Well, I am not familiar with a Z;duct persay, but I could imagine that this is analogous to a duct with two bends with no straight section in between them. THe reason this is different than a duct with two bends with a straight section inbetween is that the losses are much, ,much greater. After a bend, the the flow recovers some of the pressure loss, if there is no straight length, the separation is high off of all the corners and bends, and the recovery is low. Lokk up the SAE (soicety of aerospace engineers) losses for a duct with two bends.