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