Results on chaining a 5” batch box and two bells:

[fiedia]

I tried to chain two bells to heat two rooms with one 125mm (5”) batch rocket. The first bell housing firebox and heat riser is located in the kitchen (35 m²). The second bell is located in the living room (45 m² + stairway).

Detailed description is available here

Last year, the second bell was heated with dirty smokes from a standard stove. Incoming smoke temperature reached 750 °C max. Bell outside wall temperature reaced 80°C. The living room temperature was then around 16°C. This year, I replaced my dirty stove with a 5” batch rocket.
• First set up:
◦ Bell2 with 2 skins
◦ SIA : bell1 = 1.45m²; bell2 = 1.8m²; pipe = 0.3m²; total = 3.55 m²
◦ Smoke peak temperatures: bell2 in = 180 °C; bell2 out = 100 to 110°C
◦ Bell2 outside wall: 40 °C max

Bell2 exhaust temperature is quite low. Bell2 outside walls do not warm enough.


I tried to get better heating by removing bell2 second skin.

• Second set up:
◦ Bell2 with 1 skin
◦ SIA : bell1 = 1.45m²; bell2 = 1.8m²; pipe = 0.3m²; total = 3.55 m²
◦ Smoke peak temperatures: bell1 out = 140 to 210 °C; bell2 out = 50 to 80°C
◦ Stored heat: bell1: 68%; bell2: 17% (of the heat flowing through the heat riser)
◦ Bell2 outside wall: 50°C max
Bell2 wall temperature is better but still insufficient. Bell2 exhaust temperature is now definitely too low.




in order to improve exit smoke temperatures, I reduced the bell2 SIA by shortening its internal exit pipe.

• Third set up:
◦ SIA : bell1 = 1.45m²; bell2 = 0.9m²; pipe = 0.3m²; total = 2.65 m²
◦ Smoke peak temperatures: bell1 out = 190 to 220 °C; bell2 out = 100 to 110°C
◦ Stored heat: bell1: 66%; bell2: 14% (of the heat flowing through the heat riser)
◦ Bell2 outside wall: 50 °C max
Bell2 exit temperature is much better though still too low. Bell2 wall temperature and stored power are roughly the same (not good enough).

• Conclusion: In addition to the flue temperature problem, chaining the bells results in 75% heat in the kitchen (bell1) and 25% heat in the living room (bell2). I tried to build a first bell as small as possible according to cross section design rules (bell vs HR ratio). Therefore, bell1 can not be drastically downsized. The second bell will never get much more heat from the first one. Chaining does not look like a good solution for balanced heat return between two bells.

I thought the chimney draught would drive heat and hot smokes from one bell to the other as in standard heat exchangers. Looking at the temperature profiles, density seems to be the main driver. Hot smokes stay on top of bell1. Smokes arriving in bell2 have been cooled to much.

In order to get a better heat balance between both bells, I will join bells with pipes both on top and bottom in my next design. Hot smokes should fill up both bell tops and cooler smoke leave bells through bottom pipes.

[peterberg]

Theoretically, balancing the two bells with top and bottom pipes would even out the temperature of both. However, the first bell, containing the hot mass of the combustion core will always be hotter, just because of that fact. You aren't saying anything about whether or not the kitchen heating is satisfying. In case it is just right, a bigger heater would be in order to satisfy the heating demand in the living room.

May I suggest another solution just a little bit different from what you have in mind? This is in case the kitchen temperature has to come down. Don't balance the bells, the kitchen mass, being larger and hotter will always win. Instead, mount a bigger pipe at the top of the first bell that is feeding the second. All of the hottest gases coming out of the riser are channeled to the second bell right away. Also, mount another pipe at the bottom from the second bell to the kitchen. Not to connect to the first bell but to the chimney directly, hopefully making one 90 degree elbow redundant. Now the majority of the heating power is led to the second bell while the kitchen is still equipped with the hot combustion core.

Taking this idea a little bit further, the first bell could be narrower, its walls against the core. Only above the core there should be a (smallish) bell void in order to aid the gases in rounding the 90 degree bend to the second bell's feeding pipe. At the same time the second bell could be larger as well. Beware of this feeding pipe, it will get very hot very quickly! Maybe a tunnel of (insulated) refractory bricks would solve that problem.

In case the kitchen temperature was already good, you could do the same as above but scaling up the core as well.

[fiedia]

Thank you Peterberg for all these good advises. I will definitely have a bigger flue diameter in the next design (200mm). I also need to check different options (heat riser in the living room, i.e. first bell there). I have to take into account your new ideas to manage heat flow by reducing the first bell cross section. I have still some work to do for the next design