The adjacent ends of {TAIL ~ the inner shells} form a disconnected joint and the secondary inner shell is free to move relative to the primary inner shell upon thermal expansion of {HEAD ~ the tubes} 
The adjacent ends of { TAIL ~ the inner shells } form a disconnected joint and the secondary inner shell is free to move relative to the primary inner shell upon thermal expansion of { HEAD ~ the tubes }
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD