a heat exchanger is provided comprising a stack of heat exchanger plates formed of sheet metal having a three - dimensional structured pattern , each heat exchanger plate having a groove , a gasket being arranged in said groove and resting against an adjacent heat exchanger plate , said groove having a bottom inner surface , said inner surface bottom having at least a protrusion directed to said adjacent heat exchanger plate . it is intended to minimize the risk of a leakage . to this end in the region of said protrusion said gasket is compressed more than in a region out of said protrusion .