the soil moisture adequate?

Induction stove tops are available.  I donít know much about the elements, but they seem to require metal pots.  Also they come with warnings about pacemakers.  We do have such a device in the family, so I have no intention of experimenting with them.  Others may have more information on this also.

No social isolation for forum members, and all this ďsheltering in placeĒ is giving us all more shop time.   But it is very hard on those who have lost jobs and trying to support a family.  My thoughts and prayers go out to them.

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 #1323 on: April 28, 2020, 03:15:15 AM »
Hi MJM,  Thanks for the reply.. when I microwave the compost I don't add water to it or in a cup as there is always moisture /condensation inside when I open it ..It does smell a bit though !!  There are a couple of magnets in the microwave construction that are quite easy to remove ,and people do chuck them out when they are broken...

Willy

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1324 on: May 23, 2020, 11:13:32 PM »
Hi MJM.  this is one of the models that my friend Mitch has made for an advertising  company  !!quite cool ..sort of...

   

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1325 on: May 24, 2020, 01:25:43 PM »
Hi Willy, you seem to like living dangerously.  But if I keep my comments to fair study/ review, and stay away from politics, I hope it will keep us both out of trouble.

Itís a great model that Mitch has made, it will go well with museum visitors.

It very clearly demonstrates the dirty aspects of coal that we can all see, and compares it with the clearly much less dirty gas.  Not totally clean mind you, still some fine particulates, but people with expertise in that area will have to comment on that.  But in summary, the little model is fair and useful as far as it goes.  We canít discuss bigger issues until we all, or most of us anyway, understand the basics.

I am an engineer, so my area is in the practical application of the science to solve real world problems. Applied science rather than pure science in the research sense.  On that basis, what do I think?

Well, full disclosure, my whole working life was in hydrocarbon processing, but which way did that influence me?  I hope I can present an analysis neutral enough to pass the pub test.

The issue that is rarely included in the context of this model is Carbon Dioxide.  We canít see it, so it can justifiably be left out of a discussion about ďdirtyĒ.  But it is important in climate, and the science is clear on that.  So where does that leave us?  It is a different but closely related discussion.

Coal and gas both release energy by combustion in which hydrogen and carbon are the most significant.  Hydrogen burns with oxygen from the air to give us water, carbon to give us carbon dioxide.  The ratio of hydrogen to carbon is different for each, and gas gets more of its energy from burning hydrogen than does coal, so to that extent it is better, but not by a huge amount.  Gas power stations sometimes work on a combined cycle and can have efficiencies higher than that of a coal fired station, particularly an old coal fired station, so that definitely is in its favour.  Also, a gas powered station can respond quicker to load changes than a coal station.  But in the end, they still produce a lot of Carbon Dioxide and the climate change science canít just be ignored.

So where does that leave us?  Can we just shut them all down tomorrow?  Well for a start, we have not yet built the alternative, and if you think the present lockdown is causing chaos to the world, just think about shutting down all the coal fired power stations overnight!

We have to remember that the community has a large amount of money tied up in coal stations, regardless of the specific ownership, and I suggest that we really canít afford to just discard them.  Just like you canít afford to just send your car to the wreckers, and buy a new one, just because the new one has some nice features that you would really like, and may even be safer, the world community cannot afford to just close down the all old power stations without spreading out the pain.

But again, just like your car, they do wear out, and many of them are already old, and approaching end of life, so they are going to have to be replaced sooner or later.  That makes now a really good time to think about what we want to replace them with.  And like the old car, we canít just replace them with like for like, as while that feels like it would be easier, that old model went out of production years ago.  The old power stations have served us well to get to where we are today, but the new ones will be much more expensive to build, and in the end, despite all the offers of easy finance, interest free, accessories thrown in, all made as enticements to buy ď before its too lateĒ,  we all pay for them all one way or the other.  And what ever we choose to do, we have to plan early, a national grid size power station cannot be picked up from a big box store, unpacked and switched on preferably after reading the instructions.  In our consideration of currently on line power stations, we probably have to include some that, while construction has not yet started, could already be beyond the point of no return due to design work completion and cancellation costs on other contracts for which preparations are already well advanced.

A more practical approach is to recognise that we will have to replace them sooner or later, and plan carefully how we will go about it.  And the sooner we start to properly plan, the sooner we will understand the required decisions, including what to do when each one becomes no longer economic to run.  We canít do nothing, they will just break down more and more often until the pain of lights going out convinces us.  Why do I keep going back to thinking of the old car?

If the decision is to go for more renewables, and I think it is feasible, we have to consider what that means in terms of what we need for a stable grid, and how we go about phasing out the old.  (And I donít underestimate the difficulties of doing that with minimal pain to the community.)

That brings us back to the basic thermodynamics, starting with a very basic law of physics, conservation of energy.  We have discussed that before in this thread.  In power supply terms, it can be written as

Generation - Load = storage.

All power systems have all three, yes, even the current coal based systems, even if we donít recognise them, particularly the storage, which in different power technologies, can look very different, and behave very differently.   And that is a whole new area, one that I am more interested in discussing if it is of any interest to others.

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 #1326 on: May 25, 2020, 03:46:53 AM »
Hi, MJM. I was just sharing this from Mitch to show what he does for a day job ... he does get time to make very nice models and he has the use of lots of nice machinery.... I was not trying to be political or anything ..indeed my Morris Minor is 55 years old and is petrol and I have just replaced the exhaust valve today as they frequently burn out!!!? One can still buy all the parts and it should see me out...everybody uses the benefits of coal and gas fired power and in a way these are renewable ...if you wait long enough !!! I notice that on the adverts for electric cars they say they are pollution  free whilst driving  !! also people always go for the cheapest option for fuel , whoever they are. 


Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1327 on: May 25, 2020, 01:34:20 PM »
Hi Willy, quite happy to talk about what is above the ground, or at least only at shallow depth, but deep down is geology.  I will leave that area to them!

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1328 on: June 08, 2020, 02:39:16 AM »
Hi MJM, a new  T  question about Microwave ovens... It says that you should not put anything metal in it as it will cause a cataclysmic event ??  If you heat something up like water it takes a certain amount of time ...but if you want to heat up something really solid and dense  perhaps a lump of meat ..the same weight and temp will it use the same amount of time/electricity ??  if you use the timer but with an empty microwave will it use the same amount of electricity as it is not doing any work  !? so the question really is how do microwaves behave differently to an open /enclosed flame or an electric element.  And will a lump of metal drag out extra power from the unit , to harm it ?? , and do you use more/ less electricity given that the same amount of energy is needed ?? incidentally there are a couple of useful magnets inside microwaves ??

willy

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1329 on: June 08, 2020, 11:21:30 AM »
A certain amount of power are used for the heating of the power tube that creates the microwave radiation, so that remains constant !!!!

Actually what you have is a RADAR Transmitter, where the "Antenna" radiates into the cubicle inside the oven and like any RF Transmiter, it will consume the same amount of power when ON no matter where it ends up. If the Transmitter has a serious antenna problem the power might end up being reflected back to the power stage -> usually result, is a destroyed output stage of the transmitter. This is more or less what can happen if you put enough metal into the oven - but if you have an empty oven or filled with the 'normal' stuff (meat, water, etc.) it behaves like a normal antenna and the power drawn from the wall is constant.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1330 on: June 08, 2020, 01:32:23 PM »
Hi Admiral, I am glad you are on board to answer that one, I am no expert on microwaves.

As always all the heat consumed has to go somewhere, and I guess that at the end of the number of reflections it takes, the energy all goes into heat which escapes from the even.

When radiant energy meets a surface, my understanding is that it is always partly absorbed, partly reflected and partly transmitted through.  In the case of microwaves in the oven I hope it is mostly reflected and absorbed, as we donít want too much transmitted through the walls of the oven.  I guess the reflections continue until it is all absorbed.  The fan on my microwave ovens continues to blow warm air for quite a while after cooking finishes, with the display saying ďcoolingĒ.

But I also have in mind that the problem with putting metals in a microwave, such as an aluminium foil cover on a dish or china with a plated metallic decoration around the rim, is that sparks are induced somehow,  though I am not sure of just how that happens.  I have managed to avoid doing it so far, so itís only second hand information.

Hi Willy, The question seems to be very similar to the question of a radio transmitter and whether it makes a difference to the transmitter if there is a receiver to pick up the signal.  You might have come across this sort of equipment in your electrical training.  Although in the case of the radio transmitter, the transmitter and all possible receivers are not enclosed in a reflective walled enclosure.

I hope that makes sense and does not cause confusion after the Admirals excellent explanation.

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 #1331 on: June 09, 2020, 02:55:30 AM »
Hi MJM  & Admiral. I suppose that all thermodynamic rules are the same in every thing that is happening in our world even if it is not immediately obvious. I have noticed that grass stalks in snow always have a tube of melted snow around them ?  I have heard that when I was in the Army the radar Techs use to heat up their pork pies in the microwave tubes !! So thanks for the info....!!

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1332 on: June 09, 2020, 01:01:39 PM »
Hi Willy, it all comes down to the most basic laws of physics, conservation of energy, conservation of momentum and conservation of angular momentum.  There are a couple more more obscure ones about conservation f charge, and two laws about some basic sub-atomic particles, but for most of what we do, the sub atomic particles ones lead to the very reasonable assumption of conservation of mass.  So long as you remember that conservation of mass is only an approximation.  Mind you, itís good enough unless you are getting too realistic about your scale atomic submarine model, or are perhaps building a model Hadron Collider.

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1333 on: June 11, 2020, 01:09:27 AM »
Hi MJM, thanks for that and I thought there was a lot more to this than just 'simple' thermodynamics. also talking about microwave ovens  my toaster machine stoped working because the lever would not stay down !!  so I used a couple of the discarded microwave magnets!!  and now it is working again ......a sort of thinking outside the box remedy !!! these magnets come in pairs and are really easy to remove..

Willy


Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1334 on: June 11, 2020, 11:08:45 AM »
Careful with that solution Willy - we do NOT want you to burn down the house and you have just prevented the safety bit to switch the toaster off when it get too hot !!!!

Per