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| John Hill:
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? |
| vtsteam:
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. |
| vtsteam:
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...... |
| Brass_Machine:
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 |
| vtsteam:
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. |
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