The plug box header consists of tube sheet, plug sheet, top, bottom, and end plates welded together to form the header. Holes are drilled and grooved in the tube sheet so that the tubes can be rolled in place. Holes are drilled and threaded in the plug sheet opposite each tube to allow access to the tubes. Partitions are welded in the headers to establish the tube-side flow pattern, which generates suitable velocities in as near countercurrent flow as possible for maximum mean temperature difference. Partitions and stiffeners (partitions with flow openings) also act as structural stays. Horizontally split headers may be required to accommodate differential tube expansion in services having high fluid temperature differences per pass.
Bolted removable cover plates are used for improved access to headers in severe fouling services or where complete access to the inside of the header boxed is desired. The removable cover header is constructed similarly to the plug box header except that a flanged and gasketed solid cover plate replaces the plug sheet. Cover plate headers are limited to approximately 400 psi in design pressure.
Pipe and U-bend headers are utilized for very high pressure applications. These designs are more expensive and difficult to access the tubes, but necessary for design pressures above approximately 5000 psi.
A rolled tube-to-tubesheet joint is standard but seal-welded and strength-welded construction is also available.