We did some brief testing of this design, but did not really like the drafting. To be fair it was still very hot, 95F. It did work well in terms of lowering the exit temperatures in increasing the bell heat storage. However, it did not draft near as well as the chimney flue build. So for the moment, we are tabling this design until the fall for more realistic test.
After several days of testing, we have concluded that the heater needs to be re-designed. What we built does not facilitate enough heat capture. Because the exhaust is presently directly under the barrel, in the barrel supports, too much of the exhaust from the barrel was moving into the stove pipe without lingering and stratifying in the bell. Attempts to pull exhaust from the floor of the bell through the use of stove pipe elbows inside the bell resulted in poor stove pipe velocity and extensive back smoking.
The New Design
We are going to re-locate the stove pipe exit from the barrel supports to the bottom right hand side at the feed tube end of stove. This will require the exhaust from the barrel to travel to the other end of the bell before leaving. The additional time in transit should providing more opportunity for the “free movement of gases” to occur and stratify the temperatures better.
The stovepipe exit will be changed from a 6″ round to an over sized, 12″ x 4″ rectangle to 6″ circle adapter, placed ½” above the floor. By going to a wide, shorter shape closer to the floor of the bell, exhaust gases drafted into the stove pipe will come from the coolest portion of the bell. By increasing the cross sectional area for this drafting, stove pipe velocity should remain strong.
We are hoping for an exhaust gas in the 170° – 225°F range. This design would then offer all the benefits of a traditional rocket mass heater, without the larger footprint requirement of benches or other types of mass.
Other points of interest
This was our 1st time to breakout our various data loggers and Gas Analyzers. So we had a fair bit of learning curve on them, including a number of lost data logs :<. So I do not have lovely logs to publish here. We look forward to providing detailed logs for the new design.
It was fun to see the heat profile of all the different parts in action. Here are some of the highlights.
Burn Tunnel Temperature
We placed 2 probes in the burn tunnel, one just after the tripwire and another located where the heat riser joins. The junction between the heat riser and the burn tunnel always had the highest numbers, by about 50-100°F.
Most of the burns had sub-optimal chimney arrangements, even still the temps at the burn tunnel typically ran 1500-1600°F, with the odd rush well into the 1700′s. It will be interesting to see if the new design drives these numbers higher.
Heat riser and Barrel
The top of the heat riser at full burn might be 1200°F or so with 800°F being loss to radiation from the barrel into the surrounding space.
The roof of the bell then should theoretically been in the 350F range, but instead was only around 250°F, and the floor maxed out at 160°F. The fire bricks heated up to 230°+ in the roof of the bell. Three hours after the fire was out this number was still 180°; the CMU wall stayed at 120°F.
The cast refractory barrel supports also absorbed a LOT of heat and stayed very warm for several hours.
We are building a traditional rocket heater style build, but with an single bell underneath the burn tunnel. This should provide a lot of heat capture without increasing the footprint of the stove. It is a bell heat capture and not a flue heat capture approach. (you can read more here about the differences between the two approaches)
The exhaust is identical to a standard J-tube heater shown here, except that instead of exiting the back of the heater, it continues to drop down to a “bell” (chamber) below the burner. Because gravity and pressures naturally sort out the hot from the cold gases only the colder gases leave.
The cold gases are exhausted to the chimney from a few inches above the floor in the middle of the bell. We used a piece of stove pipe with an elbow facing the floor. The entire bell chamber, except the floor, is lined with cast refractory or firebrick. Half size bricks are mortared to the CMU walls and 3″ full bricks cover the ceiling, where most of the heat is collected.
Between the fire brick and cement filled CMU bricks there is a lot of thermal mass for heat collection. We considered using 2 chambers underneath, but there is not that much room by the time the bell is lined with firebrick, so we opted for a single chamber.
I have included 3 2″x4″ steel tubes that run through the top of the bell area to both support the fire brick ceiling and provide an avenue for experimentation by block and opening the channels. The steel tubing is wrapped with 1/8″ ceramic fiber gasket material. A piece of expanded metal painted with high temperature paint is laid on top of the steel tubing.
Materials and Construction
Construction techniques are pretty similar to the YouTube video on the 4″ build. (Yes we will have a video on this build.) The combustion system is based on the 6″ Dragon Burner. We used the 6″ Barrel Support with Bell Chamber, and the 6″ Steel and Gasket Kit. Other materials included generic 4″ thick CMU blocks for the main construction, fireclay bricks for the bell lining, mortar, fire clay, and a 55 gallon drum.