Smaller fires are often cleaner burning compared to larger fires, and higher heat transfer efficiencies allow the use of smaller fires while the time to boil stays acceptable. A pot skirt which creates a narrow channel gap reduces fuel use and time to boil.
Dr. Sam Baldwin recommended 10mm to 11mm channel gaps for household cook stoves at normal high firepowers (4kW to 5kW).
ARC uses tighter channel gaps at 5kW and we do not seem to experience problems. ARC recommends 6mm channel gaps in pot skirts for 20cm in diameter (or larger) pots.
It is interesting to note that Dr. Baldwin decreased the channel gap to 8mm with bigger institutional sized pots. We just tested a 6mm gap in a skirt surrounding a 40cm in diameter (60 liter) pot. The result, with a small open fire, was around 48% thermal efficiency.
We recommend a 6mm channel gap to achieve close to 50% thermal efficiency with a large range of pots..
The tight channel gap seems to be OK, so far. It would be great to model velocity, temperature drop, etc. as the channel gap changes! We will do it one of these days…