Thanks John
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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.
