The primary gas flow is subjected to {HEAD ~ a final treatment stage} similar to {TAIL ~ the initial heat exchange treatment} 
The primary gas flow is subjected to { HEAD ~ a final treatment stage } similar to { TAIL ~ the initial heat exchange treatment }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL