Plate type heat exchanger (20,120) for a isothermal chemical reactor (60), of the type comprising a substantially box-shaped flattened body (22), defining an internal chamber (24) and equipped with an inlet connection (28) and an outlet connection (29) for a first flow of a heat exchanger operative fluid into and from said chamber (24), at least one supplying-distributing device (26, 126) of a second fluid flow, associated with said body (22) and in fluid communication with the internal chamber (24) thereof, in order to regulate the temperature of said operative fluid, said at least one supplying-distributing device (26,126) being supported inside said internal chamber (24) and in fluid communication therewith.