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