Author Topic: Talking Thermodynamics  (Read 122301 times)

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1050 on: September 28, 2018, 10:20:27 AM »
Hi MJM, I missed taking in all of your comments  regarding temp. of outside cladding ,(26th Sept.). This one in particular, "...but suggests that even another layer might be useful." This then equates to a total of a 9mm thick cork layer of insulation on a 75 mm diameter boiler, quite a lot proportionally and of course any insulation that was inferior to cork would require a greater thickness to achieve comparable results. This I find interesting, as way back in the seventies when we were having our twelve inch gauge locos boilers inspected for their annual tickets I remember discussing minimum insulation thicknesses with the boiler inspector, as our oldest loco had no lagging at all on its belpaire firebox and we were contemplating lagging it with an asbestos plaster type lagging. This loco was a very touchy steamer with an undersize grate and 160 psi pressure, so needed precise firing and consequently any reduction in loss of heat would be useful. The Inspector stated that it would need to be at least 1/2 inch thick, any less and it would likely be a waste of time. Now, your 9mm of cork, presumably a better insulator than our asbestos plaster, is not very far off 1/2 inch. This seems to be roughly in accord with the inspectors opinion on a thickness that would be effective. Therefore, would it be reasonable to assume that most model boilers, even ones that might be considered satisfactorily lagged, are in fact deficient owing to the type material used, or, not having sufficient thickness for a given lagging material??? Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1051 on: September 28, 2018, 01:04:52 PM »
Hi Zephyrin, thank you for those comments.  I don't know if anyone else has made similar measurements, perhaps my career was unusual in preparing me with the interest and the theoretical knowledge to want to follow it through.  Of course to be really reliable, I have to do more tests to get an idea of repeatability, and preferably others should also get similar results.  With very small boilers there is a delicate balance of the requirements for heat conservation and the requirements for adequate water capacity.  It is obvious that the thickness of insulation I have used is not practical on a small locomotive where scale appearance is also required.

There is an issue niggling in the back of my mind, it is in both my text books, so I will have to go back to the calculations and see if I can tease out some answers.

Hi Willy, it is a fairly conventional marine type centre flue boiler with cross tubes in the flue, but the flue has a tee piece ear the far end, so that the stack exits through the top of the boiler just before the pressure end plate, instead of via a full diameter smoke box as on most locomotives.

I will come back to your other points tomorrow, as I have been out to dinner with family for my wife's birthday celebration, so am a bit late tonight.  Just remember that heat only travels from high temperature to lower temperature, not the other way.  I will continue from there tomorrow.

Hi Paul, adding more insulation will further reduce the heat losses, but there is definitely diminishing returns.  For the moment I have too many other projects in mind to be motivated to stripping of the wood strips and adding another layer before replacing them.  Really not a huge job, but my progress is very slow due to other activities.  With you and Zephyrin both interested in the similar area, I will after all, go back to the calculations over the next few days and see if I can work out some answers.  I might at least be able to see if there is enough potential for it to be worth while.  And also I want to do a few more runs to check the repeatability of the results.

Thank you especially for the interesting comments, and thank you to all who have looked in,

MJM460
 
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1052 on: September 28, 2018, 09:03:12 PM »
Hi MJM, a further bit to add to the heating the boiler...as the cooling of a boiler always takes far longer than the heating up of it why does the addition of the constant heat/time not make the boiler get really really hot very quickly ?? or is this another silly question ??
 Also if you wanted to win one of the IMLEE  engine efficiency  events could you use a 3 1/2" boiler in a 5" gauge loco using up to 5/8" lagging ? Just a thought. Also just a thought about the water gauge that may be counter intuitive  as the steam pressure enters the gauge from the top and is in effect pushing against the liquid in the lower half is the level the same on both sides of the backplate.? i'm thinking that as you cannot compress a liquid the steam pressure will tend to push it back into the boiler ? We know it doesn't but all the text books on fluid dynamics go to great pains to explain how FD actually works in practice !!

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1053 on: September 29, 2018, 05:15:50 AM »
Hi MJM, I assume your 3 inch boiler is a 4 bar or 60 PSI one therefore 145 C internal temperature. Now if we took your 9mm cork insulation thickness as best practice for this temp/pressure, and knowing there is 20 degree C. increase at 100 PSI (approx. 7 bar), would you consider it worthwhile adding another layer (3mm) of cork to compensate for this 20 degree increase. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1054 on: September 29, 2018, 11:14:31 AM »
Hi Willy, back to your questions roughly in order.  Some people talk about a Zeroth law of Thermodynamics - "if there is no heat transfer between to objects in contact, then they are at the same temperature".  And of course the converse is also true.  So, if the boiler could get to the flame temperature, they would be at the same temperature, there would be no further heat transfer, so the boiler could not get any hotter.  But first it takes a very long time to actually get to equal due to the small temperature difference as it gets close.

With your electric boiler, all the heat dissipated in the heating element must go into the boiler, and the element temperature just increases until it transfers all the heat.  Now if we get silly, and insulate an electric boiler with thick layers first of Kaowool, then rock wool, then encase that in polyurethane and so on until we can assume perfect insulation, then the temperature increases without limit.  However in the real world, something will break and so the temperature only increases until something breaks.  Possibly the boiler shell starts expanding like a balloon due to the lower strength of the material at high temperature,  then bursts, or the silver soldering melts, or the electric element fails, by melting like a fuse. 

However, a fired boiler, gas, spirit, coal, wood or whatever is different because you need air for combustion, and you have to vent the combustion products, or the flame is soon extinguished by lack of oxygen.  The air and the combustion gases absorb all of the heat from combustion, and so limit the temperature reached.  Not just the air necessary for combustion, but there is normally excess air required to ensure that complete combustion occurs. If less heat is transferred to the boiler, the combustion products just retain more of the heat, and carry it out up the chimney.  So, if I insulate my centreflue boiler very well, even if I do not allow any steam escape, the boiler will get to quite high temperature and pressure, thus reducing the temperature difference and hence the heat transferred between the combustion gases and the boiler.  With less heat transferred to the boiler, the combustion products remain hotter until eventually they are carrying away all the heat of combustion.  It could potentially get to the point where the copper is no longer strong enough, or the silver soldered joints.  And of course, it is for just this reason we must install a safety valve, set to limit the pressure to some safe level.  As soon as it opens, the escaping steam carries away enough heat to prevent further accumulation of heat or pressure (providing the valve is big enough).

Now with your electric boiler, it is relatively easy to use an electrical signal from a thermocouple to interrupt or modulate the electric power input to prevent the safety valve being needed at times of low steam demand, however for a gas fired boiler, we need to use the boiler pressure to press on a diaphragm and close a needle valve in the gas supply to the burner.  In principle it is simple, and many of the usual books show drawings of how it is done.  However, I have no illusions about the difficulty of tuning of spring stiffness and diaphragm size necessary to get it to work.  And the pipe work to connect it up.  Full size equipment is readily available and I believe such units are available in model sizes.  For me the problems of connecting it up to the gas supply without leakage would be more than enough to dissuade me. 

With spirit firing it is a little easier, though you might need a pilot wick to relight the burner if the fire gets too low, but at least minimal pressure to deal with.  I am not sure how you would do it with coal.

You are right, cooling is slow, but heating is quite quick, depending on the capacity of the burner.  My little burner consumes about 30 g (1 oz) of gas in about twenty minutes, so it is not very big.  It is not that cooling is inherently slower, it is just that it depends entirely on the temperature difference to the air, and of course, this temperature difference reduces as cooling proceeds.  By the time it gets to less than twenty degrees difference, even I run out of patience to watch it cool further.  In fact, a bit earlier than that.  However, for heating the temperature difference results from combustion of air and the boiler, so is greatest when the boiler is first lit up, but more than adequate for much higher heat transfer rate than the cooling process.

A level gauge is connected to the boiler in the steam space at the top and the water space at the bottom so the only pressure difference is due to the density of the water.  It is this that drives water into the bottom of the gauge to nominally the same level as the inside.  I say nominally, because in our model sizes, surface tension effects can cause the level to creep up a bit in the gauge.  We are looking for such small levels that this can be quite significant.  Other less intuitive effects also seem to occur in a vigorously boiling situation, so it is desirable, but not always practical to locate the bottom connection in a quieter part of the boiler.  No insulation on the glass of course, so the heat lost from the glass where the steam is on the inside causes some steam to condense.  This runs to the water filled section, and the tendency to increased height actually drives some of the water back into the boiler space rather than increase the level in the glass.  And more steam enters the glass from the steam space to replace the amount condensed.  The steam pressure in the glass is always the same as in the boiler

For locomotive efficiency, I don't think I would use a smaller boiler, but rather, I would consider sacrificing appearance and add external insulation, with suitable cladding for appearance of course.  But the first step would be to try and work out the sources of inefficiency, and how much heat loss was occurring to see if it was significant in the total.  I would expect that mechanical friction would be a major factor along with firing technique.  Excess air would likely be the biggest source of loss.  It is a very good trick to consistently fire a small coal fire at an efficient air fuel ratio.  I will leave it to others to tell us more about the technique.  For my part, I would start/continue my efforts to improve my engine making skills to get the smoothest running motion works possible.  I have a long way to go.

Hi Paul, I did overlook part of your insulation comment the other day.  On the question of minimum useful thickness of insulation, there is the issue of scale.  Any added insulation will normally be expected to reduce heat loss, but whether this is significant depends on the heat rate of the boiler.  Saving a few joules per second on a full size boiler would not be noticed, but it might reduce the severity of burns if the surface is accidentally touched.  Of course if the surface temperature is too high, the reduction might be a bit academic.  However that same few joules per second on the miniature locomotives that you and Zephyrin run, is a significant portion of the heat input, so quite important.  In full size, I don't believe we ever used less than half an inch thickness but part of that is simply for mechanical handling of the material. 

I don't think I would describe any model that worked as deficient, but it is always interesting to think about ways a model might be improved.  It turns out that very small differences in insulation properties make very little difference overall, once convection from the outside of the material is considered, however they sometimes have very different usefulness at different temperatures.  So cork and Kaowool have very similar insulating properties but you don't want a flame impinging on the cork.  Kaowool will stand very high temperatures on the inside of a large industrial furnace, and not easily broken during building the furnace.  Before the advent of those ceramic fibres, firebrick and refractory cements were used internally, despite relatively poor thermal properties, and other materials used outside the furnace walls to provide additional and more useful reduction of losses.  So those small models, whether insulated with cork or Kaowool would be about equal, to the best of our ability to measure, so long as the flame did not impinge on the cork.  Differences in insulation thickness are more significant so long as the boiler remains large enough to be useful and the appearance acceptable.  As always, many compromises are involved.

To determine the effect of the boiler temperature increase from 145 to 165, I would calculate the temperature difference to the atmosphere, say at 15 degrees, so 130 and 150.  So the heat loss would increase very roughly by 150/130, say 15%, but this is 15% of the already very low heat loss due to the insulation already added, and you would only save a small part of that 15 %.  So there comes a point where the diminishing returns of extra insulation soon make it not worth chasing.  Maybe the next three mm, but probably not the next one.

I think that brings us up to date with the issues raised so far.

Thanks for looking in,

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 #1055 on: September 30, 2018, 01:47:46 AM »
hi MJM, once again thanks for your extensive reply. another question... we can heat up things very quickly  but is there a way of being able to cool things down very quickly ? perhaps spraying with liquid nitrogen or something similar ? perhaps there is no need to cool things down fast though !!

Wily

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1056 on: September 30, 2018, 04:42:44 AM »
Afternoon MJM & all.........still following on with each daily post

Whilst I thought had a pretty good understanding with the ~~3 degrees C steam temperature 'improvement' with hefty insulation of my steam line to the engine, I was a little taken back with MJM's test results  :Director: with the boiler insulation from the past week

Clearly when I placed the single layer of wooden planking to my boiler, my mind did not understand :Doh: the actual importance or need of additional insulation thickness

One point for any future boiler build is to specify all threaded tapping's to have a face protrusion of ~~6 mm  to the boiler shell

I could modify a few 12 point Crows Foot Ring spanners by increasing the set then and also cut out a section.....[as per the image].....this would be the only way of tightening the fittings with the protruding insulation

Derek
« Last Edit: September 30, 2018, 05:09:53 AM by derekwarner_decoy »
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Offline 10KPete

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Re: Talking Thermodynamics
« Reply #1057 on: September 30, 2018, 04:46:35 AM »
It would be a large undertaking but the boiler could be stripped and extensions soldered on...?

Yes, large.

Pete
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #1058 on: September 30, 2018, 07:18:13 AM »
Hi MJM, When I used the term 'deficient' in the previous post I was inferring that the insulation was often, deficient, not the model. Yes, it is very often the case that people want to follow scaled prototypical dimensioning with their creations, but there can be a cost in reduced performance.  To me insulation needs to be thought of volumetrically like other boiler parameters, not as a thin more or less 2 dimensional skin. One of my dreams would be to have two locos exactly the same, but with different boiler diameters, say one with a 4'' (100 mm) dia. barrel with more or less standard model practice lagging, then the second with a boiler barrel of say 3" dia. (75 mm) but with a minimum of say 12-15mm of insulation. Then apply some standardised tests to ascertain if the reduced boiler volume etc. but superior insulation would generate anywhere near the same steaming rates. If the results confirmed that more effective insulation provides significant benefit then it might not be always necessary to try to squeeze in the biggest diameter boiler possible onto the frames. The question is, whether the improvements that you saw with your small gas fired marine/stationary boiler translates to a larger coal fired locomotive models. I think this insulation investigation is creating some real interest and curiosity amongst some modellers and over time might spark some revealing investigations regarding larger models, at least I hope so. Regards Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1059 on: September 30, 2018, 11:50:03 AM »
Hi Willy, liquid nitrogen would give a healthy temperature differential so would definitely increase the cooling rate.  In fact, as it would be a boiling liquid against the shell, it would not only provide the increased differential, but would also have a very high film coefficient compared with natural convection by atmospheric air, which would also help. 

Of course it would not be a recommended procedure.  The copper would be OK at the temperature, though there would be a danger of cracking due to temperature differentials causing stress at changes of section such at bushings or end flanges, and I don't know about the ductility of the silver soldered joint at the temperature.  Apart from the very real danger of very severe frost bite if you spill any on yourself.  So in principal yes, it would speed things up, but in practice I agree with your last comment, there is no need to speed up the cooling.

Hi Derek, part of the issue with the small diameter tubes we use as model steam pipes is that the ratio of surface area to volume is high, so heat loss is significant.  I don't know your boiler steam temperature, but the engine can only extract heat down to 100 C unless you have a condenser, so I suspect your three degrees loss avoided would deliver significantly more heat to the engine so increase the engine work output.  If your steam temperature is say 130C at the boiler, 3 degrees would be 10% of the potential work output would be lost.  So the savings are very worthwhile.

However the significant difference with any flue tube boiler is that the lost heat is actually heat from the steam space, having already transferred through the flue tube wall to the water.  So avoiding this loss by insulation does not require any further heat transfer at the flue, it is just straight extra steam available for the engine.  In these small boilers, the heat saved is significant compared with the burner heat release.  But the insulation does make getting at the fittings more difficult, and as you say, it is a very good idea to make the bushes with extended length on the outside to allow for the insulation.  It is difficult to design a spanner for fittings buried deep below the insulation surface, and tube spanners really only work for a filler plug.

Hi Pete, great to have you on board.  Yes a huge job to replace the bushes.  One approach is to make an adaptor bush to screw into the boiler bush with the appropriate thread the outer end to replace the fitting.  Has the disadvantage of an extra joint to seal, though it could be caulked with soft solder if necessary, I assume.  The better approach is to allow for insulation thickness when designing a new boiler.  In fact, that is the approach we used on full size vessels in the oil industry where pressure vessel nozzles were made deliberately longer where insulation was to be fitted.

Hi Paul, the truly definitive demonstration would be your two locomotives as described, but a lot of work unless you were planning on building two anyway.  And then make provision for replacing the less successful boiler.  But for my part, I would be happy to now try the experiment on a 4 inch boiler, and extend the results to other similar size boilers.  It is easier to do the measurements on a typical stationary boiler, though in your scale, the whole locomotive could be placed on the scale before and after to measure the water consumption.  Fuel could be done the same way especially if the fuel tank is easily removed.  Not so practical on a larger coal fired model.  However, I suspect the coal fire puts out a lot more heat than a small spirit or gas burner, so the same heat loss or savings might not be so important.  Remember, the importance of that heat saving is not because it is so large in absolute terms, but because it is large compared with the burner output.

I like your idea of designing the boiler as a whole with the insulation as an integral part if it is going to be applied.  But the other issue with a locomotive where the outside size is constrained by scale considerations, is making sure there is room for adequate water volume, especially when the model is expected to run around the track largely unattended.  So insulation has to be balanced with the possibility of just making a bigger burner.

With the interest in the insulation tests, I can see that I have to proceed with two or three more tests to demonstrate repeatability, one test demonstrates the principal but is no use if it can't be repeated, not to mention considerable embarrassment.  At least the test is quite simple.  The main assumptions are that the burner heat release rate is constant, or at least close enough, and that all the water evaporated happens during the timed steaming time.  So nothing to obtuse.   And I will also dig out the books again and see if I can produce supporting calculations.  The calculations are the best way to explore how it all works on a larger or smaller boiler to minimise unproductive testing.  So I will be busy trying to fit this in to my normal schedule for the next week or more.

Thank you everyone for your interest.  And all the comments are especially appreciated.

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1060 on: September 30, 2018, 05:14:15 PM »
Hi MJM et al, thinking about a smaller boiler with more insulation....................as there is less water in the boiler available for steam production to run an engine ,, would the continuous topping up with cold water have a detrimental effect on the efficiency of the system ??  Diminishing returns ??!!! Just a thought/

Willy

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1061 on: September 30, 2018, 11:18:00 PM »
My actual steam involvement is 5" Gauge 'scale' here in Wollongong....& I spend a great proportion of Public Running Days on the Station ...handing over scale sized buckets of Char....and filling tenders with cold tap water

This invariably is fed by axle pumps or injectors directly into hot boilers

Talking with fellow members, there is no apparent concern   :hammerbash: to preheat the boiler feed water...'as just another shovel of Char will do'

One continuous horizontal U shaped spool would have no joints to leak & could be secured or placed against the boiler shell proper and ultimately under the lagging shell so unseen?

I suspect other 5" & 7" Gauge members up North in Paul's steaming area would have similar thoughts

From this, scale sized model marine boilers would appear to be the winner here if builders 'and manufacturers' were made aware of the benefits of  insulation & the need for longer protruding boiler bushes etc.....

Derek
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1062 on: September 30, 2018, 11:34:49 PM »
hi Derek..that sounds like a good idea as a bit more time with the initial steam up but ok after that..!! I had not thought of that actually !!

Willy

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1063 on: October 01, 2018, 01:57:39 AM »
Hi MJM, Again having no experience with the burner type in your 3" boiler, so, another question. Is this burner designed purely for one output, ie. maximum, or is there a capacity to reduce its output somewhat? If so could you give me a guesstimate of what percentage of maximum it might operate effectively at? My thinking is that two of the 4" boilers from 'Minature Steam' steamed in parallel would be compact and convenient for running a larger 'early design' stationary engine at say 30 psi. 60 rpm. As 'M.S' state the 4" boiler is matched to their high speed twin cylinder model engine, two boilers in parallel should have sufficient capacity to run an engine with a swept volume of twice, (perhaps a bit more), that of one of these engine units, but with lower speeds and reduced pressure applying. Being able to adjust burner output would be necessary for this application.

Hi Derek, Yes, it has been also my observation that the loco modellers don't concern themselves much with trying to evolve more efficient boilers very much. It is easy, as you stated, to just throw in a bit more fuel, or, build bigger engines. This seems to be more the case nowadays where clubs running days are principally for the public and where operational requirements of hauling people seem to have taken over from other more sedate concerns like running extensive tests. As for steamers up here, as far as I am aware there are none near Cairns. I believe there is a 7 1/4 track down in Townsville, but thats too far at 350 klm's for me to drive nowadays. Maybe the marine/stationary modellers like yourself and MJM can help lead the way in convincing others that attempts to improve insulation are a worthwhile exercise rather than just burning more fuel.

Hi Willy, Just as an example, my tiny gauge 1 'Lion' loco boiler has about 80 cc capacity when totally filled, boiler is 35mm Dia by 100mm long with three 8mm fire tubes and generally this means, very roughly, 40 to 60 cc is the volume one would aim at maintaining. The loco has an eccentric pump that is usually on fully, (except for a slowly forming drop at the return to indicate pump is working), when hauling a train and feed water goes into the steam space at the rear end of the boiler without any preheating. The engine when running well under load usually has a feather of steam from the safety valves, so things seem to be well balanced.

Regards to all, Paul Gough.
« Last Edit: October 01, 2018, 02:01:02 AM by paul gough »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1064 on: October 01, 2018, 02:15:49 PM »
Hi Willy, you have cleverly brought us back to the connection between feedwater heating and efficiency that we have looked at before.  Some of the problem is the variety of meanings people apply to "efficiency", which are often more about performing well in some undefined way.  I like to stick with the concept of ratio of work output to input energy, although efficiency compared with some theoretical output, such as adiabatic efficiency is also a recognised technically acceptable definition. 

All the heat necessary to heat the water to the steam raising temperature has to be supplied by the burner, it does not matter whether this is a batch process such as you and I employ when we first fill the boiler with cold water, then heat it up before continuing to evaporate the water at steam temperature, or if it is a continuous process such as Paul's locomotive with an axle driven feed pump trickling in the cold water on a continuous process.  The steam tables tell us that heating the water to boiling point takes about 15% or more of the total heat required to produce steam.  The simplest way to increase efficiency is to reduce the losses to atmosphere.  The two largest sources of loss are the exhaust steam and the flue gas.  If we can find a way to extract more heat from the flue gas, or from the exhaust steam after they have been used to the maximum extent for raising steam or doing work.  One way of using some of this waste heat is to pre-heat the feedwater, so reducing the heat that has to be supplied by the burner.  So providing we are capturing some of the waste heat, and not just taking heat from the boiler, feed water heating will improve efficiency.  The low temperature of exhaust steam limits its usefulness, and the limit for the flue gas is the dew point of water in the flue gas which tends to result in an unacceptable degree of corrosion of boiler and furnace components.

Another way of increasing efficiency which is used in full size practice is to preheat the combustion air usually with flue gas but potentially some heat could be usefully extracted from the exhaust steam.  This directly reduces the amount of heat absorbed in bringing the combustion air up to the temperature exiting the burner.

And of course, as in my current experiments, we can insulate to reduces losses.

It would be nice if the feedwater could be heated by a coil in the reservoir, however, there is a problem with pumps and injectors, in that they do not work well with hot water.  The issue is called Nett Positive Suction Head, or NPSH, which is the pressure in height of water column above the equilibrium vapour pressure (or boiling point) of the water, necessary to push the water into the pump without causing vapour formation.  Effectively it means for model steam plants that the heater has to be at boiler pressure, after the pump, so the pump handles cold water.

Hi Derek, I think the pragmatic view is that "another bucket of char" is probably the best answer.  Improvement of efficiency is more about the satisfaction of doing it well, rather like a good coat of paint, though that at least is visible for all to admire.  In both cases the engine will work perfectly well without it.  Efficiency really only becomes important in full size passenger and cargo services, where the cost of fuel is a significant part of the cost of providing the service.  Or on really miniature engines where a larger burner may not be practical. 

When it comes to designing a feedwater heater, it is important to consider where the heat actually comes from.  To improve efficiency it must truly come from waste heat sources.  If it just comes from the boiler it simply reduces any effects of sudden temperature gradients where it enters the boiler, it does not actually improve efficiency.  Certainly bushes that protrude further from the shell are helpful if you apply insulation, however, I suspect the dreaded cost issue limits any commercial availability.  It is very practical if you are designing or building a new boiler.  Extended bushes would not be a significant change from the boiler integrity point of view.  Otherwise screw in adaptor bushes with a good gasket and a tight joint would fill the gap for an existing boiler.  On thinking further about it, I suspect that solder calking is not really necessary.

Hi Paul, my 3" boiler is just the smaller model of the 4" one from the same source, with the matching burner.  I probably should have bought the larger one, but I was thinking of how big a boat would be needed to accommodate it, and still hope to build one day.  But that is a whole other story.  The burner will certainly operate over quite a range of gas pressure to provide the turndown you require.  There is no fundamental difference between lowering the gas pressure at the burner by cooling of the liquid as the gas is drawn off, or by partially closing the shut off valve.  I normally light the burner as recommended by holding a match to the chimney while I open the gas valve on the gas tank.  I tend to stop opening the valve when the burner lights.  Sometimes I find the stack temperature decreasing, suggesting reduced heat input as the gas bottle cools and the pressure gets too low.  When I then open the valve fully the temperature comes up again.  So I guess I am unintentionally turning the burner down in that early heating period.  Then again as the run continues as the gas liquid continues to cool, it is very clear that the gas flow is not constant, and this is the biggest source of uncertainty in my testing.  It will take a lot more work to eliminate this source of variability.

Boilers will run perfectly well in parallel, so long as you are particularly careful to balance the water levels and steam pressure in each.  The combined steam output will supply a larger engine as you require.  This involves preferably installing a balance line below water level, which is quite separate from the feed water inlets.  My three inch boiler does not have the necessary extra bushes for a balance line plus a water inlet, so if a feed pump is required it would be best to take care that the water inlet is completely symmetrical to each boiler.  The steam lines should also be completely symmetrical, to ensure the pressure remains the same in each boiler.  You would have a single steam valve on the combined outlet line.  These points are required as very small pressure differences in pressure would result in a significant level difference in model size boilers.  And it is most unlikely that the steam generated in each boiler is exactly the same, so water balancing and pressure balancing are both necessary to avoid low or high level in one boiler.

That gauge one boiler sounds like a very well developed and well balanced unit.  You must be very proud of that one.

Thanks everyone for following along,

MJM460
 
The more I learn, the more I find that I still have to learn!