Heat exchanger

A heat exchanger suitable for placing first and second fluids in heat exchange relationship with each other comprising two arrays of header tubes, which arrays are spaced apart from each other, a plurality of heat exchange tubes each header tube in one of said arrays being in flow communication with a respective header tube in the other of said arrays via said plurality of heat exchange tubes, said heat exchange tubes being so configured and disposed as to be other than normal to the operational flow of the first of said fluids thereover, the header tubes of one of said arrays being in flow communication with a first manifold, some of the header tubes in the other of said arrays being in flow communication with a second manifold and the remainder of the header tubes in said other array being in flow communication with a third manifold, pipe and valve means being provided and so arranged that in a first mode of operation said second fluid is supplied to said first manifold only, so as to flow to both of said second and third manifolds via said header and heat exchange pipes and be subsequently exhausted from said second and third manifolds, and in a second mode of operation said second fluid is supplied to said third manifold only, so that said second fluid flows from said third manifold to said second manifold via said first manifold and said header and heat exchange pipes and is subsequently exhausted from said second manifold so that said second fluid is in operation sequentially placed in parallel flow and then contra-flow heat exchange relationship with said second fluid by its passage through said heat exchange pipes.