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| vtsteam:
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. |
| doubleboost:
I shall be watching with great interest John |
| vtsteam:
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. |
| vtsteam:
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. |
| vtsteam:
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. |
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