Double spiral devices have been employed as combustors for exceedingly lean mixtures (for example, for methane, mixtures of fuel content corresponding to less than 1/5 of the lower limit of flammability have been successfully burned), and the leanest mixture combustible is determined by losses. For any given burner an optimum flow exists as at low flows, losses are critical, and at high flows there is inadequate area for heat transfer (also at high flows, large pressure losses occur). Mathematical modeling of the spiral device demonstrates that radiation is significant even at modest elevations in temperature and that spirals of many turns help control these losses. With metal burners, conduction losses can be critical. Several parameters are presented to identify design limits for spirals including wall emissivity, conductivity, and thickness.