is it right to say that in the janto J system that it is constant csa it will act more like the water exaqmple?
Hmmmmm. If you take just the "J" portion (feed tube +burn tunnel+ heat riser), then sort of. They would be similar in that there is nothing other than friction slowing either one down. They are also similar in that both are being driven by the pressure differential between two fluids of differing densities (let's call them "Fluid A" and "Fluid B").
However, they are different in one important way: the pressure differential between the two fluids is much greater in the water system than in the J system. Specifically, in the water system, Fluid A (atmospheric air) is far less dense than Fluid B (water). In contrast, in the J, Fluid A (again, atmospheric air) is this time *more* dense than Fluid B (flue gas), but by a much smaller margin. Practically speaking, this can be demonstrated by weighing one liter of cold air (atmospheric air, Fluid A), one liter of hot air (the J's fluid B) and one liter of water (the water siphon's Fluid B).
So why is this difference in density a big deal? It is a big deal because it is the heart and soul of how gravity makes a siphon run, be it a water siphon or a chimney. Pressure is measured in kilograms per square cm. Where does pressure come from? In the scenarios here, pressure comes from the pull of gravity on the particles that make up the fluid. The more dense the fluid, the more particles (molecules) there are in a square cm. Gravity acts on each particle of the same mass with the same amount of force, but if you crowd more particles into the same space, there are more particles pressing, and the total force is greater -- the force of gravity on a single particle, multiplied by the number of particles. So, if a given fluid is more dense (more particles per square cm) there is more downward pressure on that square cm.
But how does this downward pressure translate into the function of these two siphons?
In the water siphon, you fill a hose with water, put one end in the water and the other end out. If the ends are perfectly level, the siphon will not flow, as pressure at both ends is equal. That is, with both columns of water inside the hose being the same height, the stack of particles on the one side is equal to the stack of particles on the other, and gravity is pulling the same amount on both of them, balancing out the force of each. Likewise, the pressure of the atmospheric air pushing down on the surface of the bucket of water is equal to the pressure of the atmospheric air pushing *up* at the open end of the hose, canceling the effect of each.
If gravity is pulling *down* on the atmospheric air, why would we assume that it is pushing *up* into the hose? Because, as illustrated by the "towers" of particles in the drawing, atmospheric air is a massive pool of fluid stretching upward to the end of the atmosphere. If you could stack a single column of molecules from the earth's surface to the outer edge of the atmosphere, the resulting pressure at the bottom of the column would the accumulated force of gravity acting on each particle, which in turn presses on the one below it, transferring the combined force to the one below it, and so on. At the very bottom, every particle is under this "atmospheric pressure". If you remove the column analogy and return to the "pool" analogy, the bucket of water immersed in the "pool of fluid" of the atmosphere will experience atmospheric pressure on it from all angles, including upward, because the pressure is not a function of just the particles immediately beside it, but because of the accumulated force of the column of air pushing down and pressing the bottom particles into every available void.
Consider also that every particle is not standing still, but vibrating up, down, side to side, etc. as indicated by the arrows in Figure 1. The force of the vibration of each particle is a function of various things, one of which is temperature. When you increase the temperature of a substance (including a fluid), it vibrates more energetically. When it does, it creates a greater cushion of space between itself and the particles around it. (Incidentally, if it is vibrating more energetically and crashes into another particle that is moving more slowly, it transfers a portion of its energy to that particle, slowing its own movement, which is the act of cooling.) When an entire body of fluid is heated, all the particles vibrate more energetically, pressing for more space. If the fluid is not constrained (and in neither the J nor the water siphon are any of the fluids constrained on all sides), then it expands, becoming less dense. As it becomes less dense, the same amount of fluid occupies a greater volume. With fewer particles per square cm for gravity to pull on, the net downward pressure is less.
If you refer to the "J" portion of Figure 1, I have drawn two columns of dots representing columns of air stretching up to the end of the atmosphere. Both columns are identical to one another above the horizontal dashed line representing the level of the top of the heat riser. Below that dashed line, the heat riser column is comprised entirely of hot flue gas particles which are more energetic and thus farther apart from one another, leaving fewer particles for gravity to pull down on. When contrasted with the column to the left, which is made up primarily of cool atmospheric air particles and just a few hot flue gas particles, there are many more particles in that space for gravity to pull down on. Thus, since force of gravity on the greater number of particles in the column on the left is in sum total greater than that of gravity's pull on the many fewer hot flue gas particles in the column on the right, the column of atmospheric air forces its way down into the feed tube, pushing hot flue gas particles out the other side. Notice that there is a column of atmospheric air particles also pressing down on the top of the heat riser. Why does this not stop the flue gas from escaping from the heat riser? It does slow it down, but it can't stop it, because it is shorter (that is, contains fewer particles, has less mass) than the column of dense particles on the left, and is thus pushing downward with less force. As long as combustion continues and all other variables are held equal, this flow will continue.
Now consider Figure 2. The column on the left (Column 1) is atmospheric air with its denser particles. The column on the right (Column 3) is flue gas at its final temperature after having much of the heat harvested. It is warmer than atmospheric air, and if that were the only variable, the siphon would run without a problem. However, the middle column (the two-story bell) contains a fluid that is *less dense* than either column! Herein lies the challenge. In a contest between column 2 and column 3 alone, column 3 would be more dense (heavier), and would push all of the column 2 gasses out until equilibrium was reached. However, considering the entire system together with atmospheric air, what you need to figure out is, do column 1 and column 2 have a add up to a greater combined number of particles than the number of particles in column 3? If not, there are two possible approaches to a solution.
The first is to raise the level of the dashed line (raising the chimney) so that the column of warmish (less dense) gasses is taller in comparison to the height of the atmosphere. Then the height of less dense particles is greater, meaning that there are less particles in the column for gravity to pull on, making them more easily pushed out by the now larger-in-comparison column of densely-packed particles in column 1.
The second is to add more energy into column 3. You could do that by harvesting less energy in column 2, thus leaving more energy left for column 1 (e.g., reduce ISA). Or you could harvest as much as possible in column 2 (because, after all, the point is to get heat into the lower level), then re-introduce heat into column 3 either via a bypass; by having column 3 be single-wall chimney pipe exposed to the hottest gasses in the upper bell; or some other creative method.
When you consider the Ianto J system complete with bench as compared to, say, a J followed by a bell to harvest heat, the only difference is that with constant CSA you require constant flow and thus create more friction. Otherwise it just comes down to height and temperature to determine flow
Hope that whole thing made some sense!
DCish.