In {HEAD ~ many cases}, existing ovens can be used as {TAIL ~ the heating source}, without modification 
In { HEAD ~ many cases } , existing ovens can be used as { TAIL ~ the heating source } , without modification
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH