{HEAD ~ The first tube sheet} is mounted in {TAIL ~ the primary inner shell} while the second tube sheet is connected to the secondary inner shell 
{ HEAD ~ The first tube sheet } is mounted in { TAIL ~ the primary inner shell } while the second tube sheet is connected to the secondary inner shell
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH