Heat exchanger, comprising: a plurality of heat exchange plaques and a plurality of plates stacked one by one on each other along a stacking direction (Y-Y) to define, in the space comprised between two adjacent plaques, coolant passage channels and hot passage channels to be cooled arranged perpendicularly to the stacking direction (Y-Y) and configured to be crossed by a coolant fluid and by a hot fluid to be cooled, respectively, wherein each plaque has a coolant fluid inlet opening (IC), a coolant fluid outlet opening (OC), a hot fluid inlet opening (IH) and a hot fluid outlet opening(OH) collimated with the respective openings of additional plaques to define coolant fluid inlet/outlet ducts of the (IC, OC) and hot fluid inlet/outlet ducts (IH, OH) for the fluids (C, H), wherein the coolant fluid inlet/outlet ducts (IC, OC) and the coolant fluid inlet openings (IC) and the coolant fluid outlet openings (OC) are in communication with the coolant passage channel, and wherein the hot fluid inlet/outlet ducts (IH, OH) and the hot fluid inlet openings (IH) and the coolant fluid outlet openings (OH) are in communication with the coolant passage channel, characterized by the fact that each plate is modular and comprises: a first lateral plate configured to be positioned at the point where the hot fluid inlet opening (IH) and the coolant fluid outlet opening (OC) are located, and a second lateral plate configured to be positioned at the point where the coolant fluid inlet opening (IC) and the hot fluid inlet opening (OH) are located and by the fact that said lateral plates (8, 9) are connected to each other by means of respective connecting bars (10, 11). a plurality of heat exchange plaques and a plurality of plates stacked one by one on each other along a stacking direction (Y-Y) to define, in the space comprised between two adjacent plaques, coolant passage channels and hot passage channels to be cooled arranged perpendicularly to the stacking direction (Y-Y) and configured to be crossed by a coolant fluid and by a hot fluid to be cooled, respectively, wherein each plaque has a coolant fluid inlet opening (IC), a coolant fluid outlet opening (OC), a hot fluid inlet opening (IH) and a hot fluid outlet opening(OH) collimated with the respective openings of additional plaques to define coolant fluid inlet/outlet ducts of the (IC, OC) and hot fluid inlet/outlet ducts (IH, OH) for the fluids (C, H), wherein the coolant fluid inlet/outlet ducts (IC, OC) and the coolant fluid inlet openings (IC) and the coolant fluid outlet openings (OC) are in communication with the coolant passage channel, and wherein the hot fluid inlet/outlet ducts (IH, OH) and the hot fluid inlet openings (IH) and the coolant fluid outlet openings (OH) are in communication with the coolant passage channel, characterized by the fact that each plate is modular and comprises: a first lateral plate configured to be positioned at the point where the hot fluid inlet opening (IH) and the coolant fluid outlet opening (OC) are located, and a second lateral plate configured to be positioned at the point where the coolant fluid inlet opening (IC) and the hot fluid inlet opening (OH) are located and by the fact that said lateral plates (8, 9) are connected to each other by means of respective connecting bars (10, 11). a first lateral plate configured to be positioned at the point where the hot fluid inlet opening (IH) and the coolant fluid outlet opening (OC) are located, and a second lateral plate configured to be positioned at the point where the coolant fluid inlet opening (IC) and the hot fluid inlet opening (OH) are located and by the fact that said lateral plates (8, 9) are connected to each other by means of respective connecting bars (10, 11).