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