Author Topic: Talking Thermodynamics  (Read 118377 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1200 on: August 06, 2019, 12:19:05 PM »
Hi Avtur,

Thanks for the additional input.  Glad you were able to download the information.  I assume you found the tables in Excel format as well.

I tend to agree that the temperatures should be absolute, however, as the tables generally assume an arbitrary start temperature of zero at the triple point of water, it probably does not matter too much so long as we all understand when to convert them to absolute for a calculation, and of course, remember to do it. 

Because we all have so clearly in our heads centigrade values for the freezing point and atmospheric pressure boiling point, it is probably helpful to relate things to the references we have in our heads.  273 is a bit of an awkward number to always have to add and subtract, especially if we want to include the extra decimal places.

With that lost work, I suspect that even if we ignore friction of the piston against the cylinder walls, though there will always be some, there is also energy lost in turbulence within the gas in the cylinder.  This cannot be recovered or used, we can only use the work that crosses the boundary of the space we are considering is, I think, the explanation.  As you say, we canít beat the second law.

MJM460

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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1201 on: August 11, 2019, 12:02:37 AM »
Hi MJM, thanks for the last post.. I have now been watching a new series of documentaries entries on the telly that is quite interesting . they stated that heat always travels to cold areas ...however if you have a large quantity of very cold icy water and mix it with a very small quantity of warm water the cold water will actually move into the warm water and make it less warm.!!  sorry about being pedantic !! :mischief:  also found  a chart talking about the heat losses in a coal boiler....although i don't quite understand some of the losses/movement of the losses.?.the conclusion being 19% to produce steam !! if this is correct ??...however if the coal is suddenly extinguished/removed the boiler will still produce steam for a while ?? and there will still be some calorific value left in the coal..?? I haven't read the full article so may have got it all wrong !!!
« Last Edit: August 11, 2019, 12:05:58 AM by steam guy willy »

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1202 on: August 11, 2019, 12:57:44 PM »
Hi MJM , Actually i have got it wrong it is not a boiler !!! :-[ it would appear to be a blast furnace or other ...sorry about that  :-X

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1203 on: August 11, 2019, 01:05:08 PM »
Hi Willy, I hope that last post made sense and helped further your understanding.

That sounds like a very interesting program on the BBC.  Programs on basic physics are a bit thin in the programming space, to use the current popular jargon.  I am pleased that they agree that heat moves from hot areas to cold areas.  I think the clue to the conundrum is in that description of icy water.  Remember that hot water is generally less dense than cold water so the hot water tends to rise, while the cooler water tends to sink due to the density difference.  Just as we have previously discussed with air.

But water is strange stuff.  As water is cooled it contracts until it reaches about 4 deg C.  Then it changes its behaviour and actually expands as it cools further.  This is why ice forms on the surface of the pond, while the warmer water flows underneath.  There are only a few other substances which have this same behaviour.

So at moderate temperatures, if you have some cold water and carefully pour some warmer water onto the surface, carefully meaning in a manner so it does not immediately mix up, the warmer water will tend to stay on the surface, and the density difference will slow the mixing.  However, heat will still travel from the warm water to the cooler water.  If you do it the other way and put the cold water carefully onto the surface of the warmer water, the higher density cooler water will tend to sink, and the convection movements resulting will speed the mixing so the temperature evens out more quickly.

However, if that warmer water is only about five or six degrees, so still not swimming temperature, and you put that warm water on top of icy water, the icy water could have lower density than the warm, so the cooler water will rise to the surface, mixing with the warmer water, and the heat of the warm water moves to the cooler water, leaving it cooler.  Because of the unusual density change with temperature, the normal density difference is reversed and the unexpected happens.

You can look up the density/specific volume column of the steam tables and see the range of temperatures over which this happens. 

Also when ice is further cooled, there is a temperature at which it starts to get cooler again.

In summary, heat always moves from a higher temperature body or material to lower temperature material, but less dense fluids, liquid or gas, tend to rise against cooler fluids due the difference in gravitational forces.  Heat moves under a temperature gradient, fluids move under a density gradient.

The type of diagram in your book is a common representation of where the heat goes in a furnace or other equipment.  But I believe that particular diagram has been prepared to show where the heat goes in a furnace used in steel making, so they are making coke, and producer gas, and steam.  I think you will find it is in the section on steel making processes.

A similar diagram can be prepared to show where the heat goes in a normal steam boiler.  I expect you will find it would show about 70% goes into steam, most of the remainder goes up the stack as flue gas and the remainder is lost as heat radiated through the furnace walls.  You can refine the accounting of the losses into all the detail you wish, so you can include heat loss from steam piping, heat into feed water, heat in the ash.  The important thing is that you draw a boundary around the equipment you are including, and only count heat that moves across that boundary.  So if you use some flue gas to preheat incoming air or water, that might not cross the boundary, depending on precisely where you draw the boundary, so would not be counted.

I hope that helps

MJM460

Hi Willy, your extra note arrived while I was typing.  We are now on the same page.  Well done.

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Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1204 on: August 11, 2019, 03:37:52 PM »
Willy and MJM

I have little to add. I watched a programme, one of three, on the BBC a couple of nights ago about temperature which was well put together. I think the presenter was a professor at The Open University.

Many years ago I worked as a research engineer at a steel works, a great and fascinating job which did not last. I did have a little to do with blast furnaces but I would not want to consider their thermodynamics. Obviously the air used in the furnace is pre-heated, generally using a something called a Cowper heater. This is large regenerative heater, a cylinder filled with refractory bricks which absorbs the heat from the furnace gases and then gives it up to the blast air. How the whole system, including the furnace, warms up and cools down is very complex. Once a furnace was started it was not usually shut down until it became dangerous to operate.

I could go on but I am rambling (a bad habit of mine).

AVTUR
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1205 on: August 14, 2019, 11:50:06 PM »
Hi MJM, A sort of T question ..when i have a mug of tea and squirt some honey in it ...if i mix it up it tastes of honey...but  if i dont stir it it stays the bottom until i have almost finished it.? Is it something to do with specific gravity or some thing else  also if the honey was the same temp as the tea would it self mix ??? Just a diversion whilst i am staring at the rain...

Willy

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1206 on: August 15, 2019, 03:52:14 AM »
Willy

A very nice question! And not a diversion.

AVTUR
There is no such thing as a stupid question.

Offline Steam Haulage

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Re: Talking Thermodynamics
« Reply #1207 on: August 15, 2019, 08:40:39 AM »
Hi MJM,

In your recent post about gently pouring various temperature liquids into one container to make 'separate' layers how do you account for Brownian motion?

Still following along although I thought I had deliberately forgotten all this when i retired.

Sorry for the perhaps out of kilter question but in the past I was more concerned with ensuring intimate mixing at various rates so I'm more than rusty.

Jerry
Dogs look up to you, cats look down on you, pigs treat you as equal.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1208 on: August 15, 2019, 11:22:00 AM »
Hi Willy, you have an amazing capacity to draw a physics lesson out of your tea, coffee and slushies. 

Specific gravity, or density is obviously involved, as the honey sits at the bottom, and as it is colder than the tea, convection currents are not helping.

Also, mixing honey into your tea is also resisted by viscosity of the honey, so another starting point is to try and understand viscosity.

Now, as you know, I am an oil man, not an apiarist, so I am sticking my neck out a bit.  Never the less, the chemical compositions of hydrocarbons and sugars have much in common.

Hydrocarbons are molecules consisting of carbon and hydrogen, and the properties of the carbon atom mean that molecules can build up chains which can be extremely long.  We have only talked about the simple ones, but once you have at least six carbons, you can have straight chains, branched chains and even rings.

Sugar molecules are made up of carbon, hydrogen and oxygen.  The molecules look like carbon chains with some of the molecular bond sites taken by hydrogen atoms and some by OH units, if not taken by another carbon, whereas with hydrocarbons all the sites not taken by another carbon are generally taken by hydrogens.

Normal sugar actually has two rings, each of six carbons, joined together, and with the required number of H and OH units.  And these rings can continue to join together to make very long chains, just as in hydrocarbons the chains can get extremely long.

Now with the very simple compounds, even with short chains, the molecules are quite small compared with the space they occupy and the distance they move in the normal random motion due to temperature.  Liquids such as water, propane, butane etc can move relatively freely.  We see that in the low viscosity of these liquids.

But you can readily imagine that as those chains get longer, they start getting tangled up and having to slide over each other.  At a very simplistic level, water is like those ďhundreds and thousandsĒ coloured decorations used on cakes and childrenís sandwiches (not to mention some adults).  Basically little coloured sugar balls if you use a different name.  While a very long chain tends to be like cooked spaghetti, probably best imagined as relatively short chopped strands rather than really long ones.  A spoon of the little balls spreads everywhere, while the spaghetti lands in a heap.  Donít carry the analogy too far.  If you drop the spaghetti in a pot of water, it still lands in in a heap, but if you stir it a bit with a spoon, you facilitate the separation of the strands enough to get water between the strands and it can finally spread out to fill the pot.

A bit of an over simplistic analogy I suspect, perhaps someone can suggest a better analogy.  Honey has relatively long chains of those sugar rings, perhaps not as long relatively as spaghetti, but long enough that a single molecule cannot easily get free of the other molecules over its entire length, so it is more reluctant to move like a fluid with smaller molecules, and hence its viscosity.  With the spoon, you separate the honey and give a bit more surface area for the water to mix with the honey.

As Avtur says, we tend to put aside the molecular description of materials early and then forget it, but this is another case where thinking about the molecular structure can be helpful.

Hi Steam Haulage, thank you for looking in.  I hope that you are finding some topics that interest you, and that most basically make sense.

In talking about gently putting the low density fluid on top of the higher density fluid, I was just trying to avoid the vigorous mixing that occurs if you pour one of the fluids in, say from a jug some height above the surface of the other.  Pouring in very gently reduces the mixing caused by density gradient, if the lower density fluid is gently put on top.  In chemistry lessons, long, long ago, we were taught to hold a spoon horizontal near the surface of one fluid and pour the second gently onto the spoon so if flows at low velocity off the edge. 

This principle is used in storage hot water systems.the cold water is introduced at the bottom, while the hot is extracted from the top.  When you have a shower, and the cold water coming into the tank was allowed to mix freely, the hot water would start reducing in temperature quite quickly, and you would continually have to adjust the taps for comfort.  By reducing convection mixing, you get the benefit of higher water temperature for much longer.  There are also baffles around the heating element which also help with the issue.

Brownian motion the visible evidence of another mechanism for mixing, and this occurs as well as convection.  But it is a much smaller range action by our scales of dimensions, so it takes longer to extend its influence throughout a vessel.  So not a contradiction.  So in this small scale molecular motion is not generally very effective in complete mixing, compared with convection, especially if you are in a hurry.   With very high purity products, say the propylene used for polymerisation, around 99.99% purity, a small amount off spec material, can sit in the storage tank for a long time, instead of mixing, despite the molecular motion which might be expected to mix it all up.  It generally comes out as detectable off spec product, even when the quantity is small enough not to put the whole tank off spec if it was properly mixed.  You need energy input via mechanical impellers or jet nozzles on a tank inlet to ensure thorough mixing.

I hope that helps sort things out.

Thanks to everyone looking in,

MJM460

« Last Edit: August 15, 2019, 11:31:36 AM by MJM460 »
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1209 on: August 16, 2019, 03:12:39 AM »
Hi MJM  Et Al.  Thanks for the explanation  I didn't realise that it was that complicated at the molecular level !!  I was thinking the same might be for sugar cubes.?..I will try that tomorrow as it is 03-10 am and i am about to go to bed..... I wonder if Einstien thought about things whilst drinking tea and coffee !!

Willy

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1210 on: August 16, 2019, 10:06:11 AM »
Hi MJM, Can the rate at which heat flows or is transmitted through a known material of fixed dimension with a fixed heat input at one end, under normal environmental conditions, ever vary? Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1211 on: August 16, 2019, 03:13:00 PM »
Hi Willy, the composition of sugars rapidly gets more complex than hydrocarbons, and hydrocarbons get complex enough as the length of the chains increases.  Your normal sugar cubes are basically sucrose, which has two of those ring structures joined together.  Not linked like the chains on Chrisís Marion, but sort of welded together by an oxygen atom which joins the two rings.  So a lumpy molecule but not a really long chain.  Classed as a disaccharide, it is one of the simpler sugars, although there are also monosaccharides which have only one ring, the group which includes fructose.  And the molecules would seem to be relatively regular so that they stack together in a regular crystal.  When you put the sugar cube in a drink, the sugar molecules are quite compatible with water as the H and OH units attached to each carbon tend to attract the H or O atoms of water.  But when packed into a crystal or a sugar cube some stirring helps strip off the outside molecules and allow the water to get at the ones underneath.

Also while six carbons form the basic structure of the ring, which is normally drawn flat, it is actually a distorted ring in three dimensions, but perhaps that is going too deep.

I am sure thinking over a cup of tea or coffee has a long and honourable tradition.  And I believe history records that Archimedes did some of his best thinking in the bath. 

I am not entirely happy with my spaghetti analogy, as strands of spaghetti are quite flexible and bend easily, whereas I am sure that the molecular structures are not so flexible.  But how flexible or rigid a very long chain, I donít know.  Also, strands of spaghetti are quite slippery, while the molecules with a long chain structure are a bit lumpy, so do not slide too easily.  But the long chain compared with a simple molecule of two or three atoms is the idea I was trying to suggest.

Hi Paul, good to hear from you again.

I am trying to understand the context of your question, as there seems to be two different concepts involved. 

First, conduction generally is proportional to the temperature gradient.  Mathematically, Q = k x A x (T2 - T1)/d, where Q is the heat transferred, k is the conductivity of the material, usually assumed to be a constant, A is the area perpendicular to the heat flow, and d is the distance between the surfaces through which the heat travels. T1 is the temperature of the high temperature side  and T2 the temperature of the low temperature side.

While the material conductivity, k, is generally considered a constant, it can be different at different temperatures, so with a large temperature range, allowance has to be made for this variation.  But the heat input to a system is not part of this equation.

Back to the problem, for any boundary surrounding a heat source, the if heat crossing the boundary is less than the heat input, the temperature inside the boundary will rise, so the temperature difference across the material forming the boundary will increase, so the heat transferred will increase in accordance with the formula.  The temperature of the source  will rise until the heat loss is equal to the heat generated.  Then the temperatures and temperature profiles will stabilise.  During the initial period there will be storage as well as transfer, but when the system becomes stable the heat loss will equal the heat input.  And the heat transferred will increase as the temperature profile develops, until that equilibrium is obtained.

Does that answer the question?

Thanks everyone for looking in,

MJM460

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Offline paul gough

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Re: Talking Thermodynamics
« Reply #1212 on: August 17, 2019, 12:48:32 AM »
Hi MJM, Knowing what to ask when you don't understand the process sufficiently is quite difficult. I am trying to visualize the 'heat energy flow' through a solid material. Lets say a copper rod of fixed dimension with a heat source at one end. If one could 'observe' what was happening at a single point along the bar, what would one see?? How is the energy transmitted, what enhances or impedes it, can the flow vary, could it be regulated and if it is not self evident in the answers to the previous questions, what causes the different rates of flow with different solid materials. The electric/hydraulic analogy and the bouncing molecules of steam imparting work to the face a piston are all able to be visualized once explained, but I find heat flowing through a solid material difficult to understand precisely. Regards, Paul.

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1213 on: August 17, 2019, 03:40:47 AM »
Hi MJM, Just a reply to the honey question... I tried putting a large brown sugar cube in some tea hot  water and the cube sank to the bottom and fell apart...1 Hour later it was still visually the same and did not taste sweet. once it was stirred the colour was dissipated and it tasted sweet . ...thinking about Einstein and Archimedes i wonder how quickly the advancement of science would have progressed if they had actually bathed in tea/coffee !!! :lolb: :lolb:

Thinking about Pauls question ..we know that materials expand when heated...however if a piece of a copper cube were heated between two very solid low thermal imouveable  objects  how would the copper try to expand?? would the molecules become oval on the restricted faces/plane or would the copper expand a larger/different amount on the un restricted faces ??

willy.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1214 on: August 17, 2019, 12:20:13 PM »
Hi Paul, just to formulate the question in that way shows a high level understanding of the issue.  Even my trusty Thermodynamics text book dodges the issue of providing a definition of energy, simply stating that it is such a well understood concept that a definition is not really necessary.  It then goes on to define work and heat as energy in transit.  Work crosses a boundary when a force acts through a distance, while heat is energy crossing a boundary due to a temperature difference.  Energy is not a substance that can be identified moving around.  So energy can be stored as potential energy (height), kinetic energy (velocity) or internal energy (which is evident as temperature).  And it can move either as work due to work being done, or as heat due to a temperature difference.  And energy can be changed between the various forms.  It should also be mentioned that energy is not created or destroyed, conservation of energy is one of the fundamental laws of physics.  Mind you, it can sometimes be quite a puzzle to understand where all the energy went, or came from, but if we look hard enough it can be found.

Perhaps not a very good explanation, but that is the nature of the question.  So what does it mean in the context of your question?

If we consider a block of metal at some high temperature, the atoms all have energy, which if it was possible to see them, would be evident in their vigorous motion.  In a solid it is more or less like vibration around a point.  In a cooler block, the atoms are still in motion, but the motion is smaller amplitude or velocity.  The atoms do not all have the same energy, but the temperature is the result of the average. Presumably larger numbers of the atoms are closer to the average, while small numbers depart from average by a larger amount.  Think of the bell curve of a normal distribution.

If one surface of a block is heated to some temperature perhaps by a flame, or by contact with another block already at a high temperature, then the atoms near the hot surface bouncing against the slower moving atoms deeper in the block results in some energy exchange between those atoms, with the higher velocity atoms slowing and the slower moving ones increasing in velocity.  Thus some energy is transferred by the collisions, and while the temperature gradient is maintained the process continues.  If the whole block is at the same temperature, all the atoms have on average, the same energy level, and there is no transfer of energy.

If the material is relatively simple atoms in some sort of crystal array, like most metals, the process of energy transfer proceeds quote rapidly.  Think of silver, copper or iron and so on.  But if the block is a more complex compound, it is much harder to get those molecules vibrating in a pattern that results in predictable collisions.  It still happens, but proceeds more slowly.  Or if the material has a cross linked structure, like a plastic, similar resistance to heat transfer.

So the actual rate of transfer is determined to a large degree by the molecular structure of the material.  And not so easy to regulate in the way would regulate steam or water flow for example.  It is essentially controlled by the material molecular structure and the temperature difference.

Another example where the molecular theory of matter can help our understanding.  I hope it helps answering the question.

Hi Willy, brown sugar and honey are both quite different sugars.  The brown sugar is mostly sucrose, the ordinary table sugar, and a disaccharide, but the last of the molasses has not been washed out in the final crystallisation process in its manufacture.  It also traps some moisture which sort of sticks it together.  So it needs a bit of help by stirring to mix it up well with the water. 

Honey is mostly fructose and glucose, both simple monosaccharides, of which the glucose can chain together into long chains.  But there are over 100 other compounds in honey which contribute to its unique qualities.

Sugar just gets more and more complicated as soon as you look at it all closely.  Definitely well outside my comfort zone.  I prefer the simple things.

Your block of copper is much easier to discuss.

When you heat a metal, it expands in all three directions.  You can look up the coefficient of expansion many text books, or just search the web.  If you constrain this expansion as you have described, you get very high stresses which can be calculated by Hookes law and the modulus of elasticity.  These stresses will soon exceed the yield stress of the copper, and the atoms will be pushed around in the lattice, just as if you had squashed it is a vice, or hit it with a hammer.  Each atom effectively occupies the same volume, but the atoms are pushed around and the lattice distorted.  Similarly, with steel or other metals.  This is the source of thermal expansion stresses.  A major consideration in designing pipes for refineries and gas plants. If a metal object is heated unevenly, the colder parts expand less and effectively constrain the hotter parts which are trying to expand.

I suppose if those pioneers had actually bathed in tea or coffee they would have had scold injuries in places very embarrassing to explain, and it might have held back progress considerably.

MJM460

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