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