{HEAD ~ The adjacent ends} of 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 
{ HEAD ~ The adjacent ends } of 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
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL 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