Talking Thermodynamics

[MJM460]

Hi Admiral DK, those gas turbines are truly fascinating and high tech machines with blades cast as single crystal high temperature alloys with all those cooling passages, and still the performance limit is the turbine inlet temperature, so as not to melt them, or simply abrade the blades so they become less efficient.  And all that just to produce a lot of high velocity air.  But thanks for putting it in, there will be quite a few interested.

In aircraft the turbine just turns the compressor that gets air into the engine, and the exhaust air then provides the thrust to power the aircraft. Though as you say, some have that extra stage to provide even more thrust, by providing even more air flow.  In my compressor applications, the hot air drives a further turbine called a power turbine, often on a separate free turning shaft, which turns the process compressor, or in power generation applications, a generator.  About a third of the power produced drives the front compressor just to get air into the machine, a third is mechanical power output, and a third hot exhaust gas.

It was probably a mistake for me to mention them, but I was getting at the path for the air involved in combustion.  In the combustion chamber, it is divided into primary air mixed with fuel for combustion and secondary air to ensure complete combustion, just the same as any other combustion chamber.  Well, actually they operate at much higher pressure and flow rate, but otherwise similar.  Then all those other streams further increase the mass flow through the machine in order to provide thrust, or power turbine drive and to ensure they don't melt anything on the way through.  Power output is about mass flow, but that is a whole other topic.  Sorry for the distraction.

In the discussion of Chris' poker burners, we were only discussing the actual combustion air with the necessary air fuel ratio.  Butane will only burn in fuel to air ratio of about 2 to 10% fuel.  Too rich, or too lean and it will not ignite.  So those air holes we have been talking about have to be the right size to meter the air in at an appropriate rate for the fuel supplied through the gas jet.  Alternatively, they can be a little big and have that sliding sleeve partially cover them for adjustment.

MJM460

[MJM460]

A little Chemistry-

A few days ago, when talking about Chris's poker burner for butane, I mentioned normal butane and iso butane, and the difference in vapour pressures.  It might be useful to talk a bit about what these are.  As both are chemical compounds classed as hydrocarbons, which means the molecules are predominately combinations of hydrogen and carbon atoms.   Hydrogen has a molecular weight of 1, and also has one point where it can bond to another atom to make a compound.  Carbon has a molecular weight of 12, and has four points where it can bond to other atoms.  So let's look at what happens.

The simplest hydrocarbon is methane, which has one carbon atom, with one hydrogen atom bonded to each of its sites, a total of four hydrogen atoms in each molecule.  It is symbolically represented as CH4, and is the predominant component of natural gas, whether supplied as pipeline gas or liquified as liquified natural gas or LNG.  When it burns, the carbon joins with two oxygen atoms, providing there is plenty of oxygen available, to form Carbon dioxide, or CO2.  However, if there is insufficient oxygen, it can also form Carbon monoxide, CO, which is extremely toxic to life.  You can get some CO, even if the quantity of oxygen is sufficient, but with so little excess that it is difficult for every carbon atom to find the necessary two oxygen atoms.

The four hydrogen atoms join with one oxygen molecule (which has two oxygen atoms, so O2) to make two water molecules, water being H2O.

The combination of either hydrogen or carbon with oxygen is called oxidation, and releases a lot of heat, and the flue gases contain CO2 and water in vapour form.  In addition, the oxygen normally comes as air, which is eighty percent nitrogen.  At the simplest level nitrogen goes along for the ride, but absorbs some of the heat produced to so that all flue gas components exit at the same temperature.  Thus the nitrogen limits the maximum temperature that is achieved, but it also adds to the amount of heat that is lost up the stack after the heat transfer area has allowed as much heat as possible to the water in the boiler.  If that temperature is still above 100 C, the water exits as vapour and we get the lower calorific value.  If we are able to use the heat at a lower temperature so the water condenses, then we also benefit from the latent heat as the water condenses, so we get the higher calorific value of the fuel.

Now, methane is not very useful for our hobby.  It liquifies at about -161 C, and as a gas, it has such low density, that we can't carry a worthwhile amount as compressed gas on a model at reasonable pressure.  At 40 C, to keep methane liquid, you need a pressure of roughly 35,000 kPa.  So why have I spent so much time on this?  Now you will see the delightful simplicity of dealing with the hydrocarbon series that includes methane but also, ethane, propane, butane pentane and so on.  You know have the knowledge for a basic understanding of all of them.  One more step, then let's see how it works.

You will remember that I mentioned that a carbon atom has four points that can bond to another atom.  In methane each of those bonds to a hydrogen.  But there are other possibilities.  What if one of those points bonds to another carbon atom.  Each carbon has now bonded to the other carbon atom at one of its four points, but it has three more.  A hydrogen can bond to each of the three points on each carbon, and we have a molecule with two carbons and six hydrogen atoms, C2H6, which is called ethane.  Usually present to some extent in natural gas, but the industry tries to separate ethane as a chemical feedstock.  With a boiling point of -88 C at atmospheric pressure, or around 6,000 kPa at 40 C, it is still not much use to us as a fuel for our models, so let's continue.

If we now have three carbon molecules, one joins to another carbon at each of two sites, while the other two carbons join to that first one, the middle one on opposite sides, and to three hydrogen so with the remaining three sites.  Of course the middle one has only two more sites so only joins to two hydrogens.  We can represent this as CH3-CH2-CH3, alternatively C3H8, which we call propane.  For representing on paper in those symbolic representations, it is convenient to think of those four carbon bonds as being located at 90 degrees, and so the picture forms of a straight chain of Carbon atoms with all the remaining sites occupied by hydrogen atoms.  In reality, the sites are located around a sphere, in three dimensions, so it is a bit of a wriggly chain, but close enough.  I am not trying to turn you all into molecular physicists.  And that chain with three carbons is called propane, and generically simplified to C3, at least when the context is clear that we are talking about these simple hydrocarbon chains.  Now propane we know as the gas in our bar-b-cue bottles.  It's boiling point at atmospheric pressure is about -42 C, and vapour pressure at 40 C is 1341 kPa.  When it burns, the carbon combines with oxygen to form CO2, while the hydrogens form water.  But three carbons to each 8 hydrogens gives a bigger proportion of CO2 to H2O than burning one carbon for each four hydrogens.  So methane is a bit less carbon intensive than ethane.

I hope it is obvious that there is a pattern here.  The next step is four carbons, it now has two centre atoms which then only have two sites for hydrogen.  So it is represented symbolically as
CH3-CH2-CH2-CH3, or more concisely, C4H10.  Now with four carbon atoms there are two possible arrangements, that depend on whether the second middle carbon is still arranged in a roughly straight line, or if it joins to adjacent sites to make a little branch.  You might think of it as a bit like a propane, with a carbon(with its three hydrogens) attached to the centre of the three instead of a single hydrogen.   This requires a three dimensional symbol rather than a straight line representation, but still has four carbons and 10 hydrogens.  This is known as an isomer.  There are other structures also known as isomers, it is not  a total definition, but is enough for our purposes.  The "straight line" version is known as normal butane, or n-butane, while the branched version is called iso- butane or sometimes i-butane.  The butane fuel we use for our gas fired boilers is usually a mixture of normal butane and iso butane.  Burning butane again produces CO2 and H2O, and a little more carbon intensive again.

I suspect that is enough to take in at one go, so next time, I will discuss some properties of butane that interest us when we use it to fire our boilers.

Thanks for following along,

MJM460

[paul gough]

Glad we are getting to the nitty gritty of combustion, so thanks for breaching this subject. Flame temperatures, in air, for propane, butane and methylated spirits are all very roughly 1900 degrees Celsius, so I expect for our uses, model boiler burners, we can treat them as equal on this point. What I would like to know is; are there other parameters we need to consider when trying to determine the most appropriate fuel for our requirements for a given boiler design? One of which might be, greatest heat output per unit volume of fuel burnt and hopefully greatest volume of steam generated for that volume of fuel. But, I understand gas velocities are an issue in heat transference, I am assuming/guessing gas velocities in a poker burner are higher than a ceramic bed type and a metho wick burner, so do we derive higher heat transfers and thus steam generation from a poker than a ceramic bed or a metho wick, if so what would be the degree of difference if we tried all three burners in the same boiler, assuming one to be a suitable design for all three types. I have seen figures for energy released burning ethanol as 30kJ/g, propane 50kJ/g and butane 49kJ/g. Does this necessarily mean we are going to have to burn an equivalent proportion more metho, (ethanol), despite same flame temperatures to get the same quantity of heat output and thus steam production. Hope these questions are conveyed meaningfully, took me some time to frame them, I'm afraid an old mind finds it just as difficult to ask a sensible question as to answer one!!! Regards, Paul.

[MJM460]

Hi Paul, great to see you back again.  Don't worry too much about that old mind, you have very clearly put some very difficult questions.  Those figures you have quoted are all close enough to the standard data, though I will comment a little more on methylated spirits further down the trail.  This series is background which I feel is necessary to enable to sensibly describe how our gas fuels behave in our gas tanks.  Then I will try and look at combustion, so I am glad you are finding the topic interesting.  It is pushing my limits, but I think I will be able to help you move further forward on your questions.  I am doing a little extra reading and will try your questions after I have discussed the pressure temperature characteristics of these fuels.

Just a little more chemistry, then we can get back to butane and other camping gas fuels commonly used.  for our purpose.  In the meantime, look on the containers for your fuel for the composition details, you already know some of the names to look for, and let me know what you are using, in case we are offered different mixtures in different parts of the world.

So back to that pattern.  We have already covered molecules from the very low molecular weight (also called light hydrocarbons) methane, or C1, (with its attendant hydrogens of course), C2, C3 and C4.  You can see the pattern, and it continues to pentane, C5, hexane, C6, heptane, or C7, but it's then octane, a name you have definitely heard, and so on.  The series are called paraffins, you might have heard this name in relation to the wax your grandmother used to seal jars of home made jam, maybe you still do the same.

You might have noticed another pattern.  As the number of carbons, or the molecular weight increases, the substance becomes easier to liquefy.  It condenses at a higher temperature, and the vapour pressure of the liquid, represented by that figure of 40 C, becomes lower.  Once we get to C5 and above, they are liquid at normal atmospheric temperatures, though the early ones still evaporate quite quickly in a tank without a lid.  The very heavy ones start looking like a waxy solid, I am not sure just where that starts.  The C5 through to around C10, again I am not sure exactly the upper limit, can be found in normal automotive fuels, then you go through distillate, heavy fuel oil and right up to tar.  Most of these are rarely separated into pure components, but are generally simply separated to a narrow boiling point range of components.

Of course, once you get to C5, there are more than 2 isomers, and by C6, the angles between the bonding points are such that the chain can go right around to make a ring, the characteristic of the aromatic hydrocarbons, no longer paraffins.  In fact even four carbons can form a ring, called naphthene, again not a paraffin.

The other possibility you might be wondering about is whether those carbons with four bonding sites could join to each other with more than one of the points.  The answer is yes, and double bonds, or even triple bonds are quite normal.  Well, in my world anyway.  Even C2 can form with a double bond, C2H4, called ethylene, the basic constituent of polyethylene.  And even a triple bond, C2H2 which you all know as acetylene.  These are called unsaturated hydrocarbons, as they do not have all the possible bonding points taken up with hydrogen.  You don't want either of these in your fuel tank as a rule.

Enough chemistry, we are model engine makers, not chemical engineers, so let's go back to butane, and the properties that govern how it behaves in our basic gas fired steam plants.  Now the two isomers of butane have quite similar properties, though not identical.  The gas containers here often have a mixture of both isomers of butane so we had better have a look at both.  The calorific value of each is very similar, the very small difference being due to a different energy of formation for each of the isomers.  With the same number of carbons and hydrogens to burn in the same reaction with oxygen this is not very surprising.  The big difference is in the vapour pressure.

Normal butane boils at -0.49 C at atmospheric pressure, and has a vapour pressure of 377 kPa at 40 C.  Iso-butane boils at -11.81 C at atmospheric pressure and has a vapour pressure at 40 C of 528 kPa.  All the pressures in this context are absolute pressures.  Atmospheric pressure has no relevance as these flammable gases must be kept separate from the atmosphere, except where you want it it burn.  You can divide kPa by 7 (or 6.89 if you want to be more accurate) to get psi, or just divide by 100 to get bar, depending on your preferred pressure units.

When you design a fuel tank, you have to design for the difference between atmospheric pressure on the outside and the vapour pressure in the inside.  And you need the vapour pressure for the highest temperature your tank is likely to be exposed to, perhaps out in the direct sunlight.  Here that would be at least 65 C, so much higher pressure than I have quoted for 40 C.  I prefer to buy a commercial fuel tank for safety sake, and I recommend you do the same.

The pure substances, whether normal or iso, behave in a very similar manner to water, just at different pressures and temperatures, and of course different latent heat, enthalpy and entropy values.  I will scan the chart and attach it tomorrow, it is publicly available information, but perhaps a bit harder to find than some.  Other substances with similar behaviour are all the refrigerants, the chlorinated hydrocarbons, ammonia and so on.  Depending on the size of your plant, and the temperatures you want, all these lighter paraffin series hydrocarbons are even very good refrigerants, just not so commonly used, as leakage and sparks must be absolutely avoided.  Sorry, another side track, so back to topic.

The other thing we need to know about the commercially supplied fuels is that they are not normally supplied as pure substances, but as a mixture.  I have three containers in my garage, one, as sold for those very common single or double burner portable stoves which use disposable containers, does not even specify the composition beyond liquified butane.  I suspect it is actually a mixture of normal and iso, as purification of just one isomer is more expensive.  It also claims to have an EN417 connection, but it has no threads.  I didn't know if that is really a legitimate variation, but it would not work with my fitting for transferring the fuel to the gas tank.

The others clearly label the contents, one has 44% n-butane, 29% iso-butane and 27% propane.
While the other has 75% isobutane and 25% propane.  Fortunately, we don't need to know the exact composition, though it is helpful for working out the calorific value with accuracy.  Again, fortunately, there is only 2% difference in the calorific value of propane and butane, as Paul has mentioned, and a much smaller difference between normal and iso-butane. 

While we really don't need to know the precise composition, it is useful to understand in a qualitative sort of way the general effect of the difference in composition.  And first we need to know how the components behave when you have different components mixed, as in those cans.

I have discussed before mixtures of gases, and how the total pressure is the sum of the partial pressures of each of the components.  In mixtures of gases, each component acts independently, as though the others were just not there.  But in this case, we have both gas and liquid, and the vapour pressure comes into it some how.  And in this case, the vapour pressure of the mixture ends up being somewhere between the individual components, a sort of average weighted for the composition.  So if you had say 90% n-butane and 10% propane, the mixture would be close to the vapour pressure of n-butane, but that propane will lift the vapour pressure a bit.  Similarly if it was 90% propane, the vapour pressure would be lowered a bit by the n-butane. 

You will note I have been cautious about implying any exact relationship here.  The really interesting thing here is that the vapour and liquid in equilibrium will have different compositions.  There will be a higher concentration of the lighter or higher vapour pressure component in the gas phase, and a lower concentration in the liquid, while the lower vapour pressure component will have a higher concentration in the liquid phase.  That difference is the basis of the process for separation, or fractionation, whether you are separating propane from butane, or alcohol from water.  Of course, that also means the lighter component tends to be used slightly faster than the heavier component, so the gas composition changes as the gas burns.  That's enough chemical engineering for our needs. 

I keep promising to get back to how the fuel behaves in our fuel tanks, but if you can understand the last two posts, it will be pretty easy to explain.  As this is already a long post, let's continue tomorrow.  I hope it is making sense so far.

Thanks for following,

MJM460

[steam guy willy]

Hi MJM ....talking about CO manufacture ..when i cook my tea on my gas stove  (town) my C0 meter starts off at 0 and after 4 mins goes up to 17 PPM . after about 18 mins it has reduced slowly to 0 again although the gas is still alight although at a lower setting. Also a bit more info from this old book about injectors...using two of them sometimes. Also on a lighter note we talk about hydrocarbons ...C3 ,C4 , C5,  i was wondering what the number is given to the flatulant gas Pantain that i often produce ??!!!!

[Noitoen]

C0 meters also respond to CO2.

[MJM460]

Hi Willy, great to have you back again.  Interesting that by using injectors in series, they could use some of the exhaust steam energy to reduce the amount of boiler pressure steam the main injector used to make up the feedwater.  Not sure it would be very practical in a model.  If you remember the sizes of the nozzles when we were looking at injectors, and now consider making them even smaller.  I suspect you would be in an area where very different drilling and reaming equipment would be required.

Hydrogen Sulphide is a well known gas for producing rotten egg type odours.  However, there is a whole class of sulphur compounds known as mercaptans which are added to otherwise odourless flammable gases to warn of their presence due to leakage.  Hydrogen sulphide itself is extremely toxic.  I am not sure it the mercaptans are similarly toxic, but probably the source of your observations.

The CO monitor in the kitchen is an interesting experiment.  I hope that stove has a good chimney over it.  I suspect what is happening is that when you first light the stove, the air in the chimney has uniform temperature so there is no, or at least minimal draft.  When you light the stove, producing CO2, water, and obviously also some CO, it spreads through the kitchen and is picked up by your monitor.  But the air in the chimney is also heated and the updraft starts.  When it is well established, it starts ventilating the room and sweeps out most or all of the accumulated CO.  Only surmising, of course.  You would need to monitor air currents and so on to be sure.  I am not sure if it is possible to adjust the air flow to the burner, to get more secondary air, or if the reaction rates for the combustion reactions mean some CO is just inevitable, perhaps due to cooling of the combustion gases when they get close to the kettle.

Hi Noitoen, welcome aboard.  That is an interesting comment.  I wonder what that means in these times when atmospheric readings in the cleanest areas on earth have reached 400 ppm.  Willy's reading was only 17 ppm.  Is the background level of CO2 taken into account by the calibration do you think?

Yesterday I was looking at the behaviour of a two component gas, and found that the mixture of gases has a vapour pressure in between the vapour pressure of each of the pure substances.  Now at the simplest level, but adequate for our purpose, we can think of the mixture as behaving a bit like a substance with that intermediate vapour pressure. 

Our fuel tank starts off at ambient temperature and the corresponding vapour pressure.  When we open the outlet gas valve and light the burner, we are drawing off some of the gas, which tends to reduce the vapour phase pressure.  To restore equilibrium pressure, some of the liquid evaporates.  This involves latent heat, and so requires heat input.  In the absence of heat input, the heat comes from the sensible heat of the liquid, and it gets cooler.  The temperature difference then drives heat input from the atmosphere, but the temperature difference is made evident by condensation of humidity from the atmosphere.  However, at the lower temperature, the vapour pressure is lower, and if it is a chilly day at the lake, the temperature and hence the pressure may fall so low that the burner does not get enough gas to generate the required amount of steam.  This observation is reported often enough in the modelling press, but I hope that you can now see that the explanation is simple enough.

There is a very important difference between our two component mixture and a simple pure substance.  If we have a pure substance, whether it be propane, butane or even water,  the liquid and vapour have the same composition.  However with a two component mixture, we find that the liquid and vapour each have a different composition.  There vapour will have more of the higher vapour pressure component than the whole mixture, while the liquid will have a higher concentration of the lower vapour pressure component.  As we draw off the vapour, we are burning more of the lighter component, so the composition of the remaining fuel slowly changes as we draw off gas for our burner.  This means the vapour pressure of the remaining mixture is always reducing.  Now the extent of this, and a detailed analysis of how the pressure changes with fuel consumption is beyond me.  We need a chemical engineer to join in and help out here. 

The simple message is that the liquified gas in the container is a boiling liquid in equilibrium with its vapour pressure, just as steam in a boiler.  The pressure and corresponding temperature is different for different substances, but they all behave in a similar manner.   As we draw as we draw off gas to burn, the latent heat must be supplied to continue the evaporation process.  This involves reducing the temperature of the liquid until the incoming heat from the atmosphere is sufficient to maintain the pressure. 

When our fuel is a mixture of components, the vapour pressure is somewhere between the vapour pressure of each component.  The vapour and liquid have slightly different composition, with more of the higher vapour pressure component in the vapour phase.  As we draw off gas from the mixture, the composition of the remaining liquid slowly changes, due to the concentration of the lower vapour pressure component, so that by the time the last drop of liquid remains, the vapour pressure at a given temperature is lower than it was at that same temperature at the start.

It is worth thinking about what this means to the gas in the container that we use to refill our steam plant gas tank.  We turn the container upside down, and transfer liquid to our gas tank.  As the liquid transfers, some must evaporate to maintain the pressure in the vapour space.  Again, the heat to evaporate the liquid comes from the sensible heat of the remaining liquid.  And again, the liquid and vapour are slightly different composition, but this time of course, the higher vapour pressure component stays in the can.  So each time we fill our gas tank, the average composition in the can increases in the percentage of the higher vapour pressure component.  This looks like it certainly maintains if not actually increasing the vapour pressure in the can if it is compared at the same temperature after filling steam plant fuel tank.

I hope that helps you understand the behaviour of the liquified petroleum gas or LPG in your fuel tank.  If the fall in gas pressure makes it difficult to maintain steam pressure, it might be worth exploring ways to gently heat the tank. Perhaps the tray under the boiler can be extended under the gas tank.  It may be worth experimenting with running some exhaust steam near the gas tank, but go gently, you don't want the tank heated much above about 40 C. 

Certainly, it is worth reading the composition data for the gas container you intend to buy.  In winter, you would choose, if you have the option, the one with more propane will have higher vapour pressure, similarly, more iso butane will result in a higher pressure than one with more normal butane.  In summer, you might choose a different composition, especially in a jot climate.  Yesterday it was forecast to reach 37 C, bit in my carport, under a roof and on the wall facing South, it was over 38, and the infra red thermometer showed readings up to 61 on the sandstone paving slabs in the back yard.  Not a good day to leave the gas tank out in the sun.

I am not sure that there is much more needs to be said about gas tanks and gas pressure, but feel free to ask if there is something I have overlooked.

Tomorrow, I will look at combustion issues, starting with those questions Paul wrote in yesterday.  We can see where the trail goes from there.

Thanks for dropping in,

MJM460

[steam guy willy]

Hi MJM an interesting experiment might be to attach a thermometer to the small propane/butane gas tank to see how much the temperature varies with usage !  and so another gauge to install on the loco backhead with appropriate control cocks to warm or cool it ??!! One could also use the cooling effect of the gas tank to cool the water going into the injector on hot days ??!!

[Kim]

Hi MJM,
I've been enjoying this discussion and finding it quite fascinating.  Brings back memories of the minimal chemistry I took in college!

I've got a question though about how you're saying that each compound in the mixture will have its own boiling point, which acts to bring the overall boiling point somewhere between that of the various compounds.  This makes sense to me in your description.  But when I think about adding antifreeze to water in your radiator, you want the compound to freeze at a lower temperature than just the water.  But if they each act independently, does that mean the water will still freeze at 32, and the antifreeze will freeze at a lower temp?  That can't be quite right, can it?

Kim

[steam guy willy]

Hi Kim, Unfortunately or otherwise there are allways exceptions that make the rule  ?!!

I've got a question though about how you're saying that each compound in the mixture will have its own boiling point, which acts to bring the overall boiling point somewhere between that of the various compounds.  This makes sense to me in your description.  But when I think about adding antifreeze to water in your radiator, you want the compound to freeze at a lower temperature than just the water.  But if they each act independently, does that mean the water will still freeze at 32, and the antifreeze will freeze at a lower temp?  That can't be quite right, can it?

Kim
[/quote]

[Noitoen]

About the CO sensor reading CO2, I speak from experience at work. We have a refrigerator recycling machine which shreds  the already decontaminated fridges. At first we remove the refrigerant gas and motor and then the rest is shreded in a controlled atmosphere.  We are obliged to recover the expansion gas of the insulation foam and since this gas now a days is mainly pentane, the shredder is flooded with nitrogen to minimise the risk of explosion. To control the atmosphere there is an array of sensors that measures the oxigen, pentane and Co. In case there's a fire high CO reading is an early warning and is controlled by nitrogen flooding. The atmosphere inside th shredder is circulated in a cryogenic system that freezes all "freezeable" gases and the cleened nitrogen rich atmosphere is returned to the shredding chamber. At the end of the shift and after a defrost cycle, the gas is recovered in high pressure cylinders.

On certain refrigerators the expansion gas is Co2 and when we shred to many in a row, the high concentration triggers the CO alarm. We know for sure that it's not a fire issue, it's just a false alarm.

[Admiral_dk]

I'm not exactly an gas sensor expert, but I do know that there are many different types and while some only are sensitive to one kind of molecule (gas), others are to sensitive to two or more. What kind is used in a piece of equipment depends on specifications and price.

This mean that Noitoen is quite right in his experience, while it is (almost) safe to assume (Ass-U-Me    ) that Willy's sensor only measures CO ....

[MJM460]

I am glad to see that people are finding the thread interesting.  It might have seemed a long way around, but I always feel it is more satisfying to understand the "why" rather than just be given another rule to follow.  Rules rarely always apply exactly as remembered. 

Hi Willy, on a large enough model to have the space, temperature measurement and a steam valve controlled by the operator, or better still an automatic temperature controller would be the way to go.  I tend to think in terms of perhaps a model boat, with no one on board, so something simpler and inherently safe is required.  On a really large system, you might actually draw liquid from the tank, and evaporate it using exhaust steam in a separate heat exchanger.  But perhaps not the simplest scheme for a small model, or for the inexperienced.  A full size boat, if not coal fired would be better oil fired than gas, as boats are holes in the water, purpose designed to accumulate heavy gases until they reach an ignition point.

Hi Kim, glad you are enjoying the thread and were reminded of your college chemistry.  The great thing about starting work in an ethylene plant is that you get to really understand that basic level, but don't have to get lost in the heavy detail of the rest of the chemical world. 

I hope that I have not led anyone astray by reminding us of the earlier statements about mixtures of gases, where the components act independently.  When we have two phase mixtures, meaning that there is condensed liquid and vapour in equilibrium, as in this discussion, the behaviour of the two components is not independent, and the pressure/temperature of the mixture is in between the temperature and pressure that each component would be if in separate containers.

My wife tells me I should not speak on the negative case, as people often overlook the "don't".  When I sent my kids to get my coffee, I always said "don't forget the sugar."  They always did, and I think the game became whether they could remember to forget the sugar.  Probably still sugar in the bottom of the washing machine from their pockets when they forgot.

However, we are now talking about simple mixtures of a family or series of hydrocarbons that mix quite freely.  What ever the concentration, the mixture boils over a distinct temperature range, and the vapour and liquid compositions are each different from the average for the whole container.

I am glad you asked about that antifreeze in water, because that is an example of a mixture used by many forum readers in their cars in winter.  Possibly even in the water hoppers of their engines.  In this case, the mixture behaves a little differently.  At a concentration of 65% glycol, it freezes at a single temperature of around -60, a bit like a eutectic alloy in metals, though I understand not easy to measure accurately for some reason.  But also, like metal solutions, either side of that specific concentration, the solution freezes over a range of temperature.  If you have excess water, the solution starts to form ice crystals at a higher temperature as water concentration increases, to the limit of 0 C for pure water, but is still not completely frozen until -60 C.  It forms a sort of slushy mixture at temperatures in between.  On the other hand, if you have excess glycol, the point at which freezing starts, increases as the glycol concentration increases, up to the freezing point of pure glycol, or -12 C, by separation of flakes of solid glycol.  But note the freezing point of that mixture is way lower than either glycol or water alone.  That slushy mixture provides burst protection for your pipes, providing there is room for a significant expansion as the solid ice crystals form as they can move around.  But if you want freeze protection, that is, no ice crystals to abrade seals and so on for pumping at low temperature, the higher temperature is the criteria, and you need the 65% mixture for lowest temperature protection. 

The mixture also provides a small increase in boiling temperature.  This is not linear with concentration, but a modest lift in boiling point until quite a rich mixture, then rapid rise in boiling point with further increase of concentration up to the boiling point of pure glycol.

Hi Noitoen, thanks for telling us about that recycling operation. It gives a whole extra meaning to the old phrase about whole of life costs, when a plant like that is needed to dispose of the old equipment.  Perhaps they need to use air to expand those foams.  I could specify a suitable compressor for them.  So the monitor response in that case might be due to overwhelming the sensor with CO2, compared with the more normal air with background levels of CO2.  I have also heard reports that they also respond to hydrogen from battery charging, also an environment different from that intended by the manufacturer.  I guess your plant was evacuated a few times before the cause was discovered.

Hi Admiral D K, depending on the detection principal, I guess most detectors would respond to some degree to a range of gases and it depends on what the manufacturer can do to identify a response to a particular component by the signal analysis software.  I suspect that in normal air composition, including the normal range of background CO2, it is more sensitive to the specified gas, otherwise it would be no use as a warning device.  I guess that in the domestic situation Willy has described, it would be foolish to ignore the alarm until the cause was completely understood.  Something definitely worth questioning the manufacturer on, especially these days when it can be quite easy to contact them.  Good to see that the standard definition for 'assume' is understood in your country too.

Hi again Willy, I hope those points have clarified the point about the apparent anomaly in behaviour, not an exception to a rule, but just requiring a better understanding of how it all works.  Admittedly still a bit of homework required before we all completely understand what is happening with the CO monitor.

Thanks for the quite searching questions, I hope that my replies have clarified the issues adequately.

I think that is long enough that I should not start a new topic at this point, but perhaps we can get back to Pauls questions on combustion tomorrow.

Thanks for following along,

MJM460

[paul gough]

Glad we are getting into a little detail with fuels. Some time ago I pondered the prospect of experimenting with hydrogen peroxide as an accelerator or enhancer for combustion with metho, (meths, ethanol), via its release of oxygen. As it is compatible, with ethanol and water, I conclude that it should freely mix with metho and enhance its performance as a generator of heat. I am specifically concerned with its performance using a wick type burner supplied with fuel by the standard 'chicken feed' system as commonly used on metho fired gauge one model locos, but may be applicable in other scenarios. As there may be some risk when experimenting with such things it is wise to PROCEED WITH CAUTION, research first before striking a match! This article might be a suitable starter for those who are interested, <http://www.odec.ca/projects/2007/park7l2/index.html>. MJM have you any knowledge or comment regarding the above application? I look forward to your reply on my previous enquiries. I have to say, thank you, again, for the time you must be putting in satisfying all your inquisitors. Regards, Paul.

[steam guy willy]

Hi MJM just a quick question ........wondering about thermal coal ??!!