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