Author Topic: Talking Thermodynamics  (Read 108985 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #690 on: January 31, 2018, 03:39:13 PM »
HI MJM, Here is another engine that looks interesting... and is fuelled by coal tar...I have tried to get all the text in a reasonable size to read for you...this is from Graces Guide ...the Engineer 1888....Jan 27th...An early diesel engine ??!!
« Last Edit: January 31, 2018, 03:43:58 PM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #691 on: February 01, 2018, 11:29:25 AM »
Hi Willy, I can see that you are getting full value out of that Graces Guide.  I assume you have found it in the Webb with the normal search?  Not had time to explore that yet.

It is indeed an interesting engine, internal combustion with regeneration.   With tar as a fuel, I imagine there would be a lot of cleaning out required. Interesting also that the write up looks at Carnot efficiency.  Really quite fascinating writing in that magazine, good technical content but very readable.  It will take quite a bit of study to follow the explanation completely.  Really amazing that the inventors were able to develop such a complex engine with the knowledge, data and calculating power available at the time.

It will be quite a challenge to follow the description right through, but unfortunately no time to do it at the moment.  I am having enough trouble making time for the calculations on my own engine tests.  Making progress, but I need to do some checking and construct the graphs.  Sorry not to have enough progress to report this evening.

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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #692 on: February 02, 2018, 11:34:52 AM »
Well I have some initial results from the boiler tests on my second boiler. 

You will remember that the first boiler was just a simple pot type with only the lower part enclosed in the firebox, much like some of the little Mamod models. 

The second boiler is larger, made from 2" copper tube and a little longer, and is enclosed in a full furnace and firebox with a stack for the flue gases.  The furnace is not a work of art.  I made it out of an empty coffee tin from work, as a cheap way to try my skill at the sheet metal work before I started on more expensive material.  I also wanted to try the concept.  It has no external insulation.  I have purchased some manifold insulation from the car parts store, and I am in the process of bending that up to fit on the outside of the furnace.  It can only get better from there. You can see it in the first picture attached.  The boiler is fitted with extended bushes for the filler plug and safety valve and these are used to support the boiler from the top of the casing.   

Also in the photo, you can also see two burners, the 50 mm one I made for the small boiler, and the 90 mm one I made to get more heat input to this boiler.  Both use the same fuel tank.  The thermocouple, the thermowell that accommodates the thermocouple and also acts as a filler plug are all included in the photo.  It was quite warm still when I took the photo at about 8:30 pm!  The boiler is connected to my horizontal slide valve mill engine.  I am obviously still very much a beginner at the machining and engine building, but it works, so a satisfying early effort.

I removed the boiler from the casing for the second photo, so you can see the heating area details.  Instead of being a simple pot type, it has four 1/4 inch diameter longitudinal water tubes to increase the heating area.    The steam is taken from a bush in the centre of the boiler, using a banjo fitting, and the steam pipe, of 3/16 tube passes two full turns around the firebox as a superheater before exiting the firebox to the engine via the lubricator.  The pipe is insulated for much of its length outside the furnace with silicon tape.

I knew from previous runs that it made steam quite quickly, but only when I carefully measured every 10 degrees during heat up did I realise just how quickly.  First trial reached 100 deg C in 3 min 10 sec, and was steaming at 110 deg C in 3 min 30.  I was flat out getting the readings, and I only had to tap the iPad at each temperature, so the times could be written down later.

I tried a second time after it had all cooled down.  Spectacular but not a great success.    First two readings ok then I blinked and it was already over 60, and while I did a double take at that, it was at 100 in 2 min 30 seconds!  I am wondering if I had spilt a little Meths while filling the burner, and it flared up as the burner got going, thus increasing the heat rate for a short time.

It took until 3 min 20 sec to get to 110, but it turned out that the slide valve had not seated and the exhaust separator was blowing steam.  No really useful results from that run, it was too difficult to get the readings with sufficient timing accuracy.  I think the valve might be sticking due to the oil I was using, as it had not played up previously.  I now have some real steam oil which I want to try (after a thorough clean) before another run, and before thinning the nut a little so it lets the valve move a bit easier.

However, I did try a third run, but using the little 50 mm burner for a slower heat up.  I am interested to get steam production figures both ways.  Of course the lower heat input really restricts the steam production, so it's just an experiment.  The quick heat up is not an issue when you just want steam to run your engine.  It is better to have a higher steam rate, or higher pressure for the engine, but I am wondering if the smaller burner results in less proportion of the heat lost up the stack or boiler efficiency.

Basically the bigger burner puts out about 1800 watts compared with about 600 for the smaller burner.  Clearly the extra heat in the firebox affects the burner performance in addition to it just being a larger burner.  The boiler takes about 200 g of water at each fill compared with 130 in the smaller boiler, so the bigger boiler has more heat input per kg of water.  Similarly, the larger boiler has a mass of 770 g compared with 350 g for the smaller one.  So it is not surprising that larger boiler heats up more quickly, and produces more steam.

I have some graphs showing the heat up performance (just have to reduce them to size), and will calculate the performance parameters to compare the two boilers.  Hope to be able to produce these for tomorrow.

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 #693 on: February 03, 2018, 02:50:18 AM »
Hi MJM,  lots of info there and lots of further experiments in the pipe line... would lengthening  or shortening the chimney have any effect on the way the burners perform ? Also has the temp gauge got two separate probes and what make /model is it and are they still available ?...

Offline MJM460

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Re: Talking Thermodynamics
« Reply #694 on: February 03, 2018, 11:20:52 AM »
Hi Willy, the information has been a bit slow developing, you are seeing it in real time.  I made the mistake of trying a new spreadsheet program, Libre Office.  I am sure it will be good when I am more used to it, but it did not like the way the iPad spreadsheet, Numbers, handled graphs or times.  So I had a lot of conversion to do that would not have been necessary if I used it from the beginning.  But the iPad on the knee, while watching the Tele, is too much of a convenience to ignore.

That temperature instrument is a Digitech, model QM1601.  I bought it in Jaycar, and I expect they would still have them if I wanted another.  They are the local store for electronic components and other similar stuff, but I suspect they are mainly or only Aust and NZ.  As they do mail order, it may be worth looking them up.  But it is not a special brand and probably no better or worse than a similar instrument with another label.

Yes, it has two thermocouples, and the facility to read with a resolution of 0.1 deg.  Resolution is not the same as accuracy of course, but it helps me anticipate when the whole number will change, especially when things are happening rapidly.  With two thermocouples, it has the facility to read either one, or it can read the difference in the two temperatures, so quite useful for the current experiments and similar.  Last time I checked it at zero and 100 C it seemed pretty close, but I suppose I should do it again.

I have made some progress with the heating and steaming tests for my second boiler, the one I described yesterday.  I have attached two time-temperature graphs, one with the 90 mm burner I made for this boiler, and just to experiment with heat rate, I have included a run in the same boiler with the 50 mm burner I made for the simple pot boiler.  I have just noticed that I should have used the same scale for the time axis to make the two easier to compare.

You can see many similarities with the previous curves for the smaller boiler.  I think it is clear that the burner is a bit slow to warm up then takes off at a greater heat release rate, then the temperature rise rate falls of as the boiler starts to steam with that sticky slide valve.  Perhaps I need a proper isolation valve.  Then at least I could determine with some certainty when steaming actually starts. 

I had a look at the calculations for the heat absorbed in each temperature interval, but really they were all over the place, and difficult to make any sense of.  Yet the overall result and the temperature rise seemed reasonable.  So I have not tried to produce the graphs, as it does not seem likely that I can provide any reasonable explanation for the behaviour.

I also had a look at the overall parameters, the one we were discussing in relation to K.N. Harris's
Boiler parameter.  So first I calculated the heat transfer area for the second boiler.  As with the little pot boiler, I assumed half the shell area as having water or boiling on the inside and so much more effective than the top part for heat transfer.  Then I calculated the area of those four water tubes.  It is interesting that on small boilers like this, they make a big contribution to the heating area, and in fact nearly double the area compared with the bottom half of the shell.  The ends of the shell are inside the furnace enclosure so I included half their area as well.  So 0.0165 m^2 for the shell, plus 0.0136 m^2 for the four water tubes (1/4 " tube by 170 mm long), or 0.03 m^2 in total.  This compares with only 0.0078 m^2 for the simple boiler.

Well, with nearly four times the heating area, I wanted to compare the steam production.  I was easily able to calculate the average heat output from the burner fuel consumption, about 600 watts from the small 50 mm burner, and 1800 watts from the 90 mm burner.  Clearly the heat output is about more than just length, but then I did not set out to just lengthen the same design, the larger one is a bit wider and has different size and numbers of holes in the two side sections.  It is likely that the larger heat output also causes the burner to run a bit hotter, and hence to vaporise and burn more Meths.  In fact even the small burner, when used in the larger boiler, was nearer 630 watts, so a little more than when in the small boiler.

The steam production from the 90 mm burner in the larger boiler was 0.25 g/s or 0.86 kg/h (remembering that the steaming time was only 5 min).  But this is only about 50% more than from the smaller burner in the smaller boiler, despite three times the heat output.  The heat absorbed in the steam was only 570 watts of the 1800 released by the burner, so the boiler efficiency down to  around 30 %.  Clearly those water tubes did not contribute much to the heat transfer or efficiency.

It is interesting to compare this with the little burner in the simple boiler where the steam production was about 0.17 g/s in both the simple boiler, and in the larger water tube design.  In both cases the efficiency was nearer 60%.  Clearly again, all that extra area did not add much to the heat transfer.

Finally, I calculated that boiler parameter for each of the three cases. 

About 70 kg/hr.m^2 for the simple pot boiler.

Only 20 kg/hr.m^2 for the same 50 mm burner in the larger boiler firebox.

And 30 kg/hr.m^2 for the larger burner.

That is a lot of figures to take in.  With things happening so quickly, I did not get sufficiently accurate or sufficiently many readings to justify analysis in finer detail, though I will still look at the cooling curves.

More important to think about what it all means.  I will think about that for next time, I am starting to see some valuable learning possible, if I can sort these out and follow through with some further experiment.  You see, all the theory will help understand the direction, but will not tell you some of the basic answers, unless you perform a suitable experiment to confirm it.  To really understand things, you need some theory, and the theory needs to be supported by experiment.

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 #695 on: February 04, 2018, 03:25:53 AM »
Hi MJM thanks for the new info and i will be seeing what else comes up with more tweaking ! meanwhile saw this from the Melbourne show....had me scratching my head.......

Offline MJM460

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Re: Talking Thermodynamics
« Reply #696 on: February 04, 2018, 11:47:03 AM »
Hi Willy, the thermo-acoustic engine operates on a novel principle, though I have no idea how it works.  Someone has been very clever in converting from a physical principal to a working engine.  The Melbourne show!  I would really like to go, but I believe it is on one of our big holiday weekends, and I am nearly always out of town.  It would be a great opportunity to meet up with other forum members, I must check up when it is on, and see if it is at last possible to attend.  I am sure it is good to be so busy in retirement, but some times there are distressing time clashes.  Should have taken that gap year, but if I had, I would be up to a sabbatical by now.

I felt that first boiler, at only 38 mm diameter was really too small for anything other than demonstrating a small oscillating engine.  It was more a practice for silver soldering and forming those torispherical ends.   It actually does quite well to power a double acting one.  So I intended, as a next step, to just build a larger diameter, and perhaps a bit longer, of the same design.  However, I was told that the water tubes would make so much difference that I should skip that step and put in the water tubes.  When I had a look at the extra area, compared with the cylindrical shell, I went along with the suggestion, and included those four water tubes.

Now, looking at the details of those test results, I have to ask myself what happened?  The simple pot boiler seems to offer the highest steam production per unit area.  Even if I did not include the water tube area, it would not reach the steam production per unit area of the simple boiler.  With the same burner, I expected that it might heat up a bit quicker, which it indeed did, as the extra area could be expected to absorb more heat by cooling the flue gas to a lower temperature.  In fact, now that I look more closely, it did heat up much quicker, the larger boiler has more copper (770 g compared with 350 g), contains more water, 200 g compared with 130 g, yet heated to steaming temperature of 110 C in close to 5 minutes, instead of 7 min.  Yet this does not seem to be reflected in the steam production figures.  Perhaps I need to recheck those calculations, and explore that difference a bit further.

With the larger burner, I assumed I might get similar efficiency to the pot boiler, or even a bit more, especially with the full furnace enclosure.  But as usual, experiment does not follow the laws of intuition.

What really surprises me is that during steam production, the little pot boiler, without even a full enclosure, has significantly better efficiency than the larger boiler with water tubes.  The quicker heat up time is not matched by higher steam production.  I calculated a very similar efficiency on the early runs, back when the small boiler was first built, and did a much simpler test, so I think the difference is real.  I now need to have a look at the differences between the boilers and think about possible explanations for what is going on, preferably resulting in an understanding of how to improve the boiler design.

A big difference between the boilers is that furnace enclosure.  The little boiler has a simple firebox, but it is made from sheet stainless steel.  It not only has a lower thermal conductivity than the tin plated steel of the bigger boiler, it is still quite reflective, and does not seem to be blackening very much on the inside.  But it does not enclose the top of the boiler, it only extends up to the mid point of the shell.

The tin plate of larger boiler is not blackening badly, but it is not nearly as shiny as the stainless steel.  It also has plenty of air entry opening.  The little burner looks quite lost in the firebox so it is likely that it is getting too much excess air, absorbing a lot of heat, and lowering the temperature difference available for heat transfer.  The air entries were made for the larger burner.  In addition, that large tin plate casing may be absorbing more heat than the stainless, and consequently loosing more heat to the outside, especially for the small 50 mm burner which just sits at one end of the 230 mm long enclosure.  In addition, I placed the burner near the entry opening at the opposite end from the stack, so the flue gases could rise around the boiler, and pass along to the stack end, maximising the contact with the shell.  I wanted to avoid having the combustion gas take the shorter route around the shell to the stack if I placed the burner at the stack end.

With the larger burner, things appear more mysterious.  Possibly the losses from the casing are proportionately more, and in reality, the air holes are just a guess, basically just drilled holes along the full length each side.  I also made provision for some entry around a baffle at the stack end.  Possibly too much air, for even the larger burner.

The other thing I am thinking about is the size of those water tubes.  They are made from 1/4 inch diameter tubing.   I am wondering if perhaps these are too small, perhaps resulting in the water really boiling in the tube and expanding out into the boiler, pop-pop boat fashion, rather than heating strongly but flowing by the density difference in a more even manner.  This might explain the apparent improvement in heat transfer during heat up to get that shorter heat up time, but minimal if any extra steam production.  So making steam, then bubbling the steam through the water in the main shell, rather than promoting vigorous circulation.   

Unfortunately, the only way I can see to check this is to build a new boiler with larger tubes.  Though Ramon has shown us, in his wonderful Wide-awake build, that extensive rebuilding of a boiler is possible, I think it is probably worth building a new boiler.  Not that much extra work.  Still thinking about this.  In the long run, I suspect it will be well worth exploring whether larger diameter water tubes perform in a more satisfactory manner, providing I can source 5/16" tubing from one of our local suppliers.  I am not sure that I could bend 3/8 tubes to a sufficiently small radius.  Does anyone have ideas on the merits of larger diameter tubes? 

In the mean time, I am bending up some engine manifold insulation to make a layer of insulation on the boiler casing, a simple way to check if heat losses from the casing are important.  Horrible stuff to bend, the outer perforated metal layers separate from the insulating sheet when I bend it.  But I now have the roof section bent close enough, and drilled the holes for the bushings and the stack this afternoon.  Fortunately, I already had suitable size hole saws, and they worked quite well.  I will just cut flat pieces for the sides and ends, and pop rivet them all on.  I want quick rather than beautiful, until I source some thin stainless sheet and make a new casing.  Though I have a very different interpretation of quick to Chris and others.  I like some time in the day to sleep and eat, and exercise, etc.

Then, when I have some insulation on the shell, I will experiment with restricting some of those air holes.

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 #697 on: February 05, 2018, 02:15:57 AM »
Hi MJM , I have seen other boilers with those tubes that come street out of the bottom and then curve round and go to the other end in a strait line to enter at an angle into the boiler. I think this might allow the flow to be better . I don't know in which direction  the flow would be ? . i don't know if your fairly strait tubes on your boiler would boil in the middle and try to flow in both directions at the same time ? I cannot quite see exactly how your tubes are arranged so they may be ok ...so more experimentation........
« Last Edit: February 05, 2018, 02:21:51 AM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #698 on: February 05, 2018, 11:55:51 AM »
Hi Willy, your sketch is exactly right.   I will take another photo to show it better.  Thank you for posting that.  The water tubes extend 30 mm below the boiler at one end and only 20mm at the other.   I actually placed the burner at the low end, just as you have shown.

I think this was a mistake.  The lower end should act as down comers, that is, the water should descend at that end, flow along the rising portion of the tube as it heats, and finally return to the boiler at the higher end.  This means that gravity, and density gradients all work in the same direction.  But as you have shown, with the heater at the low end, it is heating that end more strongly, so it is expanding and trying to rise in that end.  The sloping middle section is receiving more gentle heat, but the resulting density gradient is actually trying to cause flow to the higher end, the opposite direction.  Not likely to work at its best.  I suspect the strong heat at the burner end wins, but the flow will not be as strong as if the density gradient in the centre section was helping the flow.

If I put the burner at the high end, opposite to your drawing, the steam will be rising at that end due to the lower density, and as the flue gases pass to the other end the cool so the down comers will see the coolest flue gas which is what is needed.

One option would be to put a baffle to shield the down comers from the heat of the burner, and push the burner in a little further, so as to apply more heat to the sloping section.  The other option is to turn around the boiler in the casing.

Fortunately, the boiler is symmetrical with respect to the bush positions, and the steam outlet at the centre has a banjo connection, so can easily be swung around to the other side, and the whole boiler can be put back in the casing with the low end of the water tubes at the stack end where I should have put it in the first place.  So when I get that insulation on the outside complete, I will reassemble it that way.

I also had a good look at the heat up calculations.  They do in fact support the expectation that the tubes should be providing better heat transfer, during the heating up anyway.  I want to recheck the steam calculations, then I will give you the results.  It seems the behaviour during steaming might be different to the heating up behaviour, but first, I want to recheck the calculations so I can give you firm figures.

I have been out to dinner this evening after a busy day of chores and ran out of time, but hope to make some more progress tomorrow.

Thanks for following along,

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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #699 on: February 06, 2018, 11:38:33 AM »
Made some progress today.  A little progress in the shop on insulating that furnace casing.  Yesterday, the  roof pieces were cut and drilled.  Today, I cut the material for the ends and one side and pop riveted them on.  A bit rough, in keeping with the tin can enclosure, but it will enable a trial, and hopefully show some benefit of the insulation.

I also had a fresh look at the spreadsheet.  I found a couple of minor errors and tidied it all up so I am happy with it now.  Time to try and form some conclusions.

The boiler performance parameter that I calculated the other day stood up to checking. I found a few  inconsistent units, so I will list them again.

Pot boiler run 1, steam production 39 W/m^2
            Run 3, steam production 35 W/m^2

Water tube boiler run 1, steam production 19 W/m^2, burner 90 mm
                           Run 2, steam production 20 W/m^2, burner 90 mm
                           run 3, steam production 14 W/m^2, burner 50 mm

Watts/m^2 is my suggested SI alternative to K. N. Harris' boiler parameter of cubic inches per minute per square foot.  Using watts allows calculation of the steam production for any steam conditions, though in reality the steam temperature will have a small effect by changing the temperature difference.  The difference in the figures for different runs that are otherwise similar gives an idea of the repeatability of the tests.  Obviously a larger number of tests is required to get a more reliable number.

Clearly the extra area in the water tube boiler did not result in proportionally more steam production.  Just for fun, I calculated the figures for the larger boiler, based on ignoring the area of the water tubes.  The result for the larger burner were 34 and 38 W/m^2, close enough to the same as the pot boiler with the small burner.  This is what is making me wonder if the tubes are contributing anything to steam production.  With the smaller burner, the figure was 24 W/m^2.  This would seem to indicate there was not enough heat available to increase the steam production in proportion to the area.  It is instead limited by the heat available.

Next, I compared the actual steam production.  The 50 mm burner produced close enough to 0.17 g/s, which ever boiler.  The 90 mm burner produced about 0.26 g/s, more than the 50 mm burner, but not in proportion to the extra heat release from the fuel burned.  The 50 mm burner was about 610 W, while the 90 mm burner released nearly 1800 Watts.  Three times the heat released, but only 50% more steam.

Finally, I looked at the boiler efficiency, that is, the proportion of heat release actually absorbed in heating water(plus copper) or generating steam.

For the potboiler, the heat absorbed during heat up is about 30%, while during steam raising, it was about 60%.  I suspect this difference is due to the higher film coefficient of the boiling liquid.  When this same burner was placed in the larger boiler with the water tubes, the efficiency during heat up was increased to over 60%, while the efficiency during steaming was unchanged, so still about 60%.  The higher efficiency during heat up was evident in the shorter heat up time.  But the extra area did not seem to result in higher efficiency during boiling and no significant increase in steam production.  Definitely some sort of anomaly.

The larger boiler and burner showed only a bit higher efficiency than the little burner in the pot boiler during heat up.  But it really did not show any increase in efficiency during steaming.  Not easy to guess what is happening, but the explanation that seems plausible is that those tubes are not contributing in the expected manner.

I don't know if any of that makes much sense.  I think the next step is to see if that insulation of the casing, and turning around the boiler make any difference.  I doubt that I will have anything new by tomorrow, but perhaps by the weekend. 

I am also planning to build a new boiler with 5/16 water tubes, but that will take longer.  I will keep you posted on the progress.


As promised yesterday, another photo of my boiler with water tubes.  I think this view shows a little better that the water tubes are sloped.  I tried a few other angles, but none were any clearer.

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 #700 on: February 06, 2018, 05:13:28 PM »
Hi MJM, would it make any difference adjusting the height of the burner ??

Offline MJM460

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Re: Talking Thermodynamics
« Reply #701 on: February 07, 2018, 10:33:49 AM »
Hi Willy, I suspect the height of the burner is quite important.  Too high, and there is not enough space to allow the combustion to proceed to completion before the gases are cooled by heat transfer to the boiler.  Not sure about too low, probably OK so long as the setting does not allow too much additional excess air.  The base really provides an insulated well for the burner which means it will not cause extra heat loss, though just creating. A higher firebox would of course result in extra heat loss to the atmosphere.  But it would raise the whole centre of gravity, and as I would like the boiler in the end to be suitable for a boat, it is not good for stability, a very different problem.

However, to allow experimentation on this, the base for the smaller boiler was made with a trench cut out of the base under the furnace and lined with tin plate to allow the burner to be set lower relative to the boiler for just this purpose.  I can pack it to intermediate heights with strips of wood.  My early simple tests were not sensitive enough to pick up much difference over the available height range.  I dropped that idea after a couple of indecisive tests, but I think the more detailed tests I have been inspired to try by seeing your efforts on your electric boiler, are more  sensitive, and so it is worth doing some more tests.  If it is successful, I could try the same scheme on the larger boiler.  But not before I test it again with the insulation and the down comers at the stack end.  I know that is two things altered, but I am sure that both are necessary, and should have been part of the original design, so overall improvement will be interesting without it being necessary to attribute a specific proportion to each factor.

I have also been thinking about the difference in performance between those two burners.  It seems that the small one is limited by its low heat release rate, while the larger one is limited by the area available for heat transfer (indicated by the low efficiency).  I did not manage to check the stack gas temperatures, it all happened too quickly for that, but I am thinking an intermediate size burner, say 70 mm long might produce more steam, but also achieve higher efficiency.  However, only after the tests with the boiler turned end for end and the insulation on the casing.

Turned around the superheater today so the boiler can be installed the other way around when the insulation is complete.  However it was 38 today and definitely too hot in the shed.  Exercise, shopping and picking up grand kids along with a similar temperature makes it unlikely that I will get much done tomorrow either.  But it is great to have the opportunity to stay close to the kids and grandkids.

Sketch up that new boiler or burner perhaps, if I am lucky.

Thanks for looking in,

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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #702 on: February 08, 2018, 11:38:55 AM »
Hi Willy, I just had to go back and have another look at that Hargreaves Thermomotor you posted the other day.   You asked if it was an early Diesel.  As I understand it, the defining feature of the diesel is compression ignition, with fuel injection measured to give some approximation to a constant pressure expansion on the power stroke.  This engine is more like a piston version of the  cycle used in Gas Turbines, with hot tube (brick) ignition, a regenerator and heat exchangers to make it a true regenerative cycle.  Gas turbines have a separate combustion chamber, and are generally categorised as external combustion, where this one has the fuel injected into the power piston where it is ignited by hot surfaces.  However the other similarity with gas turbines is the separate cylinders for compression and expansion.  It has a few auxiliaries for startup and a rather confusing water injection and collection system that absorbs some of the heat of compression (may be a disadvantage to thermal efficiency) but then uses the hot water to cool the power piston, thus reducing loses in that area.  Possibly a zero sum game, but it's also possible that there are also subtle advantages that make it worth doing. It reduces the work required by the compressor, which absorbs a significant portion of the power from a gas turbine.  I really have not looked at that part so closely. 

But the heart of the machine is the air pump for compression (I would call it a compressor), the counter flow heat exchangers which recover exhaust heat to preheat the air prior to combustion, and the expansion piston which does the useful work including driving the compressor.  All up, quite an interesting concept.  It appears to have worked, in that working engines are reported, where as I understand that the more conventional simple reciprocating version of the gas turbine cycle are not considered very practical.  Amazing stuff hidden in that library of yours.

Just a little time in the shed today.  Managed to fix the remaining insulation surface.  A bit untidy, though some angle trim around all the corners would improve it no end.  I assume I could make that with tin plate, which might be more compatible with the metal faces of the insulation I used than aluminium or brass.  Expect to get the boiler reinstalled tomorrow, and the engine slide valve cleaned up, ready for some tests over the weekend.

Thanks for looking in,

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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #703 on: February 09, 2018, 11:25:43 AM »
Not much to report today, however, between the inevitable shopping and taking the grandkids to cricket, I managed to refitt the boiler the right way around, shorten a union nut that was too long to seat properly, clean the valve, hopefully reseating properly now, all ready to try another test.  Tomorrow, I hope.  Also a photo.  It was too dark when we got home from the cricket.  The boy did quite well, hit a couple of fours, but brother and sister would have preferred to be home with screens.  It does not look too bad from the its best angle.  I am also thinking about how to fix a thermocouple in the stack to try and get a reliable stack temperature reading.

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 #704 on: February 10, 2018, 01:15:09 AM »
Hi MJM , Interesting stuff with the  Thermomotor and has our understanding of thermodynamics changed since then or have we learned lots of new stuff? If we built the same engine with modern materials would it be even more efficient ? or was it at a peak of current theory ??.........