In some heat exchangers, particularly large steam surface condensers, it is desirable to make the tubes from a material such as titanium. One reason that titanium is desirable as the material for the tubes is its high resistance to the corrosive effects of saltwater. Heretofore, it has been conventional to roller expand the ends of the tubes into the tube sheets supporting the tubes. Roller expanding titanium tubes is an inexact art, due to the high ductility of the material, and the prevention of leakage from the shell side is difficult to guarantee. An effective seal is attained only when the tubes are welded to the tube sheet. If the tubes are of titanium, then, since titanium can only be welded to itself, it follows that the tube sheet should also be made of titanium.
In a large heat exchanger such as a steam condenser, the tube sheet is about 25 to 50 millimeters thick and the area of its major faces is about 150 to 250 square feet. There is currently no known domestic source of supply for a tube sheet of titanium and having such dimensions. Even if there was a source of supply, the cost would be prohibitive. Further, due to the high degree of cleanliness required, it would be impractical to obtain reliable welds between the tubes and the tube sheet of a large heat exchanger.
If one attempts to expand titanium tubes into a tube sheet made from another material, there is a high likelihood that the difference in the material in cooperation with the liquid flowing through the tubes will create a battery action. In such battery action, the material from the tube sheet plates onto the inner surface of the tubes, eroding the tube sheet, and creating a passageway for inleakage. After a period of time, it often becomes necessary to repair, plug, or retube the heat exchanger.
The present invention is directed to a solution of the problem of how to structurally interrelate a tube sheet and tubes in a heat exchanger wherein the tubes are made from titanium or an alloy having similar corrosion resistance in a manner which avoids a battery action, and results in reliable welds capable of being monitored.