A plate heat exchanger, with first and second heat exchanger plates (2a, 2b) arranged next to / on top of each other, which form separate flow channels (3, 6) for a first gaseous fluid and for a second gaseous fluid and through which thermal energy can be exchanged between the two fluids, wherein the dimensions of the heat exchanger plates (2a, 2b) can be designed so as to closely or exactly correspond to the dimensions of the construction space provided in total for the plate heat exchanger, wherein the flow channels for the first fluid and the flow channels for the second fluid are designed so as to alternate between the sequentially arranged first and second heat exchanger plates (2a, 2b) of the plate heat exchanger designed as a cross-flow or counter-flow heat exchanger, wherein each first heat exchanger plate comprises a collar-free outer edge portion forming the entry region of the first fluid, a collar-free outer edge portion forming the exit region of the first fluid, an outer edge portion provided with a collar and assigned to the entry region of the second fluid, an outer edge portion provided with a collar and assigned to the exit region of the second fluid, and outer edge portions (25, 26, 27, 28) between the collar-free outer edge portion forming the entry region of the first fluid on the one hand and the outer edge portion assigned to the entry region of the second fluid on the other hand, between the outer edge portion assigned to the entry region of the second fluid on the one hand and the collar-free outer edge portion forming the exit region of the first fluid on the other hand, between the collar-free outer edge portion forming the exit region of the first fluid on the one hand and the outer edge portion assigned to the exit region of the second fluid on the other hand and between the outer edge portion assigned to the exit region of the second fluid on the one hand and the collar-free outer edge portion forming the entry region of the first fluid on the other hand, wherein each second heat exchanger plate comprises a collar-free outer edge portion forming the entry region of the second fluid, a collar-free outer edge portion forming the exit region of the second fluid, an outer edge portion forming the exit region of the second fluid, an outer edge portion provided with a collar and assigned to the entry region of the first fluid, an outer edge portion provided with a collar and assigned to the exit region of the first fluid, an outer edge portion provided with a collar and assigned to the exit region of the first fluid, and outer edge portions (33, 34, 35, 36) between the collar-free outer edge portion forming the entry region of the second fluid on the one hand and the outer edge portion assigned to the entry region of the first fluid on the other hand, between the outer edge portion assigned to the entry region of the first fluid on the one hand and the collar-free outer edge portion forming the exit region of the second fluid on the other hand, between the collar-free outer edge portion forming the exit region of the second fluid on the one hand and the outer edge portion of the first fluid and the outer edge portion assigned to the exit region of the first fluid, and between the outer edge portion assigned to the exit region of the first fluid on the one hand and the collar-free outer edge portion forming the entry region of the second fluid on the other hand, wherein the outer edge portions (25, 33; 26, 36; 27, 35; 28, 34) assigned to each other and arranged between the entry regions (4, 5) and the exit regions (7, 8) of adjacent first and second heat exchanger plates (2a, 2b) are connected with each other in a fluid-tight manner, wherein each first heat exchanger plate is connected in a fluid-tight manner at their collars (37, 38) to the assigned outer edge portion (29, 30) of the adjacent second heat exchanger plate on the collar side, and wherein each second heat exchanger plate is connected in a fluid-tight manner at their collars (39, 40) to the assigned outer edge portions (21, 22) of the adjacent first heat exchanger plate on the collar side, characterised in that the heat exchanger plates (2a, 2b), at their outer edge portions (25, 33; 26, 36; 27, 35; 28, 34) assigned to each other, comprise projections (41, 42) offset relative to each other, by means of which the heat exchanger plates (2a, 2b) are designed so as to interlock at their outer edge portions (25, 33; 26, 36; 27, 35; 28, 34) assigned to each other and in that the heat exchanger plates (2a, 2b) are welded to each other except at their outer edge portions forming the entry regions and exit regions (4, 5, 7, 8).