A heat exchanger, such as a flooded evaporator, comprises a shell extending along a longitudinal axis (X), an inlet pipe and an outlet pipe, through which respectively enters (F1) and exits (F2) a refrigerant flow, and a bundle of pipes crossing the shell along the longitudinal axis (X), and comprising a refrigerant flow diffuser provided inside the shell downstream the inlet pipe, the refrigerant flow diffuser extending along the longitudinal axis (X) and comprising openings through which the refrigerant flows. The refrigerant flow diffuser comprises a moving element and a stationary element, the moving element being movable with respect to the stationary element under action of a pressure force (FP) exerted by the refrigerant flow so that the refrigerant flow going through the openings is adjusted and a differential refrigerant pressure between refrigerant pressure downstream (P2) and upstream (P1) the refrigerant flow diffuser is kept constant.