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Wood Fired Hot Tub
Something not yet mentioned, which I'm pretty sure you thought of, is to use a counter current exchanger design. This means you'd want the water to flow through the copper pipe in a downward direction through the chimney, against the flow of the smoke. If you had the water flowing up, the water and the smoke would quickly match temperatures, and the upper portion of the exchanger becomes much less effective. Alternatively, in the counter current design, you use the cooler smoke at the top of the chimney to heat the cooler water, and by the time the water gets to the bottom, you'll have the hotter smoke to heat the hotter water. A temperature differential is maintained through the entire exchanger coil this way, which is important as I'm sure you know. I learned about this in an ecology class. Fish use a counter current design to oxygenate their blood, and I'm sure it's a common engineering principle.
I agree, the change in water temp is low. However, I'm betting the change in flue gas temp is high, especially with all that water cooling it down. There's not a lot of energy per unit volume in gases, so it looses temp fast. With the fire cranking and the water cold, I bet the output at the top of the chimney isn't hot enough to burn you. All in all, I agree that the difference in design probably isn't much in this case, but I bet the counter current design works at least a little better. The rapidly cooling flue gas is part of the reason you're getting more creosote than you would in a regular chimney. Creosote forms when the water created by combustion cools to the point that it condenses on the walls of the chimney (or in our case, the copper coils), and then solidifies into creosote. The fixes for this in normal wood stoves are 2: 1) completely combust the fuel to where there is no unburned carbon left to form the deposits. This one is tricky. Stove design is important, where you want to use either secondary combustion or a catalytic converter to get complete combustion. This is a complicated and expensive way to fix the problem. Also important here is using dry wood. Wet wood adds water to the flue gas, and slows combustion to create cooler gases with much higher concentrations of unburned carbon. Wood from firewood dealers is rarely dry, even if they say it is. 20% water by weight is good, 10% is better. Find this by taking a small piece, weight it wet, and then put it in a 210° oven for 1 day and weight it again to get the dry weight. The equation is 100*(wet-dry)/dry. If you're using even somewhat wet wood, this is the bulk of your problem. 2) Keep your flue gas hot. This is done in two ways: burn a hot fire, and insulate your chimney. Contrary to popular opinion, insulated chimney pipe (aside from where it passes through the ceiling/wall), is not a safety feature for the reason most think it is. It's sole purpose, especially on the exterior chimney section, is to keep the interior flue wall temperature high to reduce condensation and creosote buildup. In your case, insulating the chimney might help, but it only solves half the problem. The water in the pipe is doing most of the cooling, and we can't insulate that.My guess is you're using wet wood if you're needing to use a shop vac to get hot temps.The flue is a convenient place to put the coil, but difficult to make effective because of the creosote. It's also the coldest part of the stove. You'll always struggle with creosote with this design. I think if you used copper pipes running across the top of the inside of the fire box, it would be more effective and less of a headache. You could use elbow fittings on the outside to send the pipe back and forth without having connections to fail inside the stove. I think this is the way I would do it if I were limited on resources.I'll make one of these someday. When I do, I plan to use a boiler design with a water jacket. I'll use 3/16" or 1/4" steel plate to weld an internal fire box which will be surrounded by a chamber (water jacket) filled with water. The hot tub water will be circulated through the chamber, and heated directly by the heat of the fire. I'm sure there'll be issues I'm not thinking of though.
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