All these products are gases, which are hot as a result of participating in the chemical action of rapid oxidation (fire). Since these gases are hot, they rise. (Remember high school science?) Given the opportunity, the gases will just rise and dissipate into the atmosphere, carrying a LOT of heat away with them.
A JUCA doesn't allow this! A sophisticated arrangement of tapering passageways is a Heat Exchange area, where the hot gases and smoke are forced to pass very near a number of metal surfaces, in order to give up some of the gases' heat to the metal by heating it.
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