Hello, I was wondering if adding a hot water coil inside of the drum would actually increase the rocket effect?
It is my understanding that the cooling of gases as they exit the riser contributes to the movement of the gases. Hot buoyant gases rising in the riser, cool gases falling in the drum. So, adding a coil for heating water inside the drum should increase the cooling effect. The water should be actively pumped to the tank, and the tank needs a boiler grade pressure relief valve (thanks allen lumley).
I'm pretty sure rockets like this have been built, what was the experience?
On my rocket stove heater (a fully hydronic system), I placed my water coils above the heat riser, so as to prevent taking away any heat from the combustion of the wood and wood gases. I also wanted to be sure that the copper coils never came in contact with direct flame as they are quite expensive to replace. The flame would probably erode away at the copper pipe over time, requiring it to be replaced. I'm not sure if it increases the rocket effect or not to be quite honest with you. It does not seem to hinder it in my experience, though, comparing it to rocket type cookstoves I've built. I use a multi-speed Taco pump to keep the water flowing and have a 210 degree pop off valve installed for safety. It is connected to my fresh water, so in the event of a pop off, cold water is pumped into the system, bringing it back down to a safe temperature. A check valve prevents the hot water from entering the cold water system.
I have tested the safety system, and it works quite nicely. It might be wise to post a warning sign near the pop off, as it releases hot water and steam at an alarming rate. If you are running a closed loop, you might even consider venting into the fire to extinguish it and preventing expensive meltdowns.
Looking at the pictures you posted, it looks like a more compact way of doing it as well as producing radiant heat. I would think this design would work quite well, just take the proper precautions with the blow off valve and you should be fine.
"above the heat riser" - is yours a tight coil of copper pipe (forming almost a solid cylinder) stacked above the brick/insulated metal riser? Then does this rise up until the 1.5"-2" gap under the top of the drum? I think this would cool the gas too soon. It should not cool until is has hit the lid and spread so it can fall down the drum, instead of wanting to fall down the riser.
I'm not using the coil to take away heat from combusting wood gases, and the flame does not contact the coil. The coil is inside the drum wall, contributing to the cooling effect of the drum. It would take away from your radiant and bench heat. I believe that this setup can provide hot water and improve the operation of the rocket mass heater. I'll have to wait until next fall when I can build it and find out.
Found this nice diagram of a hot water pressure relief setup:
I actually built mine to take all the heat out of the exhaust, hence the location of the coils. It does a pretty good job of removing the heat, the exhaust is about as warm as a dryer vent. I've got 110 feet of coil, and am probably going to put an additional 50 in there to scavenge even more. In theory, I'll have enough coil in there if the exhaust temperature matches the water inlet temperature. Because I live in a doublewide, radiant and mass heat were not options for me, especially since I have to carry insurance on the place until my mortgage is paid. By moving the furnace to the outside, I eliminated that issue. Water was the most cost effective way to pump the heat inside, so I went hydronic all the way.
I think the pictures you posted of the system you describe would work very well, I'm quite envious that you will be able to take advantage of the radiant and mass heating capabilities of a rocket stove and still manage to get hot water. I think the rocket effect would be just as strong if not better provided the coil didn't interfere too much with the flow of hot gases. Keep us posted on your progress.
I want to wrap my copper coils tightly around the exterior of the drum, and thermosyphon to a hot water tank above the ceiling. I live in a warm climate and don't need much space heating. Any issues? Of course I'll use an expansion tank and a temperature / pressure relief valve.