{TAIL ~ The tubes} form passages for flow of the hot gaseous mixture in {HEAD ~ heat exchange contact} with the gaseous fluid through the heat exchanging chamber, thereby preheating the gaseous fluid prior to reaction with the fuel 
{ TAIL ~ The tubes } form passages for flow of the hot gaseous mixture in { HEAD ~ heat exchange contact } with the gaseous fluid through the heat exchanging chamber , thereby preheating the gaseous fluid prior to reaction with the fuel
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