To provide a high performance heat exchanger having a long life.

(a) A shell is provided with an inlet tube sheet unit and an outlet tube sheet unit while the respective tube sheets are provided with a plurality of holes and the shell is provided with at least one inlet port and one outlet port for a heat exchanging medium. (b) A plurality of tubes are arranged in the shell and the inlet port end of respective tubes are attached to the inlet port tube sheet while the outlet port and of the respective tubes are attached to the outlet port tube sheet so that the axes of the tubes coincide with the axes of the holes of inlet and outlet tube sheets 4, 44. (c) A plurality of the ferrules 40, 42 are provided with an inlet end and an outlet end and the ferrule reaches the inside of the tube through the hole of the inlet tube sheet while the outlet end is positioned below the inlet tube sheet and the ferrule contains silicon nitride. A plurality of the ferrules are used in the reducing, carbonizing and/or nitriding atmospheres.