In another preferred embodiment, a forced draft blower is placed on behind the burner to force {HEAD ~ air} into {TAIL ~ the tube bundle} 
In another preferred embodiment , a forced draft blower is placed on behind the burner to force { HEAD ~ air } into { TAIL ~ the tube bundle }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL