Present invention relates to a channel system for improving the relation between pressure drop and heat, moisture and/or mass transfer of fluids flowing through said system, said channel system comprising at least one channel comprising at least a first and a second flow director, said channel having a cross-section area A and a first and a second cross-section area A1, A2 at respective flow director, said flow directors extending in a fluid flow direction and transversely to said channel, and comprising an upstream portion, deviating, in said fluid flow direction, from a channel wall of said channel inwardly into said channel, a downstream portion returning, in said fluid flow direction, towards said channel wall, and an intermediate portion located between said upstream and downstream portions, wherein said first cross-section area A1 at said first flow director is smaller than said second cross-section area A2 at said second flow director.