a heat exchanger for cooling turbine components of a gas turbofan engine in which air to be cooled is directed along a first annular , radially extending path away from the engine and cooled by bypass air . the cooled air is returned to the engine along a second radially - extending path concentric to the first path . in a preferred embodiment , the heat exchanger is a standpipe extending from the compressor outlet to the bypass air duct and includes inner and outer tubular members oriented concentrically to form inner and outer counterflowing air flow paths which also are concentrically arranged . the outer member is mounted on the combustor casing and the inner member on a diffuser within the casing . accordingly , no mechanical connection exists between the inner and outer members .