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