{HEAD ~ One or more outlet apertures} are formed between {TAIL ~ the two inner shells} for passage of the gaseous fluid out of the heat exchanging chamber 
are formed between
{ HEAD ~ One or more outlet apertures } are formed between { TAIL ~ the two inner shells } for passage of the gaseous fluid out of the heat exchanging chamber
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH