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