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.