Conventionally, plate type heat exchangers have been proposed in which a plurality of heat transfer plates are stacked between two side plates with predetermined intervals in between so that a first flow path through which a first fluid circulates and a second flow path through which a second fluid circulates are alternatively disposed in a space formed between the heat transfer plates. Further, for such conventional plate type heat exchangers, plate type heat exchangers have also been proposed in which inner fins are disposed in the flow paths in order to improve heat transfer performance, for example as described in the description “in the plate type heat exchanger in which a plurality of heat transfer plates 1, 1 . . . are stacked to form first flow paths 2, 2 . . . and second flow paths 3, 3 . . . each adjacent to the heat transfer plates 1, 1 and allow for heat exchange between the first fluid X and the second fluid Y which circulate in the first flow paths 2, 2 . . . and the second flow paths 3, 3 . . . , respectively, inner fins 4, 4 . . . that improve heat transfer and increase heat transfer surface area are disposed in the first flow paths 2, 2 . . . and the second flow paths 3, 3 . . . so that heat transfer between the heat transfer plates 1, 1 which form the flow paths 2, 3 is improved and the heat transfer surface area is also increased due to the interposed inner fins 4, 4 having high freedom of design.” (See Patent Literature 1.)
Further, for the conventional plate type heat exchangers which include the inner fins disposed in the flow paths, plate type heat exchangers have been proposed, for example as described in the description “in the core section 1 of the oil cooler, the oil flow paths 7 and the cooling water flow paths 8 are alternatively formed between the core plates 5, 6 by alternatively stacking a plurality of first core plates 5 and second core plates 6 having the essentially same shape. The fin plates 11 are disposed in each of the oil flow paths 7. The first projections 31 and the second projections 32 are disposed on the first core plate 5 and the second core plate 6, respectively, so as to project outward from the oil flow path 7 and to be located alternatively with respect to the flow of oil.” (See Patent Literature 2.)
Further, for the conventional heat exchangers which include the inner fins disposed in the flow paths, heat exchangers which include two heat transfer plates formed as one flat tube have also been proposed, for example as described in the description “a plurality of inclined grooves 7 for flowing condensate water are formed on the flat surface 1a of the flat tube 1 so as to be inclined to the longitudinal direction with the downstream end reaching a curved portion 1b of the tube, and the projection 7a is formed on the outer surface of the flat tube 1. Then, the inner fin 6 is inserted into the flat tube 1.” (See Patent Literature 3.)