A heat exchanger , comprising: one or more hot medium flow regions and one or more cooling medium flow regions ; and hot medium bars bordering the one or more hot medium flow regions and cooling medium bars bordering the one or more cooling medium flow regions , wherein at least one cooling medium bar of the cooling medium bars is joined to a pair of partition sheets , the at least one cooling medium bar comprising a base , a first leg , and a second leg , the first leg and the second leg each extending from the base , a cavity provided between the first leg and the second leg and in fluid communication with a first cooling medium flow region of the one or more cooling medium flow regions via an opening provided between the first leg and the second leg , wherein the first leg comprises a first portion , the first portion gradually decreasing in thickness toward the cooling medium flow region , and a second portion , the first portion being attached to the base on an end thereof and the second portion on an opposite end thereof, and the cavity is at least partially defined by an inner end portion adjoining a first wall portion and a second wall portion , the inner end portion having an inner end portion curved surface , the first wall portion having a first wall portion planar surface , and the second wall portion having a second wall portion planar surface , the first wall portion adjoining a third wall portion and the second wall portion adjoining a fourth wall portion , the third wall portion having a third wall portion curved surface and the fourth wall portion having a fourth wall portion curved surface , the third wall portion adjoining a fifth wall portion and the fourth wall portion adjoining a sixth wall portion , the fifth wall portion and the sixth wall portion at least partially defining the opening , characterised in that the second portion has a constant thickness. one or more hot medium flow regions and one or more cooling medium flow regions ; and hot medium bars bordering the one or more hot medium flow regions and cooling medium bars bordering the one or more cooling medium flow regions , wherein at least one cooling medium bar of the cooling medium bars is joined to a pair of partition sheets , the at least one cooling medium bar comprising a base , a first leg , and a second leg , the first leg and the second leg each extending from the base , a cavity provided between the first leg and the second leg and in fluid communication with a first cooling medium flow region of the one or more cooling medium flow regions via an opening provided between the first leg and the second leg , wherein the first leg comprises a first portion , the first portion gradually decreasing in thickness toward the cooling medium flow region , and a second portion , the first portion being attached to the base on an end thereof and the second portion on an opposite end thereof, and the cavity is at least partially defined by an inner end portion adjoining a first wall portion and a second wall portion , the inner end portion having an inner end portion curved surface , the first wall portion having a first wall portion planar surface , and the second wall portion having a second wall portion planar surface , the first wall portion adjoining a third wall portion and the second wall portion adjoining a fourth wall portion , the third wall portion having a third wall portion curved surface and the fourth wall portion having a fourth wall portion curved surface , the third wall portion adjoining a fifth wall portion and the fourth wall portion adjoining a sixth wall portion , the fifth wall portion and the sixth wall portion at least partially defining the opening , characterised in that the second portion has a constant thickness. <EOS>