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