Author Topic: Monotube Boiler  (Read 33933 times)

Offline vtsteam

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Monotube Boiler
« on: February 24, 2013, 01:45:56 PM »
Well, after probably a decade of thinking about it, and one unsatisfactory attempt at building a wood fuel monotube boiler 5 years ago, I'm going to start again. I almost hesitate to bring up monotube boiler construction here since there is a lot of criticism of monotubes in general to be found on the internet by people of wide experience in the steam engine field. And their objections are quite helpful and instructive.

Nevertheless I have to have a go at it. My primary interest in the type is due to safety concerns: the commonly agreed difference in the released energy of a monotube rupture in a system containing only a small amount of water compared to a conventional boiler vessel rupture with a large superheated water content and huge potential energy reserve.

A second reason is the simpler fabrication requirements.

And the third is the desire to use wood and solid fuel to produce electricity. It's simple and possibly selfish reason. I have a lot of wood. I don't have an oil field or refinery, or a coal mine. And possibly less selfish reasons. I think renewable fuels are a human species survival advantage over the continued  use of fossil fuels. Even if someone disagrees, I'm sure they will applaud my use of alternatives -- leaves more petroleum for them to use!! Win/win, right?

Finally, fourth reason -- I like a technical challenge.

I always believe in starting with basics -- and that means the basic assumptions about what I want to do. I think that very often the criticisms leveled at these kinds of projects are general in nature or attached to experiences with very different starting assumptions. Thus criticisms of oranges are leveled at apples -- or fruit in general. So it's important to understand in advance what the assumptions and purposes are.

As best I can express them or think about them, they are for this very special circumstance:

1.) a monotube boiler which works well with a wood heat source. I have in mind a wood burner I have developed over the years which has a relatively stable heat output and is throttleable

2.) capable of delivering a maximum of 5 shaft horsepower. I hope from a converted 5 hp 4 cycle engine I am also building. (This does not mean the engine WILL be capable of that -- just that the maximum steam capacity of the boiler should be capable of that level of steam production -- in other words this is a boiler sizing specification, not a performance prediction).

3.) that the steam temperature be optimized for 400F (200C). If this means that the engine I'm working on can not possibly attain 5 hp, then fine, this temperature is still a max design running temp. I do realize that flash steam generators can and will generate much higher temperatures and pressures briefly (or permanently!), and that they are difficult to control and stabilize. I just mean that this is the max design running temperature for steam output.

4.) that the electrical generator attached to the steam engine is a DC generator for charging a battery. This means a relatively stable steam output requirement. This is not a monotube boiler intended for a vehicle, or load following AC output requiring rapid throttle response, or highly regulated RPM for frequency stability..

5.) that the system is designed for occasional or emergency use, not 24/7 off grid electrical generation.

6.) that this be fun and interesting, educational, and reasonably safe as an experimental design process, and that if any or all of the goals and specifications are not reached, I will at least have the experience of trying and discovering interesting things along the way, even if they have been already discovered a hundred times before.

So that's what I'm about with this. I'm just a mad modder.
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Steve
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Offline doubleboost

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Re: Monotube Boiler
« Reply #1 on: February 24, 2013, 02:10:17 PM »
I shall be watching with great interest
John

Offline vtsteam

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Re: Monotube Boiler
« Reply #2 on: February 24, 2013, 03:07:24 PM »
Thanks John  :beer:.

Okay so I'm going to just type out a bunch of thoughts and observations and readings, all mixed up, because to even start on something like this it's important to just throw a bunch of junk out on the table and see what you've got -- like rooting through the scrap box of observations.

The first thing that hits me is that there are three major geometric orientations for the tubes I've seen.

The first is found in model boats, including highly developed racing steam hydroplanes. They have horizontally oriented spirals.

The implication of this is that they do not and can not maintain a "water level" since the coils are circular in the vertical plane. If the pumping speed is high enough, we can assume a fair number of the early heated sections of the coil may nevertheless be filled with water, and later sections steam, but that isn't literally a  water level.

It should be added that one modification of this form by HH Groves (and in fact one of the earliest -- dating back a century) has in addition a series of lengthwise runs.

The second major type of monotube is a vertically oriented spiral often found in steam cars. In these types people frequently speak of and attempt to maintain through various sensors and controls a literal "water level"

I don't know whether it is the compactness enforced by their use in an auto, or the influence of traditional boiler types that this vertical axis form and literal water level concept is adopted. Or perhaps the higher horsepower needs of an automobile, compared to a model boat, means that this boiler style is most applicable in larger sizes. Be that as it may, the needs of compactness, horsepower, etc do not seem to apply to my own set of requirements outlined above. So I'm open to thinking about the other, horizontal form as well as this one.

I read that many of the difficulties described by monotube experimenters center around failure to maintain water level, and the varying point of transition in the monotube to steam is frequently discussed as a cause of these failures. So that is a focal point to think about.

The third form of monotube is just a random bending of tube -- basically a purpose made rats nest of tubing, more or less rising vertically in a casing. I saw online one very small boat that had this arrangement, and the builder said he thought the chaotic arrangement was favorable to heat exchange, and worked well for him.

I guess I'm perverse enough to like that kind of thing! It appeals to me, though I don't know if I'll do things that way. But I have to admit, I'm attracted to it because in its odd way, it makes sense.

I do wonder what it does to the concept of a water level, and whether the "coils" rise progressively through the casing, or weave up and down randomly, as the horizontal coils of a model boat do in more regular fashion. Don't know the answer to that yet.

Finally there is the question of flow and counterflow for the water and steam in relation to the flow of hot gasses from the burner. Counterflow seems the most favored, though means that the tubing is subjected to greatest heat where it isn't cooled by entry water. To me this seems like a recipe for a relatively unstable system by comparison with a parallel flow system, though the latter will not be able to pick up as high a maximum temperature as the former. But if we are limiting temperature anyway, Maybe stability is more important.

I don't know. Something to think about.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #3 on: February 24, 2013, 03:31:48 PM »
So now another thought strikes me -- a parallel flow system puts the coldest water in the hottest gas, and so must effect the quickest heat exchange, initially. That exchange rate slows rapidly as the water heats into steam and the gasses cool rapidly having given up that heat. This should happen relatively early in the tube length.

If the temperature output requirement is relatively modest, then it seems to me that it can be achieved in a shorter length of monotube with parallel flow than counter flow. So the possible considerations are:

1.)Lower maximum temperature
2.)Shorter monotube
3.)Higher natural system stability
4.)Better tube protection.
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Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #4 on: February 24, 2013, 04:21:21 PM »
More thoughts.

The burning out of tubes is not a simple result of exceeding the melting temperature or exceeding the yield strength at elevated temperature of the tubes.

It can also be the result of literal burning, oxidation at high enough temperature. This was brought home to me in my early experience of metal casting in a charcoal furnace. I had used a stainless steel container as a crucible for aluminum maybe 5 times before it developed a leak at the bottom corner.

What gives, I thought? It's stainless steel -- it shouldn't melt!

Well there was an excess of oxygen near the tuyere where the blower entry was, and early on, I used to turn the blower up high to get as quick a melt as possible. What was actually happening was that I was literally burning the stainless with excess air. The same principle as the oxy-acetylene torch when cutting steel. It is possible, after a cut is started, to completely turn off the flow of acetylene, and cut with pure oxygen alone. The iron itself is the fuel used to continue the melt.

Another example is heating a copper pipe with an oxyacetylene torch when brazing. An oxidizing flame will leave the copper covered with rough scale. A reducing flame will actually remove scale and corrosion and convert it into shiny copper.

So, it seems to me that in the interest of long tube life, I would like to put the tubes in a section of the boiler where the combustion is complete, and has used up all of the oxygen drawn into the burner. Personally, I don't like putting any kind of heat exchangers in combustion chambers in general. They cool the combustion, which is rarely an advantage. I like to site them in the exhaust.

This might also make the use of copper tubing feasible -- it is often objected to on the grounds that it scales badly in the flame of the burner.

So the following considerations might apply to our new monotube boiler, unless some better ideas come up:

1.) avoid excess air
2.) locate tubes out of the combustion zone if possible, or where in the zone, make sure it is water cooled. Again this favors a parallel flow approach.
3.) copper may be feasible if oxidation can be controlled.
I love it when a Plan B comes together!
Steve
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Offline John Hill

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Re: Monotube Boiler
« Reply #5 on: February 24, 2013, 06:16:03 PM »
What scope is there to operate the boiler at pressure such that the contents of the boiler tube are liquid at all time and only flash to steam as they leave it?
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Offline vtsteam

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Re: Monotube Boiler
« Reply #6 on: February 24, 2013, 06:45:50 PM »
Hi John!

In this  particular project that isn't a goal, though I don't see why it couldn't be for another person's work. In fact the superheated water can be maintained all the way into the cylinder chamber with the use of an injector. See: http://www.flashsteam.com/

Any project of that sort would be extremely interesting and admirable to me!

My own goal is to fulfill the original set of specs and assumptions in the first post in as basic and mechanically simple a way as I can work out, with the most easily available materials I can find, at the lowest cost I can.

That brings up the topic of efficiency in a roundabout way. Efficiencies of steam production and usage form the basis of many interesting debates on the Internet.

In general the efficiency I strive for is not the ultimate thermodynamic efficiency of steam generation and power production, but the efficiency of cost and physical means per watt within the constraints of the project specs and assumptions That guides practically every choice I make.

As an example, the choice to use wood as fuel is a choice based on the fact that it is available and free for me, not on whether it is the ideal fuel for producing steam with the most efficient burner. Likewise other choices which will eventually come up.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #7 on: February 24, 2013, 08:34:35 PM »
More thoughts on flow direction.

Thinking about horizontal vs vertical coils, and the randomly oriented rat's nest style tubing in the SL Alba. (remembered the boat name, but not yet the owner apologies to him  :bow:)  it seems to me that there are two very different mechanisms here beyond just parallel and counter flow. designs

And those are gradient and non-gradient hot gas flow in relation to the tubing.

Both parallel flow and counter flow depend on an assumed gradient of heat parallel with the overall tube direction. But what happens in a randomly oriented monotube?

There is no gradient overall, though locally there will be hotter and cooler locations along the tube. Similarly in HH Groves horizontal style monotube boiler with its axial weave back and forth within the outer coils, there is no overall gradient.

This lack of gradient may be an assistance to to stability.   .......thinking about this......

I love it when a Plan B comes together!
Steve
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Offline Brass_Machine

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Re: Monotube Boiler
« Reply #8 on: February 24, 2013, 08:58:59 PM »
Well... I don't have much experience with a boiler. I have only a tiny one that came with my first steam engine kit that I built. So I will not be able offer much assistance. I do have a piece of 3" heavy walled copper tube that will be used for one at a later date.

I will be watching with great interest.

Eric
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Offline vtsteam

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Re: Monotube Boiler
« Reply #9 on: February 24, 2013, 09:34:57 PM »
In a vertical oriented automotive type monotube, one of the basic problems I've seen discussed is that when an increase in steam is suddenly called for, extra cold water is drawn into the bottom end by the reduction in pressure at the the steam chest end. This addition moves the steam transition area further up the tube, reducing the  steam generation and pressure, compounding the need for more steam. If the throttle is opened further, pressure reduces further. Unless more heat is added. Well, of course it is in most practical systems, but the lag in response as well as overheating at the top end (closest to the burner can cause further problems, including tube failure.

This seems like a classical problem of initial negative stability in the monotube boiler system. A displacement from equilibrium  increasingly reduces the tendency to equilibrium. And it seems to me that a conventional counter flow gradient monotube will tend to accentuate this negative stability. The solutions require sophisticated control mechanisms which enforce stability, and avoid damage. These are difficult to produce.

On the other hand, a randomly arranged tube would seem to reduce the unstable tendency -- or at least increase the rate of recovery (if the throttle is not opened further). In a random flow monotube, increasing the heat available would not primarily affect the "top" end (there is no top end) but would be applied throughout the monotube in a relatively even fashion.

The increased load of water created by low pressure will have a higher heat transfer rate across the tubes since more of the tube is cooler in temperature, and the water is a much better conductor than steam, Because transfer is faster, Reduction in the flue gas temperature will occur more quickly, and response to increased burner output will therefore be more rapid. In other words, response lag will be reduced.

Now what about parallel flow?

A sudden increase in water volume in the tube, would seem to me respond even more quickly to an increase in burner temperature. And the cooling effect of the added water in the tubes (now closest to the burner) would tend to lower the temperature of the steam chest end of the tube, rather than overheat it, thus more more effectively avoiding damage.

These are all theories. I really don't know the answer. And It's pretty likely to have been discussed by others before. So apologies to those who may think this is old hat.

Specifics of an individual boiler will be more important than theory in taking advantage of any theory like those thoughts above -- if it is an actual advantage at all.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #10 on: February 24, 2013, 09:45:04 PM »
Brass Machine thanks!!
I think I'd favor a conventional boiler for stationary models, too. I have a few I'd like to get working some day.

I hope this thread will at least be interesting, even if the results don't pan out.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #11 on: February 24, 2013, 09:55:24 PM »
So to conclude -- the big problems I've been thinking about in monotube boilers are tube damage from overheating, tube damage from excess air, control and stability problems in steam production, cost inefficiency, exotic materials requirements, and system complexity.

What I'm tending to favor to try to mediate the problems are:

1.) a horizontal design
2.) a parallel flow or random flow design
3.) a heat exchanger located largely past the combustion area
4.) reduction of excess air
5.) wood fuel
6.) a relatively constant load style of electric generation

These are not the common choices in monotube construction or design today, in fact added together they are pretty much the opposite of conventional wisdom. It is not the way to do things if the greatest value of heat exchange and temperature is desired from a monotube. However, it might actually be a practical response to the conditions and assumptions I gave in the first post, and so efficient in that sense. In other words, it might be good enough for who it is for.
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Steve
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Offline R.G.Y.

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Re: Monotube Boiler
« Reply #12 on: February 25, 2013, 11:23:53 AM »
Beeing a boat man I think the horizontal design will have a  lower CofG, for a longer coil as in steam hydroplan racers. :mmr:

Offline vtsteam

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Re: Monotube Boiler
« Reply #13 on: February 25, 2013, 01:15:30 PM »
Yes RGY, that's very true.  :beer:

And it can also be longer if it is horizontal and still fit tween decks. Which can mean easier heat conversion over a longer length of heat exchanger.

On a slow boat, though, we need a stack, so lose some of that layout advantage (unless there's a blower).

Fast boats like racers use ram air to provide oxygen to the burner, flow through the exchanger, and to clear exhaust.
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Offline fcheslop

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Re: Monotube Boiler
« Reply #14 on: February 25, 2013, 03:23:49 PM »
http://www.modelboatmayhem.co.uk/forum/index.php?topic=24568.0
http://www.modelboatmayhem.co.uk/forum/index.php?topic=15817.0
There maybe some info  in these threads and E T Westburys Experimental Flash Steam book is pretty good
I did play with flash steam in the early 80s before the advent of cheap electronics and found for marine use a simple figure of 8 worked well.


Good luck
best wishes
frazer
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Offline vtsteam

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Re: Monotube Boiler
« Reply #15 on: February 25, 2013, 04:14:10 PM »
Frazer, wow thank you  :bow: great information in those two threads! It's going to take me all night to read and absorb.

I've had ET Westbury's flash steam book and Benson and Rayman's experimental flash steam book for almost ten years  --  they are well worn! I have also collected Model Engineers for many years and have paid close attention to Dr Chaddock's and Jay Bamford's work and Bob Kirtley's among others. R.F. Yates also collected a lot of monotube information in his book "Model Making." And even Henry Greenlee had a go at it with a flash domestic power plant. And then of course HH Groves, and his steam aircraft, who seems to have hit many advanced concepts all at once in his engines and very early on. Kind of like Parsons did in turbines. It's like these guys put 6 different major innovations in any particular step they make, and they just work out of the box.

Anyway the above two threads are gold.

Thank you again!  :beer:
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Steve
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Offline dvbydt

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Re: Monotube Boiler
« Reply #16 on: February 25, 2013, 04:44:35 PM »
Another web site I have bookmarked from a few years ago :-

http://www.flysteam.co.uk/index.htm

Might still be of use, he mentions the same names as you did.
I want to make a small version (eventually), to run my models.

Ian

Offline doubleboost

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Re: Monotube Boiler
« Reply #17 on: February 25, 2013, 04:48:09 PM »
If
You lads dont stop posting links i will be up all nite reading :bugeye: :bugeye: :bugeye: :bugeye: :bugeye:
John

Offline fcheslop

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Re: Monotube Boiler
« Reply #18 on: February 25, 2013, 05:35:51 PM »
I don't know exactly what the boiler is for is it for a model or for a small launch?
The use of PICs has had a great effect on boiler control I keep thinking of getting back into it but would consider some kind of feed back loop.Don't know what you think ?
I seem to remember a couple of American designs in The Modern Steam Launch magazine I think the editor was Bill Fitt or Durham
Also I think Lunevalley in the UK made a monotube boiler there maybe some info on the Steam Boat Association UK site
http://www.steamboatassociation.org.uk/Default.aspx?pageId=1242242
Bobs drawings for Pisces 2 are available from My Hobby Store
cheers
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Offline vtsteam

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Re: Monotube Boiler
« Reply #19 on: February 25, 2013, 07:03:50 PM »
A generator, Frazer, the specs are in the first post.

I think electronic control using PICs is fine and interesting, very attractive and I really look forward to seeing more. But for me the need for a number of sensors and actuators and a microprocessor is a different direction than the one I want to head in. Control by wire is I think  very applicable to a system with negative stability, where highest heat exchange and high temperature performance is the goal.

I would love to see your work if you pick back up on that direction. That would be cool!

I'm taking the opposite tack which is dumbing down the system and reducing the heat exchange efficiency to try to get more control stability. Most of what I am thinking about has no application to racing or automotive steam power. And so it is kind of interesting in its own right.

As far as I know, the only surviving common practical domestic use for steam power are the pressure cooker and espresso maker. Both of those use simple mechanical control rather than electronic control of the steam itself -- and both are robust systems.

I'm a sentimentalist. I still don't have DRO's on my tools, though I'm sure I will some day. Just not ready for it yet -- having fun acting like I'm back in the last century still!
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Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #20 on: February 25, 2013, 10:29:45 PM »
Another web site I have bookmarked from a few years ago :-

http://www.flysteam.co.uk/index.htm

Might still be of use, he mentions the same names as you did.
I want to make a small version (eventually), to run my models.

Ian

dvbyvdt thanks. I have seen that site, and it's very good. In fact it had an earlier incarnation a number of years ago, and I wrote Geoff back then asking a bunch of questions about monotubes. He said he'd been working on a mechanical control mechanism and a book on monotubes. I think he even sent me a draft chapter. Besides all of the interesting stuff he has now on the website (it is much larger now than it was) one of the most important to me is the discussion of tubing diameters and the relationship of water velocity inside the monotube.

There seemed to be a magic number for diameter in both his readings of tests by Edgar Westbury in Model Engineer and his experience with a small steam launch. And that was 3/16" diameter tube. His theory was that the velocity of the water in the tube (for the sizes of engine in the small range) was critical to good performance of the monotube.

As a result of this I bought back then a roll of 3/16" stainless steel tube, which I still have. I'm very curious about this point. And we'll probably find out here whether it holds for an engine of increased displacement, like the one I'm adapting. It seems to me that 3/16" may not be a magic number -- but the velocity theory may very well be true.

Seems reasonable to me that the steam needs of a small engine might enforce an optimal velocity with 3/16" tube, and that the steam needs of a larger engine might enforce the same velocity in a larger tube. The reason that 3/16 tube might work well for a pretty good range of small engines is because the next common (imperial measure) increment in tubing size is 1/4", and while that seems only a 33% increase in nominal size, it's a cross-sectional area increase of 77%.

To achieve the same velocity the pumping capacity would need to increase that amount. So the steam volume (and possibly the engine displacement volume) would increase 77% to match.

The tube surface area has only increased 33% however, unless we lengthen it -- so that also might need to increase 33% to bring the heating surface in line with the quantity of water being pumped per minute.

So, basically, going up only one size in tubing has made a big difference in what size engine it would be appropriate for. And so it might seem that for a fairly wide range of small engines 1/4" is too large. Beyond some point, however, I bet that a next size tubing increment is appropriate. And that would logically continue up through the engine sizes.
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Offline John Hill

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Re: Monotube Boiler
« Reply #21 on: February 26, 2013, 03:15:00 PM »
Thanks for the interesting read and all those links.  I have incorporated some of what I have learned from them in my latest daft idea which is a "semi-internal steam generator" engine.
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Offline Raggle

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Re: Monotube Boiler
« Reply #22 on: March 03, 2013, 07:12:23 PM »
I hope the following is of interest, I do tend to ramble ...

About 40 years ago I made my living as a chemical descaling operator, largely on conventional steam boilers of multiple firetube type. Other work included water jackets on large compressors, cooling towers, etc., in fact anywhere a heat exchange had taken place and deposited calcium carbonate.

I'd already developed an interest in the Doble steam car and I like to think my mention of it at interview helped me to get the job.

One day I was called to see to a Stone's steam (or vapour) generator installed at a knitwear factory in Leicester. I believe it was to provide steam for one or more Hoffman presses, though it may have had other jobs.

I arrived with my full kit of pumps, 70 gallon plastic bath and hoses, and 2 or 3 carbuoys of hydrochloric acid. On being shown the item I confessed to the owner that I had never seen one before and asked if he had a manual. This he presented and left me to it.

I turned straight to the descaling section, piped it up as per instructions and got going. Most of the time is spent waiting between measuring acid strength, adding more as and when required. This gave me ample time to read the rest of the manual.

The Stone's and the Doble are very similar, possibly this later one contained some Doble patents. The fire, kerosene in the case of the Doble, maybe diesel in the Stone's, came in at the top and is electrically blown. The key to steam production is continuous pumping through the coil, which I think was 3 concentric diameters. At a high point above the coils is a steam drum/water reservoir. Steam bubbles are centrifugally seperated in this vessel, inside of which there are 2 water level sensors, high and low.

The pumps are interesting in being of diaphragm type. The diaphragm is at the end of a long pipe and its valves are at the other where the main flow occurs. Thus the diaphragm is not subjected to the full working temperature, being at some distance and merely generating pulses. The pumps are cam driven and the diaphragm can be held stationary by a solenoid. The two pumps looked identical but one is for continuous circulation, the other is the feed pump, triggered by the demands of the 2 sensors in the upper vessel (which I now recall was called the accumulator).

Feed water was preheated by a coil, probably by-pass or parallel connection to the main coil, (or maybe somewhere late down the flue, I can't remember)

I believe that for some models there is a superheater coil somewhere in the steam side, but details escape me.

When acid strength ceased to fall my job was done, other than adding lime to my acid bath to neutralise it before dumping down the drain and a number of clean water flushes and I packed up my truck without having to do the usual cleanup of my work area.

The owner reassembled the burner system and pressed the start button. 100 psi arrived in a few minutes which he pronounced good.

The unit by the way was about 3 ft high and about 3 or 4 ft diameter, not including ancillaries.

Ray
still turning handles  -  usually the wrong way

Offline vtsteam

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Re: Monotube Boiler
« Reply #23 on: March 03, 2013, 08:03:01 PM »
That is very interesting stuff, Raggle. Thank you very much for posting it.  :bow:

Completely new to me at least are the cam driven diaphragm pumps, the method of de-scaling with hydrochloric acid and watching the pH drop. Some of the other details are more widely reported but the pumps are especially interesting. In fact I'm going WOW, that is just brilliant! Almost every feed pump I've seen in, well, mostly model engineering sources for monotube steam are plunger type. Well D.H. Chaddock experimented briefly with an oscillating cylinder type -- like an oscillating steam engine in reverse.

But thank you so much! I am definitely going to think about this more.

 :mmr:
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Offline vtsteam

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Re: Monotube Boiler
« Reply #24 on: March 03, 2013, 08:32:42 PM »
Reading a second time through I'm struck by the circulation pump and accumulator, as well. And the longevity of this monotube steam generator, its industrial setting, its size. and the 100 lbs working pressure.

Thank you again for this very rare and useful information!
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #25 on: March 03, 2013, 09:54:46 PM »
Here's a photo of one found after some research.

http://www.wellman-group.com/boilers/products/stone-steam-generators

Also, just missed a scanned version of the Stone operator's manual -- the site where it was published closed down in the last 6 months! (www.locodocs.co.uk). Did a lookup of the site on the internet archive but they'd only archived the cover of the manual and not the contents  :(

I did however locate a used copy of the manual through Amazon, and it was reasonably priced enough to order it.

Wikipedia has a good article reiterating the details of steam (vapor) generators -- these were primarily used as railroad auxiliary saturated steam heat generators. Nevertheless very interesting for development of a low pressure monotube steam engine generator. I believe these were produced at least into the '60s. That makes it even more interesting.
I love it when a Plan B comes together!
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Offline vtsteam

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Re: Monotube Boiler
« Reply #26 on: March 03, 2013, 10:21:33 PM »
Speaking of railroad monotube steam generators, there is a very intriguing passage in Experimental Flash Steam  describing the Besler steam railcar, put into service on the New York New Haven and Hartford Railway Co. (which would have brought them near to my own neck of the woods) about 1937.

1000 hp, @1500 PSI "The boiler is of continuous flow, non-water level type It has no drums or headers but is a continuous tube from the boiler inlet to the throttle. The water enters at the top and passes down through a series of flat coils where it is heated and then boils in the helical coils at the bottom surrounding the combustion space, afterwards passing to the superheater coils just above the firebox and emerges as superheated steam. The boiler is 4 feet in diameter and 6 feet 5 inches in height. The oil burner is of the pressure atomising type and of special design and construction.

"The train performs a daily total mileage of 317.26 miles and operates in almost continuous service from 6:00 A.M to 10:20 P.M"

Now that is tantalizing! I have never found more information about these railcars than the above. The scale, the regular usage, the type of transport are all unique, as far as I know for monotube steam. If anyone has or turns up more information about this chapter of steam transport, I would greatly appreciate hearing about it!
I love it when a Plan B comes together!
Steve
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Offline Raggle

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Re: Monotube Boiler
« Reply #27 on: March 04, 2013, 05:55:14 PM »
The search continues! I hope I can be more help.

Meanwhile, this is the book that spiked my original interest, borrowed from my local library

http://books.google.co.uk/books/about/Doble_steam_cars_buses_lorries_and_railc.html?id=eIVTAAAAMAAJ&redir_esc=y

and again after seeing the Stone's  -  that link is UK Google page but clicking "find in a library" I find a good number have it in stock. It doesn't seem to be available to buy. I suggest you try your local area from a similar (US) google page.

I hope your machine doesn't look like this:

http://www.dangerouslaboratories.org/genboiler.htm

Thinking forward your engine might possibly work part-time driving a wood chipper so you could have a hopper feed to the boiler  :lol:

Ray
still turning handles  -  usually the wrong way

Offline vtsteam

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Re: Monotube Boiler
« Reply #28 on: March 04, 2013, 09:24:18 PM »
Hey thanks again Raggle. I haven't begun to build the monotube yet. Just finishing up the conversion engine. It is together now, and needs to be run in and a leak sealed. I checked it on compressed air briefly this eveningand it does a sharp half turn but stops past BDC, but clearly it wants to go.I need to patch the leak (in a poppet valve chamber plug), and maybe change the timing. The cutoff is intended for steam, so quite short (55 degrees) for compressed air.

I may make up a second valve with a 180 degree cutoff just to get it running on air and broken in. Then I can start on the boiler.

The link to the boiler you gave didn't work for me, but I think I remember that one anyway. No It won't be like that one.

I have a wood chip burner built already, though not up to the capacity of my conversion engine. I intend to try the burner with some smaller model engines I have before tackling a larger monotube.

Last night I altered my search terms and found a video of the Besler monotube railway car mentioned above -- it's included in a promotional film also including the better known Besler steam airplane. Besler must have had quite a bit of financial backing to venture into such disparate monotube projects.

Here's the video -- pretty amazing detail -- I think you can see the monotube boiler in the plane when the cowl is open.
 
http://archive.org/embed/BeslerCo1932
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Steve
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Offline 1hand

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Re: Monotube Boiler
« Reply #29 on: March 27, 2013, 12:19:51 AM »
Hey, I don't know much, but I like watching someone that does!

 make sure you post pics, when you start twisting metal!

 :worthless:

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Re: Monotube Boiler
« Reply #30 on: March 27, 2013, 08:12:43 AM »
Thanks 1hand! Too much "knowledge" on my part and too little doing on this subject -- talk is cheap!

Still don't have my engine running so I can move on to the boiler. But as soon as that baby turns over on its own, I'll be onto the monotube trials here. And gradually work through plans A, B, C, etc. Wonder how many letters I'll go through this time?  :lol:
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Steve
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Offline tom osselton

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Re: Monotube Boiler
« Reply #31 on: March 28, 2013, 05:40:08 PM »
Probably a to h that will give you the " AH " factor

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Re: Monotube Boiler
« Reply #32 on: May 11, 2013, 02:18:00 PM »
I'm new to this so this is my first post!
I have been working on various designs of monotube boilers over the last few years and find the views on this forum most interesting. My original challenge was to design and build a 3HP double acting steam engine and boiler from scratch to fit in a dinghy. It needed to be light so much of the engine is ally with sealed ballraces throughout. The boiler had to be monotube for safety.
The engine is fine and chugs away quite happily at about 800 RPM. It has some unusual features. The flywheel is driven by a timing belt at 3:1 above crankshaft speed, this enables the use of a flywheel that is only 1/3 of the weight and also the engine can be mounted lower in the hull reducing shaft angle and lowering the C of G. The weight of the engine assembly including condenser and scavenge pump (but not the boiler) is only 34 lbs.
The boiler on the other hand is a different matter!  I needed about 80 PSI of moderately superheated steam (about 200 C). It was important that it was limited to <220 to avoid damage to the various O rings in the engine and valve gear. I started off with a monotube boiler and economiser coil in the flue controlled by a PIC and R/C servos and using gas burners from a domestic boiler. After much plying around with the control system it was moderately successful but I was not really satisfied with it so went for a Lamont design with separate superheater coil and burner. The pump was the problem here and I ended up with a much modified central heating pump. Again moderately successful but it was getting very heavy and I was worried about the separator tank which was, after all, a small pressure vessel.
So now I am looking at going back to a single coil again. There is much information on small units for hydroplanes producing amazing quantities of steam from 3/16 dia tube and a very noisy burner, but this is not suitable for a steam boat.
From what I understand, it seems necessary to have a feed flow of at least 1m/sec for removal of vaporisation bubbles forming on the inside of the tube. This leads to very small Dia tubing and a small surface area. I have noticed in the past that if you tap the coils lightly with a small hammer, the pressure rises quite dramatically for a few seconds (until the bubbles reform again?). I am now experimenting with an ultrasonic transducer fitted near the cold end of the coil in an attempt to shake these bubbles loose. Another idea would be to use a very oversized piston type feed pump which could shuffle the water back and forth in the tubing at about 1m/sec velocity.
What do you think?

Offline dsquire

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Re: Monotube Boiler
« Reply #33 on: May 11, 2013, 03:07:15 PM »
I'm new to this so this is my first post!
I have been working on various designs of monotube boilers over the last few years and find the views on this forum most interesting. My original challenge was to design and build a 3HP double acting steam engine and boiler from scratch to fit in a dinghy. It needed to be light so much of the engine is ally with sealed ballraces throughout. The boiler had to be monotube for safety.
.
.
.
.
use a very oversized piston type feed pump which could shuffle the water back and forth in the tubing at about 1m/sec velocity.
What do you think?

Tony

I think that you have made a great start on your engine and boiler. I am sure that there will be much interest by the members here. If you would be so kind as to go here http://madmodder.net/index.php/board,3.0.html and post an introduction telling a bit about yourself, where you are from, etc. It would help us all get to know you a bit better. I hope to see more posts on your projects in the future.

Cheers  :beer:
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'til your good is better,
and your better best

Offline vtsteam

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Re: Monotube Boiler
« Reply #34 on: May 11, 2013, 05:18:11 PM »
Hello Tony!

Great info! Thanks so much for your post.

It's a funny coincidence that I'm just about to start work on monotube boiler experiments since I'm just about completed with my 4 cycle to steam conversion.

Okay, so now for the opinion piece.....

I think that the 1 meter/sec quoted  is just a theory, not necessarily written in stone. Few people if any have ever watched bubbles form inside a monotube. There are many explanations, theories, and statements of "common knowledge" made on the internet and repeated, but very little proof of these conjectures. Basically, anecdotal experience when somebody changes something and sees either an improvement or performance decrease.

Here are just a few reasons why I have doubts that we are anywhere near a state of certainty on monotube function. It seems obvious that steam bubbles can form at different internal skin temperatures, different pressures, different material heat transfer rates, different relative surface to volume ratios of the conduit, etc. There are just lots of diverse factors besides flow rate. One example? If pressure is high enough, and maintained superheated water obviously can be conveyed all the way to an injector and engine --  as in true "flash steam". Where along the length of the monotube do the bubbles form there? Nowhere. What speed is critical to attachment? No speed, as long as the pressure is maintained.

And re "bubbles" -- what do we mean by bubbles? Is the insulating value people have observed a result of actual "bubbles", or a sheets of steam, and if bubbles, how large are they -- do they fill the conduit, are they a small percentage of the conduit?

There are just millions of questions as I see it, in a subject of much greater complexity than some arbitrary rate is going to answer.

Not saying ignore the theory completely -- might be a good rule of thumb for low pressure of a specific size boiler and material tube. But I think there's room for a lot more knowledge and development.

Which is a very good thing!
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #35 on: May 11, 2013, 05:38:19 PM »
Tony to respond to your other questions, I guess virbrating the tubes could help detach small bubbles if they are truly what is happening. But it sounds like you want to move in the direction of simplicity and low weight. The more electronic gear you use and the greater the system complexity, cost, and failure modes.

Why not try to locate your bubble formation problem area in the monotube and increase pressure there by constriction, rather than having a uniform diameter tube? Then reduce pressure to your engine's tolerance level by increasing tube diameter when you approach the inlet?
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Monotube Boiler
« Reply #36 on: July 11, 2014, 12:23:21 PM »
I'm finally getting back to this project -- 18 months later and I have my 3/16" stainless tube to make a monotube. But I wanted to read up on a couple more things before bending it. I don't think stainless will take a lot of re-bending, so I want to pick the right shape first. I'm still up in the air about coils vs figure 8  in a horizontal monotube, or just random bending.

I found a really interesting article in issue # 3355 of Model Engineer, mentioning a successful monotube at the very end, with a photo, but I don't have the follow-up issue #3356 -- my stack jumps to 3357!  I must have lost it :doh:

Can anybody send me or link to a pdf (or just page photos) of  the continuation article "Model HUNT Class Destroyer" by T.B. Rose  in Vol 134, Number 3356  (Nov. 1968) ?

Or I'd be happy to buy the issue if someone has it, and no longer needs it.

Thanks!
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Steve
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Offline tom osselton

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Re: Monotube Boiler
« Reply #37 on: July 11, 2014, 04:03:09 PM »
You will probably find this interesting it's one of Jay Leno's  steam cars telling a fair bit about the steam generator.


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Re: Monotube Boiler
« Reply #38 on: July 11, 2014, 04:15:43 PM »
Hi Tom, I'll check that out tonight.

I got tired of waiting to do something on this today and just dug in. I wound a plain coil of the stainless tube on 3" dia. form. I was a little worried about kinking, but it bent fine.

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Steve
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Offline tom osselton

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Re: Monotube Boiler
« Reply #39 on: July 11, 2014, 04:34:33 PM »
Look good

Offline S. Heslop

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Re: Monotube Boiler
« Reply #40 on: July 11, 2014, 10:45:26 PM »
Great to see you're back at this! I was just reading the thread about a week ago since I was curious about monotube boilers, and thought you might've given it up. It does seem difficult to find any concrete information on them, or even boilers in general.

Offline vtsteam

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Re: Monotube Boiler
« Reply #41 on: July 12, 2014, 11:55:28 AM »
Thanks Simon. What I've got is probably too short (about 20 feet) for what I want to power, and the coil size seems big in diameter to me, but we'll pursue what we have here anyway and see where it takes us.

I didn't want to risk kinking by winding tighter, though maybe I might have been able to. What I will probably do to compensate is run a cylindrical baffle down the center of the coil to force the combustion products over the coils instead of taking a path of least resistance through the center.

I might space the coils out a little farther so I get more penetration between coils instead of the gas just running over them. There is something to be said for irregular coils rather than a nice neat cylinder like it is now, but it can look pretty crude that way. We'll see -- might end up as spaghetti just to find out what happens after testing it "neat" and cylindrical. But it's easier to go in that direction rather than the opposite: start with a rats nest and end with a neat coil.
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Steve
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Offline awemawson

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Re: Monotube Boiler
« Reply #42 on: July 12, 2014, 12:11:06 PM »
Could you lay it out like they do for ground source heat pumps. Essentially your neat coil pushed over sideways so you have overlapping loops looking like the swirls left by a vertical milling tool. Apparently it maximises the surface contact which is a similar need to your boiler.
Andrew Mawson
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Offline S. Heslop

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Re: Monotube Boiler
« Reply #43 on: July 12, 2014, 03:24:34 PM »
Thanks Simon. What I've got is probably too short (about 20 feet) for what I want to power, and the coil size seems big in diameter to me, but we'll pursue what we have here anyway and see where it takes us.

I didn't want to risk kinking by winding tighter, though maybe I might have been able to. What I will probably do to compensate is run a cylindrical baffle down the center of the coil to force the combustion products over the coils instead of taking a path of least resistance through the center.

I might space the coils out a little farther so I get more penetration between coils instead of the gas just running over them. There is something to be said for irregular coils rather than a nice neat cylinder like it is now, but it can look pretty crude that way. We'll see -- might end up as spaghetti just to find out what happens after testing it "neat" and cylindrical. But it's easier to go in that direction rather than the opposite: start with a rats nest and end with a neat coil.

Could you wind them tighter by packing them with sand and plugging the ends, or maybe even something like a wax you could melt out later?

Offline tom osselton

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Re: Monotube Boiler
« Reply #44 on: July 12, 2014, 03:48:54 PM »
I remember dad telling us about bending pipe filled with rosin so they would not kink.

Offline awemawson

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Re: Monotube Boiler
« Reply #45 on: July 12, 2014, 04:02:57 PM »
"Cerrobend" is the bees knees for pipe bending. An alloy of (I think) bismuth that melts at about 70 centigrade. Melt in a water bath, pour in, bend, melt out in a water bath.

http://www.csalloys.com/products-cerrobend-alloy.html
Andrew Mawson
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Offline tom osselton

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Re: Monotube Boiler
« Reply #46 on: July 12, 2014, 04:06:39 PM »
I was thinking of that too but haven't played with any yet.

Offline tom osselton

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Re: Monotube Boiler
« Reply #47 on: July 22, 2014, 08:12:18 PM »
I thought you might find this interesting (at the bottom of the page)

http://photos1.blogger.com/img/252/5340/640/DSCN0185.jpg

http://steamprojects.blogspot.ca/

Offline vtsteam

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Re: Monotube Boiler
« Reply #48 on: July 22, 2014, 09:45:37 PM »
Thanks Tom. Not a monotube boiler but a vertical water tube boiler with external coils. Funny thing is the PM model boiler I'm making now in the other thread is a very simple horizontal version of the same thing.

For occasional use like my model version, copper water tubes are usable. And in this specific case they are easily replaced since they are attached by screw fittings. But I do wonder how long lived they will be in a full sized marine boiler, due to oxidation. I think It will be important to control and reduce excess combustion air for the tubes to last well.
I love it when a Plan B comes together!
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Offline vtsteam

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Re: Monotube Boiler
« Reply #49 on: August 05, 2014, 08:35:41 PM »
So, returning here from the PM boiler experiment in another thread and having done it's first steam this afternoon , an interesting thing I saw today was that the copper coil under the main boiler barrel was glowing cherry red.

How is that possible I'm wondering, if the barrel is full of water and the steam pressure is less than 20 PSI?

Does that mean that there is no water in the coil (where it was red hot), just steam?

I wish I had noticed how much of the coil was red, but I didn't.

Is it completely empty of water, or is water crculating and being converted along its length into steam before reaching the red section? And would this mean it is acting like a separate monotube connected to the main boiler barrel?

Is water traveling through the coil or just vapor locked out of it?

Questions, questions....
I love it when a Plan B comes together!
Steve
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Offline mattinker

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Re: Monotube Boiler
« Reply #50 on: August 06, 2014, 02:14:39 AM »
A copper tube with water in it cannot heat to red heat. Have you ever tried to braze a copper plumbing installation with even the slightest amount of water in it? You just can't do it. It must be empty.

Regards, Matthew.

Offline vtsteam

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Re: Monotube Boiler
« Reply #51 on: August 06, 2014, 07:40:08 AM »
I'm not sure what you mean Matt by the "slightest bit of water in it". Flash boiler tubes routinely carry both water at the inlet and steam at the outlet, and may glow red along the superheated steam portion. This is commonly reported in operation.

My question was, whether there was circulation in this copper coil or something closer to vapor lock, and therefore poor circulation. I believe, but will have to check again, that only part of the coil was glowing. Which would suggest circulation and flashing into steam along its length. If all coils in the flame were red, I'd suspect a lock.

Yes I have soldered pipes before and pipes do need to be dry near the joint. I built and plumbed my house, and complete hydronic heating system including a wood furnace with copper heat exchangers and copper piping throughout.
I love it when a Plan B comes together!
Steve
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Offline mattinker

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Re: Monotube Boiler
« Reply #52 on: August 06, 2014, 04:14:02 PM »
Hi Steve,

 Well I was kinda thinking out loud this morning going out the door on my way to a job. I was pretty sure that you'd experienced the amount of energy to needed to keep a copper pipe dry whilst you braze it, it's enormous, red heat being of the order of 750C is a huge amount of energy. In a small circuit, mostly filled with water, the pressure rise would be very fast, I would have thought that if it was circulating, the pressure would have gone up rapidly above 20 psi, I may well be wrong, (probably!) but I would have though you had a vaper lock!

Regards, Matthew

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Re: Monotube Boiler
« Reply #53 on: August 06, 2014, 06:39:52 PM »
I lean toward that too, Matt, though thinking about it, it seems like even then it would have to "refresh" itself since it's below the main body and steam would be leaving it up through the body.

It couldn't drop in pressure too low before at least a little water re-entered it to build it back up. But maybe not a steady flow.

I'd like to work with it and see how good a flow can be achieved. And try different shapes. That's easy to do because the coil attaches with two flare nuts, so I can just change it out for another. This is kind of a nice design for experimenting with -- everything is screwed together and replaceable.

This has a classic helical horizontal coil of 4 turns (I believe), per the magazine article plans. I'm wondering if a straight pipe might not work better because of increased circulation, even though it has less area. I wouldn't be surprised if that were true.

 I've seen them  angle down gradually from the attach point at one end, and then sweep back up rapidly at the other. Probably what I'll try next.
I love it when a Plan B comes together!
Steve
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Offline mcflin

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Re: Monotube Boiler
« Reply #54 on: September 29, 2014, 04:29:36 PM »
This is the project I have been working on for many years ,a mono-tube boiler running on LPG and reaches 120 psi in 30 odd seconds,it still has bugs to iron out,but learning so much. ( hope the picture comes through )
« Last Edit: September 29, 2014, 09:42:58 PM by dsquire »

Offline S. Heslop

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Re: Monotube Boiler
« Reply #55 on: September 29, 2014, 04:53:37 PM »
This is the project I have been working on for many years ,a mono-tube boiler running on LPG and reaches 120 psi in 30 odd seconds,it still has bugs to iron out,but learning so much. ( hope the picture comes through )

That's fantastic! Got any more details?

A steam powered bike has been a pipe dream of mine for a while. Seems like it'd be a real challenge.

Offline tom osselton

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Re: Monotube Boiler
« Reply #56 on: September 29, 2014, 05:41:34 PM »
A steam powered bike would be cool!

Offline doubleboost

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Re: Monotube Boiler
« Reply #57 on: September 29, 2014, 05:42:34 PM »
You have my attention

Offline mcflin

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Re: Monotube Boiler
« Reply #58 on: September 29, 2014, 05:56:08 PM »
Here is a YouTube of some test runs

     


Its 1 1/2'' bore.a microprocessor measures and controls the water,which is pumped in from a coffee machine pump at up to 180 psi.But over the next 2 weeks am redesigning the burner and boiler.is all very difficult as there is no one in NZ to ask advice as not many people get to control a mono-tube boiler satisfactory
regards lyn

« Last Edit: September 29, 2014, 09:32:18 PM by dsquire »

Offline S. Heslop

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Re: Monotube Boiler
« Reply #59 on: September 29, 2014, 06:27:39 PM »
Great video. I feel like i've already trawled everything the internet had to offer on steam bikes, so it's great to see more.

You'll probably get sick of hearing this question, but I have to ask; what kind of range do you get/ expect to get with it?

Offline mcflin

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Re: Monotube Boiler
« Reply #60 on: September 30, 2014, 05:03:47 AM »
In the video the exhaust steam goes to atmosphere.where as it is normanly fed back to the water tank through a internal condenser .this gives more water to use,the tank is approx 6 lts and this determined the range.but the whole project is just a folly anyway .I just use it on our local cycleways of which we have 186 km of of road cycle paths in our area.

Offline awemawson

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Re: Monotube Boiler
« Reply #61 on: September 30, 2014, 07:09:07 AM »
"The whole project is just a folly"


No - I love it - superb style and I bet it was great fun getting it to the stage that you have. Inundate us with details please  :ddb:
Andrew Mawson
East Sussex

Offline Arbalist

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Re: Monotube Boiler
« Reply #62 on: September 30, 2014, 10:50:34 AM »
That is completely bonkers Lynn, I love it!   :clap: let's see some more when you've done the mods, and welcome to the forum!

Offline JohnHaine

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Re: Monotube Boiler
« Reply #63 on: September 30, 2014, 06:34:50 PM »
There is useful information on design and control of monotube boilers in the SBA magazine Funnel.  It may be accessible on the web, do a search in google on Funnel monotube and there are a couple of links.  I have a fairly complete set but they are a bit inaccessible.  If you are interested and can't find them on line I could have a hunt...