In most heat exchangers it is understood that the two fluids concerned shall be maintained in discrete streams; any failures, such as a tube leak, will mean that the streams will mix to greater or less extent. In many cases, such mixing can occur with little adverse effect, and any repair done need only be of such integrity as to keep any contamination within reasonable and defined limits. There are, however, the other cases where separation of the two streams must be absolute; for example, in nuclear work, one stream may contain radio-active elements which cannot be allowed to leak, again where a mixture would produce a potentially hazardous situation, or yet again, where the mixture is considerably more corrosive than either of the two separate components. It was through an instance of the last mentioned that we came to develop the system we are about to describe which was to give a simple, speedy and predictable method of plugging tubes to the high degree of integrity required.
