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Aprovecho Justa Stove

 
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In the "rocket heater book" as well as here on the forums and elsewhere on the internet it is stressed heavily that the cross sectional area of the burn tunnel be the tightest place anywhere in the system otherwise you cause a bottleneck and inefficient burn/lack of draft etc.

I am wondering if anyone is familiar with the Aprovecho "Justa Stove" design. They utilize a 5" diameter rocket (which also has a height well below what is recommended in the previously mentioned places) 5" round burn tunnel is roughly a 20" cross section. They then force the flue gasses/flame path through a 1" gap under a 12" wide cook surface or in other words... a 12" cross sectional area. So why do they not create a huge bottleneck when doing this? what am I missing?? This seems to be a proven design with much research done.

Justa Stove Plan

By the way I ask because I am working on tweaking the performance of the 3rd generation of my rocket powered maple syrup evaporator which is quite similar in design...

 
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A rocket stove has far less back pressure to overcome, compared to a rocket mass heater (RMH). The RMH has many feet of ducting running through the mass, causing resistance to air flow. I would expect that a simpler rocket stove can overcome this one restriction because there's so little beyond that to also overcome.
 
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