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